Impact of passive oxygen supplementation under varied regimes on Biofiltration-NF process

Nanofiltration (NF) membranes are effective for surface water treatment but face challenges like membrane fouling, scaling, and flux decline. This study evaluates low-rate biofiltration (BF) combined with passive oxygen supplementation as a pretreatment to boost NF performance and limit disinfection byproduct (DBP) formation. Two BF setups were tested: a down-flow system (BFd, oxic-anoxic) and an up-flow system (BFu, oxic-anoxic-oxic). The BFu system achieved higher dissolved organic carbon (DOC) removal at 69 %, compared to 65 % with BFd, due to improved biodegradation from better passive oxygen supply. When paired with NF, both configurations enhanced effluent quality and minimized membrane fouling, with BFu-NF systems showing notably less flux decline, i.e., 5.6 % and 5.21 % in BFu-NF90 and BFu-NF270, respectively, compared to BFd-NF90 and BFd-NF270. This is related to a more comprehensive breakdown of organics, as reflected by lower levels of polysaccharides and proteins in BFu-NF effluents. Effluents from BFu-NF also had reduced UV absorbance and DOC, indicating more effective removal of organic foulants. Additionally, BFu reduced DBP formation potential (DBPFP) by 18.8 % more than BFd; combined with NF, BFu systems cut DBPFP by 10-19 % more than BFd. Toxicity tests showed that BFu-NF90 and BFu-NF270 achieved toxicity reductions of 45 % and 28.7 % respectively, greater than BFd-NF90 and BFd-NF270, respectively. Overall, the results indicate that passive oxygen supplementation by BFu is more effective than BFd, and its integration with NF significantly enhances treatment efficiency, prolongs membrane lifespan, minimizes fouling, and substantially reduces the formation potential of regulated DBPs.