what is gray water

Gray water, also spelled as grey water, is water that has already been used domestically, commercially, or industrially. It includes the leftover, untreated water generated from washing machines, bathtubs, bathroom sinks, and showers. Gray water is different from black water, which is wastewater from toilets. Gray water should not be confused with rainwater, air-conditioning condensate, foundation drain water, stormwater, and swimming pool backwash and drain water, which are considered alternative onsite water. Gray water should be assumed to have some black water-type components, including pathogens. Gray water can be used for outdoor irrigation, flushing toilets, and other similar activities. However, gray water should not be used on edible crops that touch the soil because it can contain harmful bacterial contaminants, grease, or residues of detergents. Gray water should be used within 24 hours of being collected because as the residues and contaminants break down, the water can start to smell. Gray water reuse systems can help take stress off municipal water supplies by replacing some of the water that would otherwise have been used for outdoor irrigation. Gray water should be applied below the surface where possible (e.g., via drip line on top of the soil, under mulch; or in mulch-filled trenches) and not sprayed, as there is a danger of inhaling the water as an aerosol.