The Design of a Rainwater Harvesting System for Pabal, India

The Design of a Rainwater Harvesting System for

Pabal, India

Kieran James Cooke

MEng Civil Engineering

School of Civil Engineering and Geosciences

Newcastle University

June 2009

Admin

Stamp

-i-

Executive Summary The feasibility of rainwater harvesting (RWH) has been assessed for the Pabal district of

Maharashtra, India. It has been found that the most suitable system would be to implement

RWH on all domestic houses, with the rainwater satisfying the potable water demand of the

population of 3 000. This provision of water would reduce the current water shortages that have

been quantified through estimating the existing water supply and demand. These water

shortages have been found to be particularly large during Pabal’s six month dry season. A

simple system that utilises local skills and materials has been designed; consisting of metal

gutters and a concrete storage tank reinforced with bamboo. A Biosand Filter has also been

included in the design and has been estimated to be likely to improve the water quality to the

levels specified in World Health Organisation Drinking-water Guidelines. Likely changes in

precipitation and demand for water over the system’s ten year design life have been factored

into the design. The economic benefits of the RWH system, in terms of water quality and water

quantity, have been quantified and it has been shown that the proposed system is likely to be

economically viable.

-ii-

Table of Contents

1 Introduction...............................................................................................................1

1.1 Outline of project ..............................................................................................1

1.2 Background to the project .................................................................................1

1.3 Description of organisations involved in project ..............................................1

1.4 Description of Pabal..........................................................................................2

1.4.1 Current water supply situation ..................................................................2

1.4.2 Current water quality.................................................................................4

1.5 The scope and limitations..................................................................................4

2 Aims & Objectives....................................................................................................5

2.1 Aim....................................................................................................................5

2.2 Objectives..........................................................................................................5

3 Literature Review......................................................................................................6

3.1 Introduction to review.......................................................................................6

3.2 Catchment .........................................................................................................6

3.3 Conveyance.......................................................................................................7

3.4 Storage...............................................................................................................8

3.4.1 Sizing of the tank ......................................................................................8

3.4.2 Materials used .........................................................................................10

3.4.3 Comparison of above ground and underground tanks ............................10

3.4.4 Tank Components ...................................................................................11

3.5 Distribution .....................................................................................................12

3.6 Health implications of RWH...........................................................................13

3.6.1 Water quality...........................................................................................13

3.6.2 Insect vectors...........................................................................................14

3.7 Methods of improvement of water quality......................................................15

3.7.1 First flush system ....................................................................................15

3.7.2 Coarse filters ...........................................................................................16

3.7.3 Settlement in tanks ..................................................................................17

3.7.4 Treatment options....................................................................................17

3.8 Design processes .............................................................................................19

3.8.1 Available runoff ......................................................................................19

3.8.2 Calculating the demand for water ...........................................................20

-iii-

3.8.3 Required capacity of storage tank ...........................................................20

3.9 Community and management issues...............................................................21

3.10 Economics of RWH ........................................................................................21

3.11 Limitations and constraints of DRWH............................................................22

4 Proposed Methods Statement..................................................................................24

4.1 Scope of the work............................................................................................24

4.2 Programme of work.........................................................................................24

4.3 Required data ..................................................................................................24

4.3.1 Sociological.............................................................................................24

4.3.2 Economic.................................................................................................24

4.3.3 Hydrological............................................................................................25

4.3.4 Spatial......................................................................................................25

4.3.5 Water usage.............................................................................................25

4.3.6 Catchment data........................................................................................26

4.4 Design procedure ............................................................................................26

4.4.1 Calculation of demand ............................................................................26

4.4.2 Calculation of available runoff ...............................................................27

4.4.3 Tank.........................................................................................................28

4.4.4 Design of the conveyance system ...........................................................29

4.4.5 Water quality...........................................................................................30

4.5 Cost of system.................................................................................................31

4.6 Presentation of design .....................................................................................31

4.6.1 Design Report .........................................................................................31

4.6.2 Drawings .................................................................................................31

4.7 Contact with outside organisations .................................................................31

5 Method Statement ...................................................................................................32

5.1 Scope of design ...............................................................................................32

5.2 Details of buildings .........................................................................................32

5.2.1 Domestic houses .....................................................................................32

5.2.2 Non-domestic buildings ..........................................................................33

5.3 Meteorological conditions...............................................................................34

5.3.1 Current rainfall data ................................................................................34

5.3.2 Simulation of future climate ...................................................................35

5.4 Determination of the demand to be met..........................................................36

-iv-

5.4.1 Current demand.......................................................................................36

5.4.2 Future demand.........................................................................................37

5.5 Determination of the most appropriate RWH system.....................................37

5.6 Design of the storage tank...............................................................................38

5.7 Design of conveyance system.........................................................................39

5.7.1 Design rain storm ....................................................................................39

5.7.2 Gutter detailing........................................................................................40

5.8 Water Quality..................................................................................................41

5.8.1 Estimation of water quality parameters...................................................41

5.8.2 Calculation of water quality improvements due to storage of water ......41

5.9 Suitable materials ............................................................................................42

5.10 Equipment available locally............................................................................42

5.11 Geotechnical analysis......................................................................................42

5.12 Economic analysis...........................................................................................42

5.12.1 Valuation of water...................................................................................43

6 Design .....................................................................................................................45

6.1 Determination of most appropriate RWH system...........................................45

6.1.1 Current supply-demand balance..............................................................45

6.1.2 Annual rainwater harvesting potential of different options ....................46

6.2 Storage tank.....................................................................................................47

6.2.1 Sizing of storage tank..............................................................................47

6.2.2 Tank detailing..........................................................................................48

6.3 Conveyance system.........................................................................................53

6.3.1 Outline of conveyance layout..................................................................53

6.3.2 Design of roof gutters..............................................................................54

6.3.3 Transfer from roof gutters to storage tank ..............................................56

6.4 Water quality...................................................................................................57

6.4.1 Removal of debris ...................................................................................57

6.4.2 Water quality improvements due to storage............................................59

6.4.3 Filtration..................................................................................................59

6.5 Distribution .....................................................................................................62

6.6 Maintenance instructions for the villagers ......................................................63

6.6.1 Conveyance system.................................................................................63

6.6.2 Storage tank.............................................................................................63

-v-

6.6.3 Biosand filter...........................................................................................63

7 Discussion of the design .........................................................................................65

7.1 Assessment of the effectiveness of proposed water treatment........................65

7.2 Economic appraisal of RWH system ..............................................................66

7.2.1 Benefits of RWH.....................................................................................66

7.2.2 Costs of RWH .........................................................................................67

7.2.3 Calculation of payback period ................................................................68

8 Conclusions.............................................................................................................69

8.1 Overview of the design ...................................................................................69

8.2 Limitations of the design.................................................................................70

8.3 Recommendations for further work ................................................................70

9 References ...............................................................................................................72

Appendix A: Current water supply infrastructure...........................................................78

Appendix B: Meteorological Data ..................................................................................79

Appendix C: Raw water quality data ..............................................................................80

C1: Chemical water quality of rainfall........................................................................80

C2: Biological and microbiological water quality parameters of rainfall...................80

C3: Changes in water quality parameters due to roofing............................................81

Appendix D: Calculated runoff for different RWH options ...........................................82

Appendix E: Sizing of the storage tank for the current scenario ....................................83

Appendix F: Calculations................................................................................................85

Appendix G: Cost and benefits of RWH ........................................................................91

Appendix H: Bill of Quantities .......................................................................................92

-vi-

List of Figures Figure 1.1: Location of Pabal........................................................................................................ 2

Figure 1.2: Aerial image of dam near Pabal ................................................................................. 3

Figure 1.3: Dam near Pabal .......................................................................................................... 3

Figure 3.1: Process diagram for a rainwater harvesting system.................................................... 6

Figure 3.2: Galvanised steel V guttering ..................................................................................... 8

Figure 3.3: Benefits of tank sizing................................................................................................ 9

Figure 3.4: DRWH underground tank in South Africa............................................................... 11

Figure 3.5: Marley rainfall leaf slide .......................................................................................... 17

Figure 3.6: Slow sand filter......................................................................................................... 18

Figure 3.7: Rainfall intensity, cumulative rainfall availability and demand from a RWH scheme

in Bangladesh.............................................................................................................................. 20

Figure 4.1: The dimensions of the catchment area ..................................................................... 27

Figure 4.2: Comparison of the harvestable water and the demand for each month (for a site in

Biharamulo District, Kagera, Tanzania) ..................................................................................... 28

Figure 4.3 Predicted cumulative inflow and outflow from the tank (for a site in Biharamulo

District, Kagera, Tanzania) ......................................................................................................... 29

Figure 5.1: Ceramic tiled roof (background) and corrugated steel roof (foreground) ................ 32

Figure 5.2: Dimensions of a typical house in Pabal.................................................................... 33

Figure 5.3: Average monthly precipitation for Pune .................................................................. 34

Figure 5.4: Comparison of observed monthly precipitation and estimated monthly precipitation

for 2020....................................................................................................................................... 36

Figure 6.1: Monthly runoff from Roof 1 and monthly potable water demand for 2020 scenario47

Figure 6.2: Monthly runoff from Roof 2 and monthly potable water demand for 2020 scenario47

Figure 6.3: Predicted inflow and outflow for Tank 1 for 2020 scenario..................................... 48

Figure 6.4: Predicted inflow and outflow for 2 for 2020 scenario.............................................. 48

Figure 6.5: Assumed position of water storage tank................................................................... 49

Figure 6.6: Plastic lined bamboo tank......................................................................................... 49

Figure 6.7: Bamboo reinforced concrete tank............................................................................. 50

Figure 6.8: Reinforcing details for bamboo-concrete water tank................................................ 51

Figure 6.9: Elevation and plan view of Tank 1........................................................................... 51

Figure 6.10: Elevation and plan view of Tank 2......................................................................... 52

Figure 6.11: Corrugated steel cover............................................................................................ 52

Figure 6.12: Direction of runoff from roofs................................................................................ 53

Figure 6.13: Effect of sloping gutter on distance between roof and gutter................................. 53

-vii-

Figure 6.14: Arrangement of guttering and downpipe................................................................ 54

Figure 6.15: Roof gutter dimensions for Roof 1......................................................................... 55

Figure 6.16: Roof gutter dimensions for Roof 2......................................................................... 55

Figure 6.17: Runoff patterns from clay and corrugated metal roofs........................................... 55

Figure 6.18: Guttering attachments for Roof 1 and 2................................................................. 56

Figure 6.19: Gutters to convey water from roof gutters to storage tank ..................................... 57

Figure 6.20: Inlet of gutters into tank ......................................................................................... 57

Figure 6.21: Coarse filter on tank cover ..................................................................................... 58

Figure 6.22: Principle of intermittent-use slow sand filter ......................................................... 60

Figure 6.23: Plan and cross section of slow sand filter............................................................... 61

Figure 7.1: Comparison for cumulative annual costs and benefits for proposed domestic RWH

system for Roof 1........................................................................................................................ 68

Figure 7.2: Comparison for cumulative annual costs and benefits for proposed domestic RWH

system for Roof 2........................................................................................................................ 68

-viii-

List of Tables

Table 3.1: Possible materials for storage tank and corresponding capacities .................10

Table 3.2: Methods for the prevention of mosquitoes in RWH......................................15

Table 3.3: Locations for coarse filters.............................................................................16

Table 3.4: Drainage Coefficients ....................................................................................19

Table 3.5: Runoff coefficients.........................................................................................19

Table 3.6: Unit cost of different types of tanks...............................................................21

Table 4.1: Unit costs to be used for storage tank ............................................................25

Table 4.2: Runoff coefficients.........................................................................................27

Table 4.3: Spreadsheet to be used to determine storage .................................................29

Table 4.4: Typical ks values ............................................................................................30

Table 5.1: Details of non-domestic buildings .................................................................34

Table 5.2: Projections for changes in precipitation for South Asia sub-region for the

period 2010-2039 ............................................................................................................35

Table 5.3: Unit demand and daily demand for institution types in Pabal’s core ............36

Table 5.4: Projections for % change in rural population of India...................................37

Table 5.5: Population projections for Pabal....................................................................37

Table 5.6: Values for constants for rainfall intensity-duration-frequency equation for

Bhopal, India ...................................................................................................................40

Table 5.7: Geometric characteristics of the most hydraulically efficient trapezoidal

cross-section....................................................................................................................40

Table 5.8: Estimation of chemical, biological and microbiological parameters for

rooftop runoff in Pabal ....................................................................................................41

Table 5.9: Size and densities of suspended particles ......................................................42

Table 5.10: Economic value of different grades of water during dry and wet season....43

Table 6.1: Recharge for the Pabal catchment..................................................................45

Table 6.2: Estimation of volume of water available to Pabal’s core from current water

supply infrastructure........................................................................................................45

Table 6.3: Current demand for water in Pabal’s core (based on 2005 population) ........46

Table 6.4: Calculation of annual runoff for different scenarios......................................46

Table 6.5: Estimated cumulative runoff and potable water demand for 2020................46

Table 6.6: Estimated daily domestic demands that can be met from RWH ...................47

Table 6.7: Design storm parameters ...............................................................................54

-ix-

Table 6.8: Number of rainy days each month .................................................................58

Table 6.9: Terminal settling velocities for particles........................................................59

Table 6.10: The effectiveness of BioSand filters ............................................................60

Table 7.1: Comparison of guideline values specified in the WHO Guidelines for

Drinking-Water Quality with the water quality of the runoff .........................................65

Table 7.2: Comparison of water quality of runoff and final water quality .....................65

Table 7.3: Benefits from Biosand Filter..........................................................................67

Table 7.4: Construction costs for RWH system for Roof 1 ............................................67

Table 7.5: Construction costs for RWH system for Roof 2 ............................................67

Table 7.6: Calculation of payback time ..........................................................................68

-x-

Acknowledgements

I would like to thank engINdia for providing me with the chance to complete this project and for

Engineers without Borders for facilitating initial communication between engINdia and myself.

I would like to thank the following people in particular for assistance in this project:

• Lara Lewington from Engineers without Borders for the information she gave me

following her visit to Pabal in March 2009

• Pooja Wagh of engINdia for answering my queries and putting me in contact with

people from Pabal

• Chetan Shenoy and Yogesh Kulkarni of Vigyan Ashram in Pabal for the information

they provided me with

I would also like to thank my supervisor Dr Cesar Mota, Lecturer in Environmental Engineering

at the School of Civil Engineering and Geosciences, for the guidance he has given me

throughout my project.

-xi-

List of abbreviations

AGT Aboveground water storage tank

AR4 Intergovernmental Panel on Climate Change’s Fourth Assessment Report

ARTI Appropriate Rural Technologies Institute, Pune, India

DRWH Domestic Rainwater Harvesting

EWB Engineers Without Borders UK

lcd Litres per capita per day

MDGs Millennium Development Goals

mins minutes

NGOs Non-governmental organisations

NTU Nephelometric turbidity unit

RWH Rainwater Harvesting

TDS Total Dissolved Solids

TSS Total Suspended Solids

UGT Underground water storage tank

UN United Nations

VA Vigyan Ashram

WHO World Health Organisation

Kieran J Cooke Design of RWH for Pabal June 2009

- 1 -

1 Introduction

1.1 Outline of project A rainwater harvesting (RWH) system will be designed to help to alleviate the water shortage

problem currently experienced in the rural Indian village of Pabal. This water shortage is

typically between the months of February and May. However in recent years, this drought has

commenced as early as October if the monsoon season has been poor (EWB, 2008).

RWH can be defined as “the small-scale concentration, collection, storage and use of rainwater

runoff for productive purposes” (Jean-marc et al, 2007). It is specified in the project proposal

(EWB, 2008) that the RWH system that is designed must

• be low-cost, sustainable, easily maintainable and utilise locally available materials and

skills

• complement Pabal’s existing water sources and ensure reliability of the water supply

• take into account that water consumption increases with availability

1.2 Background to the project This project is being carried out at a time when the provision of safe and clean water to the

entire world’s population is high on the international political agenda. A number of initiatives

have been introduced in recent years to help achieve this aim.

Following a decade of major United Nations conferences and summits, in 2000 world leaders

agreed to the Millennium Development Goals (MDGs). The MDGs are a set of agreed targets

set for 2015 to assist development in the world’s poorest countries. Target 7c of the MDGs is to

“ reduce by half the proportion of people without sustainable access to safe drinking water and

basic sanitation” (UNDP, 2008). An improved water supply will also contribute to a number of

the other targets in Goal 1 (Eradicate extreme poverty and hunger) and Goal 3 (Promote gender

equality and empower women).

In 2002, the United Nations Economic and Social Council (UNESC) declared water as a human

right. This human right entitles everyone to “sufficient, safe, acceptable, physically accessible

and affordable water for personal and domestic uses” (UNESC, 2002).

The provision of an improved water supply for the village of Pabal through RWH, will

contribute to the achievement of the MDGs as well as fulfilling the villagers’ basic human right

of water.

1.3 Description of organisations involved in project This project has been put forward by engINdia. engINdia is an organisation which aims to

“promote appropriate and sustainable engineering solutions in developing areas”


- 2 -

(www.engindia.net, accessed 08/11/2008). The organisation is currently focusing on the village

of Pabal, following a visit of six students to the village in the summer of 2005. Through

working with the villagers, the students developed an understanding of the challenges faced by

the village that could be solved through engineering solutions. One of these challenges was the

development of a RWH system.

This project is co-ordinated by Engineers Without Borders UK (EWB), which is a student led

charity focussing on “removing barriers to development using engineering” (www.ewb-uk.org,

accessed 08/11/2008). EWB connects students wanting to undertake projects in development

issues with NGOs who have technical problems which they are keen to solve.

The proposed solution for a RWH system would be trialled at Vigyan Ashram (VA). VA is

located within Pabal and develops rural technologies whilst providing training to young people

(engINdia, 2005). The institution consists of labs, workshops and classrooms and has fostered

an interest in technological development among the local population (engINdia, 2005).

1.4 Description of Pabal Pabal lies 80 miles east of Mumbai within the state of Maharashtra in India, as shown in Figure

1.1. The village has a total population of approximately 9 000, with the core (which has a radius

of about 2 kilometres) containing about a third of these (engINdia, 2005). The remainder live in

hamlets outside of the core and are mainly farmers.

Figure 1.1: Location of Pabal (http://maps.live.com, accessed 08/11/2008)

1.4.1 Current water supply situation

The current water supply infrastructure in Pabal is shown diagrammatically in Appendix A.

Wells are abundant and the primary source of water in the area. In addition to a significant

proportion of houses within the core having their own wells, there are also communal wells.

Water from these communal wells is pumped into a storage tank, of 70 000 litres, which is

situated on the top of the entrance arch to the village (engINdia, 2005). Water is piped from this

tank to peoples’ homes. All homes have a water supply however this supply is only available for


- 3 -

20mins each day (personal communication: Lara Lewington). During this time people fill up

barrels to provide water for the rest of the day. In the dry season (November - April) these wells

run dry and government tankers’ truck 12 000 litres of water per day into the village tank. This

water is frantically withdrawn from 6.45am on a first come first served basis and is emptied

within 20 minutes.

In an attempt to increase the water levels in the wells through greater groundwater recharge, a

dam has been constructed (Figures 1.2 & 1.3). Water is also pumped from the dam to a 170 000

litre storage tank which is located close to VA and is the point of highest elevation for an

approximate radius of 30km (personal communication: Lara Lewington). The Gran Panchayat

(the local council) is responsible for turning on the pump each morning to transfer the dam

water to the tank. The tank is completely filled up each morning, however due to the frequent

occurrence of power cuts in Pabal sometimes this is not possible. Water is then piped from the

tank, utilising the height difference, to the whole of Pabal’s core (personal communication:

Lara Lewington). Individual connections to the tank have a one-off connection cost of 3 000

Rupees and a 700 Rupee per year charge. Combined connections have a 1 200 Rupees one-off

charge (EWB, 2008).

Figure 1.2: Aerial image of dam near Pabal (http://maps.google.com, accessed 01/05/2009)

Figure 1.3: Dam near Pabal (EWB, 2008)

The provision of the dam has improved the supply of water to Pabal, but due to climate change

and other unforeseen events it is unsure how long it will last for (personal communication: Lara

Lewington). Furthermore, engINdia state in the project proposal for RWH that any additional

moves to help gain a more reliable and plentiful supply of water would be beneficial (EWB,

2008).

Currently the only RWH system that is being used in Pabal is the collection of rainwater in pots

and pans. This shows that the attitude of RWH is already present.


- 4 -

1.4.2 Current water quality

engINdia during their visit in 2005 found the quality of the water being used in Pabal to be poor

(engINdia, 2005). Further evidence for the unsatisfactory water quality comes from the high rate

of water borne diseases in Pabal, with diarrhoea and gastrointestinal disease being the most

common reported ailments (engINdia, 2005). Despite these issues with water quality, water

from the dam and the wells is used for all purposes (personal communication: Chetan Shenoy).

Currently most water is consumed by the villagers with no treatment at all, apart from the

filtration that occurs as the water percolates through the soil. Some water is chlorinated and/or

boiled prior to consumption. Health problems can occur since neither chlorination nor boiling

reduces the mineral content of the water. Boiling is also a considerable fuel sink (engINdia,

2005). There are no plans for village wide chlorination and whilst ceramic filters are supposedly

available in the village, they are generally too expensive to purchase and maintain (engINdia,

2005 & EWB, 2008).

