Nitrogen removal crash of denitrification in anaerobic biofilm reactor due to dissimilatory nitrate reduction to ammonium (DNRA) for tofu processing wastewater treatment: Based on microbial community and functional genes

Nitrogen removal crash of denitrification in an anaerobic biofilm reactor (ABR) occurred during tofu processing wastewater treatment. This study discovered that the crash occurred due to dissimilatory nitrate reduction to ammonium (DNRA) based on the microbial community and their functional genes. Metagenomic analysis and enzyme activity measurements were carried out to identify variations of microbial communities and their functional genes encoded for denitrification and DNRA. Total nitrogen removal efficiency of the ABR was maintained at approximately 78 +/- 2 % during Stage 1 but decreased from 82 to 52 % during Stage 2 (nitrogen removal crash phase). Moreover, NO2--N and NH4+-N increased during this crash phase with stable NO3--N removal rate (86 +/- 2 %), indicating DNRA in the ABR. Functional gene analysis found a decrease of bacteria with nitrite reductase and nitric oxide reductase, especially NirK and NorC, excluding microorganisms, containing nitrous oxide reductase, variation during the ABR operation. Most DNRA bacteria increased and replaced denitrifies as the dominant nitrate-reducing microorganisms in Stage 2. Moreover, the activities of denitrification functional enzymes, especially nitrite reductase and nitric oxide reductase, decreased from Stage 1 to Stage 2, while the DNRA nitrite reductase showed a noticeable increase. During stages 1 and 2, the tofu processing wastewater contained a high COD:TN ratio of 7.3-9.1, directly inhibiting denitrification and promoting DNRA. The COD:TN ratio reduction in Stage 3 led to denitrification recovery and DNRA inhibition. Therefore, the nitrogen removal crash in the ABR was due to DNRA, which resulted from the high COD:TN ratio.