Greywater is non-industrial wastewater generated from domestic processes such as dish washing, laundry and bathing. Greywater comprises 50-80% of residential wastewater. Greywater comprises wastewater generated from all of the house’s sanitation equipment except for the septic tank (water from toilets is blackwater, or sewage). Greywater is distinct from blackwater in the amount and composition of its chemical and biological contaminants (from feces or toxic chemicals). Greywater gets its name from its cloudy appearance and from its status as being neither fresh (white water from groundwater or potable water), nor heavily polluted (blackwater). According to this definition, wastewater containing significant food residues or high concentrations of toxic chemicals from household cleaners, etc., may be considered “dark grey” or dirty water.

In recent years, concerns over dwindling reserves of groundwater and overloaded or costly sewage treatment plants have generated much interest in the reuse or recycling of greywater, both domestically and for use in commercial irrigation. However, concerns over potential health and environmental risks mean that many jurisdictions demand such intensive treatment systems for legal reuse of greywater that the commercial cost is higher than for fresh water. Despite these obstacles, greywater is often reused for irrigation, illegally or not. In droughtzones or areas hit by hose pipe bans (irrigation restrictions), greywater can be harvested informally by manual bucketing. In the third world, reuse of greywater is often unregulated and is common. At present, the recycling of greywater is poorly understood compared with elimination.

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