ABSTRACT
The process is generally used before rapid filtration as it will reduce the load on the filters. It is also utilized in the removal of precipitates produced as a result of softening lowland waters.
Raw water storage reservoirs are often considered the first line of defence against the transmission of waterborne diseases. Periods of storage can be up to several months during which time counts of enteric bacteria, typhoid and other pathogenic organisms fall to a significant extent. This is partially due to sedimentation, partially due to the lethal effects of both ultraviolet and visible light and partially due to the effects of predation. If large storage reservoirs are used in areas where the rivers acting as sources of abstraction receive sewage or other effluents then should the river become polluted the reservoir can maintain the volume of supply during the time it takes the river to recover. One disadvantage of storage reservoirs is the opportunity presented for the excessive growth of algae (see Chapter 18).