1.5 The scope and limitations Since one of the major problems facing Pabal is the lack of a reliable source of water, this

project has the potential to make a significant difference to the villagers of Pabal. However

dealing with the root cause of these water shortages is dependent on the state and federal

governments (engINdia, 2005). So it is important to recognise that a RWH system is a

temporary solution rather than the solution to the root of the problem. Interviews carried out by

the engINdia team with villagers, showed that the villagers were keen on technologies such as

RWH as they believed these technologies would make their home and professional lives easier

(engINdia, 2005). The engINdia team found a large variation between technologies in other

villages and that in Pabal (engINdia, 2005). This variation would cause a difference in need and

therefore it remains to be seen if the solution developed through this project will have potential

to be used elsewhere than just Pabal.


- 5 -

2 Aims & Objectives

2.1 Aim The aim of the project is to design a rainwater collection and storage system for the village of

Pabal in India at either household or community level. This system should help to alleviate the

water shortage problems that Pabal is currently experiencing.

2.2 Objectives The objectives of the project are to:

• quantify the current supply and demand of water in Pabal, comparing the supply-

demand balance in the wet and the dry season

• determine the most suitable system for RWH in terms of which catchment surfaces to

use and whether RWH would be more appropriate at community or household level

• quantify the demand that RWH will be able to meet

• determine what water quality improvements for the runoff are necessary and design

appropriate water treatment to achieve these improvements

• decide the most effective way to harvest, store and deliver the rainwater water in a low-

cost manner whilst utilising local skills and materials

• quantify likely changes in precipitation and demand over the design life of the RWH

system

• determine the cost and the economic feasibility of the scheme by quantifying the

benefits


- 6 -

3 Literature Review

3.1 Introduction to review Literature clearly outlines the advantages of rainwater harvesting over other water resource

developments. RWH provides water close the point of use and when existing catchment

surfaces are used, RWH has fewer negative environmental impacts compared to other types of

development (GDRC, 2007). In terms of water quality, rainwater is comparatively good in

contrast to other sources (Feroze Ahmed, 1999). As the users often manage the RWH system,

they are more likely to exercise water conservation with RWH than with other types of

developments (GDCR, 2007). RWH systems can be built to meet almost any requirements,

with construction, operation and maintenance not being labour intensive (GDCR, 2007).

A RWH system is divided into the following components: catchment, conveyance, storage and

delivery, as shown in the process diagram in Figure 3.1. This literature review shall evaluate

each of these components in turn and then discuss other associated issues of health implications,

economics and the design procedure of RWH systems.

Figure 3.1: Process diagram for a rainwater harvesting system (Thomas et al, 2007)

3.2 Catchment Runoff can be collected from any surfaces within the catchment that are impermeable. In the

case of rainwater, these are most commonly roofs, land surfaces or rocks; with roofs of houses

being the most widespread (Thomas, 1998). Provided the houses are one storied, roof area is

usually not a limitation in design. There is typically between 12 and 22m2 of developable roof

area per person for house occupancies of six to 12 people (Twort, 2000).

Peters (2006) suggested that the most common types of roofing materials for RWH are

corrugated galvanised metals, concrete or clay tiles and asphalt-type or wooden shingles. The

quality of the runoff is dependent on the roof material; concrete, tiled and metal roofs give the

cleanest water (Feroze Ahmed, 1999). Runoff from thatched roofs is not suitable for potable

uses, as thatching and mud discolours and contaminates the water (Smet, 2003). A thatch roof

can be covered with polyethylene to reduce contamination and discolouration. However, this

sheeting can only be used for a single season and tends to degrade in the sunlight quickly (DTU,


- 7 -

2005). Some literature recommends avoiding the use of asbestos for RWH systems (GDRC,

2007) whilst others have concluded that it does not present any health risks (Smet, 2003).

Painting and coating of catchment surfaces should be avoided, but where they are required they

must be non toxic (GDCR, 2007).

Impermeable land surfaces such as paved surfaces, plastic sheeting and cemented surfaces can

provide large catchment areas, making them particularly suitable where there is a large demand

for non-potable water (British Standards Institute, 2008). Infiltration into the ground causes a

high rate of water loss from these catchments. This loss can be reduced by clearing or altering

vegetation covering, increasing the land slope or reducing soil permeability by soil compaction

(GDRC, 2007). Preventing entry of people and animals onto ground catchments through fencing

can reduce the contamination of the runoff (GDRC, 2007).

Regardless of which catchment surface is chosen, the potential runoff must be determined.

Peters (2006) used the following terms in determining this runoff potential:

• “theoretical potential: the total amount of precipitation in the catchment

• available potential: precipitation that can be collected on roofs or other specific

catchment areas

• practical potential: water that is collected in the storage tanks for consumption under

normal conditions (due to inadequate storage, not all the roofs being guttered etc)”.

3.3 Conveyance The function of the conveyance system is to transfer the rainwater collected on the catchment

surfaces to the storage tanks (GDRC, 2007). In rooftop harvesting the conveyance system

consists of gutters and downpipes. Gutters are open channels that carry water sideways under

the edge of the roof to a point just above the water tank. Downpipes are tubes that lead water

down from the gutters to the entrance of the water tank (Thomas & Martinson, 2007).

Twort (2000) recommended that local practise, experience and field tests are used to determine

the size and location of the guttering as well as a suitable allowance for gutter overspill.

Increasing the gradient of the guttering may reduce the cost and required size but can also

increase gutter overspill (Thomas, 1998). These water losses, which are common in climates

with intense rainfall periods, may be acceptable from a water harvesting perspective but can

cause serious damage as a result of erosion. Gutter overspill can be reduced through the

installation of a downward pointing metal sheet at the lip of the roof, known as a Splash Guard.

The off-shooting water hits the Splash Guard and flows vertically downwards into the gutter

(DTU, 2005).

Suitable materials for the guttering include timber, bamboo, plastic and cement based products.

Pesticides to prevent rotting in timber and bamboo should never come in contact with drinking


- 8 -

water, instead the guttering should be regularly replaced (Smet, 2003). The low pH of rainwater

can cause corrosion and mobilisation of metals, and so consequently only galvanised metals

should be used for guttering (GDRC, 2007 & Water Aid, 2007). Metal sheets bent to form a ‘V’

and suspended by galvanised wire stitched through the roofing material, as shown in Figure 3.2,

are a form of low cost guttering proposed by Water Aid (2007).

Figure 3.2: Galvanised steel V guttering (Water Aid, 2007)

Downpipes for roof catchments or pipes to transfer water from other catchments to the storage

tank must be sized correctly. Over-sizing can cause water quality problems, whilst leakages can

occur as a result of excessive pressure in under-sized pipes (British Standards Institute, 2008).

The strength of the pipes must be sufficient to resist bursting forces which are caused by the

pressure that pipes are subjected to in operation (British Standards Institute, 2008). If the

collection surfaces are land surfaces, BS 8515 recommends that the gutters should be sealed to

prevent the ingress of contaminated water from other sources.

3.4 Storage

3.4.1 Sizing of the tank

Since the tank accounts for a large fraction of the total cost of a RWH system (Mwenge Kahinda,

2007), the required capacity of the tank must be calculated accurately. Thomas (1998) suggested

that domestic RWH (DRWH) is unlikely to be affordable unless storage costs are kept below

US$ 15 per cubic metre. The capacity of the tank will depend on the proportion of the total

water demand that a RWH system meets. RWH can be:

• the sole source

• the main source (70% of water use, (Thomas & Martinson, 2007))

• only a wet season source

• only as a source for some water uses.

RWH as the sole source is very costly and often socially unacceptable. It should only be used

where there are no other feasible alternatives to RWH (Thomas et al, 2007). It is only possible

to use RWH as the sole source in locations of little seasonality and where the mean rainfall is

over 2 000 mm/year (Thomas & Martinson, 2007).


- 9 -

An alternative source is necessary when RWH is used as the main source of water. This source

could use the same storage tank as the RWH system (Thomas & Martinson, 2007). The costs of

alternative sources are often greater than RWH, as a result of larger economic costs and greater

walking and queuing times (Thomas & Martinson, 2007). Therefore if the RWH system meets

80% of annual consumption, the total annual cost of water supply from other sources is reduced

by 60% (Thomas & Martinson, 2007).

A wet season source normally meets all the water needs in the wet season and is suitable for

climates where there is one long rainy season (Smet, 2003 & Thomas and Martinson, 2007). A

storage capacity of between 3 and 10 days consumption is advised when RWH is acting a wet

season source (Thomas, 1998). Since poor rural women often are hungry, working in the fields

and looking after sick children during the wet season, having a water supply close to their home

is particularly advantageous during this season (Smet, 2003). Thomas & Martinson (2007)

suggested that for a monsoon climate with a 6-month dry season, RWH is only feasible as a wet

season source.

The literature generally recommends that RWH satisfies the potable water demand, where only

part of the total water demand can be met. (Smet, 2003 & Thomas and Martinson, 2007).

A measure of the storage capacity of the tank suggested by Thomas (1998) is the ratio of the

volume of the tank to daily water consumption; the lower the value of this ratio, the smaller the

cost of storage but the greater the seasonal dependence on other sources. Simulations carried out

by Peters (2006) used a criterion of it being acceptable that the tank runs dry not less than once

every 50 years. This would perhaps be an appropriate standard to use in the design of a RWH

system.

Figure 3.3: Benefits of tank sizing (Mwenge Kahinda, 2007)

Figure 3.3 shows that the benefit of a tank is not strictly proportional to its size. Doubling the

tank size less than doubles the demand that is satisfied, as a small tank is emptied more

regularly than a large one. Thomas (1998) suggested that multiple storage vessels are

advantageous over a single vessel. This is because multiple vessels allow communities to

engage with new technologies in easy stages, can spread the outlay for storage over a number of

years, minimises the consequences of a tank failure and can reduce guttering costs.


- 10 -

3.4.2 Materials used

Materials that storage tanks can be constructed from and the corresponding suitable capacities

are outlined in Table 3.1. Twort (2000) recommended that tanks up to 800 litres can be made

from one piece of material and are transportable, whilst larger tanks are usually more

economical if constructed in situ.

Table 3.1: Possible materials for storage tank and corresponding capacities (Peters, 2006 & GDRC, 2007 )

Material Suitable capacity (litres)

Reinforced cast-in-situ concrete 32 000

Plastic 760 - 3000

Polyethylene 1000 - 2000

Metal 760 - 1900

Drums 170

Wooden barrels 130 -150

Peters (2006) stated that there is generally a large initial capital cost for constructing storage

tanks from concrete. Reinforced concrete tanks are favourable to those that are unreinforced as

they are repairable if they leak and can be rendered inside if poorly constructed (Twort, 2000).

Ashworth (2005) found that pathogenic removal was greater in concrete tanks compared to other

tanks. This was due to salts leaching from the concrete, causing a reduction in the pH and

therefore providing a less stable environment for pathogens.

Twort (2000) warned that plastic plates bolted together tend to fracture under the repeated

bending caused by the changing water levels. He also states that steel plates bolted together

often rust at the joints. The fracturing of plastic plates is unrepairable and the rusting of the steel

is hard to repair.

Despite the large capacity of polyethylene tanks, they are compact, easy to clean and have many

openings which can be fitted with connecting pipes (GDRC, 2007). Caution has to be taken with

tanks that are made from timber or bamboo, as the wood can become infested with termites,

bacteria and fungus (GDRC, 2007).

Another type of tank is ferrocement tanks. These consist of steel mesh and wire which are

covered on the inside and outside with a thin layer of cement (Brikké at al, 2003). These tanks

are one of the most economical types where the required skill to construct them is available or

can be trained (Twort, 2000).

3.4.3 Comparison of above ground and underground tanks

In addition to the materials and the number of tanks to use, the designer must also choose

whether to use underground (UGT) or aboveground tanks (AGT). Despite UGTs normally being


- 11 -

cheaper than AGTs (Thomas, 1998), literature on this topic has identified a number of problems

associated with UGTs compared to AGTs:

• they require a pump to extract the water.

• it is more difficult to notice and locate leakages

• they can become polluted due to groundwater seeping into the tank through a crack or

floodwater entering via the cover

• they can float out of the ground

• the danger of infants downing is greater

There is uncertainty in whether an UGT can rely on the soil for support and therefore be

constructed cheaply with thin unreinforced walls (Thomas, 1998). Mwenge Kahinda (2007)

suggested that an AGT should be used to collect rainwater from roofs, with an UGT being used

for all other catchments. An example of an UGT is show in Figure 3.4. The availability of space

and soil type should be considered in the decision of whether to use an UGT or AGT; for

example an AGT should not be built on expansive clays or sandy soils (Mwenge Kahinda,

2007). BS 8515 advised that AGTs are insulated and opaque to avoid algal blooms, freezing and

warming.

Figure 3.4: DRWH underground tank in South Africa (Mwenge Kahinda, 2007)

3.4.4 Tank Components

The cover of a tank acts as a barrier to mosquitoes, avoids animals and people falling in and

prevents algal growth through restricting sunlight. A cover can reduce the rate of pathogenic

reduction as strong sunlight has a bactericidal property (Thomas, 1998). One suitable cover is an

iron sheet supported by timber members (Handia, 2003). For UGTs the cover needs to be strong

enough to carry people and in some cases vehicles


- 12 -

BS 8515 recommended that tanks should have air vents that are screened and raised above the

surface flood level. BS 8515 also recommended the construction of an overflow tank to allow

excess water to be discharged during extreme rainfall events. This tank should be attached to the

storage tank by a pipe of equal or greater capacity than the inlet pipe to the tank and prevent

backflow. GDRC (2007) suggested the inclusion of an indicator of the amount of water in the

storage tank.

3.5 Distribution In some cases water is distributed in pipes to where it is required, whereas in other cases users

of the water collect it from the tank. For collection, a water lifting device is required for an UGT

whilst for an AGT either a tap or a water lifting device could be used (Warm et al, 2006).

Taps can break easily due to poor construction or lack of maintenance. Taps should be located

500 to 600mm above the floor of the tank to allow buckets to be placed underneath the tap

(Warm et al, 2006). It is not possible to extract the water below the level of the tap and therefore

such a tank has ‘dead storage’ (Warm et al, 2006). To eliminate this ‘dead storage’ the base of

the tank can be raised to 500mm above the ground level. However this causes sediments to be

extracted which prevents the water being used for potable uses.

Where a pump is chosen, Kerr (1989) recommended that hand pumps should be the first choice

in most cases. Hand pumps are capable of lifting enough water to meet the domestic water

demand of a small community (Mann & Williamson, 1993). The displacement pump is the most

suitable for rural communities as village craftsman are able to construct them (Mann &

Williamson, 1993). Hand pumps have the advantages of being capable of meeting the power

requirements from within the community and keeping the capital cost of pumping low (Hofkes,

1983).

Where hand pumps are not possible, mechanical pumps can be used. The pump drive can be

powered by either an electrical motor or a petrol engine. Electrical motors rely on a reliable

source of electrical power, whilst petrol engines only require a supply of petrol and lubricant

(Hofkes, 1983). However electrical motors require less maintenance and parts are more easily

obtainable than for petrol engines (Hofkes, 1983). The type of pump required depends on the

height that water must be lifted. If the height is less than 6 metres then a horizontal or vertical

direct-drive pump may be used, otherwise the pump must be submerged (Mann & Williamson,

1993).

Rope lifts for water are simpler to build than any type of pump (Mann & Williamson, 1993).

They rely on the rope being partly submerged in the water source and lifted over the pulleys at a

rate faster than the water is flowing down the rope (Mann & Williamson, 1993). Delivery rates


- 13 -

are 15 litres/min for hand driven lifts, with cycle powered lifts increasing the output by 50 to

100% (Kerr, 1989).

Buckets are perhaps the most successful water lifting device and can be used to lift the water

through the use of a windlass (Kerr, 1989). An adaptation of the standard bucket system is a

pulley system with a bucket on either end of the rope. A rope and bucket are suitable for lifting

water over a height of less than 15m (Brikké et al, 2003).

Using devices that are currently used locally will make maintenance and the acquirement of

spare parts easier and will cause the device to have greater user acceptance (Kerr, 1989). The

design of these devices must be simple to reduce the number of parts to the bare minimum and

therefore keep operation and maintenance as straightforward as possible (Kerr, 1989).

3.6 Health implications of RWH Mwenge Kahinda (2007) proposed that the health implications of RWH should be divided into

two aspects:

• concerns regarding water quality and possible direct implications due to contaminants

• the breeding of insect vectors in water storage tanks and health implications arising out

of it.

3.6.1 Water quality

In most climates only 2 litres of water is necessary for survival (Thomas, 1998), whilst in Pabal

the water consumption is 100-120 lcd (EWB, 2008). Therefore the volume of water that would

be required to meet drinking quality standards would be a small proportion of total consumption.

For the fraction of the water that is going to be used for drinking, there is some debate over

whether rainwater meets the international standards for drinking water. Mwenge Kahinda

(2007) reported that some studies have concluded that rainwater from rooftops generally meet

the guidelines, whilst others have concluded that chemical and/or microbial contaminants are

often present in level exceeding these guidelines. It is also important to compare the quality of

the rainwater with the alternative sources. Thomas & Martinson (2007) indicated that RWH

provides water that is as safe as that obtainable from protected point sources such as wells and

often has an improved taste compared to other point sources.

Contamination of rainwater can occur in the air, on the collection surface or in the water store.

The contamination which occurs in the air is negligible as there is no evidence that pathogens

are picked up and the absorption of acid gases is insignificant and well within acceptable limits

for humans (Thomas, 1998).

However the contamination occurring on the collection surface is significant and depends on

characteristics such as topography, the weather conditions, and the proximity to pollution


- 14 -

sources (Mwenge Kahinda, 2007). The level of contamination is also largely dependent on

whether the collection surface is a rooftop or the ground; with the ground being subject to

higher levels of microbial contamination (Mwenge Kahinda, 2007).

In the case of rooftop RWH, it is not the surface itself that causes the majority of the

contamination but what is on that surface, for example dust from soil, leaves from trees,

repellent insects and bird droppings (Mwenge Kahinda, 2007). This contamination can silt up or

deoxygenate water stores, discolour water and increase the risk of diarrhoeal diseases (Ashworth,

2005). There is also a suspicion that bird droppings can sometimes spread typhoid (Thomas,

1998). Contamination from collection surfaces is a particular problem in areas that have a dry

season, since material accumulates in the dry season and enters the system during the

subsequent rains.

The roofing material can also affect the water quality of the runoff. Yaziz et al (1989) found a

better quality runoff from galvanised iron roofs than concrete roofs. It was suggested that this

was due to more contaminants being deposited and entrapped on the concrete roof as a result of

its rougher surface. A similar finding was obtained by DTU (2005), but they suggested that the

difference in quality of the runoff was due to the sun’s rays having a sterilising effect on the

metal roof.

The roof can also alter the pH of the rainwater, but there are conflicting predictions on whether

it causes the rainwater to become more acidic and alkaline. Yaziz et al (1989) found that there

was an increase in pH and suggested that this was due to the build up of basic particles on the

roof surfaces. However experiments carried out by Efe (2006) found that the runoff from roofs

was acidic, but gives no suggested reason for this decrease in pH.

Mwenge Kahinda (2007) suggested that the level of contamination in the water store is

dependent on the type of water tank and the handling and management of the water. The tank

should be regularly cleaned; Environment Agency (2003) recommended at least twice a year

whilst Feroze Ahmed (1999) advised only once a year.

3.6.2 Insect vectors

Mosquitoes are of concern in RWH since they can cause diseases such as malaria (Mwenge

Kahinda, 2007). Adult mosquitoes can breed within tank or alternatively they can lay mosquito

larvae in the guttering which is then transported with the runoff into the tank (Thomas et al,

2007). Mwenge Kahinda (2007) proposed a number of measures to prevent mosquitoes, which

are detailed in Table 3.2.


- 15 -

Table 3.2: Methods for the prevention of mosquitoes in RWH (Mwenge Kahinda, 2007)

Method Solutions

Prevention of mosquitoes breeding in the tank, by killing immature mosquitoes during larval stages

• use a biological control in depressions of the tank, such as Bacillus spareicus or Bacillus thuringiensis. These organisms contain proteins which are toxic to larvae of a variety of mosquito species.

• the use of chemicals such as kerosene oil and other well tested chemicals that disperse as a thin layer on the surface of the water. This causes the larvae and pupae to drown.

Prevention of mosquitoes breeding in the surroundings of the tank

• the growing of plants around the RWH site which repel mosquitoes.

• by tightly closing the tank to ensure there are no openings for the entry of mosquitoes

• the use of a screen (with hole size less than 1mm) to bar entry of mosquito larvae into the tank

• ensuring there is no stagnating water around the RWH site, as mosquitoes might use it to breed in

• gutters should allow the free flow of water, as mosquitoes may breed in the stagnant water

3.7 Methods of improvement of water quality

3.7.1 First flush system

To address the problem of debris collected over the dry season on roofs causing contamination

of the runoff, some literature suggests throwing away the initial runoff at the beginning of the

wet season (Thomas, 1998). This runoff is commonly known as the first flush. DTU (2005)

reported that this practise is particularly popular in Asian countries. The World Health

Organisation (2003) recommended that the first 20 litres of runoff per roof should be diverted

from the storage tank at the beginning of the wet season. However Martinson & Thomas (2004)

warned that the value of 20 litres has a large number of built in assumptions that may or may

not be true. They therefore suggest that this value is likely to considerably underestimate the

necessary volume of first flush for low-income countries. Martinson & Thomas (2004) instead

recommended that runoff should be diverted each time rainfall follows three dry days and that

the necessary volume of runoff to be diverted should depend on its turbidity. They suggested a

target turbidity of 20 NTU, since the turbidity will be further reduced in the tank due to

processes such as sedimentation.

