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Intermediate Chlorination Feasibility Study, Freetown Sierra Leone

Veolia Foundation Février 2014 1

INTERMEDIATE CHLORINATION FEASIBILITY STUDY

FREETOWN, SIERRA LEONE



SUMMARY

INTRODUCTION ..................................................................................................................................... 3 1. CONTEXT OF THE STUDY AND NETWORK COMPREHENSION .... ....................................... 3

1.1. General Context ..................................................................................................................... 3 1.2. Objectives of the study ........................................................................................................... 4 1.3. Area of the study – pre-requirements .................................................................................... 4

2. FEASIBILITY STUDY.................................. ................................................................................. 6 2.1. Methodology ........................................................................................................................... 6 2.2. The life of water from GUMA reservoir to the network ........................................................... 6 2.3. Visit and choice of the site of implantation of intermediate chlorination ................................ 9

2.3.1. Spur Road Reservoir: ........................................................................................................ 9 2.3.2. Tower Hill Reservoir ........................................................................................................ 11 2.3.3. Income Tax Break Pressure Tank ................................................................................... 12 2.3.4. Advantages and disadvantages of each site: .................................................................. 13

2.4. Feasibility study to install intermediate chlorination at tower hill.......................................... 15 2.4.1. Flow to chlorinate: ............................................................................................................ 15 2.4.2. Chlorination and bacteria : ............................................................................................... 17 2.4.3. Chlorine demand ............................................................................................................. 18 2.4.4. Size of installation – choice of technology ....................................................................... 19 2.4.5. Implantation of installation ............................................................................................... 23 2.4.6. list of necessary equipments ........................................................................................... 26

2.5. Operating cost ...................................................................................................................... 27 CONCLUSION ...................................................................................................................................... 27



INTRODUCTION

In 2013, ACF asked for the support of the Veolia Foundation to study the feasibility of implementing

an intermediate chlorination system on the network of Freetown. The general purpose of this

technical support from the foundation is to validate and develop the technical design, tender

specifications (including statement of works and bill of quantities) and capacity building

requirements for the installation of an intermediate chlorination system in the existing Guma Valley

Water Company (GVWC) network to reinforce the residual free chlorine available in the water at

distribution points. This intervention has been prioritized as during the 2012 cholera outbreak, most

of the water provided by GVWC had a free residual chlorine (FRC) level of 0 mg of Cl2/l at tap level

which increased the likelihood of water as a cholera transmission route.

1. CONTEXT OF THE STUDY AND NETWORK COMPREHENSION

1.1. GENERAL CONTEXT

ACF (Action Contre la Faim) is a part of a DFID funded Urban WASH Consortium and has implemented

WASH activities in Freetown since 2010. ACF plays a facilitating role to support GVWC to access

international expertise to carry out the feasibility study and installation of the intermediate

chlorination system.

The population of Freetown is approximately 1,200,000 people and is characterized by a large

number of people living in formal and informal settlements, many of whom rely on the GVWC

network for their water supply. GVWC was established in 1961 under the Guma Valley Water

Ordinance which gave it responsibility for supplying water to the Freetown area. GVWC is a

parastatal organization of which 99% is owned by the central government and 1% by the Freetown

City Council. It is an autonomous body that does not receive financial support from the government.

GVWC recently underwent a reform program aiming to improve service delivery and cost recovery

(December 2012 – March 2013). To further support these efforts, ACF will help GVWC to install an

intermediate chlorination system in its existing network, in order to ensure sufficient levels of

residual chlorine in communities at risk of cholera outbreaks.

The low level of free residual chlorine is due to several factors including:

• Length of the transmission network (around 15 km from the treatment plant to the main

reservoir or around 3 hours for the transfer).

• Characteristics of the pipes (old, internal corrosion (DN550) and loss of internal protection).

• High water temperature (pipes not buried, etc) leading to a decrease in the stability of the

free chlorine and creates a suitable environment for bacterial growth.

• Inconsistent water supply resulting in variable levels of water in the network, allowing

pathogens to enter the pipes in areas with leakages, etc.



