Feature Article

Using Graywater for Landscape Irrigation

Separating graywater from sewage and using it for landscape irrigation makes a lot of sense.

Separating graywater from sewage and using it for landscape irrigation makes a lot of sense. The practice conserves potable water, allows irrigation during droughts, and increases the life of in-ground septic systems. Unfortunately, use of untreated graywater for irrigation has long been illegal in most of the U.S. Finally, that is beginning to change.

On November 9, 1994, California became the first state to legalize the separation and use of graywater for landscape irrigation state-wide. Following several years of study and review, Appendix J—Graywater Systems for Single-family Dwellings—was added to the California Plumbing Code. This is an adaptation of a graywater standard incorporated into the Uniform Plumbing Code of the International Association of Plumbing and Mechanical Officials (IAPMO) in 1992.

This article takes a look at simple systems designed to use graywater—with little or no treatment—for below-ground landscape irrigation. Not addressed are the more sophisticated systems that thoroughly treat and disinfect graywater for use in such applications as toilet flushing, above-ground irrigation, and snowmaking.


What is Graywater?

Figure 1: Wastewater by source

Defining graywater is not as easy as one might at first assume.

Definitions vary depending on whom you are talking to and what your applications are. According to some definitions, graywater is all the water used in a home except by toilets. This definition makes sense in buildings that use composting toilets.

Most definitions of graywater, however, including those in the new California regulations, are less inclusive. By these definitions, graywater does not include wastes from kitchen sinks, in-sink garbage disposals, and dishwashers; it is limited to washwater from showers and baths, bathroom sinks, and clothes washers. Washing dirty diapers in the clothes washer also excludes that washwater from California’s graywater definition.

The pie chart (Figure 1) shows the proportions of wastewater produced in a typical suburban house from various uses. Non-toilet bathroom and laundry wastewater accounts for just under half of all water produced.


What’s in Graywater?

The primary constituent of graywater, of course, is water. Other components, including soaps, detergents, oils, and flecks of dead skin, give graywater its biological and chemical characteristics. The primary constituents of graywater are described below:

Microorganisms and pathogens. The contaminants of greatest health concern are living organisms that can cause disease. Several generations ago in the U.S. and still in many parts of the world, hundreds of thousands of people die or are debilitated annually from typhoid fever, amebiasis, cholera, hepatitis, giardiasis, and other diseases associated with water contamination.

Sources: Manufacturers, Lighting Technology Atlas (E Source, 1997), Lighting Modern Buildings by Derek Phillips (Architectural Press, Oxford, UK, 2000)

Table 2. Energy Savings Through Reflective Roof Retrofits

Notes: All buildings single story; savings measured for summertime cooling only.

Source: Lawrence Berkeley National Laboratory, Report No. 40673

Figure 2: Comparing Graywater and Blackwater

Such pathogen organisms fall into three categories: bacteria, virus, and protozoa. In water systems, the risk of such contamination is measured using

indicator organisms, which generally exist in similar conditions as the more dangerous pathogens. Fecal coliform and enterococci bacteria are the most commonly used indicator organisms. The primary source of these microorganisms is human feces, but they can be found in graywater as well.

Biochemical oxygen demand (BOD) is a measure of the concentration of decomposable organic matter in water based on the quantity of oxygen that would be used up as the organic matter is decomposed by microorganisms. BOD5 is a measure of that oxygen depletion potential in milligrams of oxygen per liter of water (mg/l) over a five-day period. Both graywater and blackwater can have high BOD levels, but levels in graywater are often higher because the organic matter can be more easily decomposed.

Suspended solids are any solids suspended in the wastewater or graywater. Suspended solids are measured by filtering the water under standard conditions and weighing the solids. Values are given in mg/l.

Nutrients are present in both graywater and blackwater. In some analyses, total nitrogen values are broken into NO3 (nitrate) and NH3 (ammonium) components. Much higher nitrogen levels are typically found in blackwater than in graywater. The other primary nutrient, phosphorous, comes primarily from detergents and is found in higher levels in graywater.

Salts and other chemicals from soaps and detergents are also present. Powdered laundry detergents have much higher levels of salt than liquid detergents. High levels of salt, boron used in some detergents, bleaches, and various other chemicals in cleaning agents are harmful to plants irrigated with graywater.

