Long-term partial nitritation and microbial characteristics in treating low C/N ratio domestic wastewater†
Simultaneous anammox–denitrification process is capable of complete nitrogen removal. To acquire a suitable influent (NO2−-N/NH4+-N > 1.32) for it, the wastewater could be treated by partially oxidizing ammonium to nitrite. Although many studies regarding partial nitritation in treating high ammonium wastewater have been reported, very little research has been undertaken on real domestic wastewater for long term. This study presents a partial nitritation–sequencing batch reactor (PN–SBR) to treat domestic wastewater with low SCOD/TN ratios (2.3 ± 0.6). During the 600 days' operation, the NO2−-N/NH4+-N ratios in the effluent mostly varied between 1.32 and 10.07 and thus, stable partial nitritation was maintained. Moreover, during each PN–SBR cycle, ∼47.6% of organics in raw wastewater were efficiently utilized for denitrifying nitrite, by which the aeration for the oxidation of organic matters was decreased, and stable nitritation was further assured due to the lack of substrate for nitrite oxidizing bacteria. High-throughput sequencing analysis revealed that the dominant nitrifying bacteria were Nitrosomonas belonging to ammonia oxidizing bacteria, while no nitrite oxidizing bacteria were detected. During the idle period of a PN–SBR cycle, the obvious nitrite decrease without observed COD consumption might be due to the predominance of Ottowia, which functioned in fermentation and denitrification, for which nitrite was denitrified by using readily biodegradable organic matters biodegraded by Ottowia bacteria.