Yaziz et al (1989) found that diverting the first flush decreased all the water quality parameters

that were tested, apart from lead and zinc concentrations. A negative relationship between the

intensity of the rainfall and the ‘wash out’ time period for pollutants was discovered. The

suggested reason for this relationship was that the cleaning process was more efficient at larger

rainfall intensities, due to the greater energy present in the raindrops (Yaziz et al, 1989). The

experiments also showed that diversion of 0.5mm of runoff was sufficient to reduce the faecal


- 16 -

coliform count to zero; although high levels of total coliforms and plate counts were still

detected.

Simple methods of diversion of the first flush exist such as manually diverting the runoff or

using a simple automatic system where water fills a chamber of a set size until it overflows

(DTU, 2005). There are also more complex methods such as the fixed mass system, which relies

on a mass of water tipping a bucket or seesaw, and the flow rate system which balances the rate

of water intake into a suspended hollow ball against its leakage (DTU, 2005). The simpler

systems tend to be more reliable and cheaper and therefore widely used in low-cost systems.

Some literature has concluded that there are no obvious benefits to separating the runoff of the

first flush (Handia, 2003 & Peters, 2006). Smet (2003) warned that most first flush systems fail

as they are not correctly operated and maintained. An alternative to removing the first flush is

thoroughly cleaning the roof at the beginning of the raining season (Ahmed, 2003).

3.7.2 Coarse filters Coarse filters reduce the amount of roof debris entering the water tanks (Ashworth, 2005).

These filters can be positioned in the gutter, in the downpipe or at the entrance to the tank itself,

with the tank entrance being the most common in very low cost systems (DTU, 2005). The

advantages and disadvantages of each location are reviewed in Table 3.3.

Table 3.3: Locations for coarse filters (Ashworth, 2005 & DTU, 2005)

Location Advantages Disadvantages Example of device

In the gutter

• Prevents the build up of leaves in the gutter and therefore reduces mosquito breeding and cleaning requirements of the gutter.

• Large risk of people falling whilst clearing and maintaining such devices.

• Can be expensive due to the large area that needs to be covered

In the downpipe • Low space requirement

• Difficult to access for cleaning

• Blockages are not obvious

Marley rainwater leaf slide (Figure 3.5) -

catches debris in the water through a mesh

screen filter

In the entrance to

the tank

• Simple and inexpensive installation

• Very visible

• Entrance to tank is prone to accidental (or deliberate) contamination

• Reduces possibility of any further filtration at the entrance to tank

Can be as simple as a cloth.


- 17 -

Figure 3.5: Marley rainfall leaf slide (Ashworth, 2005)

Ashworth (2005) described first flush as having little benefit over coarse filters. BS 8515 gave a

set of criteria for a coarse filter:

• the filter should be water and weather resistant

• it should be removable and readily accessible for maintenance purposes

• have an efficiency of at least 90%

• should pass a maximum particle size of less than 1.25mm

3.7.3 Settlement in tanks

The storage of roof water will allow for settlement of protozoa cysts and other suspended

particles, pathogen die-of to WHO drinking water standards and considerable improvement in

the clarity of the water (Ashworth, 2005). Provided the temperature of the tank water is low and

substantial nutrient levels are not present, a storage time of two weeks can reduce bacteria

populations by 50 to 90% (Ashworth, 2005). This removal rate is dependent on the severity of

the population (Mann and Williamson, 1993).

To reduce resuspension of settled matter, the recommended minimum depth of the outlet above

the tank floor is not consistent across the literature. Ashworth (2005) specified 500mm whilst

BS8515 and GDRC (2007) recommended a distance between 100 and 150mm. Environment

Agency (2003) suggested some additional tank components. These included a smoothing inlet to

stop sediments being disturbed during heavy rainfall, a suction filter to prevent the uptake of

floating matter when water is extracted and an overflow trap to allow floating matter to be

skimmed off when water in the tank reaches a certain level. Other design considerations include

the tank having a sloped bottom to aid collection of settled matter and a sump and drain to

collect and discharge this debris.

3.7.4 Treatment options

The treatment of rainwater is not common as natural sedimentation and bacterial die-off during

storage is sufficient (Thomas, 1998). Furthermore, untreated roof runoff has been widely used

for drinking purposes for years with very few recorded serious health cases (Mwenge Kahinda,


- 18 -

2007). Where treatment of rainwater is necessary, for example where runoff is not from roofs, a

filter mechanism prior to the tank, treatment processes inside the tank and post-storage

treatment can be used.

Where filtration is used in RWH, the filter must have either a larger surface area or coarser

media than standard filters due to the large intensity of some storms (DTU, 2005). There are a

number of different types of filters that can be used, but in them all water percolates through

sand. In slow sand filtration (Figure 3.6) a layer of biological material builds up on the surface

of the sand which adds considerable biological cleansing to the mechanical filtering action

(Mann & Williamson, 1993). Rapid sand filtration uses coarser materials than slow sand

filtration and has a higher filtration rate but no biological action (Hofkes, 1983). The cleaning of

a slow sand filter is required much less frequently than a rapid sand filter, but is labour intensive

and requires the filter to be taken out of operation (Hofkes, 1983). The design of slow sand

filters are much simpler than rapid sand filters and can therefore be built with local materials

using local skills and labour (Hofkes, 1983). The complex operation of rapid sand filters makes

them unsuitable at the village scale, despite these filters requiring 40 to 50 times less land than

slow sand filters (Hofkes, 1983). Where the runoff is from hard ground surfaces Smet (2003)

advised the use of a filter consisting of a layer of sand overlying a gravel layer.

Figure 3.6: Slow sand filter (Huisman & Wood, 1974)

Chemical disinfection is the periodic addition of a disinfectant such as chlorine, chloramines

chlorine dioxide or ozone to the tank (Twort, 2000). Disinfection is well understood but requires

some management (Thomas, 1998 & Mwenge Kahinda et al, 2007). The complexity and high

cost of producing ozone described in Twort (2000) would make it unsuitable for low-cost

systems. Mann & Williamson (1993) recommended a contact time of 30 minutes for chlorine

and a residual of at least 0.3mg/l in the outflow from the tank. Suitable low cost storage for

disinfectants would be either a plastic or a metal drum. In the case of metal, it would be

necessary to coat the drum with bituminous paint to prevent corrosion (Mann & Williamson,

1993). A rubber delivery tube with a clamp or a tap can be used to dose the tank with the


- 19 -

disinfectant (Mann & Williamson, 1993). Bleaching powder which is readily available, cheap

and not dangerous could be used as an alternative to liquid chlorine (Hofkes, 1983).

In the case of rainwater from roofs, it is considered appropriate to leave any treatment to the

householder’s discretion (Thomas, 1998). This treatment can comprise of boiling, chlorination,

solar disinfection or the use of a candle filter. Despite boiling being resource intensive,

presenting a risk of accidental scalding and affecting taste, it is the easiest and most effective

way to ensure disinfection (Thomas, 1998 & GDRC, 2007).

3.8 Design processes

3.8.1 Available runoff

The runoff is calculated by the multiplication of the catchment area and the amount of rainfall.

Literature quotes rainfall in either daily, monthly or annual values generally over at least a 10

year period.

Not all the rainwater that lands on the catchment area reaches the tank. The main causes of loss

are evaporation from the catchment surface, the first flush after the dry season and in the case of

roofs, gutter overspill. Some literature quotes a single ‘efficiency figure’ to take account these

losses; for example Feroze Ahmed (1999) used 25% loss for evaporation and first flush. Other

literature deals with these water losses separately. Twort (2000) recommended 1mm is

subtracted from the daily rainfall to allow for evaporation of water from the roof. Some

literature uses a generalised efficiency value for gutter overspill, with the most common values

quoted being between 0.9 and 0.95 (Twort, 2005 & Peters, 2006). Other literature has published

runoff coefficients for roofs based on roofing material and layout (Tables 3.4 and 3.5).

Table 3.4: Drainage Coefficients (Environment Agency, 2003)

Roof type Run off co-eff

Pitched roof tiles 0.75 – 0.9

Flat roof smooth tiles 0.5

Flat roof with gravel layer 0.4 – 0.5

Table 3.5: Runoff coefficients (Smet, 2003)

Roofing material Run off co-eff (%)

Cement tiles 75%

Clay tiles <50% (dependent on production method)

Plastic and metal sheets 80-90%

Runoff coefficients for ground surfaces are quoted as being between 0.1 to 0.3, due to water

seepage into the ground. Paved surfaces have a value between 0.6 and 0.7 (DTU, 2005). Water

losses also occur when water passes through filters. The filter efficiency allows for such losses;

an efficiency of 90% is recommended by the Environment Agency (2003).


- 20 -

3.8.2 Calculating the demand for water Thomas & Martinson (2007) suggested that the demand for water from the storage tank varies

with the level of water in the tank. It has been found that demand is constant when the tank is

between one-third and two-thirds full, reduces when the tank is less than one third full and

increases when the tank is more than two-thirds full.

3.8.3 Required capacity of storage tank

Mass curve analysis appears to be a common method used in determining the necessary capacity

of the tank (Feroze Ahmed, 1999, Handia, 2003 & Smet, 2003). In a mass curve analysis the

cumulative runoff and the demand are plotted on the same axes (Figure 3.7). The maximum

difference between the runoff and demand indicates the necessary storage capacity of the tank.

Figure 3.7: Rainfall intensity, cumulative rainfall availability and demand from a RWH scheme in

Bangladesh (Feroze Ahmed, 1999)

When determining the available runoff, as well as taking into account the water losses discussed

in 3.8.1, the amount of water lost as a result of tank overspill must also be considered. This loss

will depend on size of the tank, the climate (climates with a long dry season will have the

largest overspill) and the pattern of extraction of water (Thomas & Martinson, 2007).

Alternative methods of calculating the required storage capacity are outlined in BS 8515. There

is a simplified approach where a consistent daily demand and an annual average rainfall depth

are assumed and the required storage capacity is obtained from published graphs. There is also

an intermediate approach, where 5% of the available runoff and annual non-potable water

demand are calculated and the tank capacity taken as the smaller of these values. The detailed

approach, used where there is large monthly variation in the demand or runoff, estimates the

storage capacity by constructing models of runoff and demand.


- 21 -

3.9 Community and management issues RWH systems are most effective when implemented in conjunction with water demand

management through measures such as promoting the development of efficient and affordable

devices to conserve water (Handia, 2003 & GDRC, 2007). It is also important that the

community accepts the system and members of the community are able to maintain and operate

it (Handia, 2003).

There appears to be conflicting views about whether it is more effective to implement a RWH

system at community or household level. In recent years, there has been a shift towards more

community-based approaches which emphasise participation, ownership and sustainability.

Furthermore, NGOs are more committed to community rather than household technologies for

reasons of both equity and ‘economies of scale’ (Smet, 2003 & Thomas, 1998). Despite this,

household based systems are often considered as preferable to community ones as they avoid

the difficulty in organising the operational and maintenance of a shared water supply and the

arguments about who ‘owns’ the water (Thomas & Martinson, 2007). However as a

consequence of household based systems being privately owned, it is not easy to monitor water

quality or even the quality of installations (Thomas & Martinson, 2007). Where a community

based RWH system is chosen careful consideration must be given to the choice of the site, as

the cost of construction, utility and the lifetime of the system depend on the site (Machiwal et al,

2004).

3.10 Economics of RWH Thomas & Martinson (2007) suggested two possible economic thresholds for a RWH system; a

cost of US$ 100 for a system that will meet the bulk of a demand for 100 litres of clean water

per household per day or alternatively a threshold of 0.5 ¢ per litre of water delivered.

The unit costs of storage tanks obtained from various projects are presented in Table 3.6. These

costs appear to be much higher than the affordable cost of $15 per cubic metre of storage that

Thomas (1998) suggested. Where shuttering is required to construct the tank, it typically

accounts for a third of the cost and should therefore be reused wherever possible (Handia, 2003).

Table 3.6: Unit cost of different types of tanks (Brikké et al, 2003, Peters, 2006 & Mwenge Kahinda, 2007)

Type of tank Cost ($/m3)

Ferrocement tank 47

Underground concrete tank 60

Plastic tanks 38 - 135

Stand alone concrete tank 82 - 273

To asses the economic viability of RWH, Thomas & Martinson (2007) proposed that either the

payback time should be calculated or the cost compared to that of alternative technologies. The


- 22 -

payback time is a measure of the period it takes for the cumulative annual benefits to equal the

construction costs of the system. The annual benefit is usually just taken as the cost savings as a

result of RWH, as it is difficult to obtain a value for the benefits of having additional water. To

compare rival technologies, the cost to construct each technology to meet a particular service

standard is calculated. The comparison of alternative technologies is extremely time consuming

and often not possible (Thomas & Martinson, 2007).

3.11 Limitations and constraints of DRWH Despite the numerous benefits of RWH that have been highlighted in this literature review, as

with all water supply systems there are also limitations and constraints to RWH.

A study by DTU (2002) in the humid tropics found that on average only 61% of those surveyed

accepted rainwater as potable water (for drinking and cooking). This is compared to 90% who

viewed rainwater as suitable for non-potable uses such as clothes washing and bathing. This

finding suggests that RWH may be limited to only addressing the non-potable water shortages

in some situations.

A RWH system requires space; for example a 1 000 litre tank typically requires 0.75m2 (DTU,

2002). In some locations, in particular urban locations, space is severely limited and therefore

sufficient space for RWH is not available. In the same survey carried out by DTU (2002), 54%

of households questioned expected space limitations to effect their adoption of DRWH.

As has been highlighted in 3.6.1, roofs are widely agreed to introduce less contamination into

the rainwater than ground surfaces. Therefore in cases where suitable roofs are not available,

either because the roofs are not made of an appropriate material to allow runoff or the roofs are

flat, the success of RWH is limited.

As was described in 3.4.1, in nearly all cases RWH is not able to meet the domestic water

demand throughout the year. In most circumstances it is the roof area that is the limiting factor.

DRWH is particularly limited in climates that have a long dry season. In semi arid zones

DRWH is only viable if it is highly integrated with other water sources (Thomas & Martinson,

2007). A further limitation of DRWH in terms of its reliability is that the supply of water can be

severely compromised during droughts. DRWH is more prone to droughts than water supply

systems that rely on groundwater (Thomas & Martinson, 2007)

The availability of DRWH in developing countries, in particular in rural areas, is constrained by

an absence of relevant skills and components (Thomas & Kiggundu, 2004). Many potential

private DRWH users do not have the relevant skills and knowledge to implement DRWH for

themselves. With a lack of installers, who can advise on matters such as tank sizing, install the

components and provide some maintenance services, DRWH is commonly not an option for

these private users (Thomas & Kiggundu, 2004).


- 23 -

As with most methods of improving a community’s water supply, the main reason why DRWH

is often not implemented is due to a lack of financial resources (DTU, 2002). Due to DRWH

having a large initial large capital outlay but smaller maintenance costs compared to alternative

technologies, DRWH is often disregarded where this initial capital is not available (Worm et al,

2006). With the RWH industry being relatively young, components for a RWH system are often

more highly priced than they ought to be (Thomas & Kiggundu, 2004).

In many countries there is a lack of clear policy on the development of DRWH and no

institutional arrangements to support its development (DTU, 2002). Thomas & Kiggundu (2004)

suggested that this is due to ignorance amongst relevant professionals and DRWH not being

treated as generously as other water sources of comparable performance.


- 24 -

4 Proposed Methods Statement

4.1 Scope of the work A rainwater harvesting system shall be designed for the village of Pabal in India. Suitable

catchment surfaces will be identified, conveyance and storage devices designed and appropriate

measures to improve water quality implemented.

The designed system shall provide inhabitants within the core of the village (a radius of

approximately 2 kilometres) with water throughout the year. The proportion of the total demand

that the RWH system will satisfy must be determined. This shall be based on the quantification

of the current water supply and the potential of RWH with respect to the total area of available

catchments.

4.2 Programme of work The design shall be completed by 2nd June 2009. Initially data will be collected from a variety of

sources, in order for an informed decision to be made of the most appropriate type of RWH

system for Pabal. A complete design will then be undertaken, including thorough checking to

ensure errors are identified. Finally a detailed design report shall be compiled to help engINdia

and the villagers of Pabal understand the design.

A proposed programme of work is shown as a Gant Chart in Appendix J.

4.3 Required data

4.3.1 Sociological

The total population of Pabal is 9 000 (engINdia, 2005), with a third of these living in the core

of the village.

The project proposal states that the construction of the RWH system should utilise local skills

and therefore information on the capabilities of the villagers must be gained. It will also be

necessary to know the amount of people available for labour and maintenance from within the

village. This information could be obtained from communication with the engINdia expedition

team and VA, as well as from available census data for the area.

4.3.2 Economic

The definition of ‘low cost system’ stated in the project proposal shall be defined as one that has

a construction cost of less than US$ 100 for every 100 litres of water it delivers to a household

per day.


- 25 -

The unit costs for the storage tanks shall be assumed to be the values in Table 4.1. These values

have been taken from the lower end of the range of unit costs presented in 3.10. The costs for

the pipes, guttering and filters shall be taken from relevant manufacturers’ websites.

Table 4.1: Unit costs to be used for storage tank

Type of tank Cost ($/m3)

Ferrocement tank 47

Plastic tanks 40

Underground concrete tank 60

Stand alone concrete tank 90

4.3.3 Hydrological

Due to the variation in rainfall that Pabal experiences through the year, monthly rainfall values

shall be used rather than annual values. As was advised in 3.8.1, rainfall datasets used in the

design will be at least 10 years in length. In 3.4.1 it was stated that it was acceptable for the

storage tank to run dry every 50 years. It would therefore also be useful to obtain maximum and

minimum monthly precipitation values with a return period of 50 years.

It will be attempted to obtain this data from either the Indian Metrological Department

(http://www.imd.ernet.in/) or the Indian Water Portal (http://www.indiawaterportal.org/). If

neither of these sources is able to provide suitable data, a Professor of Hydrology at the Indian

Institute of Technology (IIT), Mumbai could be contacted or VA may have suitable rainfall

records. It would be useful to have data from two sources to increase the reliability of the data

that is used.

4.3.4 Spatial

To find suitable locations within Pabal for a RWH system, maps will be required. By having

access to topographic maps of the area, a location will be able to be chosen to utilise gravity in

the flow of water.

Maps will be attempted to be obtained from Survey of India. The most detailed topographic

map available is at a scale of 1:10 000 (http://www.surveyofindia.gov.in, accessed 05/12/2008).

‘Project Maps’ which are constructed specifically for a project and allow the scale and contour

interval can be stipulated are also available. Contact will be made with Survey of India to

determine which maps are available for Pabal and suitable maps will be purchased digitally.

4.3.5 Water usage

The project proposal states that the domestic water consumption is between 100 and 120 litres

per person per day (lcd) for the wet season, with the demand halving during the dry season.

With a more constant and reliable water supply, the domestic consumption in the dry season


- 26 -

would be expected to increase. Therefore a constant domestic demand throughout the year of

120 lcd shall be assumed.

In order to determine the proportion of the total demand that the RWH system should meet, the

quantity of water provided from existing sources must be estimated. This shall be done through

communication with Mr Yogesh Kulkarni, who is the Executive Director of VA, and Mr

Santosh Gondhalekar who was involved in the construction of the dam.

The project proposal also states that the business demand for water within the core of Pabal

(café, laundry, mechanic etc) is between 30 and 800 litres per day per business. It must be

decided whether the RWH system shall meet the business demands as well as the domestic

demand. This decision shall be based on the number of businesses and the current water

provision for these businesses. This information, along with typical demands for each business

type, shall be gained from Mr Kulkarni at VA.

4.3.6 Catchment data

In the project proposal it is stated that newly constructed structures within Pabal have

corrugated steel roofs. However the proposal does not indicate what proportion of buildings are

classed as ‘newly constructed’ and what roofing material the remainder of the buildings are

constructed from. To gain this information communication will be made with Mr. Yogesh

Kulkarni and engINdia.

Also through communication with Mr. Kulkarni, it is hoped that he will be able to suggest types

of roofs (institutional and private houses) suitable for RWH within Pabal. An approximation of

the number of suitable roofs and the dimensions and types (e.g. single pitched, double pitched

etc) of these roofs will also be requested through Mr Kulkarni. The location of the roofs will be

taken from the maps obtained as specified in 4.3.4.

Soil data for the Pabal area will be required to check that the soil can resist the load exerted on it

by the storage tank. This data would also be needed in the calculation of available runoff,

should ground surfaces be used as catchments. Soil data will be obtained from Geological

Survey of India (http://www.gsi.gov.in/).

4.4 Design procedure

4.4.1 Calculation of demand The daily domestic demand will be calculated by multiplying the demand per person (lcd) by

the size of the population being served by the system. The daily business demand, if it is

decided to include this demand, will be calculated by summing the demands of the individual

businesses. The total daily demand will be the combined total of the domestic and business

demands.


