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· All water treatment methods require some expertise, regular attention and maintenance.· In general terms, a large quantity of reasonably safe water is preferable to a smaller amount of very pure water.
· The most serious threat to the safety of a water supply is faecal contamination.
· In any refugee situation, including emergencies, the use of water treatment should be restricted to those cases where such treatment is absolutely essential and where correct plant operation and maintenance can be secured and verified. This is especially applicable in most situations where refugees live in a dispersed manner, mixed with the local population or in organized rural settlements. If large numbers of refugees are concentrated in refugee camps, disinfection of drinking water should be deemed as strictly necessary; other types of treatment should be considered, in accordance with the characteristics of the raw water.
· Under normal refugee situations, treatment is carried out to improve the physical and the bacteriological characteristics of the drinking water. Only under very special circumstances would the improvement of chemical quality (desalinization, reduction of Fluor contents, etc.) be considered (See 8.25).
· It is impractical to chlorinate cloudy or turbid waters; they must be treated before disinfection.
· Water purification tablets or boiling are not generally appropriate for large-scale water treatment.
· Water quality control and treatment activities, although necessary to ensure adequate health within refugee communities, have to be combined with improved personal hygiene and environmental health practices, sometimes difficult to enforce in certain environmental, cultural or social situations.
1. The potability of any water source has to be assessed before a decision to use it for human water supply is taken. Criteria and guidelines used in this assessment have been previously discussed (See 3.10-16).
2. The importance of trying to find water sources which do not require too much treatment is obvious. When necessary, treatment undertaken should be the minimum required to ensure acceptably safe water; it should involve appropriate technological approaches and reliable methods to ensure operation and maintenance requirements within the scope of camp managers and service staff. Determining how to treat water on a large scale is best done by experts, and if possible, professional engineering advice should be sought. However, simple and practical measures can be taken before such help is available.
3. During an emergency situation, in addition to the physical measures to protect water at its source (See 4 and 6) and initial disinfection of wells and boreholes (See 6.45), there are four basic methods of treatment that may be easily applied: storage, filtration, chemical disinfection and boiling. They can be applied singly or in combination.
4. Water is sterilized by boiling. At low altitudes, water that is brought to the boiling point may be assumed to be free of pathogenic bacteria. At higher altitudes, water should, as a rule, continue to boil for at least one minute for every 500 metres of altitude above sea level, as boiling temperature reduces with altitude. Boiling, however, is a wasteful treatment method that should be avoided, especially if the energy source is fire wood collected from camp surroundings. Boiling increases the concentration of nitrates, which in large quantities are dangerous for very young babies.
5. Although filtration or chemical disinfection may be easily applied in emergency refugee situations, the design, construction, operation and maintenance of the required facilities should be entrusted to qualified technicians. This is also true for any other water treatment method.
6. Pure water is rarely found in nature (See 3). Water impurities are classified in accordance with their state: suspended, colloidal and dissolved. Running water may pick up and transport solid particles of higher density than water; the higher the velocity, the bigger the particles that may be picked up. Surface waters during floods are, therefore, at their most turbid point; they have maximum loads of suspended matter. Finer particles (colloids) may not be visible to the naked eye but could impart colour and turbidity to the water. Colloids remain in suspension even when the water is at rest. In its passage over or under the ground, water may pick up substances which are soluble. Among these dissolved solids, the most common in natural waters are bicarbonates, carbonates, sulfates, chlorides and nitrates of Calcium, Magnesium, Sodium, Potassium, Iron and Manganese. The products of decomposition of organic wastes such as nitrates and nitrites may be regarded as an indication of organic pollution. The presence of bacteriological indicators such as Escherichia coli (E. coli) provides positive proof of the faecal origin of such pollution. Algae may grow in water under certain conditions and they may impart objectionable tastes and odours to drinking water; the removal of algae is essential but often difficult. The presence of Iron or Manganese may also impart tastes or odours to water and may stain articles that are washed with it. Hardness, caused by bicarbonates, sulphates and chlorides of Calcium and Magnesium forms insoluble precipitates with soap and causes the deposition of scale. Sulphates of Magnesium and Sodium, if present in excess, act as laxatives; chlorides, in concentrations higher than 500 milligrammes per litre, give water a salty taste, while fluorides, in concentrations above 1.5 milligrammes per litre, are undesirable; in concentrations above 3 milligrammes per litre they may cause mottling of teeth. Detergents and pesticides may find an easy way into raw water and are objectionable if present in excess. Water with a high content of dissolved CO2, a low pH value and low alkalinity is corrosive and is apt to attack metals.
