Overall evaluation of microwave-assisted alkali pretreatment for enhancement of biomethane production from brewers' spent grain

In this study, microwave-assisted alkali pretreatment for brewers' spent grain (BSG) waste was comprehensively investigated using Box-Behnken design under varying operating parameters, i.e. microwave power, pretreatment time and alkali/biomass (A/B) ratio. Both quantitative and qualitative characterization analysis such as crystallinity index (CrI), specific surface area (SSA), morphology and compositional analyses were conducted on pretreated samples. Standard biomethane potential (BMP) tests were performed for each pretreated sample to observe the biomethane enhancement after pretreatment. Under optimized conditions, it was observed that the pretreatment technique has facilitated removal of 46% of lignin and 38% of hemicellulose and improved the SSA of samples from 9.55 to 161.98 m(2)/g, contributing to 52% increase in BMP value. Meanwhile, mathematical models with correlation coefficients of 97.9% and 98.2% respectively were built to predict both the soluble chemical oxygen demand (COD) yield and BMP after pretreatment. Furthermore, a simplified concept of overall evaluation mechanism for pretreatment has been introduced, taking into account waste-removal, energetic and economic indices. Under the proposed evaluation mechanism, the caustic soda price was found to play a more important role than the prices of industry electricity and wholesale natural gas.