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For the sake of high efficiency and saving operational cost for high-concentration urea wastewater treatment, a novel two-stage partial nitritation (PN)-anammox process containing simultaneous urea hydrolysis and PN in sequencing batch reactor (SBR) was investigated. Although the influent urea concentration increased from 500 to 1200 mg/L, the SBR simultaneously achieved urea removal efficiency higher than 98% and stable PN with effluent NO 2 −-N/NH 4 +-N ratio of 1.0–1.3 without any extra alkalinity addition. The intracellular hydrolysis was the dominant mechanism for urea removal and persistent free ammonia inhibition on nitrite-oxidizing bacteria was the main reason for nitrite accumulation of 97.92% in SBR. The subsequent anammox reactor showed efficient nitrogen removal performance with average ammonium removal efficiency, nitrogen removal efficiency and maximum nitrogen removal loading rate of 98.08%, 81.45% and 1.05 kg N·m−3·d−1 respectively. High-throughput sequencing results indicated Gemmatimonadetes became the most abundant bacterial phylum related to potential intracellular urea hydrolysis and displayed obvious ammonium-oxidizing bacteria enrichment and nitrite-oxidizing bacteria inhibition in SBR, and the dominant anammox bacteria (Candidatus_Kuenenia) in anammox reactor. The proposed process was proven to be promising for high-concentration urea wastewater treatment, facilitating the sustainable development of the urea industry in the future. [Display omitted] • A novel biological nitrogen removal process for urea wastewater was investigated. • Autotrophic N removal with no alkalinity added was achieved for urea wastewater. • Intracellular urea hydrolysis and stable partial nitritation was integrated in SBR. • 81.45% of total N was removed by anammox with N removal rate of 1.05 kg N·m−3·d−1. [ABSTRACT FROM AUTHOR]

In order to assess the performance of anaerobic ammonium oxidation (anammox) bioreactors, it is necessary to study the stoichiometry of the anammox reaction and pH. This study focused on the effect of the hydraulic retention time (HRT) on the effluent pH in anammox-upflow anaerobic sludge blanket (UASB) bioreactors. Anammox-UASB bioreactors with and without a recirculation system were used to investigate the effluent pH and bioreactor performance. It was concluded that under varying HRT conditions, the decrease in effluent pH did not indicate the deterioration of nitrogen removal, but did indicate that the nitrogen removal efficiency was reduced owing to a sudden increase in the nitrogen loading rate resulting from the decrease in HRT. Moreover, the results showed that the HRT directly affected the concentration of OH−, which affected the increase/decrease in effluent pH. This study demonstrated that effluent pH is a more powerful tool than previous techniques used to assess bioreactor performance. We suggest that the effluent pH could be used for preliminary assessment. • The decrease in effluent pH does not always indicate the deterioration of nitrogen removal. • The clear decrease in the effluent pH showed that the NRE decreased. • The change in the effluent pH was related to the NRE rather than the NRR. • Two UASB reactors with and without recirculation were used for effluent pH and NRE investigation. • The effect of the HRT on the effluent pH was investigated. [ABSTRACT FROM AUTHOR]