Laundry-to-Garden: How to Irrigate with Graywater
In a time when droughts and depleted aquifers are increasingly in the headlines, everyone is looking to “save water.” But besides cutting back on use, it’s also wise to use water more than once. Here’s a simple way to recycle graywater from your washing machine for irrigation.
What is graywater, exactly? Household wastewater from washing machines, bathroom sinks, showers, and bathtubs is considered “gray” because it is only lightly soiled and poses a minimal health risk. As long as you’re only putting biodegradable products down the drain, graywater is perfectly safe for irrigating plants. Kitchen sink water is technically considered graywater as well, but because of its grease content it often requires additional treatment before being used for irrigation.
In contrast, blackwater—what you flush down the toilet—requires intensive treatment before it may be reused in the landscape, which is not a feasible (or legal) DIY project.
There are brain-dead simple ways to recycle graywater—we list a few here—but they require sloshing around with a bucket or some other form of manual labor every time you want to irrigate. And there are high-tech methods as well, but these require serious plumbing knowledge and a significant financial investment. The washing machine method is in between—anyone with basic household tools and good mechanical abilities can pull it off in a weekend. It will run you a few hundred dollars for supplies.
Laundry machines are equipped with a pump to push the wastewater out of the machine and into a sewer pipe. The brilliance of this system, first developed by Art Ludwig of Oasis Design in Southern California, is that is uses a washing machine’s built-in pump to distribute the wastewater through a system of subsurface pipes to irrigate individual plants around the yard. The water flows out into mulch-filled basins around each plant, where the roots can access the moisture.
This design is most effective for watering fruit trees, berry bushes, edible vines, and other large permanent plants. It is not useful irrigating lots of little plants, like flowers beds, lawns, and annual vegetables.
A few other caveats:
- you can’t irrigate areas uphill from the washing machine (the pump isn’t strong enough)
- you also can’t irrigate plants on a slope (though you can run the water downhill from the machine to irrigate a flat area below)
- don’t use this method if you’re washing dirty diapers in the washing machine (that makes it blackwater)
- don’t apply the greywater to plantings along streams or on swampy ground (to avoid contamination)
- you may only use biodegradable products in your washing machine (this means no bleach, borax, or sodium)
Is It Legal?
Graywater systems are increasingly embraced by authorities across the country, especially in regions where water is scarce. Most greywater systems require a permit (and often a licensed plumber) to install them, but the washing machine system is unique in that is explicitly legal for homeowners to install it without a permit in five states—California, Arizona, Texas, New Mexico, and Wyoming—and with a little negotiation at your local building department, is likely to pass muster in most localities (though a permit maybe required).
Municipalities tend to be lenient in how they regulate the washing machine greywater systems because this approach does not alter your household plumbing, and the graywater remains below ground at all times, eliminating any health risks. Still, check with local authorities before proceeding. Graywater rules for a number of states beside the five above may be found here.
Step One: Calculate System Size
Before you get out your tools, pick up a pencil and paper and determine how many plants you are able to irrigate. Engineers have elaborate methods to make such calculations, but that’s not necessary for our purposes. For each load of laundry you run per week, plan on irrigating 1 to 2 fruit trees, 3 to 4 berry bushes, 3 to 4 edible vines, and 6 to 8 smaller perennial plants (such as roses, lavender, or artichokes) per load.
These guidelines are based on 20 gallons of water per load, which is typical for most washing machines. However, it’s best to check your owner’s manual for the exact water use of your model and adjust your plans accordingly (compact and water efficient machines may use half that amount).
Also, consider your soil conditions. If the soil is sandy, irrigation water drains away quickly; conversely, drainage is poor in heavy clay soil. If drainage is very poor, spread the wastewater over twice as many plants as recommended above. In extremely sandy soil, cut the number of plants in half to make sure there is enough moisture to go around.
Finally, use common sense in calculating the irrigation requirements of individual plants. A newly planted fruit tree requires a fraction of the water of a large mature tree. And drought tolerant species like olives and figs require much less water than moisture-lovers like blueberries and asparagus.
There is no need to get too scientific about the calculations—if in doubt, it’s better to overwater with this system because the mulch pits act as a sponge, allowing the roots to absorb the water they want and minimizing the risk of drowning the plants.
Step Two: Dig the Trenches
Washing machine pumps are not very powerful, so the plants to be irrigated should be within 50 feet of the machine, assuming the terrain is more or less flat. A network of flexible tubing buried in trenches below the soil will deliver water to each plant. The trenches can go up and down over modest humps and dips in the landscape, but should not run directly uphill. (the 50 feet doesn’t include any distance that the piping runs down an incline to reach the planting area, as gravity takes care of that part).
