greywater

Greywater refers to wastewater from domestic activities that does not include toilet effluent (blackwater). It typically comes from baths, showers, sinks, and laundry, and in some definitions may include kitchen or dishwasher effluent depending on local regulations. Greywater is distinct from blackwater, which contains sewage.

The composition of greywater varies with source and products used. It generally contains lower levels of pathogens than blackwater, but can carry soaps, detergents, personal-care products, skin cells, hair, lint, fats and oils, and small amounts of food residues. Chemical additives in cleaning products can affect soil and plant health and may influence treatment performance.

Reuse and applications: With appropriate treatment, greywater can be reused for landscape irrigation, subsurface irrigation, and toilet flushing. In some systems, greywater is directed to subsurface or soil absorption areas rather than surface application. Direct disposal or disposal to septic systems may be allowed in certain jurisdictions. Caution is advised to avoid contact with edible crops and to minimize odors and buildup.

Treatment approaches: Untreated greywater is not suitable for reuse. Systems range from simple, passive configurations (gravel beds, basic filtration) to more advanced setups that include filtration, settling, and disinfection (chlorination or UV) and may incorporate pumps and controls. Storage should be minimized to reduce odors and microbial growth. Kitchen greywater is typically avoided for reuse due to higher grease and solids content.

Regulation and impact: Rules and codes for greywater use vary by region and may require permits or inspections. Properly designed and maintained systems can reduce freshwater demand and wastewater discharge, but can also introduce issues such as salt buildup and nutrient loading if not managed carefully.