- 27 -

4.4.2 Calculation of available runoff

The available runoff is the volume of water that can be extracted from the storage tank for

consumption.

The following losses of runoff shall be included in the design:

• runoff not landing on suitable catchments surfaces

• initial evaporative loss

• gutter overspill

• loss due to filter efficiency

• tank overflow

The area of each catchment shall be calculated by multiplying the width of the catchment (w) by

its length (L), as shown in Figure 4.1. These areas shall then be summed to gain a value for the

total catchment surface (A), as shown in Eqn 4.1.

A = ∑ (wL) Eqn (4.1)

Figure 4.1: The dimensions of the catchment area

The daily runoff (Qdaily) shall be calculated using Eqn 4.2, where d is the monthly rainfall depth

and days is the number of days in that month. It is therefore assumed that the monthly rainfall is

evenly distributed across the month.

Adays

dQdaily ×= Eqn (4.2)

Table 4.2: Runoff coefficients

Roofing material Run off co-eff (%)

Cement tiles 75

Clay tiles 40

Plastic and metal sheets 85

Ground surfaces 30

Paved surfaces 65

The daily runoff (Qdaily) shall then be corrected for initial evaporative loss, gutter overspill and

filter efficiency in the following ways:

w

L


- 28 -

• 1mm will be deducted for initial evaporative loss per roof for each day,

• a run-off coefficient (R), which is dependent of the roofing material (Table 4.2), shall be

used to allow for gutter overspill,

• one coarse filter shall be placed in each conveyance system and a filter efficiency of

90% assumed.

Using these allowances, the corrected daily runoff (Qcorrected) can be calculated using Eqn 4.3.

)(9.0 nQRQ dailycorrected −= Eqn (4.3)

The RWH system will exclude the first 20 litres of runoff from each roof at the beginning of the

wet season (May), in accordance with WHO guidelines stated in 3.7.1. This runoff must be

subtracted from the available runoff.

Due to the calculation method used in determining the necessary storage capacity (see 4.4.3),

none or very little runoff will overflow from the tank. Therefore no allowance will be made for

tank overflow in the calculation of the available runoff.

4.4.3 Tank

As highlighted in 3.4.1, a considerable percentage of the total cost of a RWH system comes

from the construction of the storage tanks. It is therefore important that the design method keeps

the storage costs to a minimum whilst still providing sufficient storage to meet the demand.

The level of demand that can be satisfied from RWH can be determined by comparing the total

available runoff over a year with the total annual demand of the RWH system. From this annual

demand, the monthly demand that can be met is calculated. This monthly demand will then be

plotted on the same axes as the monthly available runoff, as in the example shown in Figure 4.2.

Figure 4.2: Comparison of the harvestable water and the demand for each month (for a site in

Biharamulo District, Kagera, Tanzania) (DTU, 2008)

From this graph, the point in the year where the runoff equals the demand can be identified.

Assuming that the tank is empty just prior to this point, a graph of cumulative runoff and

cumulative demand can be plotted (Figure 4.3). This graph can then be used to calculate the


- 29 -

maximum required storage. A spreadsheet, as shown in Table 4.3, shall be produced to

summarise the data obtained from the graphs and calculate the required storage volume for each

month.

Figure 4.3 Predicted cumulative inflow and outflow from the tank (for a site in Biharamulo District,

Kagera, Tanzania) (DTU, 2008)

Table 4.3: Spreadsheet to be used to determine storage

Month Monthly

runoff (m 3)

Cumulative monthly runoff

(m3)

Monthly demand

(m3)

Cumulative monthly demand

(m3)

Difference between cumulative demand

and runoff (m3)

It must be checked that the ground can resist the imposed load due to the storage tank without

excessive deformations. The worst case load, which will occur when the tank is full of water,

shall be considered. The imposed load of the storage tank will consist of the self weight of the

tank and the load exerted by the water within the tank. The density of the material that the tank

is constructed from shall be taken from a database of material properties and the density of

water shall be assumed to be 1 000 kg m-3. Using the unit weight of the relevant soil type, the

increase in vertical and horizontal stress at regular soil depths shall be considered. This change

in stress shall be used to calculate settlements and the total increase in stress compared with the

strength of the soil.

4.4.4 Design of the conveyance system

The Rational Method shall be used to determine the required diameters for the pipes.

Considering a rainfall intensity for a storm with a return period of 50 years, the Rational

Formula (Eqn 4.4) shall be used to calculate the peak discharge (Qp). The head difference

between the catchment and the tank will be calculated using the elevations of the two points.

The pipe length will also be calculated. The head loss per unit length of pipe (Sf) will then be

determined.


- 30 -

Qp = C i A Eqn (4.4)

Qp = Peak discharge (m3 hr-1)

C = run-off coefficient

i = rainfall intensity (m/hr)

A = catchment area (m2)

After the correct value of effective roughness (ks) has been selected from Table 4.4, HRS Charts

will be used to calculate the required diameter (D) of the pipes. It will be checked that the flow

is turbulent (Re> 4000) through Equation 4.5, as HRS charts are only valid for turbulent flow.

Table 4.4: Typical ks values (Chadwick & Morfett, 1998)

Pipe material ks (mm)

Brass, copper, glass, Perspex 0.003

Asbestos cement 0.03

Wrought iron 0.06

Galvanised iron 0.15

Plastic 0.03

Bitumen-lined ductile iron 0.03

Spun concrete lined ductile iron 0.03

µρDV=Re Eqn (4.5)

Re = Reynolds Number

ρ = density of water (1000 kgm-3)

D = diameter of pipe

V = velocity of flow

µ = dynamic viscosity of water (1 x10-3 kg/ms)

4.4.5 Water quality

Since a site visit will not be possible during the design of the RWH system, rainwater samples

will not be able to be collected from the site. This will prevent that the water quality parameters

of the rainfall and runoff being quantified and will mean that it will not be possible to assess

whether any improvements in water quality are necessary. Therefore, as a result of the literature

review, it shall be assumed that the water quality of the runoff meets the WHO Drinking Water

Guidelines as a result of

• the runoff passing through coarse filters

• natural sedimentation in the tanks

• the first 20 litres of runoff from each roof being excluded at the beginning of the wet

season.


- 31 -

4.5 Cost of system The unit costs detailed in 4.3.2 shall be used to determine the total cost of the RWH system.

Economic analyses, such as calculation of payback time or comparison of the cost with

alternative technologies, shall not be undertaken due to lack of necessary information and time

constraints.

4.6 Presentation of design

4.6.1 Design Report

Within the design report the assumptions that have been used in the design shall be clearly

stated. This will allow engINdia, villagers and any other organisations which use the design to

understand the design criteria that have been used and therefore the capabilities and reliability of

the RWH system. The design shall also be clearly explained, step-by-step, to allow it to be

adapted if conditions in Pabal alter or if the system is adapted for use elsewhere.

In addition to describing the design, the report shall have a section to provide guidance on

operational and maintenance issues to the users/operators of the RWH system. This will help the

users/operators to make best use of the system and aid a longer operating life for the system.

4.6.2 Drawings

Where appropriate, technical drawings will be produced to aid the understanding of the design.

This will be particularly useful for members of the village who are illiterate or do not speak

English.

To make it clear what each drawing is showing, all drawings will be clearly labelled with an

appropriate title. Drawings will also be given a unique reference number so they can be referred

to in the design report.

4.7 Contact with outside organisations It is important to maintain good communication with contacts in Pabal throughout this project.

This will help to increase the sense of ownership that the local community has of the solution

and help them feel confident in the operation and maintenance of the RWH system. Furthermore

with Pabal changing at a very rapid rate (engINdia, 2005), regular communication will ensure

that the solution is appropriate and sustainable.


- 32 -

5 Method Statement

5.1 Scope of design The scope of the design is as stated in the Proposed Method Statement. The system has been

designed for a design life of 10 years due to RWH being a temporary solution to Pabal’s water

shortages (see 1.5). The RWH system shall use rooftops as the catchment surfaces rather than

ground surfaces since this runoff is likely to be less contaminated than that from the ground (see

3.6.1). This design has focussed on the core of Pabal as this is the area of the metropolitan

district with the highest population density and where the problem with water shortages is the

greatest.

5.2 Details of buildings

5.2.1 Domestic houses

Typically 4 or 5 people share a house (www.engindia.net , accessed 19/03/2009) and therefore

an average household occupancy of 4.5 people has been assumed. Based on a population of 3

000 for the core of Pabal, it has been assumed that there are 667 houses within the core.

Approximately 50% of the houses in the core are of the traditional type, consisting of wooden

framed roofs with ceramic tiles on top (EWB, 2008). The majority of the remaining houses have

corrugated steel roofs with only a few richer households having flat concrete roofs (personal

communication: Lara Lewington). It has been assumed that houses with flat concrete roofs

make up 5% of the more modern constructed houses. Due to rainwater not draining off flat roofs

easily, these concrete roofed houses have not been included in the design for RWH. Hereafter

clay tiled roofs shall be referred to as Roof 1 and corrugated steel roofs as Roof 2.

Figure 5.1: Ceramic tiled roof (background) and corrugated steel roof (foreground) (Photo Credit: Lara

Lewington)


- 33 -

The typical dimensions of a room in a house in Pabal are 3 x 3.7m, with each house having on

average 1 to 2 rooms (www.engindia.net, accessed 19/03/2009). The height of the roof is

normally between 3.0 and 3.6m (www.engindia.net, 19/03/2009) with most houses being single

or double storey and the roof being double pitched (personal communication: Pooja Wagh).

However these details of houses are not consistent across Pabal; for example newly constructed

houses can be up to three or four storeys, the dimensions of rooms in houses for larger families

of 15 people or more are typically 3.0 x 6.0m and some houses can have up to 4 rooms

(www.engindia.net, accessed 19/03/2009). Despite these variations, a house with dimensions

shown in Figure 5.2 has been assumed. No information on roof overhang was available and

therefore in the calculation of available runoff, a conservative assumption of the roof overhang

being zero has been used.

Figure 5.2: Dimensions of a typical house in Pabal

5.2.2 Non-domestic buildings

Table 5.1 gives details of the non-domestic buildings that have been considered in terms of

RWH. Where information was not available, details have been assumed based on the

information that was known.

B

B

3.7m

0.3m

6.0m

3.3m

10°

Section A-A

A

3.0m

A A 7.4m

3.0m

3.7m

3.0m

Plan

0.3m

1.88m

Cross section of roof

Section B-B


- 34 -

Table 5.1: Details of non-domestic buildings (Personal communication: Lara Lewington)

Building Number Dimensions Detailing of roof

Shops and cafes 50 4.0 x 5.0m Single pitched (5°) corrugated steel roof

Hotels 2 15.0 x 20.0m Double pitched roof (10°) corrugated steel roof

Doctor surgeries 2 10.0 x 15.0m Single pitched (5°) corrugated steel roof.

Secondary school 1 15.0 x 30.0m Double pitched (10°) clay tiled roof

Primary school 2 10.0 x 25.0 m Double pitched (10°) clay tiled roof

5.3 Meteorological conditions

5.3.1 Current rainfall data

The monthly precipitation time series (1901-2002) obtained from the Indian Water Portal gave

an average annual rainfall of 1418 mm/yr. This was found to be inconsistent with values

obtained from other sources such as the World Weather Information Centre which quoted a

value of 722 mm/yr and from www.rainwaterharvesting.org which quoted a value of 782

mm/yr. A possible reason for this inconsistency was that the time series had been extrapolated

from a climate model rather than being from observed data. This time series was discarded but

due to insufficient time it was not possible to source an alternative monthly rainfall record.

Average annual monthly precipitation for Pune

0

20

40

60

80

100

120

140

160

180

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Ave

rag

e p

reci

pit

atio

n (

mm

)

Figure 5.3: Average monthly precipitation for Pune (data from www.rainwaterharvesting.org, accessed

04/04/2009)

Instead average monthly precipitation values for the city of Pune, which is approximately 30

miles from Pabal and the nearest location for which data could be sourced, has been used in this

design. This data is displayed in Figure 5.3 and the raw data is given in Appendix B. Based on

this data and information from the villagers, the wet season has been assumed to be from May

until October and the dry season from November to April.


- 35 -

5.3.2 Simulation of future climate

The likely changes to water resources in Pabal due to climate change have been considered in

the design. These changes have been quantified using the projections for the sub-region of

South Asia given in the Intergovernmental Panel on Climate Change’s Fourth Assessment

Report (AR4).

AR4 predicted likely increases in precipitation for the three month periods of March – May,

June – Aug and Sept – Nov (IPCC, 2007). The projection for Dec to Feb depends on the level of

emissions; A1F1, the highest future emission trajectory, predicts a decrease whilst B1, the

lowest future emission trajectory, predicts an increase (IPCC, 2007). AR4 also predicted

increases in the annual precipitation, in the inter-annual variability of daily precipitation in the

Asian summer monsoon and in the occurrence of extreme weather events such as intense

precipitation.

Table 5.2: Projections for changes in precipitation for South Asia sub-region for the period 2010-2039 (IPCC, 2007)

% change in precipitation (with reference to the baseline period of 1961 –

1990)

A1F1 B1

Dec, Jan & Feb -3 4

March, April & May 7 8

June, July & Aug 5 7

Sept, Oct & Nov 1 3

Since the design life of the RWH system is 10 years, projections for the climate in 2020 have

been considered. The projections from AR4 for the change in precipitation in the time slice

2010 – 2039, with the reference baseline period of 1961-1990, are quantified for the A1F1 and

B1 emission trajectories in Table 5.2. Due to B1 predicting the greatest increases in

precipitation, it is this emission trajectory that has been used in the design as the 2020 scenario.

The use of this trajectory shall increase the chance of the storage tank having sufficient capacity.

It is realised that the B1 trajectory may predict that a greater demand can be met from DRWH

than is possible and this must be made clear to the villagers.

The changes in precipitation over the period of 2010 to 2039 appear to be relatively linear

(IPCC, 2007). Since the time period of the current observed data (1981 – 2006) is reasonably

comparable to the baseline period used in AR4, 50% of the change stated in Table 5.2 has been

used to estimate the 2020 scenario. Assuming the % change for each three month period is

constant over each of the three months in that period, Figure 5.4 shows the estimates for

monthly precipitation in 2020 (See Appendix B for values).


- 36 -

Observed average monthly precipitation and estimated monthly precipitation in 2020

050

100150200250300350400

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month

Pre

cip

itatio

n (m

m)

Observed precipitation (1981 - 2006) Estimated precipitation (2020)

Figure 5.4: Comparison of observed monthly precipitation and estimated monthly precipitation for 2020

5.4 Determination of the demand to be met

5.4.1 Current demand

The 2005 population figure was the most recent that was available and therefore the current

demand shall be based on the 2005 population of 3 000. To encourage water conservation, a

domestic demand of 100 lcd has been used. Most households keep goats and/or chickens

(personal communication: Lara Lewington) and therefore the water demand for these animals

has also been accounted for. Typically a goat requires 4 litres per day and 100 chickens require

30 litres per day (HR Wallingford, 2003). Assuming on average each household keeps 2 goats

and 5 chickens, the water demand for this livestock will be 9.5 litres per day.

Table 5.3: Unit demand and daily demand for institution types in Pabal’s core (Twort, 2000 & HR Wallingford, 2003)

Type of business Unit demand Total demand

(l/day)

Hotels 250 l/day per bed 7 500

Schools 25 l/day per pupil and staff 7 300

Launderettes 12 m3/tonne of laundry 7 200

Shops, businesses, cafes etc

16 lcd (applied as a per capita allowance to the whole population)

48 000

Using the list of non-domestic institutions specified in Table 5.1 and unit demand figures, the

non-domestic demand has been quantified for each type of institution (Table 5.3). There are

three launderettes in the core of Pabal and since these use a considerable volume of water their

demand has been quantified separately from the generic ‘business’ demand. The unit demands

obtained from literature related to locations where there is a piped water system and waterborne


- 37 -

sanitation. Therefore these demands have been adjusted to account for the fact that Pabal uses

pour-flush toilets (www.engindia.net, accessed 02/04/2009)

5.4.2 Future demand

Due to national population growth, the number of people requiring water in Pabal’s core is

likely to increase. Furthermore as the area develops economically and socially, the domestic

demand per capita is expected to also increase. engINdia also predicts that the presence of VA

and an improved water supply will encourage people to migrate to Pabal’s core

(http://engindia.wikidot.com/population, accessed 04/04/2009).

Due to a large proportion of India’s population growth being predicted to occur in the cities, a

projection of growth of the rural population has been used to estimate the expected population

growth for Pabal. These projections have been obtained from the UN’s Population Division and

are shown in Table 5.4. To account for the migration to Pabal due to VA and the improved

water supply, an additional 7% population growth has been added onto these projections. The

estimated population of the core of Pabal at 5 year intervals is given in Table 5.5. It has been

assumed that the population growth will result in the average number of people per household

increasing to 4.8 and the number of houses in the core increasing to 744.

Table 5.4: Projections for % change in rural population of India (UN Population Division, 2007)

Year % change in rural population (compared to 2005)

2010 5.5

2015 9.7

2020 12.1

Table 5.5: Population projections for Pabal

Year Estimated population

2005 3 000

2010 3 375

2015 3 501

2020 3 573

The economic and social development of Pabal has been factored into the design by assuming

that the average domestic demand will increase to 120 lcd. This demand may be increased

further due to higher temperatures as a result of climate change but this factor has not been

included in the demand. The non-domestic demand in 2020 has been estimated by increasing

the current demand by 19.1% in accordance with the 19.1% population growth.

5.5 Determination of the most appropriate RWH system The volume of water that is available from the current water supply system in the core of Pabal

during a typical month of the wet and dry season has been estimated. To estimate the volume of


- 38 -

water available from the dam water tank, it has been assumed that the tank is filled to capacity

(170 000 litres) every day apart from two days per month when a power cut prevents the tank

being filled. The volume of water available from the wells is assumed to be insignificant during

the dry season due to negligible or zero groundwater recharge. In the wet season, the volume of

water available from the wells is quantified by the estimation of groundwater recharge over the

area of the core of Pabal (12.6 km2). Groundwater recharge is taken as the average difference

between the monthly precipitation and potential evapotranspiration. Since the supply of water

coming from government tankers is not sustainable, as there is no guarantee in the government

continuing this operation, this contribution to the water supply has been ignored. The supply of

water from the village tank has not been quantified separately since this supply comes from the

wells.

By comparing the volume of water available from this current supply and the total demand, the

demand that is currently not met has been determined. Three different options for RWH were

assessed to establish what proportion of this unsatisfied demand each option could meet. The

three options were:

• Option 1: all domestic houses would have their own RWH system to meet a proportion

of the domestic water demand.

• Option 2: the runoff from all non-domestic buildings would be collected in a communal

tank and used for domestic and/or non-domestic uses

• Option 3: all buildings in Pabal (domestic and non-domestic) would have a RWH

system where the runoff would be used for domestic and/or non-domestic uses

For each option the total available annual runoff was calculated by summing the area of all the

roofs and accounting for losses. Losses on the roof (due to spillage, leakage, infiltration,

catchment surface wetting and evaporation) were factored into the design by multiplying the

volume of rainfall by the runoff coefficient. The runoff coefficient for Roof 1 was taken as 0.55

and 0.85 for Roof 2. The runoff was also reduced by 10% due to the inefficiency of the coarse

filter and 20 litres per roof was subtracted from the runoff in May to make allowance for the

first flush system.

For the most feasible option, it was checked that this option was still viable using the estimated

precipitation and demand for 2020.

5.6 Design of the storage tank The method used for sizing the storage tank is the same as that stated in the Proposed Method

Statement. To determine the greatest required storage capacity, the required capacities based on

the demand and runoff for the current situation as well as for the 2020 scenario were considered.


- 39 -

The fluctuation in demand with the water level in the tank described in 3.8.2 was not considered

in the design.

The use of ‘average rainfall’ data in the calculation of required capacity does not account for

storm events where the precipitation is larger than average. To account for this, as well as the

increase in the frequency of extreme rainfall events due to climate change, an additional 10%

capacity has been added on to the calculated storage capacities. Due to the difference in %

runoff from Roofs 1 and 2, the sizing of the storage tanks for these two types of houses has been

carried out separately. The storage tank for Roof 1 shall be referred to as Tank 1 and the tank for

Roof 2 as Tank 2.

5.7 Design of conveyance system

5.7.1 Design rain storm

The gutters have been designed to have the capacity to transfer the discharge from the most

severe storm that can reasonably be expected. The maximum discharge will occur for rainfall

intensity of duration equal to the time of concentration, where the time of concentration is “the

time required for a drop of water to travel from the most hydro-logically remote point in the

sub-catchment to the point of collection” (Gupta, 2007). The time of concentration has been

calculated using Kirpich’s formula (Eqn 5.1). Since for design purposes the minimum rainfall

duration is 15 mins, where Tc < 15 mins the value for rainfall duration has been taken as 15 mins.

