Evaluating the effect of air flow rate on hybrid and conventional membrane bioreactors: Implications on performance, microbial activity and membrane fouling.

This study addressed the impact of air flow rate on the performance, membrane fouling behaviour and microbial community of a sequencing batch conventional membrane bioreactor (SB-MBR) and a sequencing batch hybrid membrane bioreactor (SB-HMBR) with carrier media for biofilm growth. Two different scenarios were evaluated: high (6.4 L min) and low (1.6 L min) air flow rates, associated with high (4.5 mg L) and low (1.5 mg L) dissolved oxygen (DO) concentrations and specific aeration demand per membrane area (SAD) of 0.426 and 0.106 m m h, respectively. Both reactors were subjected to alternating non-aerated and aerated conditions for organic matter (as chemical oxygen demand - COD), nitrogen and phosphate removal from a municipal wastewater. From the bacterial community analysis, the key players in nutrient removal processes were assessed. The results showed that COD removal efficiencies were above 95% in both MBRs, regardless of the aeration intensity, while complete ammonium removal was observed at the higher DO. However, nitrifying activity was adversely affected under low DO levels. High nitrification levels were re-established faster in the hybrid MBR, thanks to the presence of biofilm, where nitrifying activity was favoured and the bacterial community profile did not exhibit substantial changes upon DO reduction. A higher denitrification potential was found for the carrier-based MBR, resulting in lower effluent nitrate concentrations. Regarding phosphorus removal, a slight improvement was observed in the SB-HMBR at reduced DO, while in the SB-MBR it remained practically constant. Moreover, the specific phosphate uptake rate exhibited a significant increase, especially in the hybrid MBR, reaching 44.6 mgP gVSS h. At lower aeration rate, however, worse filterability and higher membrane fouling rates were observed, especially in the conventional MBR. Overall, the results demonstrated that the hybrid MBR better withstood the reduced air flow rate and DO as compared to the conventional counterpart.