1.2. OBJECTIVES OF THE STUDY

An initial feasibility study has already been carried out by ACF. The present study objectives are to:

• Validate the feasibility study already completed and particularly the installation site

of the intermediate chlorination,

• Confirm the technical option proposed (at current demands and projected future

demands) or propose an alternative technology; specify the power requirements and

the operations of maintenance needed with their approximate cost

• Develop a detailed design of the chlorination unit and ancillaries required to operate

it

• Prepare a bill of quantities, cost estimate, statement of works, and technical

specification in preparation for tendering

• Assess the need for training or other capacity development of GVWC staff involved in

the operation and maintenance of the system.

ACF will then produce a call for tenders to achieve the construction of this intermediate chlorination.

1.3. AREA OF THE STUDY – PRE-REQUIREMENTS

The proposed location for the intermediate chlorination system based on the initial feasibility study

was the Income Tax Zone of the network. It supplies water to approximately 142,000 people in the

downtown area of Freetown.

Fig. 1: Income Tax Zone (Atkins 2008)

Previous detailed studies of the network were carried out by Atkins Consulting in 2008 which

provided a complete map of the network, as well as functionality (at the time of the study) and

recommendations for expansion and improvements.



After discussions with GVWC, the area of the current study is extended to all the zones supplied by

gravity from Spur Road reservoir (low level zone): Spur Road Zone, Tower Hill Zone, Income Tax Zone,

Dan Street Zone, Africanus Road Zone and Wellington Zone. The sites selected to be visited are:

- Spur Road reservoir,

- Tower Hill reservoir,

- Income Tax Break pressure tank.

Fig 2: Low level supply zone

The main prerequisites needed for this study are:

- flow in the pipe where we choose to install intermediary chlorination,

- chlorine demand of the water to treat,

- location of the pipes,

- water pressure,

- availability of electrical energy in the area

- physical and organic quality of the water at the outlet of the treatment plant.

With these elements (or an estimation for those where data is not available), we chose:

- the site of implantation of intermediary chlorination,

- the technology of chlorination

After this first step, we will be able to make the feasibility study.

Spur Road

Kortwright

Tower HillIncome Tax Dan St

AfricanusRd

Wellington

Governors Lodge

Kissy

Lumley

SLAHQ

Aberdeen

Murray Town

Congo Cross

College Rd

RasmussonSt

Stadium

Circular Rd

Tower Hill

Kortwright

Income Tax

Dan St AfricanusRd

Kissy

Wellington

Governors Lodge

Bulk Transfer Main

Distribution MainPumping Station

Tower Hill WSR PRV

Bulk Meter

550mm

700mm

600mm 600mm

300mm



2. FEASIBILITY STUDY

2.1. METHODOLOGY

The methodology used to collect information for the study is:

- literature review (ATKINS main report 2008, GVWC 100 days transformation

programme report, WASH report July and august 2013, feasibility studies of

intermediary disinfection on GVWC October 2012, workshop presentation Freetown

Strategic Water Supply and sanitation Framework 13th

march 2008, Strategic water

supply and sanitation framework DFID August 2008),

- hydraulic modeling : Freetown model 2008 under Epanet software,

- site visits (GUMA dam, water treatment plant, three reservoir sites : Spur road,

Tower Hill and Income Tax),

- water samples analysis and chlorine decrease tests from the three reservoirs,

- discussions with GVWC, water directorate and CCU.

The basis of the study, after discussions and visits, are:

- finding the best place to install intermediary chlorination in terms of covered

population (most of the population should be covered by chlorine residual), injection

feasibility, energy requirements, hydraulics of the site

- finding the best technology to manage (good technology sizing, constraint of the site,

ease of use, supply in reagents).

2.2. THE LIFE OF WATER FROM GUMA RESERVOIR TO THE NETWORK

The origin of 90% of tap water in Freetown is the GUMA reservoir. The quality of the resource is

quite good. The turbidity does not exceed 6 NTU. It is a soft and low alkaline water. There are 6 levels

of intake to go to the plant.

Fig 3: GUMA Reservoir



The water treatment plant consists of 3 streams:

- Stream 1 : conventional clarification and Aquazur T filters

- Streams 2 and 3: pulsators and Aquazur T filters.

- The 3 streams are combined and then chlorinated before the treated water storage

tank.

The flow produced by plant is 75 000 m3/day.