385.0

1

77.00195.0 −

=∑= i

n

iiC SLT Eqn (5.1) (Gupta, 2007)

Tc = time of concentration (mins)

Li = overland flow length of ith stretch (m)

Si = average slope of the ith stretch of overland flow

n = number of stretches

The critical rainfall intensity has been calculated using the rainfall intensity-duration-frequency

relationship which is stated in Eqn 5.2. This relationship was developed by the Central Soil and

Water Conservation Research and Training Institute, Dehradum, India.

m

n

bD

KTi

)( += Eqn (5.2) (Jain et al, 2007)

i = rainfall intensity (cm/hr)

D = rainfall duration (hour)

T = return period (years)

K, n, b & m: constants


- 40 -

A return period of 3 years has been used, based on Parkinson & Mark (2006) recommending a

storm return frequency of 2 - 5 years for suburban residential districts in developing countries.

The constants used in Eqn 5.2 are specific for each catchment and depend on the local

metrological data. Values for the catchment of Bhopal, India have been used (Table 5.6).

Bhopal has an annual rainfall of 785mm (Gupta, 2007) and was therefore the most similar

location to Pabal for which values were available. Using the critical rainfall intensity, the

relevant runoff coefficient was applied to the Rational Formula (Eqn 4.4) to gain the critical

discharge for the gutters for Roof 1 and 2.

Table 5.6: Values for constants for rainfall intensity-duration-frequency equation for Bhopal, India (Jain et al, 2007)

K n b m

Bhopal 6.93 0.189 0.5 0.878

5.7.2 Gutter detailing

To determine the dimensions of the cross-section of the gutters, open channel flow was assumed

and Manning’s equation (Eqn 5.3) applied. The width and cross sectional area of the gutter

determine the volume of water the gutter can hold and the cost mainly depends on the perimeter

(Thomas & Martinson, 2007). Therefore in order to maximise the efficiency of the gutters, and

minimise the cost, dimensions have been chosen that maximise the width and the cross sectional

area for a given perimeter. For a trapezoidal section, the most efficient cross-section is shown in

Table 5.7. These relationships have been used in the determination of the most appropriate

dimensions of the guttering but have been rounded to the nearest 5mm for ease of manufacture.

For metal channels a value of 0.013 for Manning’s Coefficient has been used (Chow, 1985).

21

03

2SR

n

AQ

= Eqn (5.3) (Hamill, 2001)

Q = discharge (m3 s-1)

R = hydraulic radius (m)

n = Manning’s coefficient (s m-⅓)

S0 = slope of channel

A = channel cross-sectional area (m2)

Table 5.7: Geometric characteristics of the most hydraulically efficient trapezoidal cross-section (Hamill, 2001)

Area of flow (A)

Wetted perimeter

(P)

Hydraulic radius (R

= A/p)

Surface width (BS)

Hydraulic mean depth (DM = A/BS)

1.732D2 3.463D ½ D 2.309D 0.750D

x x

x

D


- 41 -

5.8 Water Quality

5.8.1 Estimation of water quality parameters

Since testing of rainwater from Pabal to determine its water quality has not been possible, water

quality parameters have been used from other studies. For the chemical composition of

rainwater, values published by Satsangi et al (1998) for the site of Gopalpura in India have been

used. Gopalpura is a rural site with similar levels of population density and proximity from

major town or cities as Pabal. A value for pH of the rainwater of 6.0-7.5 has also been taken

from Satsangi et al (1998). No data for biological water quality was available for a similar site

in India, so values have been used from the study carried out by Efe (2006) in a rural area of the

Delta State of Nigeria. Data from the study by Yaziz (1989), referred to in 3.6.1, has been used

to estimate the change in water quality parameters as a result of water coming into contact with

the roof. Yaziz (1989) carried out the study on corrugated iron roofs and concrete tiled roofs. It

shall be assumed that the effect on water quality of Roof 1 is the same as that of concrete tiled

roofs and the effect of Roof 2 the same as corrugated iron roofs. See Appendix C for details of

the data obtained from these studies. Table 5.8 estimates the water quality parameters of runoff

from Roofs 1 and 2.

Table 5.8: Estimation of chemical, biological and microbiological parameters for rooftop runoff in Pabal Rainwater Roof 1 Roof 2

pH 6.8 (a) 7.8 7.5 Turbidity (NTU) 4.7 (b) 58 24

Total solids (mg/l) 20 200 115 Total suspended solids (mg/l) 10 (b) 146 84 Total dissolved solids (mg/l) 10 (a) 50 31 Faecal coliforms (/100ml) 0 (b) 13 4 Total coliforms (/100ml) 0 (b) 75 46

(a) Satsangi et al, 1998 (b) Efe (2006)

5.8.2 Calculation of water quality improvements due to storage of water

To calculate the settlement of the suspended particles during the time that the water is stored in

the tank, Stoke’s law shall be applied (Eqn 5.4). The density and viscosity of water varies with

temperature (Parsons & Jefferson, 2006). Based on an average temperature of 25°C (see

Appendix B), the density and viscosity of water shall be taken as 996.96 kg m-3 and 0.9047 x

10-3 kg m-1 s-1 respectively (Parsons & Jefferson, 2006). In order for Stoke’s law to apply it

shall be assumed that the particles fall in a laminar flow field (Parsons & Jefferson, 2006).

Obviously the particle diameters and densities will vary in the runoff, but for the basis of the

calculations the values quoted in Table 5.9 shall be used.


- 42 -

µρρ

18

)( 2dgv p

t

−= Eqn (5.4) Parsons & Jefferson (2006)

vt = terminal settling velocity (m s-1)

ρp = particle density (kg m-3)

ρ = density of water (kg m-3)

d = particle diameter (m)

µ = viscosity of water (kg m-1 s-1)

Table 5.9: Size and densities of suspended particles (Hendricks, 2006)

Type of particle Particle specific gravity Particle size (mm)

Sand, seeds 1.2 – 2.65 ≤ 0.2

Irregular particles – mostly organics 1.0 – 1.2 < 0.5

Silica, clay, silt mineral particles 2.65 < 1

5.9 Suitable materials Bamboo has been used in the design wherever feasible due to its availability in Pabal (personal

communication: Pooja Wagh). It has been assumed that due many of the buildings in Pabal

being constructed out of concrete, clay, timber, corrugated metal, stone and lime and mud bricks,

that these too are suitable materials that can be sourced locally.

5.10 Equipment available locally It has been assumed that equipment at VA will be available to be used in the construction and

maintenance of the RWH system. The equipment at VA which may be of use in the construction

of the RWH system includes cutting machines, soldering equipment and drawing tools

(www.vigyanashram.com, accessed 01/05/2009):

5.11 Geotechnical analysis In Pabal the soil comprises of medium black soils which tend to be clayey and fine grained and

are typically 6m deep (Survey of India, 2002 & Jain, 2007). The rock underlying this soil is

basalt (personal communication: Chetan Shenoy). With this soil and rock type, the load on the

ground due to the storage tank can be assumed to be considerably less than the strength of the

ground. Therefore no geotechnical analysis of the RWH system has been carried out.

5.12 Economic analysis

Contrary to 4.5, cost benefit analysis has been undertaken due to suitable economic information

being able to be sourced. The payback period of the RWH system has been calculated since

from literature this method appears to be the most common economic appraisal technique

advised for RWH. The payback period is defined mathematically in Eqn 5.5. To assess whether

the system is ‘low cost’ the criterion stated by DTU (2002) that in South Asia the payback


- 43 -

period should not exceed 2 years has been used. This is instead of the figure stated in 4.3.2 of

US$ 100 per 100 litres, which is not flexible to different sized RWH systems.

Payback period = Cost of construction Eqn (5.5) Annual benefit

The operational and maintenance costs of DRWH are small compared to the construction costs

(Thomas & Martinson, 2007) and therefore only the construction costs have been considered in

the calculation of payback period. Costs from similar projects have been used to obtain

estimates for the construction costs of storage, guttering and water treatment. All these costs

relate to the last 3 years and therefore inflation has not been factored in. An elasticity value of

0.8 (Rees, 2000) has been used to convert the cost of storage specified in other projects to an

estimate for the storage capacities specified in this design. Where costs have been converted

from Indian Rupees to US Dollars, the exchange rate of 21/05/2009 of 0.0210719 has been used

(www.xe.com, accessed 21/05/2009).

5.12.1 Valuation of water

The benefits of the RWH system have been quantified in terms of water quantity benefits (the

benefits of householders have an increased volume and reliability of water) and water quality

benefits (the benefits of householders having water of higher quality).

The cost of water from Government tankers during the dry season is 2 Rupees per 15 litres

(EWB, 2008). Therefore water during the dry season has been valued at 0.13 Rs (US$ 0.0027)

per litre. However due to the water shortages experienced in the dry season, a unit of water

obtained during the dry season would be expected to be of higher value than a unit in the wet

season. Water has been valued at 0.00054 US$/litre for the wet season, based on the estimation

by DTU (2002) that in monsoon climates water is valued approximately 5 times higher in the

dry season compared to the wet season.

Table 5.10: Economic value of different grades of water during dry and wet season (% values obtained from DTU (2002))

Dry season Wet season

Typical % of total value of water

Value (US$/month)

Typical % of total value of water

Value (US$/month)

Potable 43 8.62 28 2.25

Cooking, boiling etc 27 5.41 22 1.76

Washing, laundry & livestock

27 5.41 45 3.61

Cleaning, horticulture and building purposes

3 0.60 5 0.40

Based on current water consumption figures of 110 lcd in the wet season and 55 lcd in the dry

season, the current value of the total water per household per month is US$ 8.02 in the wet


- 44 -

season and US$ 20.05 in the dry season. Due to the water being of variable quality, water will

not have a constant value but instead its value will depend on its use. Using estimates for the %

value of different uses from DTU (2002), the value of water used for different uses in Pabal has

been quantified (Table 5.10).


- 45 -

6 Design

6.1 Determination of most appropriate RWH system

6.1.1 Current supply-demand balance

From Table 6.1, it can be determined that the average groundwater recharge over the wet season

is 133mm/month and that zero or negligible amounts of recharge occur over the months of the

dry season. Using this average value of recharge and accounting for the other sources of water,

an estimation of the volume of water that is available during the wet and the dry season in the

core of Pabal is given in Table 6.2.

Table 6.1: Recharge for the Pabal catchment (PE data from Indian Water Portal, 2009)

Month

Average rainfall (R) (mm)

Average potential evapotranspiration (PE) (mm)

R – PE (mm)

Zero recharge/recharge

Jan 1.7 6.11 -4.41 Zero recharge Feb 1.5 6.80 -5.3 Zero recharge Mar 0.6 7.39 -6.79 Zero recharge

Apr 9.8 7.70 2.1 Recharge

(insignificant) May 30 7.52 22.48 Recharge Jun 171 6.04 164.96 Recharge Jul 171.4 4.80 166.6 Recharge Aug 139.5 4.64 134.86 Recharge Sep 141.7 5.19 136.51 Recharge Oct 85.8 6.11 79.69 Recharge Nov 21.5 6.11 15.39 Recharge

Dec 7.4 5.87 1.53 Recharge

(insignificant)

Table 6.2: Estimation of volume of water available to Pabal’s core from current water supply infrastructure

Supply (m3/month)

Dry season (Nov – Apr) Wet season (May – Oct)

Supply from dam water tank 4 760 4 760

Town/private wells 0 2 212

Total 4 760 6 972

Comparing the domestic and non-domestic demands (Table 6.3) with the available water supply

(Table 6.2), it can be estimated that in the dry season the demand is 137% larger than supply (a

supply deficit of 6 511 m3/month) and in the wet season the demand is approximately 62%

greater than supply (a supply deficit of 4 299m3/month). This approximates to the supply

demand balance during the dry season being roughly two times more severe than in the wet

season. Annually the demand that currently cannot be met by the water infrastructure is 64 872

m3.


- 46 -

Table 6.3: Current demand for water in Pabal’s core (based on 2005 population)

Monthly demand (m3/month) Annual demand (m3/yr)

Domestic 9 171 110 052

Non-domestic 2 100 25 212

TOTAL 11 271 135 264

6.1.2 Annual rainwater harvesting potential of different options

The available runoff for the three options described in 5.7 is shown in Table 6.5 (for monthly

runoff values see Appendix D). Comparing these runoffs with the current supply demand

balance shows that even if all the domestic and non-domestic buildings are used for RWH, only

13% of the demand that is currently not met could be satisfied.

Table 6.4: Calculation of annual runoff for different scenarios

Annual runoff (m 3/yr)

Option 1 (domestic houses) 6 940

Option 2 (non-domestic institutions) 1 397

Option 3 (domestic and non-domestic) 8 337

From Table 6.4 it can also be seen that the majority of the roof runoff comes from domestic

houses. Therefore the implementation of RWH on non-domestic buildings is likely not to be

economically efficient. Based on this, the most appropriate option is Option 1 where part of the

domestic demand is met by all houses having RWH. There are two possible alternatives for the

proportion of the domestic demand that could be met:

i) rainwater could be used as a potable water source (drinking and basic hygiene),

providing 5 to 7 litres per person per day throughout the year (Thomas & Martinson,

2007)

ii) rainwater could be stored and then used during the dry season when the water stress is

largest

Table 6.5: Estimated cumulative runoff and potable water demand for 2020

Current 2020

Roof 1 8.71 9.04 Estimated runoff (m3/yr)

Roof 2 13.47 14.00

Due to the poor water quality of the current water sources and the high storage cost of

conserving rainwater for the dry season, the potable water demand shall be satisfied. Table 6.5

shows the estimated runoff from Roof 1 and Roof 2 for the current situation and the 2020

scenario. Based on this available runoff, Table 6.6 indicates the daily domestic demand that

would be able to be met. This runoff should provide enough water to meet the potable water

demand, with any additional water being used for cooking and basic hygiene.


- 47 -

Table 6.6: Estimated daily domestic demands that can be met from RWH

Current 2020

Roof 1 5.4 8.3 Daily demand (lcd)

Roof 2 5.2 8.0

6.2 Storage tank

6.2.1 Sizing of storage tank

It was found that the largest storage capacity was required for the 2020 scenario and therefore

the sizing of the tank shown here shall be for this scenario (the graphs for the current situation

are in Appendix E). Figures 6.1 and 6.2 show that for both Roof 1 and 2 the first month where

the runoff is greater than the demand is June. It shall therefore be assumed that the tank will be

empty at the end of May.

Comparison of monthly runoff from Roof 1 and monthly potable water demand (2020)

0.00

0.50

1.00

1.50

2.00

2.50

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Month

Vo

lum

e (m

3 )

Monthly runoff Monthly demand

Figure 6.1: Monthly runoff from Roof 1 and monthly potable water demand for 2020 scenario

Comparison of monthly runoff from Roof 2 and monthly potable water demand (2020)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Month

Vo

lum

e (m

3 )


Figure 6.2: Monthly runoff from Roof 2 and monthly potable water demand for 2020 scenario


- 48 -

From predicting the inflow and outflow for Tanks 1 and 2 (Figures 6.3 and 6.4), it can be seen

that the largest storage requirement for both tanks occurs in October. Tank 1 will require 4.41m3

of storage whilst Tank 2 will require 6.81m3 of storage. Allowing for the fact that this capacity

is for ‘average’ conditions, the design capacities of Tank 1 and Tank 2 shall be 4.85m3 and

7.50m3 respectively. The inclusion of this additional capacity will mean that an overflow tank is

not necessary which shall save space and cost.

Comparison of cumulative monthly runoff from Roof 1 and potable water demand (2020)

0.001.002.003.004.005.006.007.008.009.00

10.00

June Ju

ly

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Janu

ary

Febru

ary

Mar

chApr

ilM

ay

Month

Vo

lum

e (m

3 )

Cumulative monthly runoff Cumulative monthly potable water demand

Figure 6.3: Predicted inflow and outflow for Tank 1 for 2020 scenario

Comparison of cumulative monthly runoff from Roof 2 and cumulative monthly potable water demand (2020)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

June Ju

ly

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Janu

ary

Febru

ary

Mar

chApr

ilM

ay

Month

Vo

lum

e (m

3 )


Figure 6.4: Predicted inflow and outflow for 2 for 2020 scenario

6.2.2 Tank detailing

Due to the problems of UGTs outlined in 3.4.3 and the risk of an UGT undermining the

structure of the house, the storage tanks shall be AGTs. Despite the advantages stated in the

3.4.1 of multiple tanks compared to a single tank, a single tank shall be used since the tank will


- 49 -

only be storing a relatively small amount of runoff. This tank shall be placed at one end of the

building against the wall to minimise space and make the collection of water from the tank as

convenient as possible for the householders. The exact position of the tank will vary between

houses depending on the layout of each house. However for the basis of this design the tank

shall be positioned at the midpoint of wall which is 3.7m in width (Figure 6.5).

Figure 6.5: Assumed position of water storage tank

The local availability of bamboo makes it a suitable material to use in the construction of the

tank. In current practise, bamboo is used as a structural component for water tanks in two ways:

• treated bamboo poles are erected inside a plinth of cement and stones, with bamboo

strips being woven in-between the poles. The inside of the structure is lined with a

plastic film (a food grain polyethylene film) (Figure 6.6). This tank has been proposed

by ARTI (DTU, 2001).

• bamboo is used as reinforcement in the tank. The filler material can either be concrete

or mud. It was found by Martinson et al (2002) that mud expands under loading which

can crack the lining and cause leakages. The effect on water quality of having mud

lining is also questionable.

Figure 6.6: Plastic lined bamboo tank (DTU, 2001)

A concrete reinforced bamboo tank shall be used since concrete is more likely to be able to be

easily sourced than plastic and leakages are less common when concrete is used compared to

mud. Bamboo will be bent to form a cylindrical reinforcement cage (Figure 6.7), using heat at

3.7m

1.85m


- 50 -

the roof of the tank to bend the bamboo (Li et al, 2002). Bamboo strips will be used to tie the

reinforcement cage together. The tank should be placed on a 0.1m thick concrete base into

which the vertical bamboo poles should be cemented. To minimise dimensional change during

curing, species of bamboo that absorb little water such as Dendrocalamus giganteus or

Bambusa vulgaris schard (Ghavami, 2005) should be used if these are available locally.

Figure 6.7: Bamboo reinforced concrete tank

The outlet of the tank shall be a tap that is located 550mm above the base of the tank. This

should allow a bucket to be placed underneath the tap and be high enough from the base to

prevent resuspension of the settled matter (see 3.7.3). The water below the tap level will not be

able to be extracted but due to the additional 10% capacity, this ‘dead storage’ shall be included

in the total storage capacity of the tank.

Assuming a diameter of bamboo of 30mm (Ghavami, 2005), vertical bamboo poles should be

placed 50mm apart with 20mm cover on either side, as shown in Figure 6.8. The horizontal

bamboo poles should also be placed 50mm apart, with the lowest horizontal pole being 50mm

above the top of the concrete base. For the concrete, a mortar mix of 1:2 cement-sand ratio by

weight is advised (Li et al, 2002). The thick consistency of the mortar mix and the closeness of

the bamboo poles should mean that the mortar can be applied by hand using a technique similar

to plastering a wall. Therefore no formwork will be required.

Circular tanks shall be used since they will be easier to clean than rectangular tanks. The height

of the tank shall be limited to 1.5m to allow householders to easily lift the lid and to make

cleaning easier. With a height of 1.5m, the diameters of Tanks 1 and 2 shall be 2.03m and

2.53m respectively with corresponding volumes of 4.85 m3 and 7.54 m3. The cross sections and

elevations of the tanks are shown in Figure 6.9 and 6.10.

Bamboo folded by heat at the top of the tank

Bamboo strips used to tie bamboo poles together

0.1m


- 51 -

Figure 6.8: Reinforcing details for bamboo-concrete water tank

Figure 6.9: Elevation and plan view of Tank 1

30mm

20mm

20mm

50mm

0.07m

2.03m

2.03m

A A

Section A-A

1.5m

0.07m

2.53m

2.53m

A A 1.5m

Section A-A


- 52 -

Figure 6.10: Elevation and plan view of Tank 2

Assuming a simple hydrostatic pressure distribution in the tank, the imposed loads (the

hydrostatic force) on the walls of Tank 1 and Tank 2 will be 35.2 kN and 43.9 kN respectively.

The maximum resistance of the concrete (assuming the strength = 10 MPa) is approximately 40

000 kN (see Appendix F for details of calculations). Therefore it can be deduced that the tank is

extremely overdesigned, with the bamboo only there to support the concrete and not to provide

any tensile resistance.

A corrugated metal cover for the tank should be made to act as a barrier to mosquitoes, avoid

animals and people falling in and prevent algal growth. Corrugated metal has been chosen as it

is lightweight which will make it easier to take the cover off for tank maintenance. The metal

should be galvanised to reduce the corrosion of the cover. The cover should be sloped at ½ % to

allow rainfall to drain off it and have an overhang of 30mm from the outside walls of the tank

(Figure 6.11). The required diameter of the covers for Tank 1 and Tank 2 are 2.23m and 2.73m

respectively. To prevent the cover being blown off in the wind, the cover should be attached to

the walls of the tank through a hook and a piece of wire.

Figure 6.11: Corrugated steel cover

Most of the failures of bamboo-reinforced concrete water tanks are as a result of a lack of

reinforcement due to the decay of the bamboo (Vadhanavikkit & Pannachet, 1987). To halt the

decay of the bamboo, a chemical solution of potassium dichromate, copper sulphate and boric

acid should be applied to the bamboo through either a pressure pump or soaking prior to

construction of the tank. This preservation technique has been developed at ARTI and claims to

make the bamboo non-biodegradable, protect it from fungal and insect attack and improve the

lifespan of bamboo by up to 15 years. (www.arti-india.org, accessed 13/05/2009).

