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) 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.
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 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
Greywater recycling
in schools in Madhya Pradesh, India
Green Roof Water
Recycling System (GROW)
