Biofilters for secondary treatment, tertiary nitrification and tertiary denitrification were investigated with regard to solids and gas accumulation. Biomass growth, solids capture by filtration and gas entrapment were quantified for the three different applications. In biofilters for COD-removal or denitrification heterotrophic growth dominates solids accumulation. In tertiary nitrifying biofilters the biomass remaining attached to the media after backwash and suspended solids deposits are the main fractions responsible for filter clogging. Depth profiles of solids accumulation indicate that top layers cause the main headloss in biofilters. As a consequence of solids accumulation increasing gas entrapment in countercurrent and denitrifying biofilters lead to a sudden headloss increase and terminate the filter runs.

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