Pre-reservoirs are small reservoirs, with a water-retention time of a few days, that reduce the phosphorus input in main reservoirs. The process of phosphorus removal involves the biochemical conversion from the dissolved to the particulate form (mainly phytoplankton) and the sedimentation of this particulate matter. The input variables are light, orthophosphate concentration, temperature of the inflowing water and discharge. The phytoplankton activity plays the most important role among the various processes. The maximization of orthophosphate elimination depends on adequate design, construction and operation of pre-reservoirs. A simple calculation procedure for the removal rates of orthophosphate-P has been developed. The efficiency of pre-reservoirs is limited, because the light intensity and the temperature during the winter period are low; on the other hand, discharge is often high in March and April (snowmelt). Although pre-reservoirs are an important tool for reservoir water-quality management, but they cannot substitute remedial action in the catchment area.

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