Unraveling the impact of salinity on biofouling on ultrafiltration membranes: A spectroscopic and microscopic view

Biofouling significantly hampers the performance and longevity of ultrafiltration (UF) membranes employed in saline wastewater treatment. This research addresses a crucial knowledge gap by meticulously investigating the influence of salinity variations (0–10 g/L NaCl) on biofouling characteristics, utilizing advanced spectroscopic and microscopic techniques. Combined blocking models elucidated the evolution of fouling mechanisms under varying salinity conditions. Higher salinity levels promoted cake layer formation, potentially circumventing initial fouling stages observed under lower salinity conditions. Quantification of confocal laser scanning microscopy images revealed a denser, thinner, and less heterogeneous cake layer at high salinity, composed of proteins, nucleic acids, and α-polysaccharides. The relatively high abundance of polysaccharides may contribute to maintaining osmotic pressure and bacterial cell viability. Moreover, the integration of the confocal Raman mapping technique and non-negative matrix factorization analysis was innovatively applied to characterize the biofouling layer, identifying the important role of carotenoids within the layer. Carotenoids were found to be more abundant in the upper regions of the biofouling layer formed at high salinity, potentially scavenging the reactive oxygen species within bacterial cells. In conclusion, this investigation offers a comprehensive understanding of the salinity-dependent alterations in the chemical composition and spatial structure of the membrane fouling layer. The findings may facilitate the development of targeted mitigation strategies to combat membrane biofouling in saline wastewater treatment applications.