English (en) | Change Language
By: Larry Yarger and Dawn Berkelaar
Published: 2005-07-20

In areas that are dry, or during times of water shortage, it makes sense to reuse water if possible. We have heard the mantra “Reduce, Reuse, Recycle.” Use of gray water (GW) is one way to do this. During a drought, when most fresh water is needed for drinking, GW may be the best water alternative for gardening.

“Gray water” (also “grey water,” “graywater” or “greywater”) is the term used to describe wastewater from dish washing, laundry, bathing, and rinsing. (Note: the term gray water does not refer to toilet waste, which is often called “black water.”) Although gray water does not need extensive treatment before it is used for irrigation, one must be careful as it can contain grease, hair, detergent, dead skin, food particles and occasionally fecal matter.

Gray water, when applied to the soil, simultaneously supports plant growth, recycles nutrients, and is itself purified through biological activity in the soil. Gray water for gardening could even reduce the use of harmful chemicals; a person might be less likely to dump toxic chemicals down the drain if gray water were used for irrigation

This article addresses agricultural, human health and environmental issues surrounding the use of gray water.

Gray Water on Soil and Crops

In many cultures where there exists a need to conserve water, people use gray water on kitchen gardens, containergrown crops and various trees and shrubs growing around the home.

Andre Schmidt with the Mennonite Central Committee in an arid region of Nicaragua wrote in June, 2001: “As water is in very short supply during the long dry season, all water sources are used for watering plants, seedlings, and vegetables around the house. Is soapy water (from bathing and washing dishes) harmful to plants? My guess is that the plants most likely to be harmed by soapy water are those grown in enclosed containers (like in pots or old tires) in which the soap collects without eventually being washed out.”

ECHO shared how one person had filtered GW through sand before irrigating. Another had used GW to directly water flowerbeds, with very good results. The question was also raised about phosphorus in the form of phosphates (PO43-), which sometimes are added to detergents to promote foaming but end up in the GW. Phosphorus is a type of fertilizer (the “P” in NPK is “phosphate”) and is beneficial to plants (but damaging in creeks and ponds in high amounts). Phosphates are no longer prevalent in soaps and detergents.

When working with gray water on a residential level, potential problems boil down to three basic issues: salt buildup, soil pH and pathogens (disease carrying microorganisms).

Any negative effects of gray water on plants are largely related to disease organisms (e.g. from laundry) and the types of cleaning products we use. These cleansers are usually soaps and detergents used in personal hygiene, laundry, household cleaning and kitchen chores.

Although soaps and detergents are biodegradable, many contain high levels of sodium (Na). Sodium laureth sulfate (SLES) and sodium lauryl sulfate (SLS) are ingredients now used to promote foaming in place of phosphates, and sodium from these compounds can be linked with problems in soil structure, salt buildup and pH. We would not expect this to be of much concern under normal household use, especially in rural areas of developing countries. It is possible that this could become a problem at an institution (e.g. if a garden at an orphanage is watered primarily from the laundry section). Just be aware of the potential in case a problem becomes apparent.

Too much sodium may cause soil to become hard and compacted. Compaction can cause rain and irrigation water to stand on the soil surface and not enter into the root zone. Elevated pH levels can “tie up” certain soil nutrients making them unavailable to the plant.

The accumulation of sodium salts (such as sodium chloride (table salt) or sodium sulfate) in the root zone reduces the amount of soil moisture available to plants and may cause wilting. Salt buildup may also prevent absorption of nutrients from the soil, resulting in nutrient deficiencies.

Tips on Gray Water Usage

One solution to salt buildup is to leach out the salts by irrigating heavily (when water is available) to dissolve and wash salts out of the soil. As Andre suspected when he asked ECHO about gray water, this is especially important when gardening in containers. When watering containers, even with gray water, do so until water flows out the drainage holes.

Due to the small amount of GW used on a small farm in a rural developing community, it is doubtful that home application of gray water to the soil would cause salt problems. Even so, the following sections describe complications you might encounter.

Cleansers with chlorine (i.e. bleach) can be toxic to plants, but household levels of bleach generally cause no problems when GW is applied to medium and fine textured soils. On coarse sandy soils with little to no organic matter, you may see root or leaf damage. Use ammonia instead of chlorine bleach for scouring or cutting grease. Use oxygenated bleach for laundry (the type recommended for colors). Use baking soda (sodium bicarbonate) sparingly (e.g. for scouring), especially if you have alkaline soils.

Avoid the use of cleansers or detergents with boron (i.e. borax or boric acid). Water with added borax may apply an excess of the micronutrient boron (B). Boron is needed only in trace amounts, and can be damaging in excess. (Note: citrus, avocado, persimmon and stone fruit trees are very sensitive to boron toxicity).