There is a chance that the bacterium Legionella pneumophila that causes Legionellosis could

colonise the water in the storage tank. The temperature of the water during the summer months

is likely to vary between 22°C and 38°C (see Appendix B for temperature data). The bacteria

0.03 m

Fixing cover to tank

Metal hook

Metal wire


- 53 -

can live in water between 20 and 50°C (with an optimal temperature of 35°C) and requires a

source of nutrients such as organic matter (WHO, 2005). This bacterium is transmitted to people

through the inhalation of small droplets of water containing the bacteria (HSE, 2008). The

bacterium’s growth is considerably increased where the water is stagnant (US Department for

Labor, 2008). Despite suitable temperature conditions and the likely presence of nutrients, the

risk of Legionellosis is considered to be low due to the cover reducing the chance of water

vapour leaving the tank and the regular withdrawal of water from the tank preventing the water

becoming stagnant.

Additional tank components of a smoothing inlet, a suction filter, an overflow trap and a sump

and drain, as described in 3.4.4., shall not be included in the design in an attempt to keep the

cost of the storage tank to a minimum.

6.3 Conveyance system

6.3.1 Outline of conveyance layout

Gutters must be included as part of the design of the RWH system, since during engINdia’s visit

to Pabal in 2005 it was found that only relatively few houses had guttering (EWB, 2008). To

maximise the collection of runoff, gutters shall be put on both sides of the roof of each house.

Figure 6.12: Direction of runoff from roofs

Figure 6.13: Effect of sloping gutter on distance between roof and gutter

It shall be assumed that the runoff on the roof is perpendicular to the length of the roof as shown

in Figure 6.12. In order to make the water flow faster and therefore provide the gutters with

extra water carrying capacity, the gutters shall be sloped towards the end of the building where

the tank is located. A further reason for having a sloping gutter is to reduce the occurrence of

pooling of water which could provide breeding sites for mosquitoes. As the gutter slopes, the

distance between the roof and the gutter shall increase (Figure 6.13). Therefore the gutter shall

be divided into three sections; the first two sections having a slope of ½ % and the third section

(nearest the tank) having a slope of 1% (Thomas & Martinson, 2007).

1.85m

6m


- 54 -

A gutter shall be also used to transmit the runoff from the roof gutter to the storage tank. The

use of a gutter rather than a pipe will make construction simpler and easier, since fittings would

be required to connect the pipe to the roof gutter. The overall layout of the conveyance system

will be as per Figure 6.14.

Figure 6.14: Arrangement of guttering and downpipe

6.3.2 Design of roof gutters

Bamboo is not a suitable material to use for the guttering as it only lasts for approximately one

rainy season due to rotting of the organic material and the porous surface provides an ideal

environment for the accumulation of bacteria (Ferdausi and Bolkland, 2000 & DTU, 2002).

Instead it is advised that the guttering is made out of metal. This metal should be galvanised to

prevent the corrosion and mobilisation of the metal. The widespread use of metal for roofing in

Pabal confirms its availability and acceptance. The easiest shapes to bend metal into are

trapezoidal and ‘V’ sections, but since ‘V’ shaped sections are easily blocked by twigs and

leaves (Thomas & Martinson, 2007), a trapezoidal section shall be used for the guttering.

Table 6.7: Design storm parameters

Parameter Value

Rainfall duration (mins)* 15

Rainfall intensity (cm/hr) 10.98

Roof 1 0.17 Critical discharge (l/s) Roof 2 0.29

* the time of concentration (Tc) > 15 mins, therefore the rainfall duration has been taken as 15 mins

The parameters of the design storm are given in Table 6.7. The most hydraulically efficient

trapezoidal cross-section that can transmit the critical discharge from Roofs 1 and 2 are shown

in Figures 6.15 and 6.16 respectively. For detailed calculations of the sizing of the cross

sections, see Appendix F. An additional 10mm have been added onto the depths calculated

Slope ½ % Slope 1 %


- 55 -

using Manning’s Equation to allow for inaccuracies such as the estimation of Manning’s n and

the averaging of the slope over the roof length.

Figure 6.15: Roof gutter dimensions for Roof 1 Figure 6.16: Roof gutter dimensions for Roof 2

The horizontal distance between the edge of the roof and the gutter must be such that the gutter

intercepts as much runoff as possible. The speed of runoff on the roof, and therefore the path the

runoff takes when it falls off the roof, depends on the roofing material. Runoff on a clay roof

doesn’t travel at high velocity and therefore tends to fall vertically from the edge of the roof,

whilst runoff from a corrugated metal roof follows a curve (Figure 6.17). Therefore the

centreline of the gutter for Roof 1 shall be in line with the edge of the roof and the centreline of

the gutter for Roof 2 shall be offset 20mm from the edge of the roof (Thomas & Martinson,

2007). The consideration of the flow path in the design will remove the need for a Splash Guard

which reduces runoff loss by causing the runoff to flow vertically into the gutter.

Figure 6.17: Runoff patterns from clay and corrugated metal roofs (adapted from Thomas & Martinson,

2007)

The method of attachment of the gutters to the building shall depend on whether the house has a

corrugated steel or clay roof. For Roof 2, holes can be made in the steel and galvanised metal

wire used to hang the gutter from the roof. This method of attachment will not possible for Roof

1 since the weight of the gutter, particularly when it’s full of water, may pull the tiles that the

gutter is hung from off the roof. With the walls of the house being made from reinforced

concrete, stone and lime or red brick and mortar and being typically 22cm thick (personal

communication: Lara Lewington), it should be possible for metal supports to be attached to the

walls to support the gutter. Whichever method of support is chosen, it is advised that the gutter

is supported at least every 0.3m along its length. The gutter attachment for both types of houses

is shown in Figure 6.18.

80mm

40mm

35 mm 40mm

40mm

30 mm

60mm

30 mm

30 mm

Runoff from clay roof

Runoff from corrugated metal roof


- 56 -

Figure 6.18: Guttering attachments for Roof 1 and 2

The slope of the gutter should cause all runoff to flow towards the end of the house where the

tank is located. However it is recommended that a piece of metal is soldered on to the gutter at

the end furthest away from the tank to ensure runoff does not flow out of the gutter.

At each point along the gutter where there is a change in slope a new piece of metal will have to

be used. To minimise leakage, these joints must be properly sealed either by soldering or using

a waterproof sealant such as bitumen or tape (Thomas & Martinson, 2007). It is expected that

soldering would be the most appropriate technique to use due to VA having soldering

equipment but this decision will be left to the villagers’ discretion. Each section of the gutter is

2m long, however if it is not possible to source sheets of metal 2m in length joins will have to

be made within each section as well.

6.3.3 Transfer from roof gutters to storage tank

Trapezoidal sections shall also be used for the gutters to transfer the runoff from the roof to the

tank. In order to transmit the critical runoff, the depths of these gutters need to be 8mm for Roof

1 and 10mm for Roof 2 (see Appendix F for calculations). These depths are considerably less

than for the roof gutters due to the larger slope between the roof and the storage tank (a slope of

approximately 50%). However for reasons of simplicity and to allow these gutters to be

soldered onto the roof gutters, the same dimensions as the roof gutters (Figures 6.15 and 6.16)

shall be used. In order for these gutters not to obstruct doors and windows, the RWH system

should be placed on the end of the house where it will cause the least obstruction. The layout of

these gutters is shown in Figure 6.19. If it is felt that supports for these gutters are required, then

the supports specified for the roof gutters for Roof 1 can be used. The gutters from both sides of

the house will meet at the centre of the cover of the storage tank. Water will be discharged into

the tank through an 80mm diameter hole in the cover of the storage tank (Figure 6.20).

Roof 2

20mm

Roof 1

Centre line of gutter in line with edge of

roof


- 57 -

Figure 6.19: Gutters to convey water from roof gutters to storage tank

Figure 6.20: Inlet of gutters into tank

6.4 Water quality

6.4.1 Removal of debris

The debris that is likely to have collected on the roofs must be removed from the runoff before

it enters the tank. As was described in 3.6.1 and 3.7.1, significant amounts of debris are likely

to be washed off the roof in the runoff at the beginning of the wet season. A first flush system

should be used to divert 20 litres of runoff at the beginning of the wet season (World Health

Organisation, 2003). This shall prevent this poor quality runoff affecting the water quality of

the water in tank.

The approach for determining the volume of runoff to divert that Martinson & Thomas (2004)

suggested (see 3.7.1) is not feasible for Pabal. Using this approach would require approximately

1.5mm of rainfall to be diverted each time rainfall is preceded by 3 dry days. It can be seen from

Table 6.8 that this would result in a considerable proportion of the rainfall for the month of Jan-

May and Oct–Dec being diverted (based on the number of rainy days < 7.5 in these months).

80mm

Storage tank lid

1.50m

1.85m

3.12m

Roof 2

1.5m

1.85m

3.01m

Roof 1


- 58 -

Approximately 20% of the annual rainfall falls during these months and therefore this approach

would significantly reduce the available runoff.

Table 6.8: Number of rainy days each month (based on 25 year average (1982 – 2006)) (www.rainwaterharvesting.org, accessed 03/04/2009)

Jan Feb March April May June July Aug Sept Oct Nov Dec

No. of rainy days

0.2 0.1 0.1 0.9 2.2 9.6 12.2 9.8 7.9 4.7 1.2 0.4

Of the four different methods of diverting the first flush presented in 3.7.1, a manual system

shall be used. The disadvantage of a manual system, of having to rely on someone to move the

pipe to divert the flow, will be acceptable with the first flush system only being used once a year.

Furthermore, with use only once a year the cost of a more complex system cannot be justified.

The householder should simply lift the tank cover off and put a bucket, which has 20 litres

marked on, underneath the gutter. This first flush runoff should not be used for drinking but

could be used for non-potable uses such as laundry and bathing.

It is likely that it will not only be at the beginning of the wet season that considerable debris will

be transported in the runoff. Due to climate change resulting in more intense less frequent

rainfall events, it is probable there will be periods in the wet season where no rainfall occurs.

Such periods will allow debris to collect on the roof. To reduce the amount of debris that enters

the tank a coarse filter, as described in 3.7.2, should be placed in the opening of the tank cover.

This filter will not only act as a barrier to debris in the runoff but also to debris which could fall

into the tank through the opening and to insects such as mosquitoes. The filter (Figure 6.21)

should consist of a fine wire mesh with holes of approximately 1.2mm in diameter (Ashworth,

2005) and could be either brought or made by intertwining pieces of fine metal wire.

Figure 6.21: Coarse filter on tank cover

1.2mm 1.2mm


- 59 -

6.4.2 Water quality improvements due to storage

As was described in 3.7.3, the storage of the runoff in the tank shall reduce the concentration of

suspended solids and the concentration of microorganisms. The short settling time of the

particles in Table 6.9 suggest that the significant majority of the suspended particles shall be

removed as a result of storage. However the natural pathogenic die-off will be limited due to

the tank cover preventing UV radiation entering the tank.

Table 6.9: Terminal settling velocities for particles

Type of particle Terminal settling velocity

(mm s-1) Time required to settle to the bottom

(seconds)

Sand, seeds 22 68

Irregular particles – mostly organics

16 94

Silica, clay, silt mineral particles

996 1.5

6.4.3 Filtration

Filtration shall be used to reduce the levels of microorganisms in the water and further decrease

the concentration of suspended solids. A suitable low cost technique is the use of a sand filter.

From the comparison of rapid and slow sand filtration in 3.7.4, slow sand filtration has been

chosen due to:

• the biological layer improving microbial reduction

• the simpler design and less frequent cleaning of slow sand filters compared to rapid

sand filters

• the complex operation of rapid sand filters.

The slow sand filters will be implemented as stand-alone units to the RWH system at the

household level. This will allow the filters to treat water from other sources as well as from

RWH. Stand alone units will also mean that the filters do not have to be constructed at the same

time as the RWH system. This will allow the villagers to see if filtration of the rainwater is

necessary and also will help to spread the cost of the system over a longer period of time if

required.

Conventional slow sand filters rely on the continual flow of water to sustain the biological layer.

Based on a filtration rate of 0.1 m hr-1 (Parsons & Jefferson, 2006) and 45 litres of water being

filtered per day (10 lcd), the maximum cross sectional area to ensure the continual flow of water

would be 0.019m2. Constructing a filter of this cross sectional area would not be economically

viable. Therefore a slow sand filtration system that is designed to be used intermittently is more

suitable.


- 60 -

In intermittently used slow sand filters the design is altered to account for the absence of the

continuous flow of water. The filter is designed so that once the water has been filtered it enters

a collector pipe on the base on the filter via holes. The collector pipe is then raised back up to

between 2 and 3cm above the sand level (Figure 6.22) to ensure that the water level is

maintained above the sand (Palmateer et al, 1998). This constant water level sustains the

biological layer by transferring oxygen between the air and the biological layer via diffusion.

This arrangement is often referred to as a Biosand Filter and was developed at the Dr D Manz

and the University of Calgary, Canada in the early 1990’s. Due to the relative recent

introduction of such filters, only limited studies into their effectiveness have been undertaken

(Table 6.10).

Figure 6.22: Principle of intermittent-use slow sand filter

Table 6.10: The effectiveness of BioSand filters

Removal Source

98.5% of E.Coli

Turbidity reduction from 6.2 to 0.9 NTU

Duke et al, 2006

95 – 98% reduction of E. Coli in a ripened filter

80 -90% virus reduction in a ripened filter

Stauber et al, 2006

97% reduction of faecal coliforms

83% reduction in total heterotrophic bacterial populations, 100% of Giardia cysts and 99.98% of Cryptosporidium oocysts when administered in concentrations varying from 10 - 100 times environmental pollution levels.

50 – 90% of organic and inorganic intoxicants

Palmateer et all, 1999

95-99% of zinc, copper, cadmium and lead

Approx 67% of iron and manganese

Fort Lewis College, 2008

The filter media shall consist of a fine sand layer, a layer of coarse sand/medium gravel and a

layer of coarse gravel. The coarse gravel layer shall promote the vertical flow of water into the

collector pipes. The coarse sand/medium gravel shall stop the fine sand from clogging the

2 – 3 cm


- 61 -

coarse gravel and the collector pipe. The grain size of the coarse sand/medium gravel should be

1-6mm and the grain size for the coarse gravel 6 – 15mm, with both layers being 5cm thick

(www.biosand.org,, accessed 17/05/2009). A grain size of approximately 0.2mm should be used

for the fine sand. It is important that sand of larger grain size is not used since this would make

the gap between the grains larger, resulting in less material becoming trapped. Smaller grain

sizes also provide a greater surface area for growth of microorganisms and adsorption to occur

on. It is also important that sand which is too fine is not used since this could clog the filter. The

sand should have preferably been screened (possibly using a metal mosquito mesh screen) and

should be washed and be organic free (Morgan, 1990 & CAWST, 2007). To avoid

contamination, this sand should not be taken from the beach, rivers or areas where the sand has

come into contact with animals or people. Filtration and biological activity in a slow sand filter

extends to approximately 0.5m below the sand surface (Twort, 2000 & Parsons and Jefferson,

2006) and therefore the thickness of the sand layer should be approximately 0.6m thick.

A concrete vessel with walls of approximately 50mm thick should be constructed to contain the

filter. A suitable mix for the concrete is 2 part Type 10 Portland cement, 3 part clean pea gravel

(6mm) and 2 part clean sand (FHCC, 2006). Concrete has been chosen instead of plastic since

concrete is more durable than plastic, it provides protection for the collector pipe by housing it

in the vessel and construction from concrete allows the household to be involved in the

construction which promotes a sense of ownership.

Figure 6.23: Plan and cross section of slow sand filter

Plan

Fine sand 0.2mm

Coarse gravel

Coarse sand/medium gravel

0.94m

0.5m

50mm

30mm

50mm

600mm

110mm

50mm

Cross-section

50mm

0.05m

0.7m

0.7m

Section A-A

A A

13mm

LID


- 62 -

Based on field and lab tests of the Biosand Filter, the most suitable flow rate to maximise filter

efficiency is 0.6 litres per minute (CAWST, 2007). This flow rate corresponds to when the

reservoir is full; as the water level drops, according to Darcy’s law the flow rate will decrease.

Using a filtration rate of 0.15 m/hr (Parsons & Jefferson, 2006), the required cross-sectional

area of the filter to achieve 0.6 litres/min is 0.24m2. Using a square cross section, the required

length of each side is 0.5m. Based on householders requiring potable water twice a day and 45

litres of water being filtered per household per day, the reservoir in the filter must have a

capacity of at least 25 litres. A depth of the reservoir (space above the sand layer) of 0.11m will

provide this capacity. Water can be poured into the filter and treated water collected

immediately. The details of the filter are shown in Figure 6.23.

To provide a flow rate of 0.6 litres per minute, the collector pipe should be 13mm in diameter

(FHCC, 2006). A PVC pipe should be used with holes every 20mm along the pipe made by saw

cuts or drilling. The pipe should be laid on the bottom of the filter in a circle with the holes

facing downwards.

A concrete lid should be constructed to be placed over the filter to prevent contamination of the

filter from the air/animals etc. A 15mm hole in the lid will allow water to be poured into the

filter without the lid needing to be removed. To avoid erosion of the sand when pouring water in,

a stone should be placed directing underneath the hole. This stone will reduce the velocity of the

water before it hits the sand.

The biological layer initially takes three weeks to reach maturity after the filter has been

constructed (CAWST, 2007). Therefore water obtained from the filter should not be used for

potable uses for the first three weeks of the filter’s operation. Biosand Filters have a typical

lifetime of 6 to 10 years (Jeuland & Whittington, 2009) and therefore a single filter should

hopefully last the complete design life of the RWH system.

6.5 Distribution After the quality of the water has been improved through settlement and filtration, it is

important that the householders use suitable practises to collect and store the water prior to

consumption. Failure to do so will risk recontamination of the water. One container should be

used to draw water from the RWH tank and pour this water into the filter whilst a separate

container should be used to collect the water from the filter. Both these containers should

• be durable, non-oxidising and easy to clean

• have a single opening that is less than 8cm in diameter (Mintz, 1995). This shall prevent

utensils or hands being put into the container since these could introduce contamination

• have a tightly fitting lid


- 63 -

The containers should also be cleaned (if possibly disinfected) frequently to remove any sources

of contamination. These guidelines are in addition to the more general considerations such as

making the container lightweight and easy to carry, inexpensive and made out of materials that

can be sourced locally. If householders feel that the water obtained from the filter is not clean

enough or contamination occurs during the transport/storage, then the water quality can be

improved by household treatment methods such as boiling and chlorination.

6.6 Maintenance instructions for the villagers

6.6.1 Conveyance system

Due to the gutters being open, it is likely that debris will collect in them. This debris could

block the flow of water causing the gutters to overflow and create pools of stagnant water that

could be used as breeding sites by mosquitoes. Therefore the householders should be instructed

to clear the gutters of debris regularly. The gutters are particularly likely to become clogged

with debris at the beginning of the wet season when the first rains wash the debris off the roof.

Householders must be made aware of the risks and necessary precautions of working at heights.

The gutters should also be checked regularly for leakages, particularly at the joints. Any

leakages that are found should be repaired as soon as possible to prevent loss of runoff. If a leak

occurs at a joint, then the joint should be re-soldered. For leaks not at joints, a waterproof

sealant or tape can be used.

6.6.2 Storage tank

Once a week any debris which has accumulated on the coarse filter should be removed. The

accumulation of debris is particularly likely at the beginning of the wet season. Once a month,

the tank cover should be lifted off and suspended matter on the surface of the water removed

using a sieve or a similar utensil. When the tank is empty or nearly empty (most probably

towards the end of May), matter that has settled to the bottom of the tank should be removed.

The tank should also be regularly checked to check that no algae are growing on the surface of

the water.

6.6.3 Biosand filter

To clean the filter the outlet should be blocked and the reservoir filled. The water should be

swirled around by hand which will cause the surface of the sand to be agitated. This will result

in the captured material being suspended in the standing water overlying the sand. This dirty

water should then be removed using a bucket and the process repeated as many times as

necessary to regain the desired flow rate. Cleaning the filter this way will cause fewer

disturbances to the biological layer compared to removing the sand and washing it. Therefore

there will not be a need to have multiple filters to allow the biological layer to regenerate after


- 64 -

cleaning. This method of cleaning stated has been taken from www.biosandfiler.org (accessed

19/05/2009).

Cleaning should only be carried out when the filter efficiency drops below the desired level

rather than as a matter of routine. Too frequent cleaning will cause the size of the pore spaces to

become too large to trap particles in the water and also causes the biological layer to be

disturbed more often than necessary.


- 65 -

7 Discussion of the design

7.1 Assessment of the effectiveness of proposed water treatment Comparing the water quality of the runoff collected from the roofs (Table 5.8) with the WHO

Drinking-Water Guidelines (Table 7.1), it can be seen that the water quality parameters that

need to be reduced are turbidity and the concentrations of total and faecal coliforms.

