81

6.5 Storage and sedimentation1

6.5.1 The technologyThe quality of raw water can be improvedconsiderably by storage. During storage,non-colloidal, suspended particles slowlysettle to the bottom of a storage tank,and solar radiation will kill some of theharmful organisms in the water. Schisto-soma larvae, for example, will die afterstorage for at least 48 hours. In contrast,colloidal particles remain in suspension.The smaller the suspended particles, thelonger the water needs to be retained inthe reservoir. If the suspended matterprecipitates very slowly, chemicals can be added to induce coagulation and flocculation.The reservoir can be constructed in several ways:

below ground level, with a lining of plastic sheeting to separate the stored waterfrom the ground;

with a lining of loam, clay or concrete; entirely from brick or concrete.

Reservoirs for sedimentation usually have two separate sections. While one is in use, theother can be cleaned. They have an intake on one side of the reservoir (or at the bot-tom), an outlet on the opposite side just beneath the water level, and a bottom outlet toflush the deposited material.

When the water quantity or quality at the source is temporarily low, a large storagereservoir can also provide an alternative temporary source of water.

Initial cost: Depends on the type of construction.

Range of depth: Usually, 0.72.0 m.

Treatment time: A few hours to several days.

Area of use: Wherever raw water contains high concentrations of suspended solids, orwhere the quality or quantity of the water at the source varies considerably.

6.5.2 Main O&M activitiesUsually, water will be let in to the storage reservoir every day or continuously, but whenthe water quality becomes too poor and there is sufficient water stored in the reservoir,the water intake may be stopped temporarily. The reservoir will have to be flushed regu-larly to remove the deposited silt the frequency for this will depend on the silt contentof the water and the reservoir depth. All valves in the system must be opened and closedat least once every two months to keep them from becoming stuck. Occasionally, thevalves may need to be repaired or replaced, and leaks in the reservoir will have to befixed. Apart from some help from the water users to clean the reservoir after it has beenflushed, the system requires little support from an established organization to maintainit.

6. WATER TREATMENT

Figure 6.5 Storage and sedimentation

1 Water Research Centre and WHO Regional Office for Europe (1989).