Clean drains with boiling water, oxygen bleach (not chlorine) or dilute hydrochloric acid (available in many pharmacies, hardware, swimming pool or paint stores) rather than lyebased drain cleaners.

Avoid detergents with “softening power,” as these contain a large portion of sodium compounds. And although one is not likely to encounter this in rural communities, stay away from “soft” water, as it is “softened” with sodium ions.

If your soil becomes alkaline (pH of 7.5 or higher) as a result of using gray water for a while, there are several minerals that will become unavailable to the plant (particularly iron, manganese, zinc and phosphorus). A soil pH changing to greater than 7.5 may also indicate that sodium may have accumulated from the detergents. Though gypsum (calcium sulfate) treatment can help, that is not a realistic approach for most of our readers.

On the other hand, acid soils (pH less than 7.0) would welcome gray water (unless, of course, you are growing acidloving plants). We recommend cleaning agents that are generally either acid or pH neutral, though most cleaning agents seem to be alkaline in nature. Mulching with leaves high in tannins, or acidic in nature, will help reduce pH.

If in doubt, consider discharging the GW into a compost pit or pile. The compost will function as a filter, and the biological activity already taking place in the compost will break down potentially harmful gray water components. The extra moisture will also speed up the composting process. This way you will still conserve the moisture in the soil. In general, soil organic matter (OM) functions as a buffer against strong changes in soil pH, such as those potentially brought on by GW. More OM is typically better for your soil.

The University of Massachusetts recommends this “rule-ofthumb” for deciding how much gray water to use on your garden: “A square foot of well-drained loamy soil can handle about a half gallon of gray water per week.” Thus in 500 square feet of garden space, you can use about 250 gallons of gray water per week (about 1000 liters per 50 square meters, or 200,000 liters per hectare per week.

The literature states that few materials in gray water cause harm to trees and shrubs when applied to the soil. The alkalinity of GW can corrode the bark of both roots and trunk, but the soil protects the roots. When irrigating trees with gray water, focus the GW on the (well-mulched) root zone, and prevent direct contact with the trunk

. Avoid rinsing the following down the drain: solvents, oils, paint thinners, and petroleum products or organic compounds used in vehicle maintenance or cleaning. These will cause the “failure” of a gray water system.

Irrigation is the most obvious use for gray water. However, irrigation needs are often seasonal. When it is not needed for irrigation, gray water can also be used to flush toilets. Do not put gray water into your toilet tank; it will stink. Instead, pour it directly into the toilet bowl using a bucket. This works well whether you use a western-style “sit-down” or a “squat” toilet.

Gray Water Systems

A bucket is the simplest, least complicated way to distribute gray water. Where gray water is used in large amounts (larger than throwing out the dishwater and the semi-weekly laundry water), for example at an institution, one might consider developing a system to treat and utilize gray water. Such a system might include a settling tank to remove solid particles,an oil trap to contain oils and grease, and a biological purification system utilizing soil, soil microorganisms and plants to further clean up the water.

Finca Agape, el Centro Desarrollo Agrícola in Honduras has published a brochure describing a system in which gray water passes through an apparatus similar to a three-chambered septic tank. The GW enters near the top of the first chamber and spills over a barrier into a second chamber. Heavy particles sink to the bottom of the first tank. From the second chamber, the water flows under a wall into a third chamber and exits from that tank. The third chamber catches oils and floating debris to be collected and discarded.

From the third chamber, the gray water flows into a separate tank where it “irrigates” plants (such as water hyacinth, papyrus and cattails) growing in sand. At the bottom of this tank, under a layer of gravel, is a perforated pipe that collects the treated water, which may be used for irrigation. It takes 2 to 3 days to complete the cycle, although “fresh” GW is added regularly. Approximately 20% of the moisture is lost due to evaporation in the process. There is ample room for adaptation to the needs of the individual household or institution.

A system developed in Senegal to treat GW uses open ponds planted with water lettuce (Pistia stratiotes), which encourages the growth of microorganisms that feed on the organic material in the water. Research has also demonstrated the use of water lettuce as a high-protein fodder for livestock, although it is considered invasive in some areas. For gray water to be stored more than three hours, the University of Georgia extension service recommends disinfecting, due to health hazards present in particulate matter and detergents. More harmful bacteria are present in standing GW than in fresh sewage. A chlorine concentration of 0.5 ppm will disinfect GW. This can be obtained by adding 0.2 ml of bleach (~10 drops) to a 20-liter (5-gallon) bucketful. Left standing overnight, the chlorine dissipates. The chlorine in laundry water is too dilute to be a useful disinfectant. Aeration and exposure to sunlight will also disinfect gray water so it can be safely stored. Old or unaerated gray water quickly becomes anaerobic and may generate hydrogen sulfide and other offensive gasses.