There have been several studies of graywater characteristics, but the results have been highly variable and unpredictable. The results of a Wisconsin study in the late ’70s showing the relative proportions of various pollutants in blackwater and graywater, are in Figure 2

below. A summary of five different studies of residential wastewater compiled by the U.S. Environmental Protection Agency is shown in Table 1 at right. Data on bacteria concentrations in graywater are shown in Table 2.

The key conclusions about graywater that we can draw from this data are as follows: 1) graywater can contain potentially dangerous microorganisms even if it does not contain sewage; 2) graywater usually contains more decomposable organic matter than blackwater but lower suspended solids; 3) wastes from a garbage disposal should be kept out of graywater because of the very high BOD and suspended solids levels; 4) nitrogen levels are usually lower in graywater than blackwater, but phosphorous levels are higher.


Advantages of Graywater Separation and Reuse

The ability to use graywater for landscape irrigation offers several important benefits: 1) increased life for in-ground septic systems; 2) more complete decomposition in septic tanks because of reduced water throughput; 3) reduced tap water requirements for irrigation; and 4) an ability to irrigate during droughts. The last benefit has been the driving force for the adoption of graywater standards in California. During extended droughts in recent years, residents in numerous municipalities were prohibited from watering lawns, many of which turned brown and died.

Greywater system from Clivus Multrum

From an environmental standpoint, the best solution is a composting toilet and a graywater system for the non-sewage wastewater.

Abby Rockefeller, of Clivus Multrum, Inc., likes to point out how ridiculous it is to take clean tap water that we’ve paid a lot of money to purify, then mix human wastes with it and dump it all down the drain. Not only are huge quantities of water wasted, but the nutrients in the excrement and urine are wasted and either contribute to groundwater pollution (in the case of on-site wastewater systems) or have to be treated at sewage treatment plants. Composting toilets solve this problem by keeping waste streams separate and turning the human wastes into a nutritive fertilizer.

For various reasons, however, composting toilets are unlikely to catch on in mainstream building markets. Even with conventional septic systems or connection to public sewers, graywater separation and use in landscaping makes sense. Indeed, the vast majority of graywater systems that will be installed over the next ten years are likely to be in buildings with conventional wastewater disposal systems.

A less tangible benefit to the separation and use of graywater by homeowners is that it fosters a tighter link with the environment. Homeowners with graywater systems are more likely to make the connection that the chemicals they put down the drain end up in the environment. This connection is all too frequently absent in our society.


Treating Graywater

The biggest difference between graywater and blackwater is that the solids in graywater can be decomposed much more easily than those in blackwater. There is less total organic matter (lower suspended solids), but the organic matter that is there can generally be broken down very easily (higher BOD5). Graywater systems can provide the necessary treatment very simply. Decomposition is extremely rapid once graywater is drained into soil and the soil bacteria go to work on it.

The same properties, however, also mean that if graywater is allowed to sit in a storage tank for any length of time, it will become smelly. Aerobic bacteria in the graywater begin decomposing the organic matter, but they quickly use up the oxygen in the water. After this occurs, anaerobic bacteria (which are also present) take over the decomposition process, and those bacteria give off odoriferous gases, such as hydrogen sulfide.

Also, we know that graywater contains common indicator organisms, such as fecal coliform, so for safety reasons humans should not come into contact with it. This means that untreated graywater should not be used in above-ground irrigation and that it should not be drained into saturated soils where it could puddle on the surface of the ground.

By keeping these aspects of graywater in mind, several important principles of graywater system design emerge: 1) get the graywater into the soil as quickly as possible instead of storing it; 2) irrigate with graywater below the surface of the ground only; 3) try to deliver the graywater to biologically active soil where the organic matter will quickly be broken down (in general, the closer to the surface, the more soil bacteria are present); 4) design the graywater irrigation field in a manner that will prevent graywater from surfacing—which entails careful sizing of the field based on soil type and expected flow; and 4) design flexibility into the graywater system so that the graywater can be channeled elsewhere (into a different drainage area or into the conventional sewage system) if soils become saturated or other conditions make using graywater unwise—see below on managing graywater systems.


Design and Components of a Graywater System

Figure 3. Graywater system schematic showing subsurface drip irrigation

The primary components of a graywater system for use in below-ground landscape irrigation are described below:

Separating Graywater from Blackwater

Graywater from bathroom sinks, showers, tubs, and clothes washers needs to be plumbed separately from other wastewater. When used in combination with composting toilets, wastes from kitchen sinks and dishwashers may also be included (in some localities) if certain precautions are taken and there is no in-sink garbage disposal. The California regulations do not permit kitchen sink and dishwasher wastewater to be treated as graywater.


