An integrated cationic etherified starch–polyferric sulfate composite flocculant for enhanced solid–liquid separation of cattle biogas slurry

Cattle biogas slurry is a complex livestock wastewater characterized by a high organic load, strong colloidal stability, and variable salinity, which make it difficult to treat and reuse for land application. In this study, a cationic etherified starch–polyferric sulfate composite flocculant (SGP) was developed and evaluated for the treatment of cattle biogas slurry. Among five starch-based formulations, SGP exhibited the best performance, achieving removal efficiencies of 87.4% for COD, 87.8% for TP, and 98.8% for SS at an optimal dosage of 7‰, corresponding to 7 g L −1 dry flocculant. Salinity-dependent tests demonstrated that SGP maintained stable flocculation performance under representative mixed-salt conditions, with limited increases in residual COD and turbidity at salinities of 5 and 10 g L −1 , indicating good tolerance to salinity fluctuations and complex coexisting ions. SGP also outperformed its individual components and the optimized PFS + PAM system, confirming the advantage of its integrated organic–inorganic composite structure. FTIR, SEM, EDS, XPS, and ζ-potential analyses supported the formation of an organic–inorganic hybrid system with modified surface charge, heterogeneous morphology, and Fe-containing species, which promoted colloidal destabilization, particle aggregation, and floc growth. Mechanistically, SGP-induced flocculation was attributed to a cooperative process involving electrostatic regulation, hydrogen-bond-assisted association, Fe-associated interactions, and auxiliary starch-chain bridging. These findings demonstrate the potential of SGP as a practical pretreatment option for cattle biogas slurry and provide useful guidance for the design of starch-based organic–inorganic composite flocculants for complex livestock wastewater treatment.