Table 7.1: Comparison of guideline values specified in the WHO Guidelines for Drinking-Water Quality (WHO, 2006) with the water quality of the runoff

Initial water quality of runoff Parameter WHO standard Roof 1 Roof 2

pH 6.5 - 8 7.8 7.5 Temperature (°C) 31 31 Turbidity (NTU) 5 58 24

Total solids (mg/l) 200 115 Total suspended solids (mg/l) 146 84 Total dissolved solids (mg/l) 600 50 31 Faecal coliforms (/100ml) 0 13 4 Total coliforms (/100ml) 0 75 46

Table 7.2: Comparison of water quality of runoff and final water quality

Initial runoff Improvements due to water

storage Improvements due to Biosand

Filter Parameter

Roof 1

Roof 2

% removal (a)

Roof 1

Roof 2

% removal (b)

Roof 1

Roof 2

Turbidity 58 24 80 11.6 5 85 1.74 0.7

Total coliforms (/100ml)

75 46 70 23 14 83 4 2.3

Faecal coliforms (/100ml)

13 4 70 4 1 97 0.1 0.04

(a) Assumed turbidity based on estimated settlement of suspended solids (6.4.2) and % pathogenic reduction taken from Ashworth (2005)

(b) % reductions taken as average values quoted in Table 6.12

The effectiveness of the proposed water treatment described in 6.4 can be assessed by

comparing the estimated final water quality (that from the Biosand Filter) (Table 7.2) with the

WHO Drinking-Water Guidelines (Table 7.1). This comparison shows that the water treatment

has successfully lowered the turbidity of the water to below the WHO guideline value. The

treatment has also been successful in reducing the concentration of faecal coliforms. On average

90% of the water samples taken from Roof 1 and 96% from Roof 2 would have no faecal

coliforms present. For a water supply that only serves a single household, WHO (2006)

classifies the quality of the water system as ‘excellent’ where more than 90% of the water

samples contain no E.coli (the most suitable indicator of faecal coliforms). However the

removal of total coliforms from the runoff is not as effective since significant concentrations of


- 66 -

total coliforms are likely to remain in the water despite the water treatment. Therefore some

household water treatment, such as current techniques of chlorination and boiling, may be

necessary in addition to the proposed water treatment specified in 6.4.

Despite it being stated in 3.7 that water treatment is commonly not required for rainwater used

for drinking, Table 7.2 shows that improvements in water quality from storage alone would be

unlikely to be sufficient. However the limitations of these estimates must be realised in terms of

the data for roof runoff being taken from sites other than Pabal and the effectiveness of the

treatment processes being estimated.

7.2 Economic appraisal of RWH system

7.2.1 Benefits of RWH

The water quantity benefits from the introduction of RWH will be:

• the RWH system will provide the households with a more continuous supply of water

than the current supply that is only available for 20 mins a day.

• the storage of the rainwater will increase the reliability of the water supply throughout

the year, particularly in the dry season.

• the harvesting of the rainwater will result in an increase in the total volume of water that

is available to the household.

Assuming the RWH system meets the potable water demand throughout the year, the value of

the potable water provided to each household in the wet and dry season will be US$ 2.25 and

US$ 8.62 per month respectively. Runoff from Roof 2 is likely to provide more water than is

required to meet the potable water demand. This additional water shall be valued at US$ 5.25

per year (based on there being an additional 2 lcd). This gives an annual water quantity benefit

of US$ 65.22 for RWH on Roof 1 and US$ 70.47 for Roof 2.

The water quality benefits will be as a result of the water gained from RWH being of higher

quality than the water obtained from current water supplies. With the water from RWH being

used for potable uses, it is likely that there will be a reduction in the number of cases of

waterborne diseases. For example Jeuland & Whittington (2009) found that a Biosand Filter

reduces cases of diarrhoea by between 20 to 60%.

For simplicity due to the most significant water quality benefits being from the Biosand Filter,

only these economic benefits have been included. Table 7.3 gives the estimated benefits per

household of having a Biosand Filter in terms of morbidity and mortality. These values have

been derived from Jeuland & Whittington (2009), which quoted the benefits for a household of

4 to 6 people in a typical developing country. The estimated benefits account for filter usage


- 67 -

declining by 1 – 5% each year (Jeuland & Whittington, 2009). The total; annual water quality

benefit will be US$ 86.40 per household.

Table 7.3: Benefits from Biosand Filter (taken from Jeuland & Whittington, 2009)

Benefits (US$/(household-month))

Morbidity 0.23

Mortality 0.74

7.2.2 Costs of RWH

The construction costs of the RWH system will comprise of the costs of storage, guttering,

coarse filter and the Biosand Filter. Tables 7.4 and 7.5 show the construction costs for the RWH

system for Roof 1 and 2 respectively.

Table 7.4: Construction costs for RWH system for Roof 1

Item Unit cost

(US$) Quantity

Total cost (US$)

Roof guttering 2.5/m (a) 12.0 m 30.00

Guttering from roof to tank 2.5/m (a) 3.01m 7.52

Coarse filter 14.75 (b) 1 14.75

Storage tank 125.00 (c) 1 125.00

Manufacturing and software costs 75.00 (d) 1 75.00

Transportation and delivery of filter media 15.00 (d) 1 15.00

Training of household in how to use it and health promotion

0.98 (d) 1 0.98

Biosand filter

TOTAL 90.98

GRAND TOTAL 268.25 (a) Martinson et al (2002) (b) www.rainwaterharvesting.org (accessed 21/05/2009) (c) Martinson et al (2002) & Rees (2000) (d) Jeuland & Whittington (2009)

Table 7.5: Construction costs for RWH system for Roof 2

Item Unit cost

(US$) Quantity

Total cost (US$)

Roof guttering 2.5/m 12.0 m 30.00

Guttering from roof to tank 2.5/m 3.12m 7.8

Coarse filter 14.75 1 14.75

Storage tank 200.00 1 200.00

Manufacturing and software costs 75.00 1 75.00

Transportation and delivery of filter media 15.00 1 15.00

Training of household in how to use it and health promotion

0.98 1 0.98

Biosand filter

TOTAL 90.98

GRAND TOTAL 343.53


- 68 -

7.2.3 Calculation of payback period

Table 7.6 shows the calculated payback period for the RWH system for Roof 1 and Roof 2. The

payback period for Roof 1 is less than the threshold of 2 years indicating that this system is

economically viable. RWH for Roof 2 is not as economically viable, since the payback period is

slightly greater than the threshold. However the payback period is only 3 months greater than

the threshold, which is not particularly significant over a design life of 10 years. Figures 7.1 and

7.2 show the comparison of the cost and cumulative annual benefits over the 10 year design life

for the systems for Roof 1 and Roof 2 (see Appendix G for actual costs and benefits).

Table 7.6: Calculation of payback time

Type of house Payback period (months)

Roof 1 21

Roof 2 27

Comparison of cumulative annual benefits and cost for domestic RWH in Pabal for Roof 1

0

200

400

600

800

1000

1200

1400

1600

0 1 2 3 4 5 6 7 8 9 10

Year

US

$

Cost Benefits

Figure 7.1: Comparison for cumulative annual benefits and cost for DRWH system for Roof 1

Comparison of cumulative annual benefits and the cost for domestic RWH in Pabal for Roof 2

0

200

400

600

800

1000

1200

1400

1600

1800

0 1 2 3 4 5 6 7 8 9 10

Year

US

$

Cost Benefits

Figure 7.2: Comparison for cumulative annual costs and benefits for DRWH system for Roof 2


- 69 -

8 Conclusions

8.1 Overview of the design The aim of the project, to design a rainwater collection and storage system for the village of

Pabal in India, has been achieved. The proposed RWH system will not only help to alleviate the

current water shortages in Pabal but also reduce the high levels of waterborne diseases that

presently exist.

The water supply from current sources has been quantified and a large difference between the

availability of water in the wet season compared to the dry season has become evident. The

demand for water within Pabal’s core has also been quantified in terms of the domestic and non-

domestic demands. By comparing the current water supply with the demand for water the

imbalance between supply and demand has become clear, with demand being considerably

larger than the supply.

As a result of three options for RWH being considered, it was determined that the most feasible

system was for all houses in the core of Pabal to have RWH. It was therefore decided that the

most appropriate way to operate RWH was at the household level. The runoff available from

RWH is expected to be able to satisfy each household’s potable water demand.

Considering the construction materials already used in Pabal and the need for the RWH system

to be low cost and use local skills and materials, a simple system consisting of metal guttering

and a reinforced concrete bamboo tank has been designed. Water quality improvements of a

first flush system, a coarse filter and a Biosand Filter have been suggested. These methods of

water treatment are expected to cause the runoff to satisfy the WHO Drinking-water Guidelines

in most cases. The Biosand Filter has sufficient capacity to filter water from other sources as

well as that from RWH.

The RWH system has been designed for a lifetime of 10 years. Considering projections for

increased precipitation due to climate change and changes in the demand as a result of expected

population growth in Pabal, a likely 2020 scenario has been developed. It has been shown that

the proposed RWH system is likely to continue to be able to satisfy the potable water demand

for all households in 2020. The storage tank has been designed to have sufficient capacity to

accommodate the additional runoff expected in 2020.

The cost of the RWH system for houses with clay tiled roofs is estimated to be US$268 and for

houses with corrugated steel roofs US$344. The benefits in terms of greater water quantity and

improved water quality have been quantified and show the system to have an approximate

payback period of 2 years.


- 70 -

8.2 Limitations of the design Due to not being able to obtain a reliable precipitation data series for the Pabal area, average

monthly precipitation values have been used in the design. It has therefore not been possible to

assess the inter-annual variability of precipitation and the associated probabilities of the

precipitation being higher or lower than the average values. So whilst future conditions in terms

of the climate and demographics have been considered and the tank designed with additional

capacity to that calculated the reliability of the RWH system cannot be quantified. This

introduces uncertainty into the success of the proposed system in terms of RWH not being able

to meet the potable water demand and/or the tank overflowing. To partly account for this fact,

conservative assumptions have been taken where appropriate.

This report has shown that in Pabal RWH only has the potential to meet the potable water

demand. This means that RWH can only partially reduce the water shortages that currently exist

in Pabal. Therefore whilst the development of RWH has many benefits, it is not the solution to

the water shortages on its own and must be part of a larger water infrastructure development and

water conservation programme.

This design has been completed purely as a desk study using information, where it was

available, from organisations with links to Pabal. Throughout the design, assumptions have been

made which may or not be accurate.

The author of this report has very limited knowledge of the social situation in Pabal. Therefore

if parts of the proposed system are found to be socially unacceptable or unlikely to be successful

within Pabal, the system should be altered accordingly.

In terms of this project contributing to the furthering of RWH as a concept, the contribution is

limited. This system has been specifically designed for Pabal and whilst the methodology used

may be able to be applied to other locations, the transferability of the design is limited. Also due

to the time constraints of this project, sufficient time has not been available to develop new

technologies for RWH. Instead technologies that are tried and tested have been adapted to

increase the chances of RWH being a success in Pabal.

8.3 Recommendations for further work The design should be thoroughly assessed by someone with more in-depth knowledge of Pabal

to allow any incorrect assumptions to be corrected. It should be clear how to correct the design

from the methodology that has been clearly stated.

Once any incorrect assumptions have been corrected, this design should be taken from a desk

study out into the field. The proposed RWH system should be explained to a sample of Pabal’s

residents to gain their views on the system and whether they think such a system would be a

success. This design should also be given to VA to determine whether they have the tools and


- 71 -

technical skills needed for the RWH system to be constructed. Water samples of roof runoff

should be taken and analysed using the water testing equipment at VA to establish whether the

proposed water treatment is necessary.

If it is decided to implement the RWH system, it is suggested that this should initially be done

on a small scale (2-5 houses). This would help to see if the system is feasible in practise and

allow the opportunity to make changes to the system before it is rolled out on a larger scale.


- 72 -

9 References Agarwal, A. (1998), “Rainwater harvesting in a new age: When modern groundwater and river

exploration has reached its limits”, Proceedings of the Congress of Water – the key to socio-

economic development and the quality of life, Stockholm, pp 5-12.

Ashworth, J. (2005), “Roof collection and storage of rainwater for drinking”, Water

Management, Vol 158, pp183 – 189.

Bates, B.C., Kundzewicz, Z.W., Wu, S. and Palutikof, J.P. (2008), Climate Change and Water,

Intergovernmental Panel on Climate Change, Geneva, 210p.

Brikké, F. and Bredero, M. (2003), Linking technology choice with operation and maintenance

in the context of community water supply and sanitation: A reference document for planners

and project staff, World Health Organisation, Geneva, 142p

British Standards Institute (2008), BS 8515: Code of practise for installation of rainwater

harvesting systems, UK, 46p

Buzunis, B.J. (1995) “Intermittently Operated Slow Sand Filtration: A New Water Treatment

Process”, MSc Thesis, University of Calgary, Canada.

CAWST (2008), “Safe Water Storage”, http://www.cawst.org/index.php?id=120, (accessed

18/05/2009).

Centre for Affordable Water and Sanitation Technology [2007] “CAWST Technical Update”,

http://www.cawst.org/assets/File/Technical_Update_March_2007.pdf (accessed 01/05/2009)

Chadwick, A. and Morfett, J. (1998), Hydraulics in Civil and Environmental Engineering, 3rd

Edition, E & FN Spon, London, 600p.

Chow, V.T.E. (1985) Open-Channel Hydraulics, McGraw Hill, Singapore, 680p.

Cruz, R.V., Harasawa, H., Lal, M., Wu, S., Anokhin, Y. et al (2007), Climate Change 2007:

Impacts, Adaptation and Vulnerability, Cambridge University Press, Cambridge, 976p.

Design Technology Unit (2002), “Very-low-cost domestic roofwater harvesting in the humid

tropics: Existing practise, The University of Warwick, 106p

Design Technology Unit (2002), “Very-low-cost domestic roofwater harvesting in the humid

tropics: constraints and problems, The University of Warwick, 38p

Design Technology Unit [2005] “DRWH Technology: Filters and Separators”,

http://www2.warwick.ac.uk/fac/sci/eng/research/dtu/rwh/technology1/technology_5/ (accessed

07/05/2009).


- 73 -

Development Technology Unit [2001] “Case Study 19: Plastic lined bamboo tanks in India”,

http://www2.warwick.ac.uk/fac/sci/eng/research/dtu/pubs/rn/rwh/cs19/ (accessed 15/05/2009).

Duke, W.F., Nordin, R.N., Baker, D. & Mazumder, A. (2006) “The use and performance of

BioSand filters in the Artibonite Valley of Hati: a field study of 107 households”, Rural and

Remote Health, Vol 6, pp 570-592.

Efe, S.I. (2006), “Quality of rainwater harvesting for rural communities of Delta State, Nigeria”,

Environmentalist, Vol 26, pp 175 – 181.

EWB (2008), “Project Proposal: Rainwater Harvesting”.

engINdia [2006] “engIndia Final Report”,

http://www.engindia.net/1%20engINdia%20Final%20Report.pdf (accessed 10/10/2008).

Environment Agency (2003), Harvesting rainwater for domestic use: an information guide,

Environment Agency, Bristol, 22p.

Ferdausi, S.A. and Bolkland, M.W. (2000) “Rainwater harvesting for application in rural

Bangladesh”, Proceedings of the 26th WEDC Conference: Water, Sanitation and Hygiene:

Challenges of the Millennium, Dhaka, pp 16-19.

Ahmed, M.F. (1999), “Rainwater harvesting potential in Bangladesh”, Proceedings of the 25th

WEDC Conference: Integrated Development for Water Supply and Sanitation, Addis Ababa, pp

363 – 365.

Fort Lewis College [2008] “Biosand Filter”

http://www.fortlewis.edu/academics/school_arts_sciences/physics_engineering/ewb_webpage/C

urrent%20Web%20Page%20Supporting%20Docs/BSF/Basic%20info%20on%20BSF.html

(accessed 18/05/2009).

Friends Who Care Charities Ltd [2006] “Biosand Filter – Information Sheet”,

http://www.friendswhocare.ca/BSF-FWC-%20Specifications.pdf (accessed 18/05/2009).

Ghavami, K. (2005), "Bamboo as reinforcement in structural concrete elements." Cement and

Concrete Composites Vol 27, pp 637-649.

Global Development Research Centre, [2007] “Rainwater Harvesting and Utilisation”

http://www.gdrc.org/uem/water/rainwater/rainwaterguide.pdf (accessed 10/11/2008)

Hamill, L. (2001), Understanding Hydraulics 2nd Edition, Palgrave, Basingstoke, 608p

Handia, L. Tembo, J. M. & Mwiindwa, C. (2003), “Potential of rainwater harvesting in urban

Zambia”, Physics and Chemistry of the Earth, Parts A/B/C, Vol 28, pp 893 -896

Health and Safety Executive [2008] “Legionnaire’s Disease”

http://www.hse.gov.uk/legionnaires (accessed 19/05/2009)


- 74 -

Hendricks, D.W. (2006) Water treatment unit processes: physical and chemical, CRC Press,

London, 1266p

Hofkes, E.H. (1983), Small Community Water Supplies; Technology of Small Water Supply

Systems in Developing Countries, John Wiley & Sons, The Netherlands, 442p

Huisman, L. & Wood, W.E. (1974), Slow Sand Filtration, World Health Organisation, Geneva,

120p

Keremane, G. B. (2005), Harvesting Rainwater, In: Water Encyclopaedia, Vol 1-5, pp 2:548 -

2.552, Lehr, J., Keeley, J. and Lehr, J. [editors], John Wiley & Sons, Hoboken, New Jersey

Indian Water Portal, [2009] “Monthly average potential evapotranspiration (1901 – 2002)”,

http://indiawaterportal.org (accessed 23/02/2009)

Jain, S. K., Agarwal, P. K., and Singh, V. P. (2007), Hydrology and Water Resources of India,

Springer, London, 1262p

Jeuland, M. and D. Whittington (2009), "Cost-benefit comparisons of investments in improved

water supply and cholera vaccination programs." Vaccine, Vol 27, pp 3109-3120.

Kerr, C. (1989), Community Water Development, Intermediate Technology Publications,

London, 279p

Lantagne, D.S., Quick, R. and Mintz, E.D. (2007), Household Water Treatment and Safe

Storage Options in Developing Countries: A Review of Current Implementation Practices, In:

Water stories: expanding opportunities in small-scale water and sanitation projects, pp 17:38-54,

Parker, M., Williams, A. and Youngblood, C. [editors], Woodrow Wilson International Centre

for Scholars, Washington DC

Zongjin, Y., Chun-Pong, L. and Tongxi, Y. (2002), "Laminate of Reformed Bamboo and

Extruded Fiber-Reinforced Cementitious Plate”, Journal of Materials in Civil Engineering, Vol

14, pp 359-365.

Machiwal, D.J., Madan, K., Singh, P.K., Mahnot, S. C., and Gupta, A. (2004), “Planning and

design of cost-effective water harvesting structures for efficient utilization of scarce water

resources in semi-arid regions of Rajasthan, India”, Water Resources Management, Vol 18, pp

219 – 235

Mann, H.T. and Williamson, D. (1993), Water Treatment and Sanitation, 3rd Edition,

Intermediate Technology Publications, Nottingham, 94p

Martinson, D. and Thomas, T. (2004), “Quantifying the first-flush phenomenon”, Proceedings

of 12th International Rainwater Catchment Systems Conference, New Delhi, pp 1-7.


- 75 -

Martinson, D. B., Ranatunga, N.U.K. & Gunaratne, A.M.C.H.A (2002), “Reducing Rainwater

Harvesting System Cost”. Proceedings of the 28th WEDC Conference, Calcutta, pp 1-4

Martinson, D.B. (2007) “Improving the viability of roofwater harvesting in low-income

countries”, PHD Thesis, University of Warwick

Mintz, E., Reiff, F. and Tauxe, R. (1995) “Safe water treatment and storage in the home: A

practical new strategy to prevent waterborne diseases”, Journal of the American Medical

Association, Vol 273, pp 948 - 953

Morgan, P. (1990), Rural Water Supplies and Sanitation, Macmillan Education Ltd, London,

358p

Mwenge Kahinda, J., Taigbenu, A. E. and Boroto, J.R. (2007), “Domestic rainwater harvesting

to improve water supply in rural South Africa”, Physics and Chemistry of the Earth, Vol 32, pp

1050 -1057.

Occupational Health and Safety Administration, [2008] “Legionnaire’s Disease”,

http://www.osha.gov/dts/osta/otm/legionnaires/index.html (accessed 20/05/2009)

Pacey, A. and Cullis, A. (1989), Rainwater Harvesting: The collection of rainfall and runoff in

rural areas, Intermediate Technology Publications, London, 100p

Palmateer, G., Manz, D., Jurkovic, A., McInnis, R., Unger, S. et al (1999), “Toxicant and

Parasite Challenge of Manz Intermittent Slow Sand Filter”, Environmental Toxicology, Vol 14,

pp 217- 225.

Parkinson, J. and Mark, O. (2006) “Urban Stormwater Management in Developing Countries”,

IWA Publishing, London, 218p.

Parsons, S.A. and Jefferson, B. (2006), “Introduction to Potable Water Treatment Processes”,

Blackwell Publishing, Oxford, 179p.

Peters, E.J. (2006), “Rainwater potential for domestic water supply in Grenada”, Water

Management, Vol 159, pp 147 – 153.

Population Division of the Department of Economic and Social Affairs of the United Nations

Secretariat, [2008] “World Population Prospects: The 2006 Revision and World Urbanization

Prospects: The 2007 Revision”, http://esa.un.org/unup (accessed 06/05/2009)

Rees, D.G., Nyakaana, S. and Thomas, T.H. (2000), Very-low-cost roofwater harvesting in East

Africa, Warwick University, 71p.