Starting from the exterior wall of your home closest to the washing machine, dig a shallow (2 to 3 inches) trench to the nearest plant that you wish to irrigate. Stop the trench at the edge of the “drip line” of the plant—directly below the outermost leaves and branches—and from there dig a trench to the drip line of the next plant, to the next, and so on.
Then dig a trench approximately 10 inches deep and 8 inches wide in a circle around the drip line of each plant. Another option that works well for smaller plants that are planted in a row is to dig a straight trench along the edge of the drip line on one side of the row (up to a maximum length of about 12 feet). The trenches will be filled with mulch to absorb the graywater.
Step Three: Lay the Pipe and Spread the Mulch
Lay 1” HDPE flexible tubing (often referred to as poly pipe) in the shallow trenches, cutting it and inserting barbed “Ts,” “elbows,” and other fittings where necessary. Run ½” tubing off the main 1” line to each of the deeper trenches, leaving a few inches of tubing sticking out into each trench. Secure the fittings with hose clamps.
Washing machine pumps are only strong enough to push water through 10 to 12 irrigation points at once. If you’re going to irrigate more plants, divide the pipe network into two zones with a three-way valve. The valve should be left accessible in a valve box so you can operate it manually.
1-gallon plastic pots provide a simple way to keep the mulch inside the trenches around each plant away from the outlet of each pipe so the water can flow unobstructed. First, cut off the bottom of the pot. Then drill a 1 ¼-inch hole into one side of the pot about 2 inches above the bottom. Fill the bottom of the trench around each plant with 4 inches of wood chips (compacted with your feet). Slide the pots over the ends of the tubing in each trench, with the bottom of the pots facing up so the top of the pot is resting on the mulch.
Then fill the remainder of the trenches around each plant with wood chips. Use a concrete stepping stone as a lid for each pot. Cover the tubing in the shallow trenches with soil, leaving a few feet exposed at the end by the house so it can be connected to the washing machine.
Step Four: Connect the Washing Machine*
Locate the flexible drain tube behind the washing machine and remove it from the rigid “standpipe” that drains the wastewater into a sewer or septic system. Connect the flexible drain tube to a 1” three-way valve using a 1” barbed male adapter and a hose clamp. Connect one side of the valve to the sewer standpipe using a 1-inch diameter PVC pipe and fittings, configuring the piping as needed so that the valve is against the wall in an accessible location near the standpipe and at least a couple inches above the height of the washing machine.
Choose the location where the piping will pass through the wall to connect with the tubing outside. Drill a test hole through the wall using a ¼-inch bit to ensure that there are no studs, wiring or piping in the way behind the wall. Then drill a 1½-inch hole through the wall. (Another option is to run the pipe through the floor and outdoors through the exterior wall of a crawlspace of basement).
Run 1-inch diameter PVC pipe from the open end of the valve through the wall, leaving about two to three inches of the pipe protruding on the outside of the wall. Use a 1” pipe strap on either side of the valve to support horizontal pipe and secure it to the wall.
Connect a 1” PVC “T” fitting to the open end of the pipe outside the wall, with the open ends of the fitting oriented vertically. Connect the bottom of the T fitting to the poly tubing on the ground with 1-inch diameter PVC pipe and fittings, as needed. Cover the last bit of poly tubing with soil.
Connect a piece of 1-inch diameter PVC pipe to the top end of the T fitting of sufficient length to rise above the height of the three-way valve indoors. Connect an air admittance valve (also known as an “auto vent”) to the top of the pipe (this device prevents foul-smelling air from circulating back through the washing machine and into the house). Seal the hole where the pipe passes through the will exterior grade caulk, both inside and outside the wall.
*Use Teflon tape on all threaded fittings. Glue unthreaded fittings together with PVC cement.
Operation and Maintenance
The system operates on its own unless you are using multiple irrigation zones. In that case, you’ll need to turn the three-way valve in the valve box outdoors once per week. Simply divide up the total number of loads you run each week into two equal sets. After the first set is completed, turn the valve to irrigate the other zone for the rest of the week.
The mulch in the trenches will slowly decompose and settle. Top it off once per year, and every few years scoop out all of the mulch and replace it (spread the old mulch around your plants). You may also need to periodically expand the diameter of the mulch trenches as the plants grow (especially for trees).
The three-way valve next to the washing machine allows you to divert the greywater into the sewer standpipe, rather than into the irrigation system, during periods of heavy rain or while making repairs. This is also useful if you wish to occasionally use bleach in the washing machine.
These are general guidelines only. For further information, greywater system supplies, troubleshooting advice, and guidance on using this method in a variety of contexts see the San Francisco Graywater Design Manual, Oasis Design, Clean Water Components, and Greywater Action.