The effect of substrate/inoculum ratio on the kinetics of methane production in swine wastewater anaerobic digestion

Methane production from swine wastewater was evaluated by using sewage sludge as inoculum in three substrate to inoculum ratios (SIRs) named A (1:1), B (3:1) and C (6:1), with the objective to identify the proportion that optimizes the performance of the process. Duplicated batch bioreactors of 1 L capacity under mesophilic conditions were used to carry out the experiment. The highest biogas yield was observed in A treatment (554 ± 75 mL/g volatile solid (VS)). Cumulative methane production decreased from 382 ± 22 to 232 ± 5 mL/g VS when SIR increased from 1:1 to 6:1. The first-order model and the modified Gompertz equation were used to model the experimental cumulative methane yield giving adjustments with coefficients of determination of 96 and 99% respectively. The effect of the SIR was analysed based on the kinetic parameters of the Gompertz equation, which are methane production potential, maximum methane production rate and lag-phase time. The best performance in terms of the kinetic parameters was obtained for treatment A; however, treatment B could still ensure a stable process. The use of higher inoculum concentration generated 463.1% higher methane production rate and required 77.3% shorter adaptation time (lag phase) in the SIR range studied. When higher SIR was used (e.g. 14:1, previous work), it could be observed that the Gompertz equation also adjusted adequately the experimental data (R2 > 0.99) although the lag-phase time did not remain in a linear relationship with SIR but exponentially above SIR = 6:1. These results demonstrated that when a low amount of inoculum was used, the adaptation time of microorganisms resulted much higher than expected delaying the methane production and extending the time needed to achieve adequate performance of the process.