Estimating Graywater Production

Residential graywater production can be estimated using based on the number of building occupants and the projected water use per occupant (25 gallons per day (95 l per day) for shower/bath/wash basin) and 15 gpd (57 l per day) for laundry). You can figure out the expected occupancy by assuming two people for the first bedroom and one for each additional bedroom. In the California regulations, this estimate of graywater production must be made to determine the necessary drain field size.


Collecting Graywater in a Surge Tank

Graywater from all suitable sources in a building should drain by gravity into a surge tank. This tank allows graywater to be emptied out of the tub or washing machine more quickly than it can be drained or pumped into the drainfield. Most surge tanks are not designed for long-term storage of graywater—which would result in anaerobic conditions. The California regulations require that the surge tank be watertight and fitted with both a vent stack and overflow drain to the sewer or septic tank. A valve is required to prevent backflow of blackwater into the tank.

As graywater drains into the surge tank it must be filtered. The filter keeps hair, lint, rice grains, and other material out of the system that could clog up the holes in the distribution piping. The filter is typically the most difficult part of the graywater system to design and the area where commercially available systems tend to differ the most. The most important feature to look for is easy cleanability. Some systems have filters that clean themselves automatically through a periodic backflow cycle so that homeowners don’t have to deal with them. Even in a well-managed graywater system, the filter will be pretty grungy and smelly.


Distributing Graywater

A pump is required to deliver the graywater to the irrigation area unless the surge tank is located above the area to be irrigated. (Gravity irrigation is generally preferable because the system can be simpler, but a pump is needed for most sites.) The pump takes water from surge tank and pumps it through distribution pipes. A key part of a graywater system design is sizing the pump based on head of lift, distance from surge tank to drain field, and maximum discharge rate of graywater sources. A pump should be selected for long-term durability, reliability, and quietness of operation. It should be set to kick on as soon as possible when water enters surge tank (some sump pumps must be fully submerged to operate). When possible, chose a pump that has been used successfully with the type of graywater system being installed. Ask the graywater system manufacturer or designer for equipment recommendations.


Designing an Irrigation Field

Figure 4: Mini-leachfield design specified by California regulations

Figure 5: Bird’s-eye view of house and subsurface irrigation system

Note: Some variations and exceptions apply; see specific regulations.
The final element of a graywater system is the drain field where the water is delivered into the ground.

California regulations permit two types of drain field: a mini-leachfield, and a subsurface irrigation system. Both of these systems require a minimum of 9” (230 mm) of earth above the graywater distribution pipes, and there must be 5 feet (1.5 m) of separation between the graywater discharge piping and the seasonal high-water table.

Some non-permitted graywater system designs have graywater delivery at the ground surface, then a layer of mulch above that. Other systems, such as that recommended by Clivus Multrum, have the gray­water drain through planting beds within the building.

In terms of environmental performance, the ideal location for graywater distribution is in the top few inches of the soil where the aerobic microorganisms to break down contaminants in the water are most concentrated. But delivery of water 8” to 18” (200 mm to 460 mm) below the ground surface will provide adequate decomposition in most soils, while minimizing health risks.

As defined by the California regulations, the mini-leachfield is designed and built like a septic system leachfield but is sized smaller and located closer to the surface (see Figure 4). The subsurface irrigation system discharges directly into the soil.

The required land area for a mini-leachfield or subsurface irrigation in California is determined based on soil characteristics and/or percolation tests. With a mini-leachfield design, the required drainage area ranges from 20 ft2 (1.9 m2) for each 100 gallons per day (gpd) (380 l per day) of graywater discharge in coarse sand or gravel to 120 ft2 (11 m2) in clay. With subsurface drip irrigation, the number of emitters required is dependent on soil characteristics, with allowable projected output ranging from 0.5 gpd (1.9 l per day) in clay to 1.8 gpd (6.8 l per day) in sand. Distances between the graywater system and property boundaries, water supplies, and various landforms are specified in the table below:

As for the actual piping, Robert Kourik, in

Gray Water Use in the Landscape, suggests 3”- or 4”-diameter (75 mm or 100 mm) perforated plastic drainpipe that comes in 10’ (3 m) sections. The slope should be less than 1” (25 mm) in 10’ (3 m) of run. Filter fabric should be used during installation to keep silt away from the perforated piping. Clivus Multrum manufactures a special piping system for graywater. One-inch perforated pipe with 516” (8 mm) holes every 6” (150 mm) is placed inside a slightly larger pipe with slit on the bottom. This arrangement is said to reduce root clogging. Some manufacturers suggest installing clean-out ports at the end of each section of pipe (typically a solid section of pipe extending above ground with a screw-on cap.

