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.
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.