Anaerobic Digestion of Dairy Manure Treated with a Tannin-Based Coagulant–Flocculant

Increasing the solids concentration in farm dairy effluent is necessary to operate a biogas plant at optimal parameters; however, conventional chemicals reduce CH4 production. This study evaluated the effects of the tannin-based coagulant–flocculant Tanfloc SG® on anaerobic digester performance using biomethane potential (BMP) tests with centrifuged solids produced at 50–300 mg Tanfloc g TS−1, and semi-continuous fermentation tests with settled solids at 150 mg g TS−1 (Tanfloc150). In the fermentation tests, the control and Tanfloc-treated solids were fed into two reactors, operating at 37 °C, at 2.1 kg VS m−3 d−1 with a 12- to 15-day hydraulic retention time. In the BMP tests, the Tanfloc-treated solids showed delayed degradation and produced 168.2–303.1 L CH4 kg VS−1, values not significantly different from the untreated solids (250.0 L CH4 kg VS−1). However, semi-continuous feeding reduced the volatile solids degradation, biogas, and specific CH4 yield by 23.3%, 17.9%, and 24.1%, while the pH, volatile fatty acids, and ammonia-N remained stable. However, Tanfloc150 reduced the biogas H2S and NH3 concentrations and increased the digestate N and S. The lower performance was associated with lower substrate biodegradability, which was consistent with the delayed degradation observed in the BMP tests. These findings highlight the importance of assessing coagulated solids under conditions representative of long-term performance.