Graywater Facts and Figures
In the United States, the rough estimate of per capita graywater production is 28 gallons per person per day (AWWA 1999), or about 10,000 gallons per person per year. For a four-person household, this would total 40,880 gallons per year of graywater. Based on the USGS estimates of 124 gallons per person a day in California (indoor and outdoor use), a four person household uses approximately 181,400 gallons each year. This same household can expect to defray 22% of their potable water needs by utilizing graywater.
This number may be higher or lower depending on the household and the water efficiency of the fixtures. Given that most households spend a minimum of 50% of their potable water on landscape irrigation, saving 22% by using graywater in the landscape is a simple answer to unnecessary potable water use in landscape.