Greywater is wastewater from showers, baths, wash basins, washing machines and kitchen sinks. It can be collected from some or all of these sources and, after treatment, used for purposes around the home such as toilet flushing or garden watering that do not require drinking water quality.

Greywater refers to all household wastewater other than wastewater from the toilet (blackwater). Typically, domestic reuse systems collect greywater and store it before reusing it to flush the toilet. More complex systems treat greywater to a standard that, it is claimed, can be used in washing machines and in the garden.

The most basic systems simply divert cooled and untreated bath water to irrigate the garden. Systems for flushing the toilet can save around a third of daily household water demand. A trial by the Environment Agency showed a range of water savings from about 5 per cent to 36 per cent. As newer properties tend to have lower toilet consumption, the maximum savings in a new build might be closer to 20 per cent. 


What about bacteria in greywater? 

Problems can arise when warm, nutrient-rich greywater is stored, as it incubates bacteria. There are currently three approaches for dealing with this problem.

  • The first is to limit the time that the greywater is stored. These systems might incorporate an electronically controlled dump valve to empty the storage tank after a period of inactivity before refilling with mains water.
  • The second approach is to use chemical disinfectants such as chlorine or bromine compounds that inhibit biological activity and extend possible storage time.  
  • The third approach is to treat the greywater in a small sewage treatment plant, either by using traditional biological methods or newer membrane filtration technology. The treated water is clear and free of unpleasant odours and contains little organic matter, allowing it to be stored and reused. However, this uses a significant amount of energy and is very expensive.

Untreated greywater can be used for watering the garden if it is used immediately after it is produced, but it should not be used on edible crops. The wastewater from kitchen skins, wash hand basins and dishwashers is not usually collected as it is too heavily contaminated.