Particle removal is one of the main objectives of rapid sand filters. The particles to be removed are considerably different regarding size, shape and nature. They are either imported from the catchment area or formed in the reservoir. With the exception of storm events in central Europe the latter are in the majority of cases the most important ones. These are different plankton organisms comprising phyto- and zoo-plankton as well as bacteria and protozoa. Other particles are formed by the precipitation of manganese, iron or calcium. There exists a great variety of methods for the quantification of these particles in raw and filtered water, the most important being turbidity measurement, particle counting, microscopic investigation, counting CFUs as well as SON (suspended organic nitrogen) and chlorophyll analysis. Which method is to be preferred depends on the local conditions and on the problem to be investigated.

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