- Author: Janet Hartin
- Author: Ben Faber
The use of graywater to irrigate landscape plants is increasing throughout the United States, particularly in California and other arid states. In the United States, "graywater" often refers to wastewater that originates from residential clothes washer, bathtubs, showers and sinks. It excludes wastewater generated from kitchen sinks, dishwashers and toilets (black water).
As of August 2009, a permit is no longer required for the installation of the following single-family or two-family residential graywater irrigation systems, if other conditions are met (California Department of Housing and Community Development): a) a simple clothes washing graywater system as long as it does not require cutting of the existing plumbing piping; and b) a single-fixture system that collects graywater from one plumbing fixture.
All other systems require a construction permit prior to creation, retrofitting, construction and installation as stated in the actual code. The full text of these standards can be viewed at the California Department of Housing and Community Development (HCD) Division of Codes and Standards website. Because these regulations may change at any time, always check with HCD and local enforcement agencies before developing plans for or installing a graywater system. Cities and counties can impose stricter guidelines than the state, so homeowners interested in installing a system should contact their local jurisdiction for specific regulations concerning graywater handling and use.
Laundry-to-landscape graywater systems are relatively simple to install and are inexpensive. The hose exiting the clothes washing machine is attached to a valve that separates graywater from water destined for the sewer. The graywater is diverted through a one-inch main irrigation line with a half-inch tubing outlets that are placed throughout the landscape, terminating in a valve box set called a "mulch basin." The basin surrounds plants being watered.
Benefits and Risks
Using graywater to irrigate landscape plants can conserve water and electricity and reduce water bills by recycling water otherwise destined for a wastewater treatment plant. Since an estimated 30 to 50 percent of home water use produces graywater, significant savings can be realized. A typical household (2.6 people) produces an average of 90 gallons of graywater each day. While most systems will not supply enough water to irrigate an entire traditionally landscaped yard, most can supply 1/2 to 3/4 of the water required by a drip-irrigated, water-efficient landscape with limited or no turf.
The potential risks should be carefully evaluated before deciding to install a graywater recycling system. Graywater varies substantially in quality and potential risks from site to site. Many household cleaning products contain dyes, bleach, chlorine, sodium, boron and phosphate, which can pose significant human and environmental health concerns and injure or kill plants at high dosages over a short period or smaller dosages over a long period.
Impact of Graywater on Human Health
Because of recent changes regarding graywater reuse under California and other state statutes, research pertaining to long-term impacts and risks of graywater reuse on human health, plant health, soil chemistry and ground and surface water quality is very limited.
Research examining the microbial constituency of graywater indicates that direct contact with graywater can pose a health risk to humans. Pathogens can enter graywater through food sources in the kitchen (which is why graywater generated from kitchen sinks and dishwashers is not recommended). Because pathogens can also enter graywater through fecal matter, avoid water contaminated by dirty diapers.
Graywater should not be applied directly to edible plants or root crops. To be safe, it should be applied only to nonedible ornamental plants. Avoid splashing graywater on neighboring edible plants. Also, graywater should not be applied using sprinkler systems, since droplets containing harmful microbes can become suspended in the air and breathed.
Impact of Graywater on Soil Chemistry and Water Quality
Limited research exists that addresses the fate of microorganisms found in graywater and the impacts on indigenous soil microorganisms, soil chemistry and water quality. Soil, rock and other materials that serve as filters can significantly diminish the threat of water pollution from graywater use.
Impact of Graywater on Plant Health
Because graywater is often rich in nutrients required for plant growth, ornamental plants may benefit from its use. However, numerous studies indicate that graywater may contain significant levels of sodium and other salts that are harmful to plants. In general, evergreens are more salt-sensitive than deciduous trees.
For more information on using graywater in urban landscapes, please contact Janet Hartin, environmental horticulture advisor, at jshartin@ucanr.edu or (951) 313-2023. For information about UC Cooperative Extension's other programs, click here.