Optimising an outdoor membrane photobioreactor for tertiary sewage treatment

The operation of an outdoor membrane photobioreactor plant which treated the effluent of an anaerobic membrane bioreactor was optimised. Biomass retention times of 4.5, 6, and 9 days were tested. At a biomass retention time of 4.5 days, maximum nitrogen recovery rate:light irradiance ratios, photosynthetic efficiencies and carbon biofixations of 51.7 ± 14.3 mg N·mol^-1, 4.4 ± 1.6% and 0.50 ± 0.05 kg CO₂·m³_influent, respectively, were attained. Minimum membrane fouling rates were achieved when operating at the shortest biomass retention time because of the lower solid concentration and the negligible amount of cyanobacteria and protozoa. Hydraulic retention times of 3.5, 2, and 1.5 days were tested at the optimum biomass retention times of 4.5 days under non-nutrient limited conditions, showing no significant differences in the nutrient recovery rates, photosynthetic efficiencies and membrane fouling rates. However, nitrogen recovery rate:light irradiance ratios and photosynthetic efficiency significantly decreased when hydraulic retention time was further shortened to 1 day, probably due to a rise in the substrate turbidity which reduced the light availability in the culture. Optimal carbon biofixations and theoretical energy recoveries from the biomass were obtained at hydraulic retention time of 3.5 days, which accounted for 0.55 ± 0.05 kg CO₂·m^-3_influent and 0.443 ± 0.103 kWh·m^-3_influent, respectively.