· All refugee sites must be provided as soon as possible with adequate water storage facilities.
· Water storage may be the only means of ensuring a constant availability of water to cover the needs of a camp population at a given site and therefore could become the main source of supply.
· The use of local technology for the design and construction of storage tanks or reservoirs should always be pursued. In many refugee emergencies, however, the use of prefabricated tanks may be the only wau of ensuring the availability of water where needed in the quickest way.
· As storage tanks are a main component of water supply systems, their design should satisfy all the system's technical requirements.
1. In nearly all water supply systems it will be necessary to store water between the source and distribution points. Substantial water storage may be needed and will always be an advantage in monitoring, collecting, treating (See 9.8.10) and distributing safe water as well as for the provision of a reserve to meet the various needs during emergency and long-term use.
2. In any given situation, storage tanks may be located in four different locations within the water supply system:
i) At the water collection point (raw water tanks at surface water intakes, run-off water collection and storage facilities such as "birkas" or haffirs, at rainfall water collection points, etc.)
ii) At central storage tanks, before or after treatment, to balance the supply from the sources with the needs and in many cases, to provide the system with enough hydraulic head to allow for gravity-fed distribution (See 6.1; 10.16).
iii) At distribution points, which may include public standposts, other service points (health or feeding centres, camp administration facilities, and sometimes at staff houses).
iv) At refugee household level. At this level, use is normally made of small containers; in this case, an effort should be made to ensure a clear distinction between the containers used to obtain and transport water form distribution points and those used for storage (See 4.5; 10.9).
3. Whatever type of storage is needed, adequate enclosure should be provided to prevent any contamination from humans, animals, dust or from any other source. A tight cover and dark storage also prevent algal growth and the breeding of mosquito larvae.
4. Under certain circumstances, notably in areas with pronounced dry and rainy seasons, and where alternative sources are limited, the construction of reservoirs to collect water to be used during the dry season may be an option, despite the dangers of pollution and of mosquito breeding. To locate the right site for these type of structures requires a good knowledge of regional environmental conditions and local technological approaches; considerable engineering experience is also needed for the design of the most appropriate structures. An erosion-protected overflow spillway should always be provided to allow for the evacuation of excess water. Enough attention should also be given to the need to control excessive weed growth on the banks of water ponds, haffirs or valley dams. Water losses in open ponds due to evaporation are considerable and efforts should be made to minimize this: well-located wind-breaks will prove useful for this purpose. With time, the loss of storage capacity due to siltation is also important; silt-traps in their inlet structures will lower siltation rates and facilitate maintenance. Water quality in these ponds degrades very easily; fencing-off the reservoirs to avoid access by people and animals should be regarded as strictly necessary; pumping and pipeline facilities to transport water to conveniently-located service tanks and distribution points should, therefore, be required.
5. Service tanks are used to store water which is ready for distribution. Their size and location should be decided by experienced engineers based on the location of highest consumption points and the overall distribution network, the topography of the terrain and its ability to provide support to the tank's foundations as well as on the technique, materials and design to be used for their construction. Although brick and stone masonry tanks are most appropriate for larger storage volumes, they may be difficult and time consuming to build under certain emergency conditions. Reinforced concrete tanks are common in many areas and have the advantage of being possible to build of virtually any size; while they are very durable, their construction is time consuming and may not be the right solution for water storage needs during refugee emergencies. A number of types of simple, air-portable, plastic or butyl rubber storage tanks (known as pillow tanks, onion tanks or bladder tanks, depending on their design and shape) are available and can be speedily supplied to any given location to meet the most urgent storage requirements during emergencies. Metal storage tanks may be made from different materials, the most common being galvanized iron sheets; their size is limited by the tendency of the material to deform unless a reinforcement framework, made out of wood or steel, is incorporated into their design. The use of corrugated metal sheets makes it possible to construct self-supporting prefabricated structures which are easy to transport, erect and commission to respond to emergency needs; some of these tanks are supplied in kit form, and may be obtained in different sizes (10, 45,70 and 95 cubic metres) to meet different storage needs. The kit may contain other fittings and material to allow for appropriate pipeline connections and for water chlorination. The use of pre-fabricated fibreglass tanks may be advisable during refugee emergencies, especially when water has been found to be corrosive.
