| Summary: | <i>Schmutzdecke</i>, the biofilm formed on the top of the sand bed in household slow sand filters (HSSF) is a key factor for the filters’ high efficiency in removing particles and microorganisms from water. This paper aims to investigate the extracellular polymeric substances composition (carbohydrates and proteins), biomass, dissolved oxygen, and microbial community in two types of HSSFs and identify a correlation between them and their efficiency. A continuous- and an intermittent-HSSF (C-HSSF and I-HSSF) were studied to treat river water for 48 days. Their efficiencies for bacteria (<i>E. coli</i> and total coliforms), turbidity, and apparent color removals were analyzed. Results clearly showed an increase of carbohydrates (from 21.4/22.5 to 101.2/93.9 mg·g<sup>−1</sup> for C-/I-HSSF) and proteins (from 34.9 to 217/307.8 mg g<sup>−1</sup> for C-/I-HSSF), total solids (from 0.03/<0.03 to 0.11/0.19 g L<sup>−1</sup> for C-/I-HSSF), dissolved oxygen depletion inside the filter (6.00 and 5.15 mg L<sup>−1</sup> for C- and I-HSSF) and diversity of microorganisms over time, pointing out the <i>schmutzdecke</i> development. A clear improvement on the HSSFs’ efficiency was observed during operation, i.e., <i>E. coli</i> removal of 3.23 log and 2.98 log for total coliforms, turbidity from 60 to 95%, and apparent color from 50 to 90%.
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