A comparison of algal productivity and nutrient removal capacity between algal CSTR and algal MBR at the same light level under practical and optimal conditions

• High-density algae cultivation in algal membrane bioreactors (A-MBR) was demonstrated.
• The A-MBR system showed superior algal productivity and nutrient removal capacity.
• Areal biomass productivity in the A-MBR was 1.7 times of that in algal CSTR.
• Nutrient removal capacities of the A-MBR were about three times of those in the algal CSTR.
• High biomass concentrations helped algae-facilitated phosphate precipitation and biomass settling.

High rate algal ponds often require large land area and face difficulties in algae harvesting due to low biomass concentration and poor settleability of algae. By uncoupling solids retention time (SRT) and hydraulic retention time (HRT), algal membrane bioreactor (A-MBR) is a promising technology for high-density algae cultivation and wastewater polishing. In this bench-scale study, the performance of the A-MBR with a tanks-in-series configuration was compared with that of a continuous stirred tank reactor (CSTR) in terms of algal productivity, nutrient removal capacity, and overall performance under practical and optimal conditions (1-d HRT, 10-d SRT for A-MBR and 4-d HRT for CSTR). At the fluorescent light intensity of 200 ± 15 μmol/m2/s, algal biomass concentrations in the A-MBR and algal CSTR were 1327 ± 17 mg COD/L and 267 ± 6 mg COD/L, respectively. The areal algal productivities were 6.0 ± 0.1 g VSS/m2/d and 3.5 ± 0.1 g VSS/m2/d in the A-MBR and algal CSTR, respectively. Nutrient removal capacities of the A-MBR were 7.75 ± 0.54 g N/m3/d and 2.46 ± 0.01 g P/m3/d which were about three times of those in the algal CSTR (1.96 ± 0.10 g N/m3/d and 0.58 ± 0.02 g P/m3/d). Meanwhile, algal biomass from both systems had high phosphorus content (1.93 ± 0.13% in A-MBR and 1.88 ± 0.06% in algal CSTR) due to algae-facilitated phosphate precipitation. This result demonstrated that algal productivity and nutrient removal capacity could be significantly improved in the A-MBR system with a tanks-in-series configuration.