Effect of Calcium Ions on the Removal Efficiency and Membrane Fouling of Sulfamethoxazole from Secondary Effluent by Slow Filtration–Nanofiltration of Biofilm

Sulfonamide antibiotics, exemplified by sulfamethoxazole (SMZ), are commonly identified in the secondary effluent of sewage plants, which introduces potential risks to the reuse of reclaimed water. The objective of this study was to examine the efficacy and mechanism of the combined biofilm slow filtration–nanofiltration (NF) process in removing pollutants from secondary effluent, to investigate the impact of Ca2+ concentration on the removal efficiency of SMZ in secondary effluent, and to analyze the interface mechanism between pollutants with different surface properties and the NF membrane surface. The findings demonstrated that the integrated biofilm slow filtration–NF process was capable of efficaciously removing SMZ, dissolved organic carbon, and ionic strength from the secondary effluent, with removal rates of 92.8%, 79.4%, and 75.2%, respectively. The dominant microbial groups (Proteobacteria, Acidobacteria, Planctomycetes, Stenotrophobacter, Dokdonella, and Nitrospira) present in the biofilm on the surface of slow filtration played an active role in the degradation of SMZ. The combined biofilm slow filtration–NF process demonstrated optimal efficacy for SMZ removal at a Ca2+ concentration of 2 mmol/L. However, as Ca2+ levels increased, the membrane pore blockage and filter cake layer pollution intensified, accompanied by a decline in the interface energy between the pollutant and the NF membrane.