Optimization of soaking in aqueous ammonia pretreatment for anaerobic digestion of African maize bran

In this study, maize bran biomass was subjected to soaking in aqueous ammonia (SAA) pretreatment to reduce and eliminate lignin, a major component that hinders biological digestibility. The effect of SAA pretreatment on the mesophilic anaerobic digestion of maize bran were investigated by using statistical techniques on batch anaerobic test. Response surface methodology by the central-composite design has verified the effects and the interactions of three variables, pretreatment temperature (10–110 °C), pretreatment time (1–11 h) and solid to liquid (S/L) ratio (0.068–0.307) on the responses: methane yield (MY), methane productivity rate (MPR), delignification and sugar recovery were monitored keenly. The optimal regions predicted via overlay plots for the multiple response falls within the following ranges: pretreatment temperatures between 55 and 70 °C, pretreatment time of 5 to 8 h and S/L ratio of 0.14 to 0.185. The optimal conditions were found at 72.5 °C, an S/L ratio of 0.14, and 7.02 h resulting to an MY of 293.8 mL/g COD, and an MPR of 106.0 mL CH₄/g VSS-day. The model equations for the multiple responses were within p < 0.05 meaning it is significant and is fitted to the data sets. Validation experiments has confirmed that the pretreatment conditions with optimal responses presented are in close agreement comparing the ones obtained from the experiment and that as estimated by the model.