Slow sand filtration is an established technique for the treatment of drinking water. However, clogging of these filters requires extensive maintenance. The clogging and hydraulic characteristics of slow sand filters operated under high flow rates were investigated in a drinking water plant that processes pre-treated lake water. Reasons for the clogging were evaluated by measuring physical, chemical and biological parameters of the interstitial water and the filter matrix. The biomass in the filters was characterised by quantifying bacterial abundance and activity as well as the concentration of extracellular polymeric substances (EPS). The results of this study showed that the clogging effects were to a large extent attributed to the presence of EPS. This microbial biomass reduced the pore space in the highly clogged parts of the filters by at least 7%, whereas the reduction due to particle deposition was not larger than 7%. Although the most severe clogging occurred in the top 5–10 cm of the filters where bacterial abundance and activity were highest, deeper layers of the filters were clogged, too.

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