| Denitrification process is limited by insufficient organic carbon in conventional wastewater treatment plants during the treatment of domestic wastewater with low C/N. Compared with the traditional biological nitrogen removal (BNR) process, shortcut biological nitrogen removal (SBNR) process resulted in saving25%of oxygen supply for nitrification while reducing40%carbon source requirement for denitrification. It has unique advantage for treating wastewater with low C/N. In this work, lab-scale continuous-flow A/O process with biological nitrogen removal (BNR) reactor and hybrid biological nitrogen removal (HBNR) reactor was adopted. The objectives of the study were to achieve shortcut biological nitrogen removal from wastewater with low ammonium concentration and low C/N ratio in two reactors, to investigate the properties of EPS fractions extracted from activated sludge and biofihn in the course of establishing shortcut biological nitrogen removal process, to discuss the mechanisms responsible for the achievement of shortcut nitrogen removal, and to evaluate the performance and mechanisms of shock resistance to short-term changes in C/N ratio, organic nitrogen, Cu2+of the influent in the HBNR. The primary results of this study are summarized as follow:(1) Nitrogen removal via nitrite in HBNR and BNR reactors applied to conventional continuous-flow A/O process treating low-strength ammonium wastewater was successfully achieved by controlling DO concentration of1.0～1.5mg/L in the aerobic tank. For BNR, ammonium removal efficiency (NH4%) maintained at high level(above90%), while average nitrite accumulation rate was64.5%and TN removal efficiency was74.4%under steady state. For HBNR, average ammonium removal efficiency was95.7%, the nitrite accumulation rate was63.6%, and TN removal efficiency was88.1%.In comparison to BNR, TN removal efficiency of HBNR was significantly higher than during the whole experiment. Based on mass balance for nitrogen, fates for nitrogen in HBNR and BNR were distinct from one another. In BNR most TN was removed by denitrification in the anoxic tank, whereas TN removal was dominated by simultaneous nitrification-denitrification (SND) via nitrite in the aerobic tank of HBNR.(2) EPS content and abundance of nitrifying bacteria in BNR and HBNR during start-up and steady operation periods were investigated. In both reactors, a clear change of EPS content was observed during the achievement of SBNR process, which was considered as a protective response of bacteria to the change of environment. SVI had positive correlations with LB-EPS content and protein content of LB-EPS of activated sludge. The nitrite pathway negatively impacted on the sludge settleability. Although sludge bulking happened in SBNR process, it did not influence the nitrogen removal. SOUR and qPCR assays demonstrated that AOB were dominant in activated sludge and biofihn after SBNR process established, while low DO concentration promoted AOB growth. NOB was not completely washout from the reactors, but the abundance of NOB evidently deceased in stable operation period, compared to complete nitrification period.(3) The effects of C/N ratios, organic nitrogen and Cu2+concentrations in the influent on the performance of HBNR in short-term shocking were examined. TN removal efficiency declined to15.3%and25.3%for C/N ratios of2and4, respectively. LB-EPS contents of bioflim and activated sludge were affected by the change of influent C/N ratio. The HBNR has good resistance to the shock of organic nitrogen in the wastewater, but EPS contents of biofilm and activated sludge obviously increased. TN removal efficiency declined to5.2%,9.9%and28.7%after shocking with Cu2+concentrations of1mg/L,2mg/L and5mg/L, respectively. SND was inhibited since the concentration of Cu2+was2mg/L. When the concentration of Cu2+was increased to5mg/L, TN removal efficiency was further decreased, and the HBNR can not be recovered. |
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