Electrochemical enhancement of anaerobic digestion with phosphogypsum addition and methane production

This study systematically evaluated the effect of phosphogypsum (PG) addition on the anaerobic digestion of pig manure under electrochemical assistance (constant voltage of 0.6 V, with graphite electrodes and 15 cm spacing) in batch reactors over 26 days. The results revealed a significant concentration-dependent inhibition window at 2-3% PG. Compared to the control (CK, no PG) and low-dose (0.5-1%) or high-dose (4%) PG groups, which achieved cumulative methane yields of 12000-12600 mL and chemical oxygen demand (COD) reduction over 70%, the 2-3% PG treatments showed severe metabolic suppression. Mechanism analysis demonstrated that the inhibition originated from intense sulfate reduction triggered, preferential precipitation of Ca2+ as CaS, leaving high soluble Ca2+ that disrupted microbial aggregation and elevated ionic stress. Strong immobilisation of bioavailable Zn as ZnS, depriving methanogens of a vital trace element. Electrochemical regulation could not override this threshold effect, but at 4% PG the excess Ca2+ fully precipitated sulfides and restored methanogenic activity, as evidenced by well-crystallised CaCO3 in the digestate. These findings provide a mechanistic framework for safe PG valorisation, low addition amount of PG maintains efficient biogas production, COD reduction and nutrient stabilization, offering a robust strategy for co-managing PG and livestock waste under electrified bioprocesses.