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

2.1.3   Treatment of Graywater

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There are four reasons why graywater may need to be treated: 

• To remove substances which may be harmful to plants;

• To remove substances which may be harmful to health;

• To remove substances which may be harmful to the wider environment;

• To remove substances which may clog the graywater system.

No Treatment

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Often, the best solution is to design, install, and maintain a system so that the graywater needs no treatment at all. The reason why graywater is being considered in the first place is due to its relatively clean nature. Many of the substances contained in graywater are not harmful to plants, and some are actually plant nutrients. Some of the substances found in graywater are actually very well treated by a healthy layer of soil, and if human contact between the graywater and humans can be minimized (by eliminating the need for user intervention in the system, and by reducing the possibility of contact at the irrigation stage), then the risk to human health is very small.

Figure 2.1 is a schematic of a simple graywater system that has been successfully operating in Amman for over 2 years. The graywater source has been chosen to minimize the amount of contamination that enters the system – only water from one bathroom – bathtub shower, handbasin – is used. The system is gravity fed throughout, eliminating the need for pumps. The diameter of the irrigation pipe is 50mm, and the distribution holes have been increased to 5mm to reduce the risk of clogging. Not even filtering has been required in this system, although there must be an element of solid material (such as skin and hair) passing through the system. The soil has been able to deal with this organic material, and also with whatever soap constituents may be present. No detrimental effect on the plants has been observed in the 2-year period of operation.

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A simple first level of treatment - a filter - may however be advisable in some cases to prevent solid material (hair, lint, food particles) from entering the graywater system. In its simplest form, this could be a stocking or sock filter on the inlet pipe to the storage tank – figure 2.2, or a more complex sand/gravel media filter as shown in figure 2.3.

A number of sources (such as Ludwig 1994 -2000) propose the use of a natural mulch basin filled with stones and organic mulch (leaves, tree bark, etc) to filter and treat the graywater. Rather than containing inert material such as sand, a mulch basin of this type provides a medium for the natural digestion of organic substances, and removes solid material from the graywater.

Similar to this are slow sand filters - shallow layers of stone, medium gravel, and pea gravel beneath a deep layer of sand, or multi-media filters - filled with a variety of media in order of increasing size. Slow sand filters are subject to clogging and very slow percolation rates, and require regular cleaning and replacement of the top layer of media. Some authors therefore do not recommend their use in domestic contexts. Multi-media filters require cleaning less frequently.

Commercially available water filters also are sometimes used for graywater.  These include activated charcoal, cellulose, or ceramic cartridges. However, these are generally designed for higher quality water and may require excessive maintenance if used with graywater. If the kitchen sink water were to be used, a grease trap – to remove fats and grease - may be necessary to prevent filter clogging, although this requires a degree of user intervention.

Settlement and Floatation

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A settling tank is sometimes recommended as a means of removing solids from the graywater. Substances denser than water will gradually fall out of suspension to the bottom of the tank. On the other hand, grease, oils, and other small particles will float to form a surface scum layer. The remaining liquid can then be reused. A settling tank also has the advantage of allowing hot water to cool before reuse. Figure 2.4 shows a simple graywater settling/storage tank.

By contrast, figure 2.5 shows a complex automated graywater system with storage, filtration and backwashing, pumping and associated controls and valving. This system, discussed in ‘Create an Oasis with Greywater’ (Ludwig 1994 - 2000) was found to be reliable, but costly – between $1,100 (about 780 JD) and $8,000 (about 5,680 JD) to install. Few applications will warrant such complexity and expense.

Disinfection

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Disinfection is often mooted as a necessary method of treatment. The addition of chlorine tablets or iodine to a graywater storage tank (a chlorine concentration of 0.5 parts per million), or the use of UV light or ozone will disinfect the graywater, thus reducing the amount of bacteria present, provided that the bulk of solid or organic material previously has been removed by filtering or settlement. This will have the effect of reducing odors from graywater that has been left standing and has begun to digest. However, unless there is risk of human contact with the graywater – something that should be avoided in all responsible graywater reuse systems – there is no particular advantage to this reduction of bacterial content. Human-borne bacteria cannot survive for long periods outside the human body, and will soon die in the soil. There is no health risk to plants from such bacteria, although ironically, the presence of chlorine or other disinfection agents in the graywater may cause problems to plants and soil. Water disinfected with chlorine should be left at least overnight, to allow evaporation of the chlorine.

