Water, Engineering and Development Centre

Household use of grey water, wastewater and rainwater

Mike Smith

People-centred solutions for sustainable development since 1971

1. Operating principles2. Experience

Grey-water recyclingRainwater collection

3. Water quantities4. Water quality issues

Grey-waterRainwater

5. Economic aspects6. Summary

To sewers

Piped water Washbasins, showers,

washing machines

Treatment unitRainwater

Storage

Toilets

Overflow

Schematic principles of a domestic grey-water

recycling system

Treatment wastes

Top-up water if needed

Treated grey-water could be used for:

• Non-potable domestic purposes

• Irrigation *

* See, for example, “Draft guidelines for the reuse of Greywater in Western Australia” (2002)

Examples of domestic re-useLoughborough University (UK)Treated grey-water and rainwater used for toilet flushing

in a student accommodation block.

Annecy (France)Treated grey-water used for toilet flushing in an apartment block, and landscape irrigation.

Hockerton Housing project (UK)Rainwater used for drinking water. Treatment is by

filtration and UV disinfection.

Nottingham Eco-home (UK)Rainwater used for toilet flushing and laundry. Storage is the only treatment provided.

Water quantities

Percentages of water used for toilet flushing:

Typical domestic range 29% to 47%

Millennium Dome 48%

Typical office 63%

Four UK universities 39% to 54%

Typical water usage at LU student accommodation block

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1 3 5 7 9 11 13 15 17 19 21 23Time (hours)

Wa

ter

us

ag

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lcd

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WC

Personal washing

Data from a study, by Thames Water and Cranfield University, of grey-water re-use in five homes

House 1 2 3 4 5

% of total 36% - 21% 9% 17%water saved

Mean per capita 43 102 181 125 79consumption (lpcd)

Source: Hills, Birks, Diaper, Jeffrey, (2003)

Water quality issuesCharacteristics of greywater from various studies

ParameterMetcalf and Eddy (1991)

Lazarova (2001)

Smith et al. (2001)

Surendran and Wheatley (1998)

Rose et al. (1991)

Laine (2001)Christova-Boal

et al. (1996)

Greywater type

Sewage Dark V. Light1 Light and Dark2 Light3 Light Light

BOD5 (mg/L) 110–400 275–580 33216–252 (light) 472–536 (dark)

* 129–155 76–200

COD (mg/L) 250–1,000 471–915 95 424–433 725–936 * 367–587 *

SS (mg/L) 100–350 71–215 3640–76 (light) 68

(dark)* 58–153 48–120

NH3-N (mg/L) 12–50 0.6–18.8 * 0.5–1.6 4.6–10.7 0.15–3.2 * <0.1–15

TKN (mg/L) * 3.9–22.8 4 * 0.6–5.2 6.6–10.4 4.6–20

TP (mg/L) 4–15 5 – 26.7 *1.6–45.5 (light) 15.6–101 (dark)

4–35 * 0.11–1.8

TC (CFU/100 ml)

105–106 1.8 × 106 – 1.8 × 108

2.4 × 103 – >2.4 × 106

5 × 104 – 6 × 106 (light) 7 × 105

(dark)6.1 × 108 6.8 × 103 – 9.4

× 103 500 – 2.4 × 107

FC (CFU/100 ml)

104–105 3.0 × 105 – 1.6 × 108 *

32–600 (light) 728 dark)

1.8 × 104 – 7.9 × 106 *

169 – 3.3 × 103

E. coli (CFU/100 ml)

*7.6 × 105 – 2.04

× 107 0 – >2.4 × 106 * * 10 – 1.5 × 103 *

Grey-water is similar in quality to settled sewage. (Better than Raw sewage.)

BOD, Suspended solids and bacterial numbers are high, and grey-water will quickly smell unless treated.

Treatment is needed to:• Minimise health risks from contact and aerosols;• Improve appearance;• Reduce odour;• Minimise biological growth, scaling and corrosion.

1. Storage of Greywater. Including storage capacity to accommodate sludge accumulation.

2. Balancing and screening: pH correction, and screening to remove suspended solids, including hair.

3. Aerobic treatment: This may include aeration, sedimentation, flotation, dilution, and addition of chemicals such as alum, lime, or chlorine.

4. Slow filtration: Filters may use foam, slow sand filters, carbon filters or membranes.

5. Storage of partially-treated grey-water.

Handling of grey-water

Rainwater quality

The quality of rainwater depends upon the cleanliness of the collecting surface.

Recent studies show that storage, and flow through a series of tanks, significantly improves the bacterial quality of rainwater.

Loughborough University 2005

Thermotolerant Coliform Reduction

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/ 10

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Tank 1 Tank 2 Tank 3 Tank 4

Tank 1

Tank 2

Tank 4

Tank 3

Typical thermotolerant coliform count in tank series

Valley Road, Loughborough (2005)

Nottingham Eco-home (2005)

Nottingham Eco-home (2005)

Economics and justification

For individual houses, grey-water recycling offers negligible economic benefits.

For Loughborough University, retrofit costs could be recovered in 8 or 9 years.

Benefits can be considered in terms of reduced demand on limited water resources, and reduced sewage flows.

Full benefits depend on specific social, technical, economic and climatic conditions.

Perceptions of users

Surveys suggest that users generally have a positive attitude to using recycled water for toilet flushing.

Householders want a simple, reliable, facility, requiring little or no maintenance. (They want a ‘fit and forget’ unit.)

Re-use of grey-water and rainwater for toilet flushing is more attractive where a maintenance team is available.

Summary

Currently, the cost and convenience of piped water supplies in UK does not encourage recycling of grey-water.

Incentives for recycling could include:• Personal or ethical preferences• Environmental concerns• Legislation• Tax incentives