Grey water is literally the water that comes out in the wash. Not to be confused with black water, which is wastewater that comes from the toilet and garbage disposal, grey water is the wastewater that comes from the laundry, kitchen, bathroom faucets, baths, and showers. Using grey water recycling and treatment systems is an important component of green building methods and practices. Recycling grey water has become more attractive to owners of commercial and residential buildings for the savings in energy and potential environmental benefits.

Recycling grey water reduces the demand on sewage treatment plants as well as water treatment plants. An estimated 50 to 80% of all wastewater a residence generates is grey water, which can be put to use in irrigation of the landscaping. Grey water is preferred for this type of recycling, as black water requires much more advanced treatment procedures. Ninety percent of nitrogen in wastewater comes from black water, and nitrogen is one of the most difficult pollutants to remove from water, making it nearly impossible to recycle black water for residential or commercial applications.

There are different methods of treating grey water on the market, and as the technology improves, more become available to the general public. Since grey water treatment is a specialized process and requires expert installation and maintenance, it is often cost prohibitive for the average homeowner. Among the current methods are reedbed filtering systems, living wall and wetpark systems, and distillation. Benefits of recycling grey water include recharging the groundwater, reducing energy usage, and improving the landscape.

Long term studies have not been conducted on the potential environmental impact of using grey water on the topsoil, and city and county officials have not quite kept pace with the developments in technology. As a result, there are few regulations on the books that govern the use of recycled grey water. If one decides to install a grey water recycling system, the household should use biodegradable, organic soaps and detergents to minimize the impact on the environment.