Other types of systems of varied complexity also have been noted in the literature, but the evidence consistently points to simpler systems being more reliable, with most complex systems being abandoned due to high maintenance costs, and disinterest by the householder due to the amount of intervention required. 

The suitability of the type of system is largely determined by the quality of the incoming graywater, its end use, and the degree of care and intervention desired by the user. The minimal  treatment system is perhaps best suited to a user who will take an active interest in controlling what goes into the graywater, and who takes a fairly active role in caring and tending for their own plants. The cleaner the graywater to begin with, the less treatment required. A more complex, catch-all system is more suited to an establishment where less attention is paid to what goes into the graywater, and how it is used. However, this relatively complex system requires more maintenance, and is more expensive.

2.1.4   Storage of Graywater

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Differences of opinion also exist with regard to whether graywater should be stored or not. There is a common misconception that stored graywater becomes more and more contaminated. This is because of the odors that may develop through time as the organic content in the graywater begins to digest. Actually, this process of digestion is a form of anaerobic treatment of the graywater, and the resulting water will contain less organic material and generally be 'cleaner'. However, the odors may be an unpleasant side effect and should be controlled by venting the graywater storage tank away from places where the odor would become a nuisance. In many countries, domestic plumbing will have odor traps that are vented to the roof to prevent odors from entering the house through the piping system. However, provided the graywater storage tank itself is sealed, there ought to be no significant odor problem.

Even if the graywater is to be stored for a substantial period of time, some authors consider treatment to be unnecessary, provided odors are controlled. Others, however, recommend it, and suggest filtration to reduce the amount of organic material present, and disinfection to reduce the amount of microorganisms present. Adding two tablespoons of chlorine bleach per 5 liters of water provides one manner of achieving this goal.

Simpler systems will not need storage of the graywater. Direct reuse without storage is favored as it minimizes the problems of microorganism growth and odor. However, even if storage is not required, each graywater system should be capable of handling sudden foreseeable inputs of graywater (for example from a bath being let out, or a washing machine rinse cycle) without overloading or saturating the soil. In these cases, many authors recommend a surge tank – a small facility to allow the flow to surge, while releasing it gradually to the soil. A simple sketch of a surge facility is shown in figure 2.6.

Even if no storage is provided, an odor problem may also arise if graywater is allowed to pool in parts of the pipe bends, tanks or other parts of the network. If a graywater system becomes unused for a period of time (for example the householders go on holiday) then there is risk that pools of graywater in the system will begin to digest anaerobically and cause unpleasant odors. Most sources recommend that all pipes be at a gradient, and that all tank bases, etc be angled, with provision for drainage, so that the entire system can be emptied of water, if necessary.

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As noted above, this investigation concentrates on the use of graywater for household irrigation. However, in some countries, graywater is also used for toilet flushing. A simple way of using graywater for toilet flushing is illustrated in figure 2.7. This hand basin toilet idea is popular in Japan and is installed in many Japanese homes, as well as in commercial areas. This system incorporates a hand basin at the top of the cistern, with a tap for hand washing. The tap operates automatically when the toilet flushes to simultaneously refill the cistern and to allow hand washing. However, it should be noted that in applications where graywater captured from other household sources has been used for toilet flushing, unpleasant odor and discoloration of the toilet bowl have been reported.

In the UK, graywater reuse is not common, but a recent study (Leggett et al. 2001a; Leggett et al. 2001b) estimated that about 150 graywater treatment units were being used to provide water for toilet flushing. This study looked at various collection and storage designs, with disinfection as the main means of treatment, and found that useful water savings were obtained, but that maintenance and reliability issues were significant.  

Cyprus is encouraging households to reuse graywater for domestic landscaping and toilet flushing (Kambanellas 1999). 

No consensus yet exists regarding the quality of water necessary for toilet flushing. Although there is no contact between the user and the graywater, and the water becomes contaminated upon use anyway, there are concerns over transmission of diseases due to splashing, or as a result of cross-contamination with the potable water supply, which would be required as a backup supply. For this reason, disinfection of the graywater before use is usually recommended.