As with many aspects of living and working in a rural agricultural setting, gray water can expose humans to potentially harmful elements. Most farmers know, for example, that animal manures can carry disease organisms, and we need to be careful how we manage this rich resource. We should consider GW in a similar light. When properly handled, GW is a valuable and safe resource. If misused, GW can spread typhoid, dysentery, hepatitis, cholera, giardia and other bacterial, protozoan, nematode and viral diseases.

Human Health and the Environment

As with many aspects of living and working in a rural agricultural setting, gray water can expose humans to potentially harmful elements. Most farmers know, for example, that animal manures can carry disease organisms, and we need to be careful how we manage this rich resource. We should consider GW in a similar light. When properly handled, GW is a valuable and safe resource. If misused, GW can spread typhoid, dysentery, hepatitis, cholera, giardia and other bacterial, protozoan, nematode and viral diseases.

Although chemically inert, the insoluble debris (organic and inorganic) is usually the carrier for most pathogens in gray water. Green Plumbers’ website states that GW contains “large numbers of bacteria that may include disease causing microorganisms,” although this was not specifically quantified. Contaminants documented in polluted water include (among others): E. coli, Vibrio cholerae (cholera), Salmonella typhii (typhoid fever, food poisoning), Candida, Aspergillus (aflatoxin), Giardia, Cryptosporidium and Entameba histolytica (amebic dysentery). These may also be found in GW, depending on the source of the GW. For example, is river or canal or pond water being used for washing?

Fecal matter contains the largest concentration of pathogens. So if GW contains water used to clean diapers or soiled bedclothes, at least E. coli, and potentially all of the above pathogens will be present in the GW. In such a case, the GW should be passed through a GW system, and not used to irrigate directly. The simplest technique would be to pass this GW into an active compost pile or pit to disinfect it.

Gray water is naturally disinfected on the soil surface by oxidation and sunlight (UV) exposure, and soil microbes further deactivate harmful compounds and microorganisms, although this process takes time.

The virus content of GW is still a topic under intensive study. Viruses are probably the main impediment to human contact or direct human use of GW. Much has yet to be learned here.

Irrigation of vegetables and forages with gray water is widely debated. Articles written for use in the United States (which tend to be extremely cautious) warn against using GW to irrigate vegetables—only fruit trees and ornamentals. Reports say that it is “unsafe,” especially for root crops and leafy vegetables. However, an article in Spore (Number 114, December 2004) lauds the use of GW on vegetables in Senegal and reports how it has increased production there.

Penn State University recommends that, rather than on leafy vegetables, gray water be used on fruiting vegetables (e.g. tomatoes), particularly those that have been mulched heavily between the rows (the latter would be less likely to make actual contact with GW). Organic mulch filters the GW, retains moisture, and increases decomposition of wastes. Use GW only on established plants, as seedlings are sensitive. We have yet to find a qualitative analysis of GW as it relates to leafy and root crop vegetables, but unless there is a significant time lapse and adequate sunshine and rainfall are anticipated, such direct application of GW is questionable.

There is little concern about contaminating groundwater with gray water. However, it is good to discharge GW into the soil or a compost-filled basin if not collecting it for irrigation. Locate your GW treatment area as you would a latrine. Keep it well away (at least 10 meters) from your well or water source

Personal experience in nations around the world and a lack of literature to the contrary is evidence that farmers, although not extensively, do irrigate with gray water and without major health problems. In the US, although under different cultural and economic pressures, there has been no documented caseof sickness or disease from GW use. In some places, water of questionable origin that is sprinkled on fruits and vegetables to “freshen” them up for sale in the market poses greater risk of sickness than irrigation with the same water.

Especially in the tropics and subtropics, note that mosquito borne diseases may increase if standing water (gray or otherwise) is allowed to accumulate where you irrigate.


The use of gray water is a technology with tremendous
potential. Through our review of the literature, we have found that we can make some general recommendations: ·

GW is especially valuable in dry climates where water is not readily available, is expensive or must be carried considerable distances. ·

GW recycling is a well thought out system of discarding wastewater to reduce potential disease danger and environmental pollution. ·

If you have a concentrated source of disease organisms for your GW, such as dirty diapers, treat it more like black water. It needs exposure to UV, oxygen (O2) and soil microorganisms. ·

Organic matter in the form of compost and mulch is important to buffer the potential alkaline nature of GW, break up the potential soil compacting properties of salts and sodium-containing compounds, and promote the growth of microorganisms to break down and deactivate harmful compounds in the GW. ·

Be aware of the cleaning products you use, as some plants may be sensitive to the ingredients, as well as to the alkaline nature of the GW.

Set up your gray water system carefully in the context of your needs and resources. Maintain it as you would any other aspect of your farm, and such a gray water system potentially will become an added resource and otherwise a blessing!

Cite as:

Yarger, L. and Berkelaar, D. 2005. Gray Water and Crop Irrigation. ECHO Development Notes no. 88