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Conventional toilet flush water is mains supplied water unnecessarily treated to drinking water quality standard, an expensive and energy intensive process. Greywater recycling is an innovative alternative whereby treated greywater is used principally for toilet flushing (which represents about a third of water use in a typical UK household (1) ) but also for watering gardens.
Greywater is wastewater from showers, baths, wash basins, washing machines and kitchen sinks although for recycling purposes kitchen sink and washing machine water is normally excluded because it is too greasy and/or contains too many detergents to allow cost effective treatment.
 Unlike rainwater, greywater requires filtration to remove hair, skin and soap products from the water and chemical or biological treatment prior to reuse. The potential level of human contact with the water in its end use will determine what level of treatment is required. For example greywater used for hosing down vehicles will require a high water quality because the risk of human contact with the water is greater in highly pressurised systems. Similarly black water (toilet effluent diluted by flushing water) is not recycled because of the even higher level of treatment needed before it is safe for human contact. Public acceptance is also a major barrier here.
Public acceptance
Perhaps the two biggest barriers to widespread uptake of greywater recycling are public concern about the risk to health and system maintenance requirements. The health concerns are twofold: firstly the health risk from contact with greywater in the normal operation of the system and secondly the health risk posed by the breakdown or ineffective operation of the treatment system. Greywater recycling systems are designed for minimal user contact with the greywater. Aerosols from toilet flushing are the only potential contact most users will have with the water and this is unlikely to have health implications if the water has been properly treated. It can be minimised even further by closing the toilet lid prior to flushing(2).
 There is a health risk however where treatment systems have broken down or not been maintained correctly so that untreated water (which may have been stored for long periods) comes into contact with users. Where untreated greywater has a long residence-time in the system the risk is greater. If there are pathogens such as enteric viruses, giardia, cryptosporidium, salmonella and campylobacter(2) present in the wastewater from affected individuals, lengthy periods of poor storage could result in the water turning septic and posing a health risk. The untreated greywater awaiting treatment should instead be stored in a dark, cool container and continually stirred to prevent anaerobic conditions. Figure 1 below summarises some of the major factors that will affect the health risk posed from greywater recycling.
Figure 1: Factors affecting the risk to health from greywater recycling.
Despite these risks, there are numerous safeguards which together diminish the health risks almost completely:
Ultraviolet, chemical and/or biological disinfection
Periodic inspection and cleaning of the system to ensure the water is being adequately disinfected
Clear identification of pipework as carrying greywater and incompatibility with mains pipework
Pale colouring added to the recycled water to differentiate it from potable water (see GROW case study below)
User training covering how the system works and good practice to adopt to minimise potential risks
A manual divert option whereby excessively contaminated water does not have to enter the recycling system
Multi-occupancy buildings are likely to have greater water circulation ensuring the greywater used is fresh rather than having had a long storage residence time in the system.
Case Studies
Read about greywater recycling in schools in Madhya Pradesh, India
Read about the Green Roof Water Recycling System (GROW )
References
1. Environment Agency (2001) Conserving Water in Buildings: Fact Sheet 3.
2. D. Christov A-Boal, D et al (1995). Installation and Evaluation of Domestic Greywater Reuse Systems: Executive Summary. Victoria University of Technology (Australia)
3. A Study of Domestic Greywater Recycling (2000) National Water Demand Management Centre, Environment Agency
4. Draft National Guidelines for Water Recycling. (2005) Environment Protection and Heritage Council. Australia http://www.ephc.gov.au/ephc/water_recycling.html
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