Roles of SRT and HRT of an algal membrane bioreactor system with a tanks-in-series configuration for secondary wastewater effluent polishing

This study systematically determined the effect of uncoupling between solids retention time (SRT) and hydraulic retention time (HRT) and the SRT/HRT ratio on algal growth and nutrient (N and P) removal in an algal membrane bioreactor (A-MBR) with a tanks-in-series configuration. Under 12-h light and 12-h dark conditions with a light intensity of 150 μmol m−2 s−1 at the algal mixed liquor surface, the A-MBR system was operated at a fixed SRT (5 d) by varying HRT from 12 to 6 h, and later at a fixed HRT (12 h) by reducing SRT from 10 d to 5 d. In synthetic secondary wastewater effluent treatment, the highest removal efficiencies of total N (TN, 73.4 ± 6.3%) and total P (TP, 91.3 ± 3.8%) were observed at a 10-d SRT and a 24-h HRT, while the highest algal productivity of 131.7 g m−3 d−1 (or 22.4 g m−2 d−1) was observed at the 5-d SRT and 6-h HRT. The highest algal biomass productivity was not coincident with the highest N removal efficiency. For real secondary wastewater effluent polishing, a SRT of 10 d and a HRT of 24 h were applied based on the best nutrient removal performance observed earlier, resulting in the permeate containing 0.09 ± 0.05 mg/L TP and 0.45 ± 0.08 mg/L TN, with average removal efficiencies of 94.9 ± 3.6% and 95.3 ± 0.9%, respectively. The use of secondary wastewater effluent increased algal productivity by about 45%. The results demonstrate that control of SRT, HRT and SRT/HRT ratio is important in maximizing algal productivity and nutrient removal from secondary wastewater effluent.