The treatments are: pre-chlorination, correction of pH with lime and coagulation with aluminum

sulfate, flocculation, settling, filtration on sand and disinfection with gas chlorine.

The performance of the water treatment plant is not very high: around 30% on turbidity, whereas

with these treatments we should normally reach a minimum of 70% and obtain less than 1.5 NTU all

the time.

The filtration is not effective (loss of sand, movements of floor panels, not adapted backwash phases

and speeds).

This low efficiency has a consequence: it increases the chlorine demand of the water and can explain

the difficulties to maintain chlorine residual all along the network to Spur Road.

The chemical quality of treated water has another consequence: distributing cor…….rosive water

which can react with the pipe and therefore dissolve iron and increase leakage.

The other data we have on treated water are the following:

6,4 < pH < 7,4

0,6 < Cl2 < 1 mg/l

Fig 4: GUMA Water treatment plant



Fig 5: view on pulsators and sand filters

The treatment plant needs a stage of neutralization before distribution.

An improvement of the treatments (processes) would allow a more effective chlorine disinfection

and to maintain more easily a residual of chlorine in the network.



2.3. VISIT AND CHOICE OF THE SITE OF IMPLANTATION OF INTE RMEDIATE CHLORINATION

The main characteristics of the sites are below:

Site Income Tax BPT Tower Hill Reservoir Spur road Reservo ir

Capacity (m3) 20 5100 18400

Maximum flow at the outlet (m3/h) - estimation based on pipe size.

570 800 2250

Average flow at the outlet (m3/h) - calculation by Epanet model.

620 680 3800

Served population 147 500 106 500 378 600

Situation of reservoir (GPS)N 08.48582

W 013.23180N 08.48243

W 013.23393N 08.46936

W 013.26736

Situation of pipe (if different) the sameN 08.48147

W 013.23468?

access to pipe in the reservoirOutlet pipe is underground.

Possibility to chlorine directly in the reservoir.

Access to inlet pipe before junction Tower Hill Income Tax

There is 2 outlet pipes, 2 chambers, but it's very difficult

to know where they join. Underground.

depth of the pipe - 4,20 m550 : 5,20 m700 : 1,20 m

diameter of pipe (DN) 450 550 550 and 700

Residual free chlorine (mg/l Cl2) 0,02 (?) 0 0,18

Residual total chlorine (mg/l Cl2) 0,1 0,09 0,28

pH 7,5 7,1 6,7

Temperature (°C) 26 25 25

disponible area for building possible but difficult OK OK

Situation of electrical power on the road upstairs near (400 V - 50 Hz - 250 A) on place

2.3.1. Spur Road Reservoir:

This reservoir has a capacity of 18400 m3 and distributes to the entire low level supply zone.

The population covered by this reservoir is 378 600 inhabitants and around 35 600 m3/d. There is

45% of leakage.

There are two reservoirs and a pumping station which supplies Governors Lodge. The capacity is

significant and the average staying time is 4 hours 20 minutes but is very variable. So, if we want to

put an intermediate chlorination, it would be on the outlet. There are 2 outlets but we cannot see

and understand where they go through at the outlet, where they join and where they split. The only



solution to install a chlorination system would be to have two points of injection and a large

installation of chlorination.

Fig 6: Spur Road reservoir and pumping station

Fig 7: Spur Road outlet pipes chambers



2.3.2. Tower Hill Reservoir

This reservoir has a capacity of 5 100 m3 and is supplied by Spur Road reservoir. It is composed by 4

compartments with an inlet for each. There are also 3 outlets on the same outlet pipe. There is a

pumping station which supplies Kortwright zone. The average residence time in the reservoir is 7

hours 30 minutes.

The population covered by this reservoir is 106 500 inhabitants and the flow is around 15 800 m3/d.

There is 30% of leakage.

Fig 8: Tower Hill Reservoir

It is not possible to put the intermediate chlorination on the outlet and not possible in the reservoir

either.

But there is a valve chamber just before the reservoir with a junction of 2 pipes: one supplies Tower

Hill reservoir and the other one supplies Income Tax. It would be possible to install the chlorination

before the junction.

Another solution is to create a derivation from Income Tax pipe to the reservoir of Tower Hill.

