Presence of algae in our sand filters.
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Potassium permanganate preoxidation may induce this brownish deposit on the surface of the sand filter. In that case, it is normal and even a good sign. This is probably linked to an increase in pre-oxidation and to a pH above 7 in the filters. If it were algae, it would quickly clog the filter. Covering the decanter and filters would then be the best solution
Basically, the algae come from the raw water (stopped by filtration; the growth of algae in the filter is pretty minimal, except obviously on the surface because exposed to light). When there is low turbidity and color, it means there is a lot of algae. Therefore, large quantities of algae present in the filters usually results in green water during washing and not brown. You mention "brown micro-algae"; is that a hypothesis or an observation? The most probable cause here is indeed precipitation, i.e., flocculation of manganese oxide generates small floc but can have a certain binding power because the floc is caught between the grains of sand. It is rare, however, except when there are large concentrations of Mn in the raw water and/or a high dose of permanganate
Another possibility is a polymer residual too high resulting from dosage too high. This is a frequent cause of the clogging of filters at the surface. Obviously, there would be an interaction between the polymer and any MnO2 floc, which makes the polymer less sticky but the floc more consistent and therefore more easily trapped in the first few inches of the filter.