6. The choice of tank type and design should always be entrusted to an experienced engineer. Several features, however, should always be present in storage tanks to allow for their fullest and safer utilization in a water supply system. Their foundations and structure should be sound as water is heavy and even the smallest structural weakness would cause the tank to either leak or completely fail. All tanks should be provided with an outlet hole situated some 20 cm from the bottom; an overflow and vent pipe (with an appropriate screen to avoid the entrance of small animals) and the inlet pipe should always be at the top and at the opposite end to the outlet, to allow for water mixing and aeration; the tank's bottom should slope towards its lowest point, where a drain should be installed for cleaning and flushing the tank. The drain's outlet should be piped away from the tank to avoid the creation of unsanitary conditions around the tank or the destabilization of the tank's foundations by excess water. A manhole, with an appropriate cover and ladders, should be provided to allow access to the interior for cleaning and inspection purposes; storage tanks should be fenced-off to avoid free access to people, and in many cases, there could be a need to have them guarded to protect the structures and pipe-work from vandalism or to avoid theft of water (See Fig 32). Elevated tanks (water towers) are mainly used to gain the necessary pressure head to allow an efficient gravity flow into the distribution network, to stabilize pressures within the system and to facilitate meeting fluctuations in water demand; if the topography of the camp and its surroundings is adequate (hills of adequate elevation), surface reservoirs should be preferred for this purpose as the economic limit of height and volume of water towers is determined when the supporting structure for an elevated tank becomes more expensive than alternative conduction pipelines.
Fig. 32 Details of a Reservoir Tank
7. It has been previously suggested that water supply systems should be provided with maximum storage capacities. Practical limits, some of them technical, but also budgetary ones, should, however, be taken into account when planning this important feature of any water supply system. Among the most important technical aspects to bear in mind are the dependability of the source and the possible fluctuations of its output, which may be seasonal (See 6.20) or related to other causes such as well interference (See 6.27; 6.38; 6.55) or well-efficiency (See 6.41). The possibility of conveying water from the storage tanks to distribution points should also be considered, as the need to use towers to elevate tanks for the obtention of the necessary hydraulic heads to achieve this would, of course, limit the size of the tanks that may be built with the available funds (water towers should be used only when strictly required as they are expensive and difficult to construct and maintain) (See 9.6). If the population to be supplied is small (say smaller than 2000 people), the aim should be to store a volume equal to at least one day's water demand (See 3.1-9). For economic reasons, larger camp populations will have less storage capacity but, under no circumstances should this capacity be smaller than 1/6 of the camp's daily water demand. In camps with a population of more than 5000 people, the total storage capacity could be obtained through a battery of smaller (and less expensive) reservoirs, strategically located to facilitate the construction of a hydraulically efficient and well balanced water distribution network to provide an even coverage of the total camp population.
8. It is always advisable to provide individual storage facilities to service centres (such as health or supplementary feeding facilities, administrative buildings and even staff houses). Special circumstances (distribution points located along gravity-fed mains, for instance) could make it advisable to provide individual storage facilities for each public distribution standpost. In these cases the aim should be to have a volume of storage equal to the daily water consumption at each of the individual water points. For this purpose, the use of properly adapted oil drums, metallic, rubber or fibreglass tanks may be considered. When water is being conveyed to the camp by water tanker (bowser), care should be taken to avoid water pollution and waste when filling the tanks. As soon as the piped distribution system is operational, these tanks may be connected to it, after the installation of appropriate float valves to avoid overflows and waste. These individual tanks should be located high enough to ensure an efficient gravity flow and other head requirements (See 10.9) to all taps. In view of complicated operation and maintenance requirements of this type of systems, their use should be restricted to only very special cases.
9. Average size refugee households (5 to 7 people) should be able to store at least 20 litres of water at any given time. However, the ultimate goal of providing a storage capacity of at least 10 litres per person should be clear at the onset of emergency assistance operations (See 4.5). As household storage containers should not be used to transport water from watering points, enough containers to perform this task should also be provided. The best type of storage containers are narrow neck water bottles or jerrycans; they should have a lid. Water pollution is more difficult to avoid with open containers such as buckets or saucepans; their use for household water storage should, therefore, be discouraged. It is advisable to disinfect household storage containers at least once a week, as silt or other materials may collect in them.