Using Household Gray Water - July 11, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Since we recently discussed the topic of water harvesting (June 27, 2001), lets explore another source of household irrigation water: gray water. In January 2001, the Arizona Department of Environmental Quality (ADEQ) adopted new regulations for household gray water use allowing private citizens to design and use household gray water irrigation systems provided they comply with a few rules (Title 18, Chapter 9 of the Arizona Administrative Code, January 16, 2001). Read on to learn more.

Gray water is that wastewater that flows from bathroom sinks, baths, showers, and washing machines. Under the new provisions, households generating less than 400 gallons per day do not need a permit to use gray water for irrigation purposes. Kitchen sink water and dishwasher water use is not allowed under the new provisions due to contamination risks. Water originating from toilets is called black water and should definitely not be recycled. On the average, older houses generate about 46 gallons of gray water per person per day and new construction generates about 35 gallons per person per day. In a four-person household, that's 140 to 180 gallons per day (likely more if you have teenagers).

In addition to the sources of water that are allowable for use, the new regulations require that household gray water systems comply with the following rules: 1) your house may not be on a flood plain; 2) the gray water may only be used for irrigation on your property; 3) the gray water cannot be used for food plants (with the exception of citrus and nut trees); 4) you must be connected to a septic system or sewer and be able to divert the gray water stream in case the system has problems; and 5) you must have a cover over the gray water storage tank for mosquito control and safety.

The new gray water regulations have some other guidelines that must be followed. Gray water must be filtered to trap hair and lint. You may simply use a nylon stocking as a cheap, replaceable filter. Gray water cannot be applied through a spray-type irrigation system: only flood and drip are allowable. Do not use washing machine gray water that has been used to launder diapers or for dyeing fabric. Divert water form these sources to sewers or septic systems. Use PVC or ABS (white or black plastic) pipe for your collection and irrigation system. Do not reuse gray water that contains hazardous chemicals from photo labs, car parts, or oily rags. Likewise, do not reuse gray water if a family member has an infectious disease, diarrhea, hepatitis, or suffers from internal parasites.

After reading and thinking about these rules, it becomes obvious that it is much easier to design a gray water system for new construction than to retrofit an existing home. In fact, it may be cost prohibitive to completely retrofit an existing home built on a concrete slab. Wood floored homes with crawl spaces are better candidates for retrofitting. In any case, valves would need to be installed on drains and collection systems and storage tanks will likely be needed.

The simplest legal gray water systems that I have seen are those that collect washing machine water into a large barrel. The inflow pipe has a simple filter on the end. The outflow pipe simply has a valve at the bottom of the barrel to which a hose can be connected. Expanding on this idea, showers, bathtubs, and bathroom sinks can be added to the stream. One should keep in mind that storage capacity should be adequate to hold maximum amounts generated and plants to be irrigated should be downhill from the storage tank. If the plants are uphill from the collection area, then a pump will be necessary to deliver the recycled irrigation water.

If you are considering gray water use, think about a year-round schedule of plant irrigation needs. Your four-person household could easily generate 1,000 gallons of gray water per week. Can you store or use this much gray water? Do you have landscape plants that require regular irrigation in the winter? Evergreens are certainly good candidates for winter irrigation. Another consideration would be excess application of irrigation water (i.e. too much water that could percolate beyond the root zone of the plant). A contingency plan to divert water to a septic or sewer system may be necessary during the winter.

The rules specifying application only to ornamentals, citrus, and nut trees are designed for human safety. Citrus and nuts have thick protective coatings that would pose a minimal risk of harboring or transmitting disease organisms. Vegetable crops are highly susceptible to contamination by pathogens. Soap and detergent residue could also cause damage to tender plants.

Given our area's water woes, gray water is a resource worthy of consideration. Even though the ADEQ has made it easier to do, there are many things to consider when assessing your need for a gray water collection and irrigation system. If you have specific questions about the new gray water regulations, you may call ADEQ at (800) 234-5677.

The University of Arizona Cooperative Extension has publications and information on gardening and irrigation. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: July 4, 2001
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