Spatial distribution of heterotrophs and nitrifiers in a submerged biofilter for nitrification

The present paper describes the changes in biofilm density and specific activities of carbon, ammonia and nitrite oxidizers within a nitrifying up-flow biological aerated filter (UBAF) as a function of the C:N ratio in the wastewater entering the filter. The filter was initially fed for several months with synthetic wastewater containing only ammonia and micro-nutrients and no external organic carbon source which favored the filter colonization by a fairly stable nitrifying biofilm. In the experimentation conditions the reactor resisted the entrance of up 200 mg COD/l without losing nitrification efficiency. This limit corresponds to a COD:NH 4 +-N ratio of four. Over 200 mg COD/l entering the reactor nitrification could no longer reach 100%. For COD:NH 4 +-N ratios higher than 4 it appeared to be two different zones within the filter. One part located at the entrance where organic matter and ammonia entering the filter are removed at rates of 3.85 kg TOC/m 3 day and 0.19 kg N/m 3 day, respectively. The second part was characterized for showing removal rates of 0.42 kg TOC/m 3 day and 0.96 kg N/m 3 day. A valley in the dissolved oxygen concentration is revealed as a practical indication of the frontier between the two zones. The spatial distribution of heterotrophic and nitrifying populations was quantified in terms of oxygen uptake rates (OUR) or specific activities at different filter heights and for increasing COD concentrations entering the reactor. Specific activities of three microbial groups, ammonia oxidizers, nitrite oxidizers and heterotrophs aerobes revealed a clear microbial segregation along the filter depending on the COD concentration entering the reactor.