7. There are no set rules as to the acceptable quality for potable water, but certain guidelines have been laid down (See Annex C). If these guidelines are not exceeded, no action is necessary. Short-term deviations above the guideline values do not necessarily mean that the water is unsuitable for human consumption; the amount by which, and the period for which, any guideline value may be exceeded without affecting public health depends on the specific substance involved. In developing drinking water standards for any refugee situation it is necessary to take into account its geographical, socio-economic, dietary and environmental conditions.
8. A common treatment plant consists of many processes: screening, coagulation, flocculation, sedimentation, filtration and disinfection, each of which performs one main function although it may incidentally assist with some other. Water impurities are removed in order of size, the bigger ones being eliminated first. Not all water contains all the impurities, therefore not all water requires all the treatment processes. Whenever necessary, impurities are removed as follows:
i) floating objects by screening;
ii) algae (if present) by straining;
iii) excessive iron, manganese and hardness by chemical precipitation;
iv) normal suspended solids by settling (sedimentation);
v) excessive bacterial pollution by pre-chlorination;
vi) the remaining fine particles and some more bacteria by filtration;
vii) final bacteria, surviving filtration, by chlorination.
All these processes overlap to some extent and many need auxiliary processes to be fully effective.
9. It is unusual that raw water is pumped directly from its source to the treatment plant. The use of intermediate processes, which can collectively be called pre-treatment, increases the effectivity and life-span of the treatment plant. Raw water storage, pre-chlorination, aeration, algal control, straining, preliminary settling, coagulation, mixing and flocculation are all pre-treatment processes. Each performs a particular function and unless the quality problem they are intended to resolve is part of the raw water's characteristics, they should be omitted.
10. In general terms, the quality of water that is left undisturbed in containers, tanks or reservoirs improves since some pathogenic micro-organisms die and heavy matter in suspension settles (sedimentation). Efforts to provide maximum storage capacity at refugee sites at the onset of emergency assistance operations is a logical step. This task may not always be accomplished, however, especially when water demand is large (large refugee populations) or when water is limited in quantity. To bring about a substantial improvement in water quality, storage should be possible for at least 12 to 24 hours; the longer the period of storage and the higher the temperature, the greater the improvement. Storage periods of up to two weeks are recommended as necessary to achieve maximum improvement in raw water by storage. The fact that other organisms are encouraged to develop in stagnant water should, however, not be forgotten. Storage of raw water may create a silt problem; reservoirs tend to silt up very quickly in the absence of some sort of a silt trap. The cost of building reservoirs large enough to be effective for water quality improvement is fairly high and, on this basis, they should normally be omitted from the treatment processes.
11. The practice of injecting chlorine into the raw water soon after it is abstracted from its source (normally a surface stream) is called pre-chlorination. This step is usually omitted for reasonable quality water and is normally more effective in low turbidity water having a high bacteria content. The amount of chlorine used is fairly high (2-5 mg./l). Chlorine oxydizes organic matter, iron or manganese during the time water spends in settling basins; it will also reduce colour and slime formation. As much greater quantities of chlorine are used than in post-chlorination and complete water disinfection may very seldom be accomplished with it, pre-chlorination should not be regarded as a substitute for post-chlorination (See 8.21) but as a safeguard to be adopted only when extremely polluted (but fairly clear) raw water has to be used in emergency situations.
