Effect of COD/N ratio on nitrogen removal and microbial communities of CANON process in membrane bioreactors

• COD/N in low ratio could improve nitrogen removal of CANON.
• The suppressing threshold of COD/N ratio on CANON was 1.7.
• Biodiversity of AOB and AAOB both decreased with COD increasing.
• Denitrifiers and AAOB showed different relationship under different COD/N ratio.
• Strategies for treating sewage with different COD/N ratio were proposed.

In this study, the effect of COD/N ratio on completely autotrophic nitrogen removal over nitrite (CANON) process was investigated in five identical membrane bioreactors. The five reactors were simultaneously seeded for 1 L CANON sludge and be operated for more than two months under same conditions, with influent COD/N ratio of 0, 0.5, 1, 2 and 4, respectively. DGGE was used to analyze the microbial communities of aerobic ammonia-oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing bacteria (AAOB) in five reactors. Results revealed the harmonious work of CANON and denitrification with low COD concentration, whereas too high COD concentration suppressed both AOB and AAOB. AOB and AAOB biodiversity both decreased with COD increasing, which then led to worse nitrogen removal. The suppressing threshold of COD/N ratio for CANON was 1.7. CANON was feasible for treating low COD/N sewage, while the high sewage should be converted by anaerobic biogas producing process in advance.