Elaboration of nitrogen loss pathways for full-cycle partial nitritation in household toilet wastewater based on empirical model establishment

Nitrogen loss (N loss) has been found in the Partial Nitritation (PN) reaction through the pathways of autotrophic/heterotrophic denitrification and oxidation of NH2OH, possibly causing the generation of greenhouse gas N2O. The pathway of N loss depends on the feedstock characteristics, the PN types, and the operating parameters. The N loss patterns of the PN reactor for treating the household toilet wastewater (TW) have not been clearly investigated. In this study, by conducting the batch tests on four working parameters that may affect the N loss from the PN reaction of TW, the N loss patterns were found. During the whole reaction cycle from the beginning of low dissolved oxygen (DO) to the end of high free nitrous acid, the N loss percentage was generally 14.2–21.8 %. Accounting for 70 % of the full-cycle N loss occurred in the low-DO period mostly by the heterotrophic denitrification pathway, with the remaining 30 % in the high-DO period by direct conversion of NH2OH to N2O. The effects of temperature and initial C/N on the amount of N loss was more pronounced among the four studied factors and the latter was more sensitive. By integrating the quantitative influence of temperature and initial C/N, an empirical equation was obtained and fitted to the N loss from a practical PN reactor treating the household TW. The establishment of this model will deepen the understanding of peer researchers on this kind of PN’s N loss patterns.