Mechanisms of NO and NO production by enriched nitrifying sludge in a sequencing batch reactor: Effects of hydroxylamine.

NO and NO as important greenhouse gases andtheir production mechanisms during nitrification are not completely understood. This study aimed to analyze the effect of hydroxylamine (NHOH) on NO and NO produced by nitrifying bacteria from activated sludge in a sequencing batch reactor (SBR). Experimental results showed that when nitrite (NO) accumulated during aerobic ammonia (NH) oxidation, NO was the main product. The total amount of NO and NO produced by NHOH oxidation was positively correlated with dissolved oxygen (DO) levels. The imbalance of NH oxidation caused by NHOH addition was more conducive to the generation of NO and NO under high DO conditions. When NHOH was added into the reactor with NO as the substrate, the production of NO and NO under high DO levels was mainly related to NHOH oxidation. Under low DO conditions, NO and NO from the biotic/abiotic hybrid pathways were more significant in the reactor of the coexistence of NO and NHOH, which could be mainly caused by the pathways of nitrifier denitrification and abiotic reaction. Besides, limited amount of NO and NO was generated by heterotrophic denitrification pathway during autotrophic nitrification. The implications for the above results are important for understanding the production of NO and NO under NHOH stress in nitrifying sludge reactor.