Biogas production from winery wastewater: Effect of the substrate-inoculum ratio on fly ash addition and iron availability

The effect of substrate-inoculum ratio and fly ash (FA) concentration on the anaerobic digestion of winery wastewater was studied. Two substrate-inoculum ratios (0.50 and 1 g soluble COD/g VSS) and five fly ash concentrations (25, 50, 75, 100 and 150 mg/L) were tested under mesophilic conditions. The results showed that 0.50 g soluble COD/g VSS was the best substrate-inoculum ratio. Regarding the FA, 100 mg/L improved the specific methane production by 79% compared to the control assay. In comparison, 150 mg/L produced an inhibition of methane production, with a 55% decrease in specific methane production with respect to the control assay. According to the bioavailability study, FA had a similar effect to a system with Fe-EDTA on anaerobic digestion, which could indicate that the Fe present in FA was present in the solution and bioavailable to microorganisms. Therefore, a possible mechanism for the action of FA on anaerobic digestion was proposed. Finally, the kinetic analysis showed that FA improved the maximum specific methane production rate and decreased the lag phase.