Grow With the Flow: Legal Uses of Graywater

Enlarge Image Image Gallery Used water from inside discharges into covered troughs of sand, then seeps into the surrounding flower gardens of this Maryland home. --JOHN HANSON/NUTRICYCLE

As many regions’ water costs rise, more people are asking if the water that flows down their drains after bathing and washing—known as graywater—can be used to water gardens. Increasingly, state permitting authorities are saying yes—with conditions.

In Arizona and New Mexico, homeowners can drain graywater right onto their lawns and landscapes. In states where laws are more stringent, especially California, underground graywater irrigation systems—some involving sophisticated water sensors that direct water to where it’s most needed—are popping up. You can help the earth and reduce bills by installing a system yourself. Though regulations vary by location, setting up a fairly simple water-reuse system in your home is becoming easier and more common.

More than soapy water: A graywater overview

Preparing graywater requires a few basic steps: draining it from the house to your graywater system via pipes kept separate from toilet drains; filtering out fibers and greases; then disinfecting the water and treating its carbon. You can take care of the last two parts—disinfecting and treating carbon—by setting up a system in which graywater drains under a few inches of soil, gravel and plant roots. The plants and soil will naturally treat the carbon and disinfect the water.

Though kept separate from what’s flushed down the toilet—called "blackwater"—graywater still can contain bacteria and pathogens that could cause illness, although the small amounts present in most graywater are a low risk, according to a University of Massachusetts study. Graywater also contains carbon from oils, soaps and skin. As in all organic compounds, that carbon will decompose, potentially causing odors and clogging the air spaces in the ground. Health officials advise draining graywater under three to 18 inches of soil, where soil bacteria decompose carbon and destroy pathogens—and where plant roots can drink it up.

State regulations for graywater vary widely, so check with your municipality to be sure your system is legal. Some states consider kitchen-sink and dishwasher drainage blackwater because it contains grease, nutrients and food bits.In most states, graywater cannot be used above ground without a special permit. In nearly all states, a graywater permit requires submitting results of a soils test and an approved plan.