In both cases, the area covered by chlorine would be quite large (covered population 254 000

inhabitants).



Fig 9: Valve chamber before Tower Hill Reservoir

2.3.3. Income Tax Break Pressure Tank

This reservoir has a capacity of 20 m3 and is supplied by Spur Road reservoir. It supplies only Income

Tax zone. The residence time in the tank is very little (around 1 minute).

The population covered by this reservoir is 147 500 inhabitants and the flow is around 16 900 m3/d.

There is 45% of leakage.

Fig 10: Income Tax BPT



Fig 11: Income Tax BPT inlet

The only solution to put the intermediate chlorination is directly in the tank. Though it would be

difficult to built the installation, to do the maintenance and to deliver reagents because of the

situation of the tank on this sloping area.

2.3.4. Advantages and disadvantages of each site:

Installing an intermediate chlorination system at Income Tax BPT has the disadvantage to cover only

Income Tax Zone. Guma Valley Water Company wants to cover the maximum network as possible. So

this solution could not be kept.

Spur Road has the big advantage to cover the entire low level network area. Though at least the two

outlets will have to be chlorinated and then the chlorination installation will have a big capacity and

Site Advantages Disadvantages

Income Tax BPT Little installation of chlorination.Possibility to chlorinate directly in the tank.

Chlorination of only Income Tax Zone.Site is a steep slope, so difficult to build and to

deliver the reagents. Electricity ?

Tower Hill Reservoir

Chlorination of a large part of low level network (because we can put the chlorination before the

reservoir before the junction Tower Hill Reservoir - pipe to Income Tax).

Availability of land. Electricity power (pumping station).

The distance between chlorine injection chamber and the top of the reservoir where we can install

the chlorination installation (180 m).Security of the injection pipe between installation

and injection (crossing shanty town).

Spur road Reservoir

All the low level network covered (but there is chlorine residual at the outlet, so actions on the

inlet and/or on the water treatment plant can increase the chlorine residual on the Spur Road

Zone).Availability of land.

Electricity power (pumping station).

Obligation to chlorine the outlet.Large amount of chlorine required.

Two outlets to chlorinate (difficult to have access to the area where they join).

Chlorinate at this step to have chlorine residual at the end of the network seems very difficult because the inhabitants of Spur Road Zone

should have high level of chlorine.



quite big reagents consumption. As there is some residual chlorine at the outlet, we can expect that

other actions on the inlet and on the treatment plant could have an influence to have chlorine

residual on the Spur Road Zone.

Tower Hill has the advantage to cover a large part of the low level network if we put the

intermediate chlorination before the junction Tower Hill Reservoir – Income Tax pipe. Many zones

could be covered by chlorine: Tower Hill Zone, Kortwright, Income Tax zone, Dan street zone and the

east of the city. There is no chlorine residual at the inlet of Tower Hill reservoir. So putting an

intermediate chlorination at this step of the network can cover a big part of the network with

chlorine. The chlorine residual needed would be lower than at Spur Road. The only difficulty of the

site is to secure the pipes for electricity and chlorine solution between the installation and the

injection chamber. So the advantages of this site are:

- area covered by chlorine,

- space on the GVWC site to build the intermediate chlorination,

- choice for the technology of chlorination to use,

- electrical energy available to provide for installation.

There are more advantages to build the intermediate chlorination on this site than on the others.

To complete comparison data between each site, you will find the table above:

Site Income Tax BPT Tower Hill Reservoir Spur road Reservo ir

Served population under reservoir 147 500 106 500 378 600

Population covered by chlorination if we implant it in this area

147 500 254 000 378 600

Necessity to chlorine this network +++ +++ +

Covered network + ++ +++

disponible area - ++ ++

Technical feasibility of implantation--

(slope, no contact time in the tank, no energy).

++(energy, possibility to chlorine Tower Hill network and pipe to

Income Tax).

-energy, 2 outlets so 2 points of

injection (diificulty to know where we can inject).

possible existing technology of chlorination

calcium hypochloritecalcium hypochloriteélectrochlorination

électrochlorinationCl2 gas

Investment cost ++ ++ ---

Operationnal cost ++ + ---

So, if we want to have at the same time:

- a good cover of the network in chlorine,

- the guarantee to inject chlorine at the good point,

- the possibility to choose easy technical technology,

- the availability of electricity to provide for installation,

- the security of the installation,

- the best compromise efficiency / installation and exploitation costs,

Tower Hill is the site which presents the best compromise.



