Anaerobic digestion of brewery spent grain in a semi-continuous bioreactor: inhibition by phenolic degradation products

In this study anaerobic digestion of selected lignocellulosic substrate, namely brewery spent grain (BSG), was studied. In order to facilitate anaerobic digestion several types of pretreatment methods were tested such as: mechanical, chemical (alkali and acid) and thermo-chemical. The anaerobic digestion experiments were carried out in a semi-continuous stirred bioreactors with the organic loading rates between 2.9 and 3.9 kgCOD m-3 d-1 (1.9 and 2.5 kgVSS m-3 d-1 respectively) and corresponding hydraulic retention times of 33-39 days. Biogas production and composition, pH, COD, TSS and VSS, short chain fatty acids and phenolic compounds were measured. A significant inhibition of biogas production occurred, depending on the type of substrate pretreatment. There are indications that p-cresol is responsible for process inhibition when its concentration in the reaction mixture exceeds critical value between 115 and 240 mg L-1. Anaerobic digestion of chemically pretreated BSG (acid and alkali) and untreated-raw BSG was inhibited between the days 56 and 63 of the experiment, followed by thermo-chemically pretreated BSG on day 112 and mechanically pretreated BSG on day 126. Analyses of the substrates showed no phenolic compounds either in raw-untreated BSG or pretreated substrates, therefore the recorded p-cresol is an intermediate degradation product, responsible for process inhibition.