raw water storageReading time:
Storing raw water can be of interest in the event of a prolonged drought (lowered river flow often accompanied by a change in water quality). The amount of raw water stored must be sufficient to meet the demand for water for the longest drought period forecast.
Storage is also of value when there is a danger of accidental pollution capable of deteriorating the raw water quality to a point where it is unacceptable to the treatment plant. In such a case, pumping from the surface water source can be suspended and the stored water supply used, thereby preventing any interruption in production. The storage volume will then be determined based on the pollution risks upstream from the raw water intake and upon the maximum time that direct raw water pumping may be expected to be suspended.
Finally, storage is of interest when the surface water properties undergo rapid and frequent variations (suspended solids, NH4+…) that call for adjustments to operating conditions that are not always compatible with reality.
However, when geographic and climactic conditions are favorable to planktonic life, storing raw water does have certain drawbacks. When storage time is short (a few days), algae and actinomycetes may proliferate and their metabolites can impart an unpleasant taste and/or smell to the water, and removing these can prove very expensive. When retention time in the storage tank is longer (1 month), a zooplankton population may develop that is capable of reducing some of these inconveniences. Accordingly, some water properties may improve: lowered suspended solids, reduced ammonia concentration and less bacterial contamination.
Raw water storage requires an extensive footprint and this will be costly, perhaps even unfeasible in urban environments; additionally, it may be necessary to clean the storage tank at regular intervals.
Finally, the construction of large water storage facilities (reservoirs…) calls for special precautions when filling with water in order to avoid any sudden eutrophication phenomena: all vegetation must be removed and burned outside the filling area, all topsoil must be stripped from the area together with any possible pre-existing contaminant deposits (landfill sites, chemical disposal sites…).