2.4. FEASIBILITY STUDY TO INSTALL INTERMEDIATE CHLORINATI ON AT TOWER HILL

The main difficulties are to know the flow in the pipes, the chlorine demand and how it could

decrease in the network.

2.4.1. Flow to chlorinate:

Based on the diameter of the pipe before the junction (DN 550), the maximum flow in the pipe is 800

m3/h. To confirm this data, we used the hydraulic modeling of the network (EPANET model)

established by ATKINS in 2008.

Simulation under Epanet: Freetown Network



Zones of network under influence of Tower Hill, Income Tax, Dan Street, Africanus road and

Wellington

Network to and under Tower Hill



The average flow determined for each influence area downstream each reservoir is presented in the

below table:

Secteur Qmoy secteur Qmoy cumulé Capacité Tps séj moyen Qrempl

m3/h m3/h m3 hh:mm m3/h

Transmission 80,4 4312,1 - - -

-> Spur_Road 808,0 4231,6 18 400 4:20 2574,0

-> Governor's_Lodge 459,0 459,0 5 250 11:26 400,0

-> Tower_Hill 602,1 683,8 5 100 7:27 632,2

-> Kortright 81,8 81,8 2 200 2:54 270,0

-> Income_Tax 619,7 619,7 20 0:01 610,4

-> Inutilisé 0,0 0,0 4 300 #DIV/0! -

-> Dan_Street 346,3 346,3 4 300 12:24 486,0

-> Africanus_Road 20,5 81,7 700 8:34 28,8

-> Kissy 61,2 61,2 2 300 13:35 122,4

-> Wellington 1233,1 1233,1 5 000 4:03 738,4

Total général 4312,1 47 570

The model works well about the influence areas under each reservoir. But it is very difficult to

simulate the flows because the network is not all the time under pressure and the reservoirs are

sometimes empty. If the network was all the time under pressure, the average flow would be, at the

point we want to implant the rechlorination, around 1300 m3/h and the maximum flow around 1700

m3/h. The pipes size and the duration when the reservoirs are full in the day allow us to minimize

these flows. More realistic flows are:

Average flow : 800 m3/h

Maximum flow : 1000 m3/h

We propose to study the size of the installation based on these data.

2.4.2. Chlorination and bacteria : Monitoring the bacteriological quality of drinking water is needed to control the presence of

microorganisms that can cause illness or disease.

Pathogenic microorganisms that occur in polluted water include protozoa, bacteria and enteric

viruses. The most common disease this can cause is the gastrointestinal illness of diarrhoea.

Although gastrointestinal illness is usually non-life threatening in normal healthy adults, the risk of

death increases among vulnerable groups such as infants, the elderly and immunosuppressed

individuals.

Since it is not practical (as tests are extremely costly) or technically feasible to monitor for all

pathogens in drinking water, the microbiological quality of drinking water is evaluated based on

indicator microorganisms, such as total coliforms and E.coli.

Chlorine destroys disease-causing germs and helps make water safe to drink. Chlorine destroys

waterborne germs by penetrating their slime coatings, cell walls and resistant shells. Chlorine either

kills the germs or renders them incapable of reproducing. Chlorine is highly effective against most

disease-causing germs found in drinking water sources.

So the preservation of a residual of chlorine in the network is an effective way to prevent a

proliferation of bacteria and to guarantee a bacteriological quality of the water of drink.



2.4.3. Chlorine demand

When we introduce some chlorine into the water, the residual decreases in time due to the chemical

reactions which develop with the organic and mineral compounds. It is the demand in chlorine.

To determine the chlorine demand of the water, we have taken samples at each reservoir and we

made chlorine demands for different concentrations of chlorine at the beginning of experience.

To realize the chlorine demand, the operating mode was :

1- take 4 samples of each reservoir in brown glass beaker or bottles of 1 liter,

2- inject in it an increasing dose of chlorinated solution,

3- for example, here, we tested 0.5 – 1 – 1.5 – 2 mg/l of initial concentration.