With less porous soils, you should provide several different drain areas with valves so that you can alternate distribution to let the soils dry out periodically.


Controlling the Flow

Appropriate valves, controls, and labels are important components of any graywater system. As mentioned, the surge tank should be provided with an overflow pipe that feeds into either the septic tank or the sewer line. The building owner should be able to manually switch over to the septic tank or sewer line if soils become saturated, or if conditions necessitate switching off the graywater collection system. A check valve between pump and discharge piping is usually needed so that filtered graywater can only flow toward the drainfield—not back. Finally, all gray­water pipes and tanks should be marked with warning labels to prevent future plumbing modifications from creating health risks. For example, the words “Danger—Unsafe Water” can be affixed to graywater lines. Pipes in buildings must be marked along their entire length. All of these recommendations are required under the California standards.


Management of Graywater Systems

To function properly, a graywater system for landscape irrigation must be properly managed. Building owners should understand the system’s operation, and they may need to make some changes in their habits. Because anything used in the shower or laundry will end up in the irrigation water, care should be taken in selecting cleaning products. Most soaps and shampoos are relatively safe for plants, but laundry detergents and other cleaning products vary tremendously. The following general recommendations can be passed along to building owners when designing or installing a graywater system:

  • Use liquid detergents rather than powders, because most powders are very high in sodium and salts, which can injure plants;
  • Avoid cleaning products with boron, which can be toxic to plants;
  • Avoid use of chlorine bleach;
  • Avoid caustic drain cleaners and other chemicals with unknown effect on plants;
  • Landscape the graywater irrigation field with plants that are tolerant of alkaline (basic) soil; acid-loving plants should be avoided.
  • When cloth diapers are being washing in a washing machine, the drain should be disconnected from the graywater system.
  • If highly contagious illnesses are present, the graywater system should be switched off, and all wastewater should be disposed of using the conventional system.
  • Graywater should not be used for irrigating vegetable gardens.

Looking Ahead

The acceptance by California of graywater separation and use is a tremendous step forward for proponents of graywater. Some have criticized the California regulations for requiring that graywater be delivered too deep underground, but regulatory standards are compromises, and California’s approach seems a reasonable compromise between optimal ecological performance and safety. Indeed, Appendix J is more flexible than the graywater provisions of the Uniform Plumbing Code, upon which it was based.

Just how quickly other states and municipalities follow California’s lead is hard to predict. There may be some wait-and-see among health officials and regulators in other states before similar regulations are adopted; lots of good data on graywater system performance should become available fairly quickly.

In locations where graywater separation and use is not yet permitted, systems will still have to be approved on a case-by-case basis. Designers, engineers, and builders interested in incorporating graywater systems into buildings should start by meeting with local health officials. The references below will provide good background information on graywater system design as well as the specific California regulations.


For more information:

Graywater Guid
California Department of Water Resources
P.O. Box 942836
Sacramento, CA 9423
916/327-1815 (fax)

Introduction to graywater use in the home landscape and details on California’s graywater regulations. Free.
Gray Water Use in the Landscape: How to Help Your Landscape Prosper by Robert Kourik


Edible Publications
P.O. Box 1841
Santa Rosa, CA 95402

Practical booklet on graywater system design and construction; $6 postpaid.


Ted Adams, Graywater Consultant
2800 Painted Cave Rd.
Santa Barbara, CA 93105

Consultant on graywater system design.


Clivus Multrum, Inc.
104 Mount Auburn St.
Harvard Square
Cambridge, MA 01238
800/425-4887; 617/491-0051
617/491-0053 (fax)

Manufacturer of graywater filter, tank, and irrigation piping. Ask for booklet on graywater.
Automatic Grey Water Apparatus


AGWA Systems, Inc.
801 South Flower Street
Burbank, CA 91502

Manufacturer of commercial and residential graywater systems.


ReWater Systems, Inc.
438 Addison Ave.
Palo Alto, CA 94301
415/321-7868 (fax)

Manufacturer of residential graywater systems.



Published March 1, 1995

(1995, March 1). Using Graywater for Landscape Irrigation. Retrieved from https://www.buildinggreen.com/feature/using-graywater-landscape-irrigation