12. Aeration is practiced to add oxygen from the atmosphere to water and to liberate undesirable gases such as carbon dioxide or hydrogen sulphide. It is commonly done by splashing the water over trays or by blowing air bubbles through the water. It is a viable and cheap means of controlling tastes, odours and corrosion but its results may not be considered complete in all cases. Among the equipment normally used for aeration, the most common are some special nozzles which direct thin jets of water into metallic plates to produce fine sprays exposing water to the atmosphere; cascade-type aerators which create turbulence in thin streams of water flowing down; tray-type aerators consisting of some five perforated trays, increasing in size from top to bottom, where water (falling from tray to tray) is exposed to air; and diffused air aerators, which are tanks where air is bubbled upwards from diffuser pipes laid on their floor. The latter method is the most efficient; the amount of air needed may be regulated; the tanks are normally about 4 metres deep and have a retention time of about 15 minutes. Among all the methods, however, trays are the most commonly used because of their low cost, simple operation and reasonably high efficiency.
13. Algal control is necessary to eliminate outbreaks of these organisms which are usually classified as plants and which proliferate in rivers and reservoirs. These outbreaks tend to be sporadic or seasonal but normally severe and can cause trouble to waterworks' operators. Fairly alkaline waters, with an appreciable concentration of nitrates or phosphates, are likely to develop important algal colonies. Although heavy pollution may impede the growth of algae, water treatment, by itself, may encourage it (once pollution has been eliminated). Chlorine doses of up to 1 mg./l may kill the algae (See 9.8.23). Algal growth is inhibited by Copper sulphate in concentrations of 0.3 mg./l; these doses are, however, toxic to some fish species and may therefore not be acceptable in some circumstances. Strainers are widely used to remove algae, some of them functioning as rapid sand filters (See 8.18) which, if their filtration medium is coarse, are known as "roughing filters" (See 8.14). Other devices, called "microstrainers", which are mainly of proprietary make, are excellent, provided that the water is relatively free of silt.
14. Where sediment loads in raw water can reach concentrations of more than 1000 mg./l, it is helpful to put in small, non-chemically assisted, horizontal flow basins immediately upstream of other treatment works, such as normal sedimentation basins, to increase the effectiveness of the treatment process, minimize plant maintenance and save on the use of chemicals. These facilities are called "pre-settlement basins". Alternatively, horizontal roughing filters may be used to improve the quality of raw water that will undergo further treatment through slow sand filtration devices; they are rectangular boxes similar to the basins used in plain sedimentation (See 8.16); their raw water inlet is situated on one side of the box, their outlet at the opposite side (Fig 29). In the main direction of flow, water passes through various layers of graded coarse material (in the sequence coarse-fine-coarse). Vertical depths of filtration are in the range of 0.8 to 1.5 m.; suitable filtration rates are in the range of 0.4 to 1.0 m/h); the total length of the filter would vary between 4 and 10 metres. Pre-settlement basins and horizontal roughing filters are sometimes built as a remedy, where changing raw water characteristics have put in jeopardy existing waterworks facilities (a common occurrence in developing countries).
