Highlights from the Alliance for Water Efficiency website:
What does it mean?
, (sometimes spelled gray water, grey water or greywater) is untreated wastewater resulting from lavatory wash basins, laundry and bathing. It never contains wastewater from toilets, urinals or any industrial process. Wastewater from kitchen sinks is also often excluded because of the high food and grease content. In most cases, graywater is recovered and used at single-family homes; although green building industry is promoting graywater in multi-family, commercial, and institutional settings as well. In its most common usage, graywater is often coarsely filtered to remove suspended solids, but not treated with sanitizing chemicals and biocides such as chlorine. The lack of biocides dictates that the water must be used immediately, and subsurface irrigation is the predominate use for this water."
How much can I save?
"More than half of the water used inside the home is potential graywater as it drains into the wastewater system. A typical home with older fixtures could generate 35,000 gallons (132.5 m3) of graywater per year while a newer more efficient home could generate 25,000 gallons (94.6 m3) of graywater per year (Aquacraft, Inc. 1999, 2004, 2008). The key to using this resource is separating graywater from the blackwater (water flushed down toilets and urinal) and using the water in a safe and productive manner."
How is it installed?
"Capturing the water requires separate drainlines for the fixtures and appliances generating the graywater. A system to temporarily store the graywater is necessary, along with a pump to convey the graywater to the desired location. Separating the drainlines is relatively easy during new construction, while retrofitting an existing home can be very expensive."
"In many retrofit projects, homeowners have chosen to only capture the wastewater from the clothes washer, because the clothes washer drain line is not permanently connected to the home sewer system, and the washer already has an intergrated pump to convey the graywater to an alternate storage vessel. An older top loading clothes washer alone can generate more than 14,000 gallons (53 m3) of effluent (graywater) per year; adequate for most residential subsurface irrigation needs. This is enough water to irrigate more than 800 square feet (74.3 square meters) of non-turf plants (trees, shrubs, flower gardens, etc) in most climates."
The source of this information, along with more details about graywater is available at the Alliance for Water Effficency