Investigation of biogas production by co-digestion of brewer’s grains and sludge digestate using a centered mixing plan

Brewers' grains are the most abundant waste product in the brewing industry. In this study, brewers' grains were used in co-digestion with digestate from sewage treatment plant (STP) sludge to produce biogas by anaerobic digestion efficiently. Samples of brewers' grains and sewage sludge were exhaustively characterized to determine physicochemical parameters. Co-digestion of spent grains with sewage sludge (WWTP) was done using experiments based on an augmented-centered mixing design. Mathematical modeling was carried out using Minitab version 19 to select variables up to order 4 using step-by-step selection. Multiple regression is performed using the least squares method. Only statistically significant terms are retained in the model. The description parameters R² and prediction parameters Q² are used to evaluate model efficiency. The results of the analysis of these raw materials showed, respectively, that the organic matter (OM) content was 99% and 39.39%, while the total organic carbon (TOC) content was 45.5% and 19.70%, and the pH values were 5.6 and 6.64. The mathematical model for biogas volume production was descriptive (R² = 99%) and highly predictive (Q² = 98%). After optimization, the maximum biogas volume was 6045 ml over six days of production. The methane content of the biogas was 74.18%, with no hydrogen sulfide content. The digestate has good fertilizing properties and can be used for crops. Therefore, anaerobic digestion (AD) with biogas collection and fertilizer production from BSG and SD is a viable option that could be integrated into future breweries for waste management.