The Effect of Alkaline Pretreatment of Spent Malting Grains for Biogas Production, Mathematical Modeling and Process Optimization

Abstract

Spent grains from brewing industries are considered a suitable source for biogas production due to their high accessibility and affordability. However, lignocellulosic material, due to its fiber content (cellulose, hemicellulose, and lignin), meets implementation issues in the anaerobic digestion process. To enhance the efficiency of anaerobic digestion of brewery spent grain, an alkaline treatment was performed using sodium hydroxide (NaOH). The parameters of alkaline treatment were temperature (T) and soda-spent grain ratio (S/D). The characterization of the raw spent grains indicates that the organic matter (OM), total organic carbon (TOC), nitrogen (N), protein (P), N/C ratio, and pH were 99%, 57.39%, 3%, 18.75%, 19.13%, and 5.6, respectively. The respective contents of hemicellulose, cellulose, and lignin were 33.3%, 30.4%, and 6.5%. The pre-treatment process yielded a 5% reduction in hemicellulose and a 43% reduction in cellulose while increasing the volume of biogas produced from 0 to 577 mL. After pre-treatment, two biodigesters produced volumes of 23.03 mL/per day (100 °C with a soda/spent grain ratio of 0.01%) and 22 mL/per day (28 °C with a soda/spent grain ratio of 0.05%). The mathematical model showed an interaction between temperature and S/D ratio. The predictive modeling analysis presented that the maximum volume of biogas can be obtained as 23.3 mL/per day after pre-treatment of spent grains at 100 °C with a NaOH/spent grain ratio of 0.01.