Gray Water

Reduce Water Use by Irrigating with Gray Water

Bird's eye view

Water savings, if local inspectors allow

Gray water is what comes from sink and shower drains and the washing machine. While it's not exactly potable, gray water has many fewer bacterial contaminants than toilet waste, or black waterPotentially contaminated wastewater from the toilet, kitchen sink, or other sources. Black water should not be reused without going through a complete treatment system. , and can be diverted for irrigation rather than sent to an overloaded municipal waste-water plant. The result is a substantial water savings.

But make sure to check with local building inspectors before installing a gray-water collection system. Not all of them will approve it.

See below for:

Key Materials

Keep the equipment simple

A variety of equipment has been devised to recycle gray water. Simple systems are always best. Gray-water systems with complicated mechanical filtration components are often not worth the investment, requiring far more money and energy than sending gray water to the municipal sewage plant.

Design Notes

Kitchen gray water is handled differently

Because it contains organic matter and grease, kitchen waste water must be treated differently than gray water from other sources. With additional filtering, however, it can be reused. The most common design requires a filtered, three-chamber holding tank, similar to a septic tank, that allows the grease to settle out.

Afterward the water passes through a sand filter before treatment in a planter bed or discharge to the biologically active layer of soil.

Builder Tips

Washing machine rinse water can be reused

If occupants are willing to monitor their clothes washer operation, it may be possible to recycle gray water used for washing clothes. This method involves switching the drain hose from the drainpipe to a collection container during the final rinse. Tubing at the bottom of the container connects to the washer supply line, and the water is used during the next wash cycle.

It's usually not possible to use gray water to flush toilets without expensive treatment. If untreated, the gray water cannot sit in the tank without fouling the system.

The Code

Gas-tight, corrosion-resistant tank required

The International Residential Code includes a section (Appendix O) governing gray water recycling systems. Among the provisions is section AO101.11, which requires that "Gray water shall be collected in an approved reservoir constructed of durable, nonabsorbent and corrosion-resistant materials. The reservoir shall be a closed and gas-tight vessel. Access openings shall be provided to allow inspection and cleaning of the reservoir interior."


The Water Conservation Alliance of Southern Arizona estimates that a gray-water system can save between 30,000 and 50,000 gallons of water per year in an average household.

Among the many benefits of reusing water: a proportionate reduction in the demand for fresh water, less strain on private septic fields and municipal waste treatment facilities, and recharging groundwater supplies. While some precautions are prudent, plants and soil alike benefit from the nutrients in gray water.

Gray water is not the same as reclaimed water

Many municipalities offer treated wastewater (sewage) to the public. In fact Los Angeles County's sanitation districts have provided reclaimed water (recycled water) for landscape irrigation in parks and golf courses since 1929.

This is very different from gathering gray water from your sink drain. Thoroughly treated to remove solids and certain impurities, disinfected, pressurized and then reused, some cities — Olympia, Washington, for instance — use it not only for irrigation, but also for commercial processes, such as cooling power plants, filling decorative fountains and ponds, pressure washing, stream-flow and wetland enhancement, etc. Tucson, Arizona even offers customers reclaimed water for drinking.

The practice is controversial. Recycled water sometimes contains higher levels of nutrients such as nitrogen, phosphorus and oxygen which may help fertilize plants.

However, disinfection by chlorination, an important part of wastewater treatment, despite initially lowering numbers of sewage-related bacteria, may substantially increase proportions of antibiotic-resistant, potentially pathogenic organisms. Staphyloccus aureus bacteria (responsible for MRSA), for example, become more virulent and drug-resistant after chlorination.

Parasites such as giardia and cryptosporidium are not killed by chlorination. Viable bacteria in sprinklers can travel more than 1000 feet in the air. Reclaimed water is regulated by states, not the EPA, sometimes using outdated standards. Many pathogens cannot be detected by currently used tests, and current standards don’t evaluate interactions of heavy metals and pharmaceuticals which may foster the development of drug resistant pathogens in recycled water.

Of course, graywaterWastewater from a building that does not include flush-water from toilets and (as most commonly defined) water from kitchen sinks or dishwashers. In some places, graywater can be collected and used for subsurface irrigation. , though not from sewage, can also contain harmful elements.


LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H WE1 (Water Efficiency) offers 1 point for installation of a gray water system.

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. 0 Under Ch. 8 — Water Efficiency: 4 pts. for each toilet set up for gray water recovery and reuse (802.1).


Reuse water, don't waste it

To collect and use gray water for irrigation, drain lines from bathroom sinks, tubs, showers, and the washing machine empty to a centrally located holding tank, typically in the basement or crawl space. A filter at the holding tank screens out particles. Water is either drained or pumped from the tank to the irrigation lines.

A properly designed system has overflow protection for both the holding tank and the irrigation lines. The overflow valve for the holding tank feeds water directly to the sewer line if the filter clogs. Overflow protection for the irrigation lines can take two forms: a diverter valve directs water to secondary irrigation loops or, if the irrigated area becomes saturated, to the sewer line.

Plumbing codes vary. Options for reusing gray water vary according to local plumbing codes. If the building inspector allows, you can decide between systems that capture all of a house’s gray water or just the drain water from specific sources.

Although the bacteria in gray water are generally aerobic, gray water can’t be stored for longer than 24 hours without using up all of the oxygen in the water and encouraging the growth of smelly anaerobic bacteria.