Fig. 29 Features of a Horizontal
Flow Roughing
Filter
15. The main chemical means of dealing with the improvement of surface waters is coagulation. Chemical coagulation removes turbidity-producing colloids such as clay particles, bacteria and other organic matter and colourants resulting from the decaying vegetation, animal or industrial wastes. It is directly followed by flocculation, a process whereby the products of coagulation are made to agglomerate and form "floes" of sufficient size and specific weight to allow removal by sedimentation or filtration. As the use of chemicals should be avoided as much as possible, coagulation should be used only when strictly necessary. The most widely used coagulant is Aluminium sulphate, commonly known as alum; Iron salts (such as ferric chloride) can be used, despite their higher cost, when broader pH ranges for coagulation are required. (N.B. pH values for alum's effectiveness range from 5 to 8, while those for the Iron salts range from 4 to 9). These coagulants react with the alkalinity of the water and hydrolyze in it; if the required alkalinity is not present in the raw water it should be added through dosage of lime or Sodium carbonate. The optimum dosing, pH, concentration of coagulant and the most effective order in which to add the various chemicals will be determined with a jar-test which should be carried out by qualified technicians and requires the collection of water samples and the use of specialized laboratory equipment (See 9.12.8). During emergency situations, before jar-tests are done and if there is a need to lower the turbidity of raw water, dosing alum at 50 mg./l is recommended. Dosing is usually done in the form of solutions prepared in special tanks with a holding capacity of 10 or more hours of coagulant feeding requirements; two tanks are required as a minimum (one in operation, the other for the preparation of new solution). To accomplish flocculation, mixing is necessary and may be accomplished hydraulically, in turbulent flow conditions at specially made structures such as weirs or "flocculation chambers", or at the suction side of the pump; it can also be accomplished mechanically (manually or with paddles, rakes, turbines, propellers, etc.). Normally, water should be retained in flocculation tanks for at least 30 minutes to ensure maximum flocculation. Coagulation and flocculation processes should be done (only if required) before sedimentation, filtration and disinfection. The sedimentation basin should be designed in such a way that the last floes settle before the filtration units.
16. The process to eliminate all impurities present as suspended particles which are carried along by flowing raw waters but which will settle in quiescent or semi-quiescent conditions is called sedimentation. It is usually considered the minimum treatment for turbid surface waters; if 24 hours can be alotted for sedimentation, clarified waters can be directly chlorinated. The sun's bactericidal effect has also been documented. Below a certain particle size, depending on the material concerned, settling velocity becomes very small and therefore sedimentation becomes unfeasible. This is the case for colloidal matter, which, as it has been discussed, requires coagulation and flocculation before the sedimentation process. Sedimentation facilities normally operate under continuous flow to:
i) achieve quiescent conditions in the settling zone;
ii) ensure uniform flow across the settling zone;
iii) obtain uniform concentration of solids as flow enters the settling zone;
iv) ensure that solids entering the sludge zone are not re-suspended.
The efficiency of these structures basically depends on the ratio between the influent flow rate and the surface area of the tank; their design should be based on the settling velocities of the particles to be removed, a factor that should be assessed by qualified technicians and requires the collection of water samples and specialized laboratory equipment (See 12.8). The main tanks found in practice are shown in Fig. 30. Horizontal tanks are compact; sludge is removed from them under hydrostatic head. Circular tanks offer the advantage of simpler scraping mechanisms but are not so compact. The vertical flow tanks, like the one shown in the figure, operate with a sludge blanket which serves to strain out particles smaller than those that could be removed by sedimentation alone at the flow rates employed.
Fig. 30a Type of Sedimentation
Tanks (horizontal flow)
Fig. 30b Type of Sedimentation
Tanks (vertical flow)
Fig. 30c Type of Sedimentation
Tanks (radial
flow)
17. Filtration of suspensions through porous media, usually sand, is an important stage in the treatment of potable water to achieve final clarification. It follows the settlement process and, to a certain extent, could be considered complementary: the more effective the settlement, the less the filters have to do. It is the final stage in water clarification and unless clear groundwater is used, it should be regarded as essential. The process consists of passing the water through a bed of sand or any other suitable porous medium. The sand retains suspended matter while permitting the water to pass; if the process is effective, the filtrate should be clear and sparkling in appearance. There are limits to the capacity of filters to achieve this final degree of clarity; pre-treatment and sedimentation processes are used to improve the water quality to levels more easily handled by filters.
18. One of the most commonly found filters is the rapid gravity sand filter, it can handle low turbidity waters and for this reason is normally operated with coagulants and often follows settling basins. The Rapid pressure filter has many characteristics similar to those of the gravity type, but is enclosed in steel pressure vessels and is used where hydraulic conditions in the system make its adoption desirable; it equally depends on coagulants for its action, although it does not always follow settling basins. A refinement of the rapid gravity filters may be called the mixed media filter where media of different densities are used; as a result a very coarse upper layer of light weight material (pumice, anthracite) provides void space to store impurities removed from incoming water. The rapid filter requires, in general, a raw water input of fairly good quality and is therefore limited in its application to only very particular situations, which normally do not include emergency response.
