N2O emission from a partial nitrification-anammox process and identification of a key biological process of N2O emission from anammox granules

Emission of nitrous oxide (N₂O) during biological wastewater treatment is of growing concern. The emission of N₂O from a lab-scale two-reactor partial nitrification (PN)-anammox reactor was therefore determined in this study. The average emission of N₂O 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 N₂O was emitted from the PN reactor. The total amount of N₂O emission from the PN reactor was correlated to nitrite (NO₂⁻) 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 N₂O emission from the anammox process (i.e., granules). In order to characterize N₂O emission from the anammox granules, the in situ N₂O 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 N₂O production zone was located in the inner part of the anammox granule, whereas the active ammonium consumption zone was located above the N₂O 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 N₂O 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 N₂O emission from the anammox reactor (i.e., granules).