Feasibility of one-step nanofiltration driven by ultra-low pressure (LPOSNF) for algae-laden brackish water treatment: insights from the relationship between algal physiological state and membrane fouling development

The co-occurrence of algal blooms and saline tide poses a challenging threat to decentralized water supply in the estuarine zones of warm coastal areas. One-step nanofiltration shows considerable potential for addressing this issue but remains constrained by specific membrane fouling. In this study, one-step nanofiltration driven by ultra-low pressure (LPOSNF) was innovatively proposed to efficiently treat algae-laden brackish water with the comparison of conventional nanofiltration systems operated under regular pressure conditions. The weaker decline in membrane flux and the smaller proportion of membrane fouling in total resistance indicated the superior permeation of LPOSNF. Although the real rejection of pollutants was higher under elevated pressure conditions, LPOSNF achieved satisfactory permeate quality through the combined effects of dynamic membranes and improved algae integrity. The analysis of algal physiological state confirmed the damage of excessive pressure on algal structure, which reduced algal activity and accelerated the release of AOM. OCT, CLSM, and UV–Vis analysis validated the facilitated accumulation of different bio-components (especially polysaccharides with chain-like structure and high viscosity) within Hi-P membrane fouling layers. Abundant bacteria involved in pollutant removal (e.g., Stenotrophomonas, Chitinophaga, and Gimesia) and eukaryotes contributing to porous fouling layers (e.g., Amoeba and Hartmannella) were also identified by HTS. SEM imaged the compacted morphology of membrane fouling layers by high pressure, while it was incomplete and partially covered by scattered foulants in LPOSNF. Besides, LPOSNF achieved much lower energy consumption (3.5 × 10⁻² kWh/m³) and carbon emission (3.4 × 10⁻² kg CO₂e/m³). The normalized membrane flux by specific energy consumption of LPOSNF was 42 % ~ 98 % higher than that of conventional nanofiltration. These findings provide comprehensive and robust evidence for the outstanding applicability of LPOSNF in decentralized water treatment without frequent membrane cleaning.