Graywater Reuse in Other Countries and its Applicability to Jordan -- continued --

3.4  Effects on Soils

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The main effects of graywater on soils are: 

• A tendency to raise soil alkalinity and salinity;

• A reduction in the ability of soil to absorb and retain water. 

Increases in alkalinity will arise due to the presence of sodium, potassium or calcium salts in the graywater, particularly from laundry detergents. The effects on plants of variations in the pH of a soil have been discussed above. Water retention also is affected by some forms of sodium – an effect measured by a parameter known as the sodium adsorption ratio – SAR. A sandy, well-drained soil will be less affected by graywater application than a poorly drained clay soil.  

Guidance for quantities of graywater use is given in several publications. For example, it is stated that a well-drained square meter of loamy garden soil, rich in organic matter, is capable of handling 200 liters of household graywater each week. Sandy, lighter soils can absorb more water, and heavier soils with a high clay content absorb less (Greenhouse People’s Environmental Centre).  

If a soil has been irrigated with graywater for an extended period, sodium levels may build up, resulting in poor drainage and potential damage to plants. High levels of sodium may be detected by conducting a pH test of the soil. A pH of 7.5 or above may suggest that the soil has become overloaded with sodium. The Greenhouse People’s Environmental Centre recommends the application of gypsum (calcium sulfate) to the soil in order to reduce the pH levels. A rate of 100g per square meter each month is suggested, until the pH of the soil drops to 7. (see www.awgypsum.com/gypsum.htm).  

Fortunately, dilution of graywater by rainfall or fresh water irrigation helps flush the soil of sodium, excess salts, and other soil contaminants that might be building up. In places such as Arizona, there are two rainfall seasons that, despite the arid conditions, allow for leaching of these substances from the soil. The application of thick compost mulches is recommended by some sources to help speed up the natural decomposition of waste residues, although this will have little effect on inorganic salts.

3.5  Effects on Human Health

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Contrary to popular belief, undisinfected graywater is not as hazardous to health as is sometimes thought. The main risks arise from physical contact with the graywater, and from eating fruit or vegetables that have been irrigated with the graywater. Some graywater may contain concentrations of human excretions that can be a mode of transmission of infectious disease. Infection theoretically could occur after contact with the eyes and nostrils, inhalation of mist from spray irrigation, or ingestion through crops contaminated by spray or surface irrigation. However, there are no recorded incidents of serious effects to human health from the reuse of graywater. 

Human exposure to graywater is also much less an issue than is commonly thought. Firstly, since all domestic graywater was initially produced by humans, most of it already has come into contact with humans anyway – for example in the shower, or wash basin. However, it may contain pathogens and contaminants that could be a risk to human health. There may be particular risks to those from outside the family household where the graywater was produced. There are two ways to minimize this risk – one is by extensively treating the graywater to remove bacteria and other pathogens. This is how the quality of treated wastewater (blackwater) is increased to allow it to be used for unrestricted purposes. However, this is unduly expensive for householders. The second way of minimizing risk is simply to eliminate contact between the householders and the graywater. For this reason, most sources recommend a graywater collection and distribution system that does not require regular user intervention. Also, irrigation by sprinkler is prohibited, to avoid the danger of airborne graywater coming into contact with people. Some jurisdictions even prohibit surface irrigation, although others do allow it provided quantities are controlled, and soils saturation does not occur. Irrigation of lawns and other areas where children or animals may play, other than sub-surface drip irrigation generally is discouraged, also to reduce the chance of contact. In addition, graywater should not be used to wash down patios, walkways, or driveways. It should also not be used for dust control, cooling, spray irrigation or any other use that would result in air-borne droplets or mist. 

Most authorities emphasize that the introduction into the graywater of pathogens from the washing of heavily soiled laundry and diapers, or of clothes with blood or vomit should be avoided, and that where there is a particularly infectious illness in the household (e.g. diarrhea, hepatitis, measles or intestinal parasites), the graywater should be disposed of through the blackwater sewerage system and not reused.  

Also, large graywater storage containers may pose a safety hazard to children. Therefore, storage containers must be tightly covered to prevent easy access, and to keep away mosquitoes, other insects, and small rodents.

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The overriding positive environmental impact of graywater reuse is the reduction in demand for fresh water. Many sources argue against the production of high quality potable water for purposes that do not require such high quality water – e.g. irrigation of plants. By reusing graywater, a reduction in demand for expensive-to-treat potable water results.  

One of the main environmental risks from graywater reuse is that of groundwater pollution. Depending on the geology of the area in question, it is possible that some of the substances found in graywater could find their way into the groundwater reserves underlying the area of reuse. If the groundwater were to contain substances that would pollute or otherwise have a detrimental effect on the groundwater, contamination may result. However, two factors reduce the importance of this. First of all, the overall quantities of graywater in domestic contexts are very small – most households in the west produce no more than 200-400 liters of graywater per day. Most of the water and nutrients in the graywater will be taken up by the plants themselves. Other substances in the graywater (e.g. organic matter and bacteria) will be broken down by the topsoil. Under normal circumstances, very little of the graywater in question will actually reach the groundwater.  

Additionally, one of the assumptions behind the reuse of graywater is that the householders take moderate care over what enters the graywater in the first place. If large doses of poisonous chemicals were being deposited into the graywater, then not only might groundwater be at risk, but the plants also would be in danger. If the graywater is of a high enough quality for it to be suitable for plant irrigation, then it is very unlikely to be a serious threat to the groundwater. 

However, some jurisdictions do take risks to groundwater into account when permitting the reuse of graywater. For example, in Arizona, graywater may only be used in locations where groundwater is greater than 1.5 m below the ground surface (Little 2001).

3.7   Irrigation

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As noted earlier, many of the potential risks to human health and other possible unfavorable side-effects of graywater reuse (for example odors, encouraging breeding of mosquitos, etc) are reduced or eliminated by prohibiting the use of sprinkler irrigation, and by the avoidance of surface pooling of the graywater. Different jurisdictions regulate this in different ways, but generally specify irrigation to be by controlled surface flooding or by drip irrigation.  

In hot climates, it is generally recommended that irrigation be carried out during the cool parts of the day (or night) to minimize water loss by evaporation. It is important that the graywater be applied no faster than the soil can absorb it, to avoid saturation and pooling of the graywater. Usually, plants are healthier when the soil is allowed to dry out between irrigations. Therefore, for best results, one should wait until the soil in the root zone is half dried out before re-irrigating.  

One important potential issue in using graywater for irrigation is the danger of clogging the irrigation network from particles in the graywater. This can be eliminated by either removing solid particles from the water (by filtering or settlement) or by increasing the diameter of the holes in the irrigation pipe. It is recommended that drip irrigation hoses (with small outlets) are not used for graywater irrigation unless the solid particulates have been removed.

Some sources have reported a build up of algal growth in the irrigation pipework – the natural result of the presence of nutrients in the graywater. This does not pose a risk to either plants or humans, but should be taken into account in the design of the system. Algae may be removed by periodic chlorination of the graywater, although care should be taken in such circumstances to avoid harm to the plants. 

Based on this preceding information, a short set of guidelines has been prepared to provide advice on graywater reuse in Jordan. Many of the important recommendations noted in the preceding discussion have been included in these guidelines.  However, these guidelines will be further revised following field trials and assessments being conducted in Jordan.

Click here to view CSBE preliminary guidelines for using graywater for irrigation.