That's why the easiest and least expensive use for gray water is irrigation. The irrigation lines should be in the biologically active portion of the soil — no more than 9 in. to 12 in. below the surface.



Gray water must pass slowly through healthy topsoil for natural purification to occur. Although it doesn’t need extensive chemical or biological treatment before using in the garden, it often contains grease, hair, detergent, cosmetics, dead skin, food particles, even fecal matter and should be used with care.

Use only on well-established ornamental plants, shrubs, and trees, never on seedlings. Since it’s alkaline, never use it on acid-loving plants, such as rhododendrons and azaleas. If irrigating food plants, fresh water is always preferable. Restrict application to soil around vegetables of which only the above-ground part is eaten, never on leafy vegetables or root crops.

Apply gray water directly to the soil over a broad area. Avoid use on slopes or concentration in one area. Don’t use overhead sprinklers or allow gray water to splash and contact the above-ground portion of plants. Don’t use gray water in a drip irrigation system, since it can easily clog the pipe’s emitters. The simplest method is by hauling it in a bucket from your source. Rotate gray water with fresh water use to help leach contaminants. Thick compost mulches help speed the natural decomposition of waste residues.

A square foot of well-drained, loamy soil can handle about a half-gallon of gray water per week. So, a 500-square-foot garden can tolerate up to 250 gallons of gray water per week. Given a choice, use shower and bathtub water first, followed in decreasing order of desirability by water from the bathroom sink, utility sink, washing machine, kitchen sink and dishwasher.


Instead of irrigation lines, a planter box can be used to filter and clean gray water.

Filtered gray water is pumped into the top layer of the planter box, which contains at least 2 ft. of humus-rich topsoil. Beneath the topsoil are two layers of sand: common cement-mix sand and, below that, coarse sand. A filter keeps the sand from clogging the pea gravel that makes up the planter’s bottom layer.

Outdoor planters can leach treated water directly into the ground, or the water can be piped to a leach field. A planter can even be used indoors as long as it's plumbed to carry off the treated water.


Water Conservation Alliance of Southern Arizona

Image Credits:

  1. Daniel Morrison
  2. Dan Thornton
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Oct 12, 2017 6:53 AM ET

Response to Kent Thompson
by Martin Holladay

Thanks for posting some helpful suggestions and links.

Oct 11, 2017 4:40 PM ET

A Different Perspective
by Kent Thompson

I'd like to express my sincere appreciation for this site! It's such a wonderful resource on so many topics. In that spirit, here are my comments on this article:

I've been a greywater installer in California for the past 5 years. This article is either a little out of date or represents the views of manufacturers and installers of some of the more complicated greywater systems.

For the single family home, systems that have a tank or filter are usually a bad idea, unless simpler options have been examined and discarded. For larger scale residential, these more complicated systems can make sense.

Let's consider filters. As compared to your potable water supply, greywater is comparatively much dirtier and filters need to be changed monthly to avoid clogging. Do homeowners want to clean a nasty filter once a month? Most do not. This results in a system failing prematurely. A much more robust system results from piping the unfiltered greywater directly to landscape where it can be treated by the soil.

Why have a tank? Better to let the greywater flow to the landscape and be stored in the soil or temporarily in a mulch basin prior to infiltration. It avoids the cost of a tank and the potential smells of partial drainage or sitting too long.

Bacteria and parasites, while a concern in designing and installing the systems, are in practice not a problem if the greywater is released subsurface. To my knowledge there are no known cases of properly designed greywater systems making people sick.

For additional reading, I recommend Laura Allen's website and books:

Or Art Ludwig's website or book:

Check out the 'laundry to landscape' or 'branched drain' type systems. They are a great place to start.

To answer some of the (quite old) questions in the prior comments...the main pollutants you'd want to exclude from greywater are sodium, chlorine, and boron. Unfortunately these are in many common household products. Either changing products or using your system's diverter valve can preserve water quality.

My understanding is that soil treatment is quite effective. Check out Art's site for more and better information:

Thanks for hosting an article on greywater!

Kent Thompson

Aug 29, 2010 5:37 PM ET

Gray water dumping
by Anonymous

I live on the Big Island of Hawaii - Kona side. We have a cess pool. We never ever had a cess pool before and we have a number of questions.
Today we are cleaning with Dawn dish washing detergent and Borax.
Is it safe to dump the dirty water in the yard or is it better I dump the water in the toilet?
What about bleach?
What about use of a garbage disposal?
Toilet paper best kept out and then what to do with it burn it or land fill it? (We here mexicans in mexico figured this one out long ago - apparently they do not to use toilet paper but if not then what?

We are trying to remediate mold without using hash cleaners but have reverted to bleach known for treating mold/mildew. We are looking into assessing what the best ventilation could be for our situation while we are cleaning. Any recommendations?

What is the best envornmentally sensitve approach as to what to put in and not put in a cess pool?
We are wondering if septic would be more environmentally sound practice?

Sep 19, 2009 9:20 PM ET

removal of pathogens from treated wate water
by Anonymous

waste water treatment specification for ground water recharging

Sep 19, 2009 9:17 PM ET

Ground water recharging with treated waste water
by Anonymous

What is the disinfection treatment level and specifications of treated water for the purpose mentioned?

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