Tertiary wastewater treatment in membrane photobioreactor using microalgae: Comparison of forward osmosis & microfiltration

•Forward osmosis and microfiltration based membrane photobioreactors were operated.•Bioreactors exhibited excellent N and P removal efficiency.•Membrane rejection played important role in osmotic membrane photobioreactor.•More than 2 g/L microalgae were accumulated in the bioreactor.•Operating costs for FO-based system was 32-45% higher.

Discharge of wastewater with high nitrogen and phosphorus content is a major cause of eutrophication. In this study, a microfiltration-based membrane photobioreactor (MPBR) and forward osmosis-based osmotic membrane photobioreactor (OMPBR) have been operated with Chlorella vulgaris for continuous tertiary wastewater treatment. Both the bioreactors exhibited good biomass accumulation (over 2 g/L), although the OMPBR achieved better nutrients removal due to high rejection properties of the membranes. At 2 days HRT, the OMPBR achieved nitrogen and phosphorus removal efficiencies of 86-99% and 100%, respectively, whereas the corresponding values in the MPBR were 48-97% and 46%, respectively. Based on the energy input, the total operating costs for OMPBR were 32-45% higher than that of the MPBR, and filtration cost for OMPBR was 3.5-4.5 folds higher than that of the MPBR. These results indicate that the integration of membrane filtration with photobioreactors is promising in microalgae-based tertiary wastewater treatment.