4- Measure residual chlorine for different contact times during 14 hours or more.

5- The chlorine demand is the chlorine rate you have to dose to have an objective chlorine

residual after your contact time (contact time in a tank or in the network).

The results are below:

chlorine decrease Spur Road

0

0,5

1

1,5

2

0 5 10 15 20 25 30

time (h)

chlo

rine

resi

dual

(m

g/l)

For Spur Road, the speed of chlorine decrease of the water is -1.21 mg/l/d so, if we want to chlorine

the water at Spur road, we need to have a minimum rate of 2.5 g/m3 to cover all the area under Spur

road (it depends on the residence time in the network).

Bécher 1L Bécher 1L Bécher 1L Bécher 1L

Traitement à0.5 g/m3

Traitement à1 g/m3

Traitement à1.5 g/m3

Traitement à2 g/m3



chlorine decrease Tower Hill

0

0,5

1

1,5

2

0 5 10 15 20 25 30

time (h)

chlo

rine

resi

dual

(m

g/l)

The results show that chlorine decreases quite quickly (the speed of chlorine decrease of the water is

-0.52 mg/l/d). Chlorine demand is not very high. A chlorination rate of 1 g/m3 should be enough.

chlorine decrease Income Tax

0

0,5

1

1,5

2

0 5 10 15 20 25 30

time (h)

chlo

rine

resi

dual

(m

g/l)

For Income Tax, the speed of chlorine decrease of the water is -0.38 mg/l/d. A chlorination rate of

0.75 mg/l should be needed.

2.4.4. Size of installation – choice of technology

The hydraulic model is not usable to know the residence time of water in the pipe. We don’t know

exactly the time between Tower Hill and the end of the network and the chlorine demand of the

network either. It is impossible to simulate it with the model because the network is not all the time

under pressure.

If we evaluate a maximum time of 48 hours between Tower Hill and the end of network, we can size

the installation of chlorination on the basis of 1g/m3 of chlorine.

If the maximum flow is 1000 m3/h and the rate of treatment 1 g/m

3, we need to produce 1000 g/h of

chlorine maximum.



2.4.4.1. Usable existing technologies :

1- Gaseous chlorine :

The gaseous chlorine and the chlorinated compounds are very widely used for the disinfection. In

solution, the chlorine can appear under various states and react with different organic molecules or

mineral.

The principle consists of the injection of gaseous chlorine in a driving water which conveys the

chlorine to the injection point.

Example of gaseous chlorination system

This technology is efficient but needs very strict safety regulations. Chlorine gas is a highly toxic

substance for both humans and the environment, and must be stored carefully to avoid leakage.

2- Electrochlorination

The process involved in electrochlorination is the desalination of water to produce a chlorinated

solution. This happens when saltwater is inserted into electrolytic cells. The saltwater is streamed

through a channel of decreasing thickness. One side of the channel is a cathode, the other is an

anode. As the water flows through the anode/cathode channel, a low voltage DC current is applied.

When this happens, the electrolysis is triggered and sodium hypochlorite is instantly produced as

well as hydrogen gas (H2). The hydrogen rich sodium hypochlorite then travels to a tank that removes

the hydrogen gas. The dehydrogenization mechanism varies from device to device but the process is

generally the same. After hydrogen has been removed from the solution, the hypochlorite is stored

in a tank as the finished product.[1]

No chemicals other than ordinary salt, or sodium chloride (NaCl),

are used throughout the entirety of the process. Although the actual chemical processes involved are

complex, they can be simply represented by the following equation:

NaCl + H2O + ENERGY → NaOCl + H2

In words this reads, energy is added to sodium chloride (table salt) in water, resulting in sodium

hypochlorite and hydrogen gas.

The solution is then pumped to the injection point.



Exemple of electrochlorination system

3- Calcium hypochlorite

Calcium hypochlorite is a yellow white solid which has a strong smell of chlorine. It is not highly soluble in water and is more preferably used in soft to medium-hard water. It has two forms: dry and hydrated. The hydrated form is safer to handle.