19. The slow sand filter is a simple filtration device which is increasingly being used in refugee camps and rural areas in view of its simple operation and maintenance requirements. Its construction, also very simple, may be carried out using widely available materials; a medium coarse sand, similar to the one use for concrete mixtures, is often a good enough filtration medium. Filters may also be obtained in prefabricated versions ("filtration package kits") which have proven their value in many emergency refugee camps during the last decade. During the slow sand filtration process the water quality improves considerably not only in its physical characteristics but also due to the reduction of the number of micro-organisms (bacteria, viruses, cysts), the removal of colloidal matter and changes in its chemical characteristics. Bacteriological changes are due to the development of a thin and active layer of algae, plankton, bacteria and other forms of life on the surface of the sand bed called the schmutzdecke, where these micro-organisms break down organic matter. While rapid sand filters require cleaning by rather complicated backwashing operations, slow sand filters are cleaned by the relatively simple periodical removal of the top of the filter bed, including the schmutzdecke. The design of a slow sand filter is a complex engineering problem that should be left to specialists. Its capacity should be such that no serious water shortages occur at the camp at any time; the quality of the supplied water should under no circumstances deteriorate below safety limits (See 3.13; 8.7) and provisions should be made therefore to deal with possible future deterioration of the raw water quality (See 8.14), breakdowns of critical elements in the system and malfunctioning due to operational failure or unfavourable conditions (low temperatures do not allow slow sand filters to operate effectively) (See 11.9). The dimensions of the filter should be decided upon after its mode of operation and output have been established to achieve a filtration rate of about 0.1 m/h, bearing in mind that it is desirable that the filters are operated for part of the day at a so-called declining head filtration (which may be achieved by closing the raw water inlet valve at the end of the day's working shifts while keeping the filter outlet valve open). The use of at least two filters in any water supply system is recommended to maintain the supply of treated water even during the time one of the filters requires cleaning or another type of maintenance. Prefabricated filtration package kits are available on the international market which allow a quick and relatively easy installation of slow sand filtration plants even in remote locations. The most typical kit consists of two raw water storage tanks and two slow sand filtration tanks and may be fitted with adequate pumping sets, if needed. Both filters would function simultaneously except during the process of cleaning, when one unit may be left in operation while the other one is cleaned. Some of these filters are provided with a synthetic filter fabric which is located at the top of the filter bed and allows a quick cleaning process, since the need to scrape off the upper sand layer each time is eliminated. These kits do not normally provide the sand, which has to be obtained, washed and graded locally. Assembling this kit would require only a few hours and may be carried out by unskilled labour with minimal supervision.
Fig. 31 Slow Sand Filtration
Installations
20. Other types of sand filter include the packed drum filter that can be improvised if drums and sand are available and may be a very good way of providing limited quantities of safer water quickly to cover small water demands (at household or health post levels, for instance). In these filters, water passes down through sand on a 5 cm. layer of gravel and is drawn off at a rate that should not exceed 60 litres per hour for a 200 litre drum; infiltrated water equal to the amount drawn off is added to the top. The river bed filter consists of a well (See 6.18) or infiltration galleries (See 6.58) that may be constructed in permeable river beds and may be used to treat large amounts of water; they are likely to be difficult to construct.
