Development of the combined nitritation–anammox process in an upflow anaerobic sludge blanket (UASB) reactor with anammox granules

Abstract

The combined nitritation–anaerobic ammonium oxidation (anammox) process represents a promising innovative technology for biological nitrogen removal, especially for treating wastewater with low chemical oxygen demand (COD) to ammonium ratios. In the present study, a laboratory-scale upflow anaerobic sludge blanket (UASB) reactor, which has been operated under low oxygen levels (⩽0.5mgL−1) for over 280days, was shown to remove about 88% of the supplied total nitrogen loads without nitrite/nitrate accumulation. During the start-up period, the reactor was initially inoculated with enriched anammox granules and fed with synthetic wastewater, where the ammonium concentration was fixed but nitrite concentration was gradually decreased to zero. As the influent NO2−-N/NH4+-N ratio decreased, O2 supply was increased gradually to facilitate the conversion of ammonium to nitrite by ammonium oxidation bacteria (AOB). Our results showed that the nitritation–anammox granules with porous structures were enriched and featured good settling ability. Microbial community composition was investigated by fluorescence in situ hybridization (FISH), which demonstrated the coexistence of AOB and anammox bacteria in the granules. The two groups of bacteria exhibited an overlapping growth style, which can improve ammonium availability to anammox bacteria and facilitate the immediate consumption of nitrite (produced by AOB) by anammox bacteria.