Calcium hypochlorite reacts with carbon dioxide to form calcium carbonate and release dichlorine

monoxide:

Ca(ClO)2 + CO2 → CaCO3 + Cl2O↑

A calcium hypochlorite solution is basic. This is due to the hydrolysis performed by the hypochlorite

ion, as hypochlorous acid is weak, but calcium hydroxide is a strong base. As a result, the

hypochlorite ion is a strong conjugate base, and the calcium ion is a weak conjugate acid:

ClO−+ H2O → HClO + OH

−

Example of calcium hypochlorite system

The solution is then pumped to the injection point.



2.4.4.2. Choice of technology:

The existing chlorination systems are compared below:

Technology Cl2 gas Electrochlorination Calcium hypochlorite

Investment costs + --- ++

Reactive costs --- - (pure salt needed) +

availibility of reactive + ? +

Exploitation costs + -- ++

needed energy ++ -- ++

security of installation / safety for the staff

--- + +

technical know-how ++ --- ++

The principal difficulty to use chlorine gas is the security of installation in urban zone, the transport

of reagents in urban zones and the security of staff for maintenance.

With electrochlorination, there are two principal difficulties: the availability of reactive (it is very

important to use pure salt) and the electrical power needed to run the installation.

Installation of chlorination using calcium hypochlorite has fewer disadvantages. We need less energy,

less technical know-how and less maintenance too compared with electrochlorination.

If we install chlorination at Spur road, electro-chlorination is the only possible technology because of

the flow to treat and the security in urban zone.

If we install chlorination at Income Tax, the best technology is calcium hypochlorite because it needs

less electrical energy and is simpler to manage.

We studied two technologies for the intermediate chlorination at Tower Hill: electro-chlorination and

calcium hypochlorite.



You will find a comparison of the two technologies in the board below.

The electro-chlorination is very successful but it is very expensive and this type of installation needs

good competence to drive it. Previous experiences have shown that repairing breakdowns in these

installations is very complex.

On the contrary, calcium hypochlorite is very simple. The only inconvenience is that if there is a big

chlorine demand sustained over many hours, it is not sure that we could have a 120 g/l Cl2 solution

all the time because it is necessary to have a time between fabrication and injection. With the

proposed installation, if we treat 750 m3/h, there is 20 hours of autonomy but there is continuous

manufacturing. So the autonomy of installation between two loads in reagents is around 3 days.

This solution needs a lower electrical power. It would be possible to use batteries to ensure a

constant supply.

It is why we favor chlorination with calcium hypochlorite installation. The other advantage is the

supply in reagents from Guinea.

2.4.5. Implantation of installation To chlorinate both arrival to Tower Hill reservoir and pipe to Income Tax, we have two solutions

which are described below:

Electrochlorination Calcium hypochlorite

Chlorine product max (g Cl2/h) 1000 1000

Power needed (kVA) 7.5 1.5

fabrication water pressure needed (minimum in meters) 20 -

Cost of installation (USD FT) departure of France 95000 USD 10000 USD

needed area (LxlxH) 2000x600x2000 1400x1000x1100

maintenance

good local aeration needed (exothermic reaction), bad quality of

chlorine at the start.Competence in electronic and

automatism required.

simple. No particular competenciesrequired. Only intervention on

injection pump and injection point if necessary.

reagents pure salt (99,95 % NaCl minimum) Calcium hypochlorite tablets

packaging Bags of 25 kgs Barrel of 25 kgs 20 gram tablets

country of provenance (reagents) France Guinea

estimate cost (reagents) 6,72 USD/kg

estimate cost (transport) 1290 USD



Solution 1 :

This solution consist in implanting a point of chlorination just before the junction Tower Hill reservoir

– Income Tax pipe.

It needs to enlarge the valve chamber to put the point of injection, static mixer and the flowmeter.

The pipes of injection and electric cables will come from the old pumping station (which will be



renovated to install the installation of chlorination and the stock of reagents) all along the feed pipe

of Tower Hill reservoir.

Solution 2 :

This solution consists in deleting the pipe which rises to the reservoir of Tower Hill and creating a

deviation from Income Tax pipe to the reservoir of Tower Hill.



A new valve chamber will be required, along with new pipework rising up the hill to the inlet point

and returning to the original Income Tax pipe, installation of a flowmeter and an injection point on

the reservoir.

