Gray water, also spelled as grey water, is water that already has been used domestically, commercially and industrially. This includes the leftover, untreated water generated from clothes washers, bathtubs and bathroom sinks.

This water source is a common way to recycle water and stretch urban water supplies. As part of this, gray water ‘harvesting’ (the collecting of gray water from sinks, showers, etc.) is increasingly popular, especially as a way to flush toilets.

However, some household water, such as wastewater from toilets, kitchen sinks, dishwashers, or laundry water from soiled diapers, is not suitable for reuse because it may contain bacterial contaminants, grease or residues of detergents that are harmful to plants.

Similarly, gray water cannot be used to irrigate root crops or other edible crops that touch the soil. But gray water is fine for watering fruit trees and landscaping plants.

Graywater in California

California has legislated household use of gray water for irrigation since 1992. Since, the state has developed standards  for plumbing design and equipment to ensure that gray water is safe for intended uses.

In 2010, for instance, California simplified its plumbing code to make it easier for homeowners to divert household wastewater for use in their yards.

Communities, too, have addressed gray water. San Diego, for example, voted to ease its graywater rules in 2013. However, cities or counties can also adopt more restrictive standards if they wish.

According to official figures, a typical California household produces more than 10,000 gallons of gray water between May and October.