Note: You’ll see graywater spelled with both an a and an e (greywater). Graywater is the common spelling in the U.S. and greywater is the common spelling in the U.K.
Graywater systems are also sometimes referred to as “laundry to landscape.”
Much of the “wastewater” generated from inside our homes is relatively clean water containing only trace amounts of soap and dirt. In many jurisdictions, this water can be beneficially re-used to water plants and trees in our yards. This conserves water and reduces the amount of wastewater that is sent to treatment plants and then to the ocean.
What is Graywater?
Graywater is untreated wastewater that has not been contaminated by any toilet discharge. Graywater includes wastewater from bathtubs, showers, bathroom sinks, clothes washing machines and laundry sinks. In most jurisdictions the official definition of graywater excludes wastewater from kitchen sinks, dishwashers, photo lab sinks or laundry water from soiled diapers.
What is a Graywater System?
Graywater systems are onsite wastewater systems that use graywater for subsurface landscape irrigation through the use of mulch basins, disposal trenches or subsurface drip irrigation fields. There are basically two types of graywater systems: gravity fed manual systems and package systems. The manual systems do not require electricity or pumps because they work on gravity taking the graywater to the area needed. They may require a larger yard area to install the system outside. Packaged systems require electricity but are self-contained and can be installed indoors. With each option codes and local ordinances should be considered.
Graywater reuse increases our drinking water supply, decreases water and wastewater utility bills, and reduces the burden on wastewater treatment facilities. In concert with water-wise landscaping, rainwater harvesting, and conservation, using graywater as a resource helps reduce dependency on imported water and protects the urban watershed and the ocean.
General precautions are not to use any detergents or bleaches in your sink, bath and laundry that may ultimately be harmful to plants. To further ensure safety, graywater cannot be used on the edible portions of vegetables and must be used for sub-surface irrigation in order to reduce human contact or ponding. You generally cannot store graywater, only divert the amount needed to water your garden.
In 2015, the National Academy of Sciences released a draft pre-publication copy of a report Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits. That report is summarized and discussed in this article, which states:
Graywater reuse is not a new strategy, but for many years plumbing codes required graywater to be combined with blackwater (wastewater from toilets) and treated through the same system as sewage. In the past decade, however, many states have revised their laws, reflecting the growing interest in graywater reuse. As of 2014, 26 states allowed some form of graywater reuse.
Simply reusing graywater to flush toilets can reduce home indoor water use by 24 percent, on average. Using treated graywater to meet water demand for toilet flushing and laundry has the potential to reduce demand by nearly 36 percent. Graywater reuse in new multiresidential buildings offers clear economies of scale, but we need more data on the cost of such systems.
Permitting requirements to install or operate a graywater irrigation system vary by jurisdiction, so consult your local state/county/city before proceeding. Here are links to a few state, county and city websites that have information on graywater systems:
Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessment of Risks, Costs, and Benefits. National Academy of Sciences, 2015.