Greywater is the term used to denote waste water from sinks, tubs, showers, and washing machines that is not as contaminated as that drained from toilets and can be recycled for some other uses if not stored for more than about a 24hr period. In homes with a water conservation scheme, it is usually used for irrigation of landscape and gardens and in some cases for toilet flushing.
Systems for greywater reuse vary greatly in design but the basic principle is to divert drain water normally sent to the standard sewerage or septic system to a secondary greywater drain system with some type of filter for removal of hair and solids, a buffer/surge storage or temporary storage which helps moderate temperature, and then whatever system is reusing it. Usually a diverter valve is placed at each fixture or at some strategic point for a group of close-by fixtures for this purpose and may be manually operated or driven by an electric actuator for push-button control. Various pumps may also be used in some systems as the greywater flow may need to work against gravity in some applications.
Systems for landscape irrigation are the most common forms of greywater reuse systems and will typically consist of a buffer tank with screen/filter element and then a set of buried sub-irrigation channels or mulch basins (trenches filled with mulch) which disperse the water to the nearby landscape. With fine filtering, manifold-based drip irrigations systems driven by pump are sometimes used and this is particularly suited to a raised bed container garden scheme. However, the underground dispersion is often favored for its elimination of filters and their frequent maintenance.
Toilet flushing systems employ a storage tank with filter, pump, disinfectant system and a fill-control sensor which determines when grey and fresh water can be used to fill the toilet tank. Systems will usually put the storage tank in a concealed location in close proximity to the toilet and the nearest sources of supply -usually bathroom sink and shower. Compact systems are often designed to be installed in bathroom under-sink cabinet space. These are some of the simplest and most inexpensive of greywater recycling systems, but only handle a modest capacity.
Kitchen sink greywater is the most problematic of greywater sources due to the large volume of food waste solids common with it. Some system designs may isolate or preclude this source.
It is also critical that households using greywater recycling for landscape or gardening use low-toxic or chemical-free alternatives to common cleaners, detergents, soaps, and shampoos as the large assortment of chemicals common to the conventional forms of these can be toxic to plants.