21. Disinfection serves to destroy pathogenic organisms which may cause various types of water-borne diseases and it can be considered as the final stage in the water treatment process. Although water disinfection can be accomplished by the addition of certain chemicals, by ozone, by ultraviolet light or by boiling (See 8.4.) the vast majority of waterworks, including those for the supply of emergency refugee camps, use chlorine or chlorine compounds. Bleaching powder, also known as chlorinated lime, is a mixture of Calcium hydroxide, Calcium chloride and Calcium hypochlorite which may contain between 20% and 35% of available chlorine, i.e. 20-35 parts by weight of chlorine per 100 parts by weight of bleaching powder. Although bulky and relatively unstable, bleaching powder is easy to handle; it is sold in drums; once the drum is opened it loses its chlorine relatively quickly: if the container is opened once a day for 10 minutes it loses some 5% of its initial available chlorine over a span of 40 days, but if it is left open all the time for the same period almost 20% will be lost; chlorine solutions made from bleaching powder, may be stored in containers kept in the dark for periods not longer than ten days. The lime content of bleaching powder is insoluble and a solution should be well mixed and allowed to settle before dosing, to avoid clogging of valves or feed lines. If 2 kg of bleaching powder, with a 25% available chlorine, is mixed with 20 litres of water, it will result in a 2.5 % solution of chlorine. HTH (high test hypochlorite material) is easily available on the international market under different brand names and contains 60-70% available chlorine; it is granular, much more stable than bleaching powder (it deteriorates much less during storage) and due, to the fact that it is quite soluble, relatively clear solutions may be prepared if the concentration of the solution is kept below 5 % (the strength of the solution should be between 2% and 4%). If 0.84 kg HTH with 60% available chlorine is mixed with 20 litres of water, the result will be a 2.5% solution of chlorine (two drops of this solution may effectively disinfect one litre of relatively clean water and leave approximately 0.5 mg/l residual of chlorine; four or more drops may be needed for cloudy waters). Chlorine compounds should be stored in a dark, cool, dry and well ventilated place in closed containers resistant to corrosion (IATA's air transport regulations for corrosive and toxic substances require special containers; these are the most desirable containers for storage in any given circumstance); chlorine gas is poisonous and may provoke fire or explosions if present in high concentrations, due to exothermic chemical reactions.
22. When added to water, chlorine reacts to form hypochlorous acid and hypochlorite. These two compound together represent the "free available chlorine" and are a powerful bactericide; if ammonia is present in the water chloramines will be formed, the type of which depends on the water's pH and its ammonia concentration. Chloramines are also powerful bactericides. At normal pH values (5-8), the total quantity of chloramines is known as the "combined available chlorine".
23. Because chlorine is an oxidizing agent, part of the chlorine applied will be used by other constituents of the water (Chlorine demand); enough chlorine must therefore be applied for reaction with such constituents and the pathogenic organisms (See 8.13). That is why chlorination should normally be done after the water has undergone other treatment processes such as sedimentation and filtration, to ensure minimum use of chlorine by anything other than bacteria.
24. Care must be taken to ensure strict control of chlorination processes and, particularly, to test the water for chemical residual levels after each disinfection and before distribution. Chlorine residual must be measured only after an appropriate contact time. After chlorination, and once chlorine has reacted, oxydizing the other constituents of the water (30 minutes are considered appropriate), there should still be at least 0.5 parts per million (or mg./l) of "free available chlorine" left in solution. The amount of chlorine required to achieve this concentration is usually a broad indication of the level of pollution of the water. If the amount of free available chlorine is higher than 1.0 mg./l, people may reject the water because of its unpleasant taste. A pocket size chloroscope (chlorine comparator kit, preferably of the "DPD" type) is required to test for residual chlorine levels; it consists of two tubes, each containing a measured quantity of the water under test, which can be compared by eye for colour. One of the two tube samples is coloured by the addition of a chlorine sensitive reagent (o-toludine, a common reagent, should be avoided, as it decomposes in hot climates; it is also a poor indicator if water has been over-chlorinated), the other by a range of standard glass slides; the chlorine concentration can be read off directly after matching the colour of the tube with the added reagent with that of the nearest standard. This test is simple and all treatment plant attendants should be trained to use it frequently to check the water quality; any water leaving the plant with a residual chlorine content of 0.4 mg./l of free residual chlorine can be regarded as safe. The dosage of chlorine should be of constant concern; no water should normally be distributed when chlorination equipment is not working (chlorination equipment should be fully duplicated in any water treatment plant).