Reese, D. (2008), Rainwater Harvesting: Technical Brief, Practical Action, Rugby, 12p

Anon [2008], “Rooftop Rainwater Harvesting”

http://www.rainwaterharvesting.org/downloads/rooftop_rwh.pdf (accessed 21/05/2009)


- 76 -

Satsangi, G.S., Lakhani, A., Khare, P., Singh, S.P., Kumari K.M. et al. (1998), “Composition of

rain water at a semi-arid rural site in India”, Atmospheric Environment, Vol 32, pp 3783-3793.

Smet, J. [2003] “Domestic Rainwater Harvesting” http://www.lboro.ac.uk/well/resources/fact-

sheets/fact-sheets-htm/drh.html (accessed 20/11/2008)

Smethurst, G. (1988), Basic Water Treatment for Applications World-Wide, Thomas Telford,

London, 216p.

Staunber, C.E., Elliott, M.A., Koksal, F., Ortiz, G.M., DiGiano, F.A. et al (2006),

“Characterisation of the biosand filter for E. coli reductions from household drinking water

under controlled laboratory and field use conditions”, Water Science and Technology, Vol 54,

pp 1-7.

Survey of India, [2002] “India-Soils”, http://www.surveyofindia.gov.in/soi_geo.html (accessed

19/02/2009)

Thomas, T. and Kiggundu, N. (2004) “Constraints to domestic roofwater harvesting uptake in

Uganda: An assessment”, Proceedings of the 30th WEDC International Conference: People

Centred Approaches to Water and Environmental Sanitation, Lao PDR, pp 450-453.

Thomas, T. (1998), “Domestic water supply using rainwater harvesting”, Building Research and

Information, Vol 26, pp 94-101.

Thomas, T.H. and Martinson, D.B. (2007), Roofwater Harvesting: A Handbook for

Practitioners, IRC International Water and Sanitation Centre, Delft, 153p.

Twort, A.C. (2000), Water Supply, 5th Edition, London, UK, 608p.

United Nations Development Programme, [2008] “Millennium Development Goals”

http://www.undp.org/mdg (accessed 23/02/2009)

United Nations Economic and Social Council, [2002] “Substantive issues arising in the

implementation of the international covenant on economic, social and cultural rights; General

Comment no. 15”

www.unhchr.ch/tbs/doc.nsf/0/a5458d1d1bbd713fc1256cc400389e94/$FILE/G0340229.pdf

(accessed 20/01/2009)

Vadhanavikkit, C. and Pannachet, Y. (1987) “Investigations of Bamboo, Reinforced Concrete

Water Tanks”, Proceedings of the 3rd International Rainwater Cistern Systems Conference,

Khon Kaen, Paper C13

Water Aid (2007), Technology Notes, Water Aid, London, 46p.

Whitehead, V. (2000), The Manufacture of Direct Action Handpumps for use with Domestic

Rainwater Harvesting Tanks, The University of Warwick, 28p.


- 77 -

World Health Organisation, [2005], “Legionellosis”

http://www.who.int/mediacentre/factsheets/fs285/en/ (accessed 20/05/2009)

World Health Organisation (2006), Guidelines for Drinking-water Quality, Vol 1, Geneva, 595p

Worm, J., Hattum, T.V. and Kat-Reynen, C. (2006), Rainwater Harvesting for Domestic Use,

Agromisa Foundation, Netherlands, 84p

Yaziz, M. I., Gunting, H., Sapari, N. and Ghazali, A.W. (1989). “Variations in rainwater quality

from roof catchments”, Water Research, Vol 23, pp 761-765.

- 78 -

Appendix A: Current water supply infrastructure

Groundwater recharge

Private wells

Village well

Dam

Rainwater

170,000 litre storage tank

70,000 litre storage tank

12,000 litres per day tanked

(during dry season)

Pabal’s core & village

Pabal’s core & village

- 79 -

Appendix B: Meteorological Data

Table B1: Average monthly rainfall for a 25 year period (1982 – 2006) (www.rainwaterharvesting.org, accessed 04/04/2009)

Month Jan Feb March April May June July Aug Sept Oct Nov Dec Total

Precipitation (mm) 1.7 1.5 0.6 9.8 30 171.0 171.4 139.5 141.7 85.8 21.5 7.4 781.9

Table B2: Monthly predicted precipitation for Pabal in 2020 (observed data from www.rainwaterharvesting.org & predicted changes from IPCC, 2007)

Month Jan Feb March April May June July Aug Sept Oct Nov Dec Total

Average rainfall (based on 25 yr average: 1981 -2006) (mm) 1.7 1.5 0.6 9.8 30 171 171.4 139.5 141.7 85.8 21.5 7.4 781.9

Predicted % change in precipitation by 2020 (with reference to baseline period of 1961- 1990)

2.0 2.0 4.0 4.0 4.0 3.5 3.5 3.5 1.5 1.5 1.5 2.0

Predicted precipitation in 2020 (mm) 1.7 1.5 0.6 10.2 31.2 177.0 177.4 144.4 143.8 87.1 21.8 7.7 804.5

Table B3: Monthly minimum and maximum temperatures for Pune, Maharashtra (http://www.worldweather.org/066/c00535.htm, accessed 18/05/2009)

Month Jan Feb March April May June July Aug Sept Oct Nov Dec

Min (°C) 11.4 12.7 16.5 20.7 22.5 22.9 22.0 21.4 20.7 18.8 14.7 12.0

Max (°C) 30.3 32.8 36.0 38.1 37.2 32.1 28.3 27.5 29.3 31.8 30.5 29.6

- 80 -

Appendix C: Raw water quality data

C1: Chemical water quality of rainfall The chemical composition of the rainwater was determined from water samples taken from tube

wells during the monsoon of 1996 at Golalpura, Agra in India. Table C1 shows the similarity

between Gopalpura and Pabal in terms of population density and distance to nearest major town

or city. The average chemical composition of the rainwater is given in Table C2.

Table C.1: Comparison of site for chemical composition with the site of Pabal (engINdia, 2005 & Satsangi et al, 1998)

Location Pabal Gopalpura

Population density (per km2) 716 825

Radial distance to nearest major towns or cities (km) 30 35

Table C.2: Concentration of ions in rainwater at Gopalpura, Agra (Satsangi et al, 1998)

Species Mean (µeq l-1) Mean concentration (mg/l)

F+ 28.99 0.55

Cl- 38.20 1.35

NO3- 42.85 2.66

SO42- 19.31 0.93

NH4+ 48.06 0.87

Ca2+ 152.60 3.05

K+ 3.52 0.14

Na+ 20.65 0.47

Mg2+ 94.25 1.15

HCO3- 43.78 2.67

HCOO- 4.35 0.20

CH3COO- 3.03 0.18

NB: The total dissolved solids (TDS) of the rainwater has been estimated by assuming that TDS

consists of calcium, magnesium, potassium, sodium, bicarbonates, chlorides and sulphates

(WHO, 2006)

C2: Biological and microbiological water quality parameters of rainfall The biological water quality parameters have been estimated by using values obtained from a

study in the Delta State of Nigeria (Table C3) (Efe, 2006).

Parameter Value Temperature 0.7°C to 1.0°C lower than atmospheric temperature

Turbidity (NTU) 4.7 Total suspended solids (mg/l) 10

Faecal coliforms (/100ml) 0 Total coliforms (/100ml) 0

Table C3: Water quality parameters for rural communities of the Delta State, Nigeria (Efe, 2006)

- 81 -

C3: Changes in water quality parameters due to roofing Table C4 shows water quality parameters for runoff from galvanised-iron roof and concrete-

tiled roof for roofs in a university campus in Selangor, Malaysia. The water quality of the

rainfall relates to the same location and time period and therefore the change due to each roofing

material can be calculated.

Galvanised-iron roof Concrete tile roof Control (rainwater) Value Change Value Change

pH 5.9 6.6 + 0.7 6.9 + 1 Temperature (°C) 27.5 28.1 + 0.6 28.1 + 0.6

Conductivity (µs/cm) 13.7 97.0 + 83.3 135.2 + 121.5 Turbidity (NTU) 3.0 22 + 19 56 + 53

Total solids (mg/l) 24.0 119 + 95 204 + 180 Suspended solids (mg/l) 17.0 91 + 74 153 + 136 Dissolved solids (mg/l) 7.0 28 + 21 47 + 40

Faecal coliforms (/100ml) 0 4 + 4 13 + 13 Total coliforms (/100ml) 0 46 + 46 75 + 75

Table C.4: Comparison of water quality of rainwater and runoff from galvanised iron and concrete tiled roofs (Yaziz et al, 1989)

- 82 -

Appendix D: Calculated runoff for different RWH opt ions

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

Average precipitation (mm) 1.7 1.5 0.6 9.8 30.0 171.0 171.4 139.5 141.7 85.8 21.5 7.4 781.9

Option 1 (Domestic houses) 15.1 13.3 5.3 87.2 253.5 1520.7 1524.3 1240.6 1260.1 763.0 191.2 65.8 6940.1

Option 2 (Non-domestic buildings) 3.0 2.7 1.1 17.5 52.5 305.8 306.5 249.5 253.4 153.4 38.4 13.2 1397.1 Runoff (m3/month)

Option 3 (Domestic and non domestic buildings

18.2 16.0 6.4 104.7 306.0 1826.5 1830.8 1490.0 1513.5 916.5 229.6 79.0 8337.2

- 83 -

Appendix E: Sizing of the storage tank for the current scenario

Comparison of monthly runoff from Roof 1 and monthly potable water demand (current)

0.00

0.50

1.00

1.50

2.00

2.50

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Month

Vo

lum

e (m

3 )


Figure E.1: Monthly runoff from Roof 1 and monthly potable water demand for current situation

Comparison of monthly runoff from Roof 2 and monthly potable water demand (current)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Janu

ary

Febru

ary

Mar

chApr

ilM

ayJu

ne July

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Month

Vo

lum

e (m

3 )


Figure E.2: Monthly runoff from Roof 2 and monthly potable water demand for current situation

From E.1 and E.2, it can be seen that the June is the first month when the runoff is greater than

the demand and therefore it can be assumed that the tanks are empty at the end of May.

- 84 -

Comparison of cumulative monthly runoff from Roof 1 and cumulative monthly potable water demand (current)

0.001.002.003.004.005.006.007.008.009.00

10.00

June Ju

ly

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Janu

ary

Febru

ary

Mar

chApr

ilM

ay

Month

Vo

lum

e (m

3 )


Figure E.3: Comparison of inflow and outflow for Tank 1 for the current situation

Comparison of cumulative monthly runoff from Roof 2 and cumulative monthly potable water demand (current)

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

June Ju

ly

Augus

t

Septe

mbe

r

Octobe

r

Novem

ber

Decem

ber

Janu

ary

Febru

ary

Mar

chApr

ilM

ay

Month

Vo

lum

e (m

3 )


Figure E.4: Comparison of inflow and outflow for Tank 2 for the current situation

From Figures E.3 and E.4, it can be seen that the largest requirement occurs in October and is

4.99m3 for Tank 1 and 6.63m3 for Tank 2.

- 85 -

Appendix F: Calculations

- 86 -

Project: Design of RWH for Pabal Date: 09/05/2009

Title of calculation: Bamboo reinforced storage tank

Sheet no: 1 of 1

For Tank 1:

Hydrostatic force = (1.5/2) x 9.81 x 1000 x (Π x 2.03)/2 x 1.5

= 35.2 kN

Resistance of conc = 0.07 x (Π x 2.03)/2 x 1.5 x (10 x 103)

= 33 482 kN

For Tank 2:

Hydrostatic force = (1.5/2) x 9.81 x 1000 x (Π x 2.53)/2 x 1.5

= 43.9 kN

Resistance of conc = 0.07 x (Π x 2.53)/2 x 1.5 x (10 x 103)

= 41 728 kN

Calculate the required volume of concrete for Tank 1:

Vol of vertical bamboo = [((Π x 0.032)/4) x 1.5] x 132

= 0.140 m3

Vol of horizontal bamboo = ((Π x 0.032)/4) x (2.10 x Π) x 30

= 0.140 m3

Vol of conc for tank = [(Π x 2.172) – (Π x 2.032)]/4 x 1.5 – 0.140 – 0.140

= 0.41 m 3

Vol of conc for base = (Π x 2.172)/4 x 0.1

Calculate the required volume of concrete for Tank 2:

Vol of vertical bamboo = [((Π x 0.032)/4) x 1.5] x 164

= 0.174 m3

Vol of horizontal bamboo = ((Π x 0.032)/4) x (2.60 x Π) x 30

= 0.173 m3

Vol of conc for tank = [(Π x 2.672) – (Π x 2.532)]/4 x 1.5 – 0.174 – 0.173

= 0.51 m 3

Vol of conc for base = (Π x 2.672)/4 x 0.1

= 0.56 m 3

- 87 -


Title of calculation: Calculation of critical runoff

Sheet no: 1 of 2 Title of calculation: Calculation of critical runoff

Sheet no: 2 of 2

For gutter from roof gutter � storage tank (Clay tiled roof)

Length of pipe in XY plane = ( ) 22 85.15.10.3 +−

= 2.38 m

Length of pipe in YZ plane = ( ) 22 085.15.10.3 +−

= 1.85 m

Length of pipe = 22 85.138.2 +

= 3.01 m

Change in head = 3.0 - 1.5

= 1.5 m

Head loss per unit length (s0) = 1.5 ⁄ 3.01

= 0.50

Calculate the duration of the critical rainfall event:

Roof gutter Along

the roof Section (1)

Section (2)

Section (3)

Roof gutter �� storage

tank

Overland flow length

(m) 1.87 2.00 2.00 2.00 3.01

Slope 0.160 0.005 0.005 0.01 0.50

Duration

Time of conc (Tc) (mins)

0.06 0.26 0.26 0.20 0.06 0.83

For gutter from roof gutter � storage tank (Corrugated steel roof):


= 2.38 m


= 2.01 m


= 3.12 m


= 1.5 m

- 88 -


Title of calculation: Calculation of critical runoff


Sheet no: 2 of 2


= 0.48

Calculate the duration of the critical rainfall event:

Roof gutter Along

the roof Section (1)

Section (2)

Section (3)

Roof gutter �� storage

tank

Overland flow length

(m) 1.87 2.00 2.00 2.00 3.12

Slope 0.160 0.005 0.005 0.01 0.48

Duration

Time of conc (Tc) (mins)

0.06 0.26 0.26 0.20 0.06 0.83

Since Tc < 15 mins for both roof types, assume the critical rainfall event has a duration of 15 mins

Calculate the intensity of the critical rainfall event:

88.0

19.0

)50.025.0(

00.393.6

+×=i

i = 10.98 cm hr-1

Calculate the critical discharge:

The critical discharge will be calculated for half the roof area (1.87 x 6m) due to the roof being double pitched.

Corrugated steel roof: Qmax = 1.87 x 6 x 0.1098 x 0.85

= 1.05 m3 s-1

= 0.29 l s-1

Clay tiled roof: Qmax = 1.87 x 6 x 0.1098 x 0.5

= 0.62 m3 s-1 = 0.17 l s-1

- 89 -


Title of calculation: Design of cross-section of roof guttering


Sheet no: 2 of 2 For the most efficient trapezoidal cross-section to transmit the critical discharge the parameters are the Manning’s equation are:

Q (l/s) R (mm) n * So ** A (mm2) D (mm)

Metal 0.290 11.453 0.013 0.0067 908.887 22.908

Clay 0.170 9.375 0.013 0.0067 608.904 18.750

* this is the Manning’s Co-efficient for painted metal channels (Chow, 1985)

** So has been taken as the average slope across the three sections of guttering

D

b

a

Dba

Area

+=2

For the corrugated steel roof:

b = 2.309 x 22

= 51.8 mm

Area =908.9 = 222

8.51 ×

+a

a = 30.8 mm

For the clay tiled roof;

b = 2.309 x 18.8

= 43.4 mm

Area = 608.9 = 8.182

4.43 ×

+a

a = 21.4 mm

- 90 -


Title of calculation: Design of cross-section of gutters from roof to tank


Sheet no: 2 of 2 Corrugated steel roof:


= 2.38 m


= 2.01 m


= 3.12 m


= 1.5 m


= 0.48

Using Manning’s equation and to find the most hydraulically efficient trapezoidal cross-section to transmit critical runoff: Q (l/s) R (mm) n So A (mm2) D (mm)

0.290 0.005 0.013 0.5900 168.987 9.878

Clay tiled roof:


= 2.38 m


= 1.85 m


= 3.01 m


= 1.5 m


= 0.50

Using Manning’s equation and to find the most hydraulically efficient trapezoidal cross-section to transmit critical runoff: Q (l/s) R (mm) n So A (mm2) D (mm)

0.171 0.004 0.013 0.6000 112.875 8.07

- 91 -

Appendix G: Cost and benefits of RWH Table G.1: Comparison of costs and benefits of RWH system for Roof 1

Year 0 1 2 3 4 5 6 7 8 9 10

Construction Cost (US$) 272.96

Water quantity 65.22 65.22 65.22 65.22 65.22 65.22 65.22 65.22 65.22 65.22

Water quality 86.4 86.4 86.4 86.4 86.4 86.4 86.4 86.4 86.4 86.4 Benefits (US$)

Total 151.62 303.24 454.86 606.48 758.1 909.72 1061.34 1212.96 1364.58 1516.2

Cost – Benefits (US$) 272.96 121.34 -30.28 -181.9 -333.52 -485.14 -636.76 -788.38 -940 -1091.62 -1243.24

Table G.2: Comparison of costs and benefits of RWH system for Roof 2 Year 0 1 2 3 4 5 6 7 8 9 10

Construction 343.53

Water quantity 70.47 70.47 70.47 70.47 70.47 70.47 70.47 70.47 70.47 70.47

Water quality 86.4 86.4 86.4 86.4 86.4 86.4 86.4 86.4 86.4 86.4 Benefits (US$)

Total benefit 156.87 313.74 470.61 627.48 784.35 941.22 1098.09 1254.96 1411.83 1568.7

Cost – Benefits (US$) 343.53 186.66 29.79 -127.08 -283.95 -440.82 -597.69 -754.56 -911.43 -1068.3 -1225.17

- 92 -

Appendix H: Bill of Quantities Table H1: Bill of quantities for RWH system for Roof 1

Item no. Item description Quantity

1. Guttering 1.1 Galvanised metal (0.09m width) 15.1m 1.2 Metal brackets 40

2. Storage tank

2.1 Chemically treated bamboo poles (1.7m high, 0.03m wide*) (for

vertical members) 132

2.2 Chemically treated bamboo poles (6.4m high, 0.03m wide) (for

horizontal members) 30

2.3 Bamboo strips (to tie reinforcement cage together)** 792 0.41 m3 (for

tank) 2.4

Low grade concrete of mortar mix of 1:2 cement-sand ratio by weight (can have strength > 10 MPa) 0.37 m3 (for

base)

2.5 Circular corrugated steel (slope of ½ % from centre): diameter:2.23

m 1

2.6 Wire and hooks 25 Tap 1

3. Biosand Filter 0.12 m3 (for

vessel) 3.1

Concrete (2 part Type 10 Portland cement, 3 part clean pea gravel (6mm) and 2 part clean sand) 0.25 m3 (for

lid) 3.2 Coarse gravel (grain size 6 – 15mm) 0.013 m3 3.3 Coarse sand/medium gravel (1 – 6mm) 0.013 m3 3.4 Fine sand (grain size ≈ 0.2mm) 0.15 m3 3.5 Large stone 1 3.6 PVC pipe (diameter: 13mm) 2.5m

Table H.2: Bill of quantities for RWH system for Roof 2


1. Guttering 1.1 Galvanised metal (0.12m width) 15.2m 1.2 Metal wire for hanging roof gutter 40 pieces

2. Storage tank

2.1 Chemically treated bamboo poles (1.7m high, 0.03m wide*) (for

vertical members) 164

2.2 Chemically treated bamboo poles (8.0m high, 0.03m wide) (for

horizontal members) 30

2.3 Bamboo strips (to tie reinforcement cage together)** 984 0.51 m3 (for

tank) 2.4

Low grade concrete of mortar mix of 1:2 cement-sand ratio by weight (can have strength > 10 MPa) 0.56 m3 (for

base)

2.5 Circular corrugated steel (slope of ½ % from centre): diameter =

2.73 m 1

2.6 Wire and hooks 30 2.7 Tap 1

- 93 -


3. Biosand filter 0.12 m3 (for

vessel) 3.1

Concrete (2 part Type 10 Portland cement, 3 part clean pea gravel (6mm) and 2 part clean sand) 0.25 m3 (for

lid) 3.2 Coarse gravel (grain size 6 – 15mm) 0.013 m3 3.3 Coarse sand/medium gravel (1 – 6mm) 0.013 m3 3.4 Fine sand (grain size ≈ 0.2mm) 0.15 m3 3.5 Large stone 1 3.6 PVC pipe (diameter: 13mm) 2.5m

* this height will allow the bamboo to be bent at the base and roof of the tank ** based on tying every 5th vertical bamboo pole to each horizontal pole

Admin

Stamp

Documents

The Design of a Rainwater Harvesting System for Pabal, India