Cation-modified starch-based bioflocculant treatment of swine manure digestate: performance, mechanism, and economic analysis

Aiming at green and efficient treatment of livestock biogas slurry, this study used high-concentration swine-manure digestate produced from a continuous stirred-tank reactor (CSTR) as the target matrix. The study conducted screening of ten commercial bioflocculants and a performance comparison between the optimal bioflocculant and cationic polyacrylamide (PAM). A performance-cost comprehensive evaluation system was constructed by integrating response-surface equations with cost equations. The flocculation mechanism was elucidated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy - energy-dispersive spectroscopy (SEM-EDS) characterization, and adsorption kinetic analysis. The results showed that the commercially available cation-modified starch-based bioflocculant A exhibited the best flocculation performance for CSTR digestate. It achieved suspended solids (SS) and total phosphorus removal efficiencies of 94.0% and 93.5%, respectively, with a treatment cost of 3.9 CNY/t, which was comparable to PAM (97.3%, 97.5%, and 4.2 CNY/t), indicating its potential as a substitute. The comprehensive evaluation system identified the optimal scheme at dosages of 150 mg/L flocculant A and 500 mg/L CaO, with an initial SS concentration of 25 000 mg/L and pH 7.07, yielding a comprehensive score of 0.96. Mechanistic analysis indicated that flocculant A contained quaternary ammonium groups; the kinetics were well described by a pseudo-second-order model (R²≥0.999). The adsorption process could be divided into a rapid stage dominated by charge neutralization and a slower stage dominated by adsorption bridging, reflecting a multi-mechanism coupled flocculation process. The results demonstrate that pretreatment of CSTR digestate can employ cation-modified starch-based flocculant A in combination with CaO to replace PAM, providing a feasible green alternative for large-scale livestock wastewater treatment.