Growing with Graywater

Graywater from a washing machine discharges into a gravel bed planted with tropical plants. A clear overhang shelters the system from rain that might flood it. Graphic: Daniel Harper

As the prospect of drought looms in some regions, more people are asking if graywater—the water that drains from their tubs, showers, and washing machines—can be used to water gardens. Increasingly, state permitting authorities are saying yes.

In California, Arizona, and New Mexico, homeowners can irrigate their lawns and landscapes with up to 250 gallons of graywater daily without applying for a special permit, as long as their permitting authority gives the nod. In states where laws are more stringent, underground graywater irrigation systems are popping up.

You can take a load off your local water supply and reduce your water bills by installing a graywater system, either yourself or with a wastewater system installer or landscape contractor.

Key to getting the nod from your local water authority is (1) installing a diverter valve that allows you to switch the graywater discharge from your irrigation system to the sewer or septic system, and (2) discharging the graywater under soil so it has no contact with pets or people.

A graywater overview

Preparing graywater for use 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 (optional but advisable)

• destroying any pathogens and treating the carbon

• then piping it to irrigate plants or for another use

An easy way to take care of the last 3 parts—disinfecting and treating carbon—is setting up a system in which graywater drains under at least a few inches of gravel and plant roots. Beneficial organisms living in the plant roots and gravel naturally treat the carbon and disinfect the water.

Although 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 present a low risk for this, 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.

Across the country, 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 states other than California, Arizona, and New Mexico, a graywater permit requires submitting results of a soils test and an approved plan.

Laws are changing fast throughout the country; if your state or town doesn’t address graywater, help pave the way by applying for a special permit.

If you’re renovating a bathroom or building a house, consider installing graywater drainage pipes—even if you can’t or don’t plan to use graywater now. Paint them purple, which signifies to plumbers they contain reused water. In the future, water recycling likely will become the norm as this resource gets too precious to throw away!

Typical graywater system components

Most graywater systems feature these elements:

• A surge tank to which all graywater first drains. This tanks equalizes and cools graywater flow so it doesn’t inundate the system and plants with a deluge of hot water. Typically a plastic tank, septic tank, or 55-gallon drum, this also serves as a grease trap if the scum that floats to the top—greases, oil, hair, particles, etc.—is periodically skimmed.

• A filter to remove clogging particles such as hair. You can buy a filter (see "Resources," below) or make one with a nylon stocking. For grease and sludge, use a grease trap—essentially a box with a baffle that holds back scum so it can be skimmed out.

• Porous substrate, fluffy mulch or aerated tanks to promote fast-acting aerobic biological decomposition.

• Irrigation components such as perforated pipe and drip-irrigation lines that get graywater to the plants.

• Thirsty plants to use up nutrients and provide root systems that support microbes, which decompose carbon and germs in graywater.

• A diverter valve, which is essentially a switch that lets you divert graywater to the sewer or septic system if your system is overloaded or if chemicals such as toxic cleansers have been drained. 

Some systems

Here are 4 examples of systems that treat graywater while distributing it to plants:

• Gravel-filled trenches: An easily installed system, this is typically composed of trenches filled with gravel and lined with perforated pipe distributes graywater. One design pre-approved in California specifies a one-foot depth to allow root access and evaporation. Other variations engineered for different purposes are small leachfields, Washwater Gardens (a planted evapotranspiration system), and subsurface wetlands. 

• Drip irrigation: Distributing graywater through 3/4-inch irrigation tube lines with tiny holes or "emitters," requires the graywater to be filtered and treated to assure graywater's carbon doesn't clog the tubing and emitters. One way to do that is to filter the graywater through a sand filter, such as a swimming pool filter, or aerate with an airstone (available from aquarium stores) to provide aerobic bacteria that eat up the carbon.

• Mulch basins: Mulch-basin systems direct graywater to shallow basins full of woodchips or mulch, ideally next to trees and other plans that can use it. The basins must be checked and their contents dug out and replenished with new mulch every year or so.

• Ponds and surface wetlands: Discharging graywater to open air should be done with care. Be sure to add plants and aeration (keep the water moving), and limit exposure to living creatures. These systems are hard to permit in most states.

What about recycling for use in the house? This requires more intensive treatment, filtration, and disinfection, and so it's beyond most homeowners' budgets and interest. Toilets flushed with unfiltered graywater clog with accumulations of soaps and oils.

How to get a permit

In California, a 2010 law allows you to use up to 250 gallons of graywater daily from one fixture without obtaining a permit (but this is still at the discretion of your local water authority). If you need to get a permit to graywater in states requiring one, your chances of success are higher if you:

• present an underground (called a "subsurface") system

• use a diverter valve to assure you can switch graywater flow to a septic system or sewer

• prove your soil will absorb water by getting a percolation test

• show that your system is sufficiently distanced from groundwater and shores

For updates on San Francisco Bay area graywater regulations, check the East Bay Municipal Utility District website.

Call Ennovationz for guidance on graywater systems.