Coupled nutrient removal from secondary effluent and algal biomass production in membrane photobioreactor (MPBR): Effect of HRT and long-term operation

In this study, the effect of hydraulic retention time (HRT) on algal biomass production and nutrient removal was investigated in membrane photobioreactors (MPBRs) in which Chlorella vulgaris was cultured with secondary effluent. The results indicated that decreasing the HRT improved the algal biomass productivity. MPBR that operated at an HRT of 2.0 d obtained the largest productivity of algal biomass. For nutrient removal, MPBR that operated at longer HRT showed a greater reduction in influent nutrients. An MPBR that operated at an HRT of 2.0 d was then chosen to study the long-term operation of the reactor. Microalgae suspensions were continuously harvested from the MPBR at a certain rate, which was calculated using an equation based on the nitrogen balance of the reactor. Therefore, the MPBR ran stably for 130 days, with an HRT and biomass retention time (BRT) of 2.0 d and 21.1 d, respectively. The algal biomass concentration in the MPBR remained at a relatively stable level of 1.035-1.524 g L−1 throughout the cultivation period. The nutrient in the influent was also effectively removed, with effluent nutrient concentrations ranging from 1.76 to 3.82 mg L−1 and 0.01 to 0.14 mg L−1 for DIN and DIP, respectively. Our study of membrane fouling showed that the membrane resistance, cake resistance and inner resistance contributed 6.7%, 39.1%, and 54.2% of the total resistance in MPBR, respectively.