25. As it has been previously suggested, the treatment of water in emergency refugee situations should be kept to the minimum required to ensure its safety. When refugees are living in rural environments, where the main water sources are dug wells or spring catchments, efforts should be directed to clean, disinfect and to protect these installations from further pollution since the onset of the emergency and to continue monitoring water quality to ensure the effectiveness of these protective measures; further source disinfection campaigns may be necessary in the long term. In other situations, where refugees are living in large concentrations, in refugee camps or mixed with the local population in villages or towns, regular water disinfection should be regarded as strictly necessary; other simple treatment measures should be carried out if the quality of the raw water supply would require them as a way of to ensuring the effectiveness of disinfection (See 8.11 and 8.23). In these cases, processes to be used would normally include slow sand filtration and even pre-treatment processes, necessary for the due functioning of the sand filters; the aim of these treatment processes is that of improving the physical and bacteriological quality of the drinking water. The improvement of the chemical characteristics of raw water should, however, be decided only after careful consideration of potential health hazards and other risks involved in the provision of drinking water with questionable chemical components, combined with an analysis of alternative solutions, capital costs and long-term operation and maintenance requirements. This exercise would not normally be possible during emergency refugee situations and should wait until the emergency is over, when preparations for longer term care and maintenance activities are under way (See 12.20). From a health point of view this approach should not normally present major problems, as most chemical-related water quality problems do not cause serious health hazards if the water is consumed during short periods (real refugee emergencies are normally short in duration). In the case where a refugee situation evolves, which makes it necessary to prolong care and maintenance assistance programmes for an undefined period, problematic chemical characteristics should have already been recognized and, if important enough, should be addressed in a technically sound and cost-effective manner. In this case, expert advice (which should include expertise in the fields of Public Health, Environmental Health and Engineering) should be sought. Among those processes normally used to improve chemical water quality, water softening, the removal of Iron or Manganese, the control of fluorides or nitrates and the removal of detergents and pesticides should be mentioned. Water desalinization practices are, by their nature and high costs, out of reach of refugee assistance programmes and should be fully discouraged under all circumstances; the search for alternative water sources should be the only solution to high salinity in refugee water supplies.
26. The wastes of water treatment plants normally used during refugee emergencies are mainly heavy sludges consisting of highly concentrated suspensions of solids in a liquid which may or may not have chemicals, depending on the type of treatment plant. Sludges without chemicals come from primary settling tanks, roughing filters (See 8.14) and sand washers attached to slow sand filtration plants (See 8.19). Total daily volumes of sludge in treatment plants may normally be 5 % of the daily plant's throughput. This sludge is inoffensive and may be returned to the river with no treatment, if the river is large enough. If the river is small, it may be necessary to dry in special drying beds before transporting it to the final dump site, which could be a land-fill site or a rubbish dump. If chemicals are present in sludge coming, for example, from sedimentation basins where coagulation takes place. (See 8.15-16), its collection, treatment and disposal become more problematic; dewatering is more difficult, recovery of chemicals is not cost-effective and the disposal of partially treated sludge may create big nuisances. If allowed to accumulate, the sludge putrefies; this process occurs very quickly in warm climates. Although "lagooning" is a traditional method of sludge treatment, sludge lagoons are complicated to build and to operate; they require large plots of land, and the end product is a very sticky stuff that should be picked up and carted away to a dump site. The use of concentration tanks and drying beds is also common. This is normally done at two settling basins (one being filled and the other one being cleaned at any given moment); the thickened sludge is transported (sometimes by pumping) to drying beds similar to lagoons but with permeable sand and gravel bottoms for efficient drainage and, when dry, it is picked up and transported away to dump sites. Choosing the right site for a sludge dump requires the same care and considerations as if it was for rubbish, in view of the need to avoid contamination of surface or groundwater. Contrary to a common belief, these sludges have no manurial value.
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