Sequencing batch reactor (SBR) and anoxic and oxic process (A/O) display opposite performance for pollutant removal in treating digested effluent of swine wastewater with low and high COD/N ratios

Sequencing batch reactor (SBR) and anoxic/oxic process (A/O) are the predominant process for the treatment of digested effluent of swine wastewater. However, which is optimal process for their treatment? The question remains unclear. In this study, lab-scale SBR and A/O anoxic/oxic process were employed to treat digested effluent with addition of different proportion (v/v 10%–40%) of raw swine wastewater in order to make clear performance difference of the pollutant removal. The results showed that the NH4+-N and TN removal efficiencies of the A/O were more than 5.5 and 6.0 percentage points higher than that of the SBR respectively when the proportion of added raw wastewater was 20% (low COD/N ratios), due to the limitations caused by the low BOD5 concentration and the higher dissolved oxygen concentration during the anoxic phase. While, the NH4+-N and TN removal efficiencies of the SBR were more than 3.3 and 14.3 percentage points higher than that of the A/O respectively when the added raw wastewater proportion was 40% (high COD/N ratios), owing to limitation by the lower mixed liquor recycling ratio in the A/O. The sludge activity tests and microbial community analysis showed that the SBR displayed higher ammonia-oxidizing and anammox activity, whereas the A/O revealed higher denitrifying activity. These results uncovered that there are differences when A/O and SBR were employed to treat digested effluent and provide good insights for process selection when treating different C/N ratios wastewater.