In situ enrichment of anammox bacteria in anoxic biofilms are possible due to the stable and long-term accumulation of nitrite during denitrification

In situ enrichment of anammox bacteria in anoxic biofilms has been observed, but the specific conditions for anammox competition with denitrification for nitrite are not yet fully understood. Therefore, an anoxic sequencing batch biofilm reactor (SBBR) was used to investigate nitrite production during denitrification. In each SBBR cycle, with nearly 80% of nitrate reduced in 3 h, over 9.7 mg/L nitrite was gradually accumulated and maintained for a long time, despite temperatures gradually decreasing from 32 to 5 degrees C. The long-term existence of nitrite was due to the low biofilm nitrite reduction rate (1.2 mgN gVSS(-1) h(-1)), which was about 10-fold less than the nitrate reduction rate. Accordingly, nitrite reduction via denitrifiers was continuously suppressed, which was favorable for nitrite reduction through the anammox pathway. Indeed, anammox bacteria were successfully enriched here (Candidatus Brocadia, 0.1%). This study confirms the potential of anoxic biofilm in enriching anammox bacteria and provides insight into understanding.