Preliminary Guidelines for Using Graywater for Irrigation
Prepared by CSBE Team, January 2003
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Graywater is the output from bathtubs, showers, sinks, floor drains, and washing machines, which although soiled, is not as contaminated as toilet water, and therefore may be used for irrigation of plants with little or no treatment, provided some simple safeguards are met.
When reusing graywater, a number of issues need to be taken into consideration. The system should be as simple and easy to use and maintain as possible. The system also should minimize risks to human health, either by providing for adequate treatment of the graywater, or by minimizing contact between the graywater and humans (and animals). The system also should minimize the risks to plants, which may arise from some of the constituents of the graywater, particularly chemicals from soaps or detergents (such as boron, bleach, and sodium), which could adversely affect plant health.
The following are basic, preliminary guidelines for the development of graywater irrigation schemes.
Sources of Graywater
1. In order to reuse graywater from a particular building, a ‘dual plumbing' system is required to separate the usable graywater from the more contaminated ‘blackwater.' The outputs from toilets, bidets, and kitchen sinks are not suitable for use in irrigation without proper treatment, and should be taken to the foul sewer or the septic tank. Only wastewater from ‘cleaner' sources, such as baths, showers, hand basins, and floor drains should be included in the graywater system.
2. Care should be taken to limit the release of inappropriate substances into the graywater system. Heavily soiled or bloodstained clothes, diapers, animals, etc should not be washed in sinks draining to the graywater system. Chemicals such as bleach, cleaning agents, paints, etc should not be disposed of into the graywater system, nor should any substance that may cause blockage, or detrimentally affect the plants to be irrigated with the graywater. Detergents (like those used in washing machines) have a detrimental affect on some plants because of their contents of sodium compounds. Consequently, if laundry wash water is to be used for irrigation, a degree of treatment or occasional irrigation with cleaner water may be required (this is the subject of further investigation). If possible, environmentally friendly soaps - such as those made of potassium or magnesium compounds - should be used in order to minimize the amount of sodium applied to the plants. These do not harm the plants, and even provide them with a source of nutrients. **
3. When designing a graywater system, an estimate of the size of the graywater resource should be made. How much graywater is produced in one week, and how is this distributed? A dwelling that receives municipal water for 1 day each week and has a 1 cubic-meter storage tank will produce most of its graywater during the day when the municipal water supply is on, and will produce relatively little graywater during the remaining days. Users therefore may wish to store the graywater to ensure a more even distribution. On the other hand, a house that receives municipal water a few days each week, and/or has a large underground tank, will produce graywater more evenly during the week. The graywater demand should be estimated (i.e. the amount and type of plants to be irrigated) to ensure that the demand and supply are reasonably well matched.
4. It is recommended that each household use its own graywater for its own purposes, rather than sharing graywater with other households. This avoids potential conflicts, and increases confidence over the quality of the graywater.
Treatment of Graywater
5. Relatively clean graywater needs little treatment if it is to be used simply to irrigate trees or shrubs via a sub-surface irrigation system - in other words, if the likelihood of human contact is low. Such graywater may be released directly into the irrigation network. If there is a chance of a sudden release of graywater (for example from a bath being drained, or a washing machine being emptied), then the system should be designed to handle such surges to avoid ponding on the soil surface. This can effectively be achieved through the use of a small surge tank.
6. Graywater that contains lint, hair, or other solid material may cause periodic clogging of the irrigation system. A simple filter may be required - for example a screen mesh in the surge or storage tank. This filter may need periodic cleaning.
7. Graywater should not be applied directly to plants if it is hot. If hot laundry wash water is to be used, means to store the graywater temporarily, in order for it to cool, should be provided.
8. Graywater may be stored, if desired. However, this may give rise to unpleasant odors. The storage tank therefore should be covered to prevent the escape of odors to areas where they would cause annoyance, but also should be vented appropriately to allow odors to escape to the atmosphere, away from areas frequented by people (for example to the roof). It should be possible to completely drain storage tanks every now and then to avoid pooling of graywater and possible contaminant buildup. If no filter is used, the outlet pipe should be located higher than the base of the tank to allow solid material to settle.
9. All pipes containing graywater should have a slight gradient to prevent graywater from lying in the pipes, and should be designed to avoid traps where graywater can lie.
10. Means of diverting the graywater to the sewer system or septic tank should be provided, in case of accidental release of harmful substances (bleach, nappy rinse, etc) into the graywater system. This will prevent contamination of the graywater irrigation system.
11. If a potable water supply is to be included (in order to dilute or supplement the graywater supply), there should be no risk of cross-contamination between the potable water and the graywater. If a potable input pipe is included in a graywater storage tank, a minimum air gap of 300mm should be maintained between the potable water pipe and the maximum level of the graywater. A non-return valve should be provided in the potable water pipe as secondary protection to prevent accidental return flow into the potable water supply. The potable water supply could be governed by a level-sensitive valve to cut off the flow of potable water when it is not required. An overflow pipe also should be provided.
Use of Graywater
12. Graywater is best suited to the irrigation of plants, trees, and shrubs. Ideally, the area to be irrigated should be at a lower level than the graywater outputs so that the entire system can operate by gravity, and the need for a pump is avoided.
13. Drip irrigation hoses with small holes may clog due to the presence of solid material in the graywater, or following the growth of algae in the hose. Therefore, holes of at least 3mm diameter should be provided.
14. For untreated graywater, the possibility of human contact should be avoided. Graywater therefore should not be used for the irrigation of lawns, unless they are for ornamental purposes only and are not used by children or household animals, or are irrigated by sub-surface irrigation systems, which reduce the risk of human contact. However, surface irrigation is permitted provided the user is careful to avoid contact with the graywater. Irrigation by sprinkler should not be used.
15. Irrigation of ornamental and fruit trees is permitted. Irrigation of vegetables that will be cooked before they are eaten is also permitted, provided the graywater makes no contact with the vegetable. However, irrigation of vegetables that have contact with the ground (such as potatoes) or that are likely to be eaten raw (such as lettuce, carrots, and tomatoes), should be avoided, as should irrigation of leafy edible plants (such as mint and parsley). Graywater is best suited for the irrigation of mature plants (not saplings), which have considerable tolerance to salinity, sodium compounds, and high pH levels. A list of such plants is being prepared by CSBE.
These guidelines have been developed in accordance with advice and practice in other countries. The CSBE Graywater Reuse Project eventually will provide enough information to develop guidelines that are specifically appropriate to Jordan. Although provided in good faith, these guidelines should be regarded as preliminary.
For additional information on using graywater for irrigation, see Val Little, Graywater Guidelines (Tucson: Water Conservation Alliance of Southern Arizona, 2002). This document is available online through the Water CASA web site.
Also see the Oasis Design web site, which has extensive information on the subject of graywater.
* image courtesy of Water CASA.
** The Inter-Islamic Network on Water Resources Development and Management (INWRDAM), an Amman-based autonomous intergovernmental organization, has developed environmentally friendly soaps and has initiated their production. For information concerning these soaps, please contact INWRDAM (tel. (06) 533 2993; fax. (06) 533 2969; firstname.lastname@example.org; www.nic.gov.jo/inwrdam)