This solution is easier to secure because all the strategic equipments and the lines of injection and

power are on the site of the reservoir.

2.4.5.1. Work to do for this solution :

A. Implantation of two tees of connection and a valve of isolation on the pipe of distribution

towards Income Tax,

B. From the tee upstream, implementation of a pipe of diversion towards the reservoir of

Tower Hill,

C. On the top of the reservoir, implementation of flowmeter to regulate chlorination, injection

point of chlorine. Isolation of flowmeter by valves downstream/upstream (1). A check valve

will be installed to ensure that the flowmeter does not drain if the water distribution stops

(2),

D. Return to the Income Tax network by a new pipe installed in parallel of B pipe,

E. Connection to supply network of Tower Hill downstream of the flowmeter with isolation

valve (3).

F. Installation of chlorination device with calcium hypochlorite (with chlorine regulation and

dosing pumps) in the old pump station. The electric cupboard should also be installed in this

room.

G. Sheaths of connection between the place of installation of the chlorination and the point of

injection and the flowmeter for hydraulic and electric connection.

H. Sheaths of connection from pumping station to chlorination for power supply of the electric

cupboard.

It is necessary to secure the storage of reagents, so, if the ordered quantities are quite high, the

chlorination room should be enlarged.

2.4.6. list of necessary equipments

A. - valve chamber on Income Tax network,

- isolation valve in the valve chamber (DN 450),

- Tee (2) in the valve chamber (DN 450).

B, C, D, E.

- pipe of derivation in cast iron (DN 450),

- isolation valves (2) for flowmeter (DN 450),

- check valve (DN 450),

- flowmeter (DN 400),

- injection cane,

- isolation valve to Tower Hill (DN 450).

F

- calcium hypochlorite installation,

- reagents storage,

- dosing pumps (2),



- chlorine analyzer.

G

- hydraulic connection to injection,

- electric connection to flowmeter.

H

- electric connection between pumping station and chlorine room.

2.5. OPERATING COST

The estimating operating cost can be broken down in the following way:

- workforce for operating (1/2 h per day to verify installation and load in reagents + 1h per

week for maintenance),

- reagents (15 kg/day max),

- electric power

So the estimate operating cost should be:

- workforce for operating : 600 USD/year

- reagents (1 supply every 2 months) : 48000 USD/year.

- Maintenance (change of probe – 1/year, change lid, membrane on pump – 1/year) : 500

USD/year

- Electric power 5 kVA or less if battery: 5000 USD/year.

CONCLUSION GVWC needs to install an intermediate chlorination in its existing network in order to ensure

sufficient levels of residual chlorine in communities at risk of cholera outbreaks. So ACF has the

mission to install this intermediate chlorination in Freetown. Veolia foundation support ACF in this

mission.

The purpose of this study was to define:

o The location for the chlorination (considering tower hill and spur road as well)

o The technology to be used

o The power requirements for it

o The operation and maintenance costs for GVWC

After visiting all the potential sites, the choice would be to install the intermediate chlorination at

Tower Hill Reservoir because we can chlorinate water just before the reservoir and have chlorine

residual down to Tower Hill network and Income Tax network and farther.

For that, we have two solutions of setting-up the installation:

* First solution: create a chamber before the existing valve chamber to put chlorination point and

flowmeter (the advantage is to not change the hydraulic configuration of the network, the



inconvenient is to have a big length of chlorine pipe and energy between the installation and the

chamber),

* second solution : abandon the current pipe which rises to the reservoir and create a diversion

on the pipe which go to Income Tax (the advantage is to have an installation more secure, energy

and installation of chlorination close to the injection point, the inconvenient is to create the pipe

between Income Tax pipe and Tower Hill reservoir).

The 2nd

solution seems better (it is safer due to the local environment).

The choice of the technology to be used goes to calcium hypochlorite chlorination. Indeed, it is easier

to use and less subject to breakdowns.

The needed power and operation and maintenance costs for GVWC are précised too in this report.

With all these elements, ACF should be able to establish the tender documents.

Documents

Feasibility Study on Intermediate Chlorination in … · Chlorination and bacteria : ... an intermediate chlorination system on the network of ... Feasibility Study on Intermediate