In California, “graywater” (also spelled greywater) is water that comes from: bathroom sinks, showers, tubs and clothes washing machines.
Water used in the kitchen sink is considered graywater in some states, blackwater in others, and dark grey in yet others due to the higher levels of grease, fat, and bacteria found in the water. In California, kitchen sink water is considered blackwater. As such, it’s not an allowable source for graywater harvesting.
Graywater is sometimes confused with rainwater (water collected from roofs and other surfaces) which is often stored for later use.
Graywater is also confused with “blackwater” (aka, sewage). Blackwater is water contaminated with fecal material, such as flushed toilet water and laundry loads of dirty diapers.
Graywater is also not “recycled” or “reclaimed” water, which is water from sewer treatment plants treated to potable standards.
Although graywater usually contains traces of dirt, food, grease, hair, and household cleaning products, it is a safe and even beneficial source of irrigation water. Plant-friendly products should be used. It also provides an alternative supply for other non-potable water needs such as toilet flushing and clothes washing.
Graywater is an important component of integrated water management. When used in concert with conservation and efficiency, water wise landscaping, and rainwater harvesting, it can greatly lower potable water demand as well as the need for costly wastewater treatment.