graywater

Graywater is wastewater from domestic activities that does not include toilet waste. It typically originates from showers, baths, sinks, and laundry machines. In some places kitchen wastewater is considered graywater, while in others it is excluded because of fats, oils, and food solids.

Compared with blackwater, graywater carries a lower risk of pathogens but can still contain soaps, skin oils, hair, and trace residues, and it may harbor microbes if the source water was contaminated. Handling and treatment are important to reduce odors and prevent buildup of salts or nutrients in soils.

Uses: When treated or properly screened, graywater can be reused for landscape irrigation, subsurface irrigation, or toilet flushing. It is generally not used for edible crops or hand-contact areas, and kitchen graywater often requires more treatment or exclusion.

Treatment and systems: Small-scale systems may rely on filtration, settling, and simple storage. More elaborate setups include secondary treatment, disinfection, or constructed wetlands. Plumbing and landscaping codes often require non-potable (purple) piping and separate supply lines from potable water.

Regulations and considerations: Regulations vary widely by jurisdiction and may require permits, inspections, and adherence to guidelines to avoid health risks or soil and water quality impacts. Prohibitions commonly target graywater from kitchens, laundries with certain detergents, or proximity to wells and streams. Users should consider soil type, climate, and plant compatibility; high salinity or certain chemicals in household products can harm soil or crops.

Environmental impact and benefits: Properly managed graywater reuse reduces freshwater withdrawals and can provide nutrients to vegetation, but improper use can cause odors, corrosion, and soil moisture or groundwater issues.