Mechanistic insights into ultrafiltration fouling mitigation by coagulation pretreatment in livestock biogas slurry treatment

The treatment and utilization of livestock biogas slurry (BS) is significantly hindered by severe membrane fouling associated with high turbidity and complex dissolved organic matter (DOM). This study established an integrated coagulation-ultrafiltration (UF) pretreatment system using aluminum sulfate or ferric chloride as coagulants to provide stable influent for subsequent membrane concentration. Results demonstrated that while raw BS caused a rapid increase in transmembrane pressure (TMP) during UF, optimized coagulation at 3 g/L maintained TMP below 10 kPa over 6 h. Specifically, reversible resistance ( R re ) decreased by 92.9–99.0%, and irreversible resistance ( R ir ) decreased by 65.9–79.0%. Particle-related characteristics were closely associated with R re , whereas R ir was governed by multiple interacting factors. Mechanistically, interfacial thermodynamic analysis revealed that optimized coagulation increased the interaction free energies for both membrane-foulant adhesion and foulant-foulant cohesion. This weakened the driving forces for foulant attachment and compaction, resulting in a thinner and looser fouling layer. Nutrient behavior was highly selective during coagulation-UF process. Nitrogen and potassium were largely preserved in the liquid phase, whereas phosphorus exhibited over 99% apparent concentration decrease under the optimized conditions, resulting in an overall nutrient concentration decrease of approximately 12%. Extended 24-h UF and downstream nanofiltration (NF) experiments further supported the improved fouling mitigation benefits of optimized coagulation. These results provided a mechanistic insight and a practical strategy for fouling-controlled BS pretreatment toward subsequent water reuse and nutrient management.