The types of algae and the concentration in wastewater depend on residence time, climate and weather, amount of pollutants entering the pond, and dimensions of the pond. Normally, small unicellular types of algae develop first, e.g., Chlorella. Because of their physical dimensions they are difficult to remove by the processes listed in Table 8.1.1. Longer residence times lead to the development of larger algae and other plankton, which is more readily removed. The algae concentration affects the choice of removal process and the rate of treatment. Because of their light density, the dried weight of suspended solids is not an efficient measure of concentration. Algae are normally measured in volumetric or areal standard units (Anon. 1971). In surface water supplies, concentrations may be as high as 30,000 cells per milliliter (ml), this can be much higher in nutrient-rich waste treatment effluents. A combination of processes may be the best treatment, e.g., copper sulfate addition and microstraining, as used on surface water supplies in London, England.
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