Divergent accumulation of membrane biofouling by slight elevation of nitrogen and phosphorus in drinking water treatment: Performances and mechanisms

Abstract

Membrane biofouling has been intensively investigated over many years; however, little attention has been paid to the role of slightly elevated nutrients in the fouling formation, especially during drinking water treatment. In this study, we evaluated the respective contribution of slightly elevated nitrogen (ca. 0.5 mg·L−1) and phosphorus (ca. 0.1 mg·L−1) concentrations to membrane biofouling formation and deciphered the associated mechanisms. The results demonstrated that the slight concentration elevation of nitrogen did not substantially decrease the permeate flux, but approximately 50% of the permeate flux was reduced by the elevated phosphorus. The fouling layer was thickened about 4–fold due to the slight elevation of phosphorus and this resulted in the best removal performance of fluorescent organics. In contrast, the fouling layer structure was not markedly changed by the elevated nitrogen. The concentrations of proteins and polysaccharides in the fouling layers increased to different degrees, with phosphorus playing a more pronounced role than nitrogen. The molecular experiments revealed that the filamentous Arthronema dominated the nitrogen–enriched fouling layer, but they decreased to nearly zero in the phosphorus–enriched fouling layer, and the Xanthobacter, which was capable of secreting large amounts of EPS, was enhanced. The growth of algae Chlorellales was stimulated by the elevated nitrogen, while Sphaeropleales was dominant with elevated phosphorus, and for protozoa, Vannella was increased by nitrogen, whereas members of Hartmannella were enhanced by phosphorus. Our findings indicate that the impact of slight increases in concentration of nutrients, especially phosphorus, should be taken into account in drinking water treatment.