Domestic grey water is the lightly used water from sinks, showers, baths and laundries - it is any water that has been used once and can be used again. Grey water is not water that’s been in contact with fecal matter, such as water from the toilet or washing diapers. On average, grey water makes up 60% of the used water in a home.  

More specifically, the California Plumbing Code defines grey water as: untreated wastewater that has not been contaminated by any toilet discharge, has not been affected by infectious, contaminated, or unhealthy bodily wastes, and does not present a threat from contamination by unhealthful processing , manufacturing, or operating wastes. “Grey water” includes, but is not limited to, waste water from bathtubs, showers, bathroom wash basins, clothes washing machines, and laundry tubs, but does not include waste water from kitchen sinks or dishwashers