Three fixed bed downflow biological aerated filters were operated at the rates of 5, 10 and 20 m/day under tertiary treatment conditions. A high NH4+−N oxidation rate of 1.75 kg−N/m3 /day was achieved with an effective residence time of 15 minutes. The biofilm biomass density increased with increasing load while the density gradient along the filter bed became narrower with higher filtration rate. Specific biological activity was inversely proportional to the biomass density. Specific overall biological activity and specific nitrifying activity were found to be higher towards the effluent end of filters operated at the rates of 20 and 10 m/day whereas the highest specific nitrification activity was found to be at the feed end of the slowest filter. Introduction of easily biodegradable organic substrate affected the nitrification activity in the slower filters. The above findings suggest that the location of concentrated nitrification activity is filtration rate dependent. Presence of nitrifiers close to the feed end in a fixed bed downflow filter could be a disadvantage in the presence of faster growing heterotrophic activity under influent fluctuating conditions.

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