N 2O emission from a partial nitrification–anammox process and identification of a key biological process of N 2O emission from anammox granules

Abstract

Emission of nitrous oxide (N 2O) during biological wastewater treatment is of growing concern. The emission of N 2O from a lab-scale two-reactor partial nitrification (PN)–anammox reactor was therefore determined in this study. The average emission of N 2O from the PN and anammox process was 4.0 ± 1.5% (9.6 ± 3.2% of the removed nitrogen) and 0.1 ± 0.07% (0.14 ± 0.09% of the removed nitrogen) of the incoming nitrogen load, respectively. Thus, a larger part (97.5%) of N 2O was emitted from the PN reactor. The total amount of N 2O emission from the PN reactor was correlated to nitrite (NO 2 −) concentration in the PN effluent rather than DO concentration. In addition, further studies were performed to indentify a key biological process that is responsible for N 2O emission from the anammox process (i.e., granules). In order to characterize N 2O emission from the anammox granules, the in situ N 2O production rate was determined by using microelectrodes for the first time, which was related to the spatial organization of microbial community of the granule as determined by fluorescence in situ hybridization (FISH). Microelectrode measurement revealed that the active N 2O production zone was located in the inner part of the anammox granule, whereas the active ammonium consumption zone was located above the N 2O production zone. Anammox bacteria were present throughout the granule, whereas ammonium-oxidizing bacteria (AOB) were restricted to only the granule surface. In addition, addition of penicillin G that inhibits most of the heterotrophic denitrifiers and AOB completely inhibited N 2O production in batch experiments. Based on these results obtained, denitrification by putative heterotrophic denitrifiers present in the inner part of the granule was considered the most probable cause of N 2O emission from the anammox reactor (i.e., granules).