A greywater system is a plumbing system which is designed to enable greywater recycling, the reuse of certain household waste water. A greywater system can vary widely in scope and size; for example, some people simply divert greywater to a tank which is used to water the garden, while others have more elaborate systems which permit the use of greywater for things like flushing the toilet. Such systems can be custom-built for a home, or purchased in kit form.
Greywater, or graywater, is waste water which does not contain hazardous components. As a general rule, this includes all waste from a home except water from the toilet and kitchen sink. Toilet and kitchen waste water are known as blackwater, because this water can potentially house pathogens which could make reuse dangerous. Such water needs to be treated before it can be reused or released.
The rest of the waste water from a home, however, is often perfectly usable, if a bit soapy. People who use a greywater system may choose to use biodegradable soaps and to avoid harsh chemicals so that their greywater is as harmless as possible.
Greywater systems include the connections to the various sources of greywater in a home, like sinks, showers, and washing machines, along with a central holding tank for the greywater. Along the way to the holding tank, the greywater usually passes through a filter which traps large particulates, keeping the water relatively clear.
Once in a holding tank, there are a number of options for greywater. Many people use greywater for gardening, reusing the water rather than using fresh potable water to water the garden. In areas where drought warnings are in effect, this reuse is often actively encouraged by city agencies who would like to see an overall reduction in water use. As a general rule, greywater is not harmful for gardens, although it can be a bit alkaline, which is not appreciated by all plants.
In addition to being used to water a garden, a greywater system can also recycle greywater to various locations in the house where water does not need to be potable or spotlessly clean. The toilet is an obvious choice for greywater recycling. In other households, additional filters may be added to a greywater system so that the water can be used again for washing and other household tasks. Special purification systems can even render greywater safe to drink.