Graywater Irrigation

By Kim Wilson, UC Master Gardener

As California’s water resources decrease due to drought and population growth, homeowners are seeking ways to conserve water.  It has been estimated that 30-50% of water used in our homes produces graywater.  Using graywater to irrigate landscape plants reduces the waste in our septic systems and the energy used in processing the waste.  Graywater use also conserves water by decreasing clean water use in our landscapes.

Although graywater use has clear benefits, the homeowner should be aware of certain guidelines in its applications and handling.  Graywater may contain microbes such as bacteria and viruses or contain organic substances which attract microorganisms.  Although there have been no confirmed cases of illness caused by graywater use, caution and correct handling is advised.

Types of Wastewater:

• Graywater is untreated wastewater from showers, bathtubs, bathroom sinks and washing machines.
• Dark Graywater is water from kitchen sinks and dishwashers which can contain pathogens or organic material which promotes microbe growth.
• Black Water is water from toilets or washing machines used to clean diapers or clothes soiled with paint, hair dye, automotive fluids or other hazardous material.

 *Dark graywater and black water cannot be used to irrigate landscape.

Graywater Irrigation of Plants

• Use graywater to irrigate landscape plants. Do not use on edible plants, including root crops or edible parts of any plant. Fruit trees can be irrigated with graywater but avoid any splashing of water on fruit.  Bottom line:  do not eat any plant or fruit which has been in contact with graywater on plants surface or in soil.
• Do not irrigate turf grass or groundcovers with graywater because water and any potential organisms may linger on surface.
• Do not use on potted plants.

Graywater Irrigation Guidelines

• Wear gloves to prevent direct contact with graywater.
• To prevent aerosol containing possible microbes, do not use in a sprinkler system.
• Do not allow run off onto sidewalks or to pool and thus reduce exposure to people and animals.
• Do not store graywater more than 24 hours.
• Avoid using graywater in same hoses that can be used to wash or play.
• Drip irrigation may require a filter to prevent clogs from hair, lint or other debris.
• Avoid spraying edible plants or splashing on your skin.

Detergents

• To reduce toxicity risks to plants choose cleaning products which are boron and chlorine free and pH balanced.
• Soaps (avoid antibacterial soaps) and shampoos are not as much an issue as detergents.
• Liquid detergents contain less sodium than powdered soaps.
• Avoid detergents which contain dyes, bleach, whiteners, softeners, fragrances or advertise enzymatic action.
• Avoid ingredients such as borax, borate, salts, sodium, chlorine, petroleum distillates and peroxygen. Products with hydrogen peroxide can be used.  
• Phosphates in detergents are not harmful to plants and may actually serve as a fertilizer.

Water Softeners

In households that use water softeners, the graywater will be high in sodium and is not usable for irrigation. This is true for water softeners which use the more common sodium chloride in the water softening process. However, some water softeners can also use potassium chloride as a substitute.  This will produce graywater which is not toxic to plants.  Check with the manufacturer of your water softener to make sure it is potassium chloride compatible.

Effects on Plants

• Graywater is often alkaline and contains high levels of sodium, chorine and boron which can be harmful to plants.
• Acid loving plants and salt sensitive plants are the least tolerant of graywater.
• Deciduous trees fare better than evergreens. However, many plants will do well with this type of irrigation.
• Monitor your plants for any signs of stress and toxicity such as brown leaf burn at leaf tips and margins, yellowing or bronzing of leaves.

Plants that do well with graywater irrigation:

Oleander, bougainvillea, fan and date palms, rosemary, roses, agapanthus, Bermuda grass, honeysuckle, Australian tea trees, oaks, Arizona cypress, cottonwood, olive, juniper, purple hopseed, manzanita, ceanothus, rushes, coffeeberry, toyon, western redbud, California wax myrtle, penstemon.

Plants that don’t tolerate graywater irrigation:

Rhododendron, bleeding hearts, wood sorrel, hydrangeas, azaleas, violets, impatiens, begonias, ferns, foxgloves, gardenias, philodendrons, camellias, primroses, crape myrtle, redwoods, star jasmine, holly, deodar cedar.

Setting up a Graywater System

• Permits are not required for graywater systems which divert water for clothes washing machines, do not require altering of house plumbing is for a one- or two-unit residential building and follows the California Plumbing Code.
• Permits required if home plumbing is altered such as cutting into drainage plumbing, for graywater systems which collect from showers, sinks or baths, systems with a pump or tank.
• Contact your city or county officials to verify if permits needed and that your system meets the California Plumbing Code.

References

Hartin, Janet and Ben Faber. 2015. Use Of Graywater In Urban Landscapes In California. PDF. UC ANR. Drought Tip: Use of Graywater in Urban Landscapes in California

San Luis Obispo Guide To The Use Of Graywater. 2009. PDF.  San Luis Obispo: SLO-COAT. http://www.mbnep.org/wp-content/uploads/2014/12/Graywater_08_20_09.pdf.

Shades Of Graywater.  UC Master Gardeners Program of Marin County. https://marinmg.ucanr.edu/files/192094.pdf

Use of Greywater in Urban Landscapes in California. UC ANR. http://ucanr.edu/sites/mgslo/files/230135.pdf

Using Household Waste Water On Plants. UC Vegetable Research and Information Center. USING HOUSEHOLD WASTE WATER ON PLANTS

Water Harvesting: Greywater Use at Your Home. City of Paso Robles. https://www.prcity.com/522/Water-Harvesting