Inhibition of anaerobic digestion by dissolved lignin derived from alkaline pre-treatment of an aquatic macrophyte

Delignification by chemical pre-treatment improves the anaerobic digestibility of lignocellulosic biomass, but the by-products can potentially inhibit the digestion process. The present study investigates the inhibitory effect of dissolved lignin on three steps of anaerobic digestion: methanogenesis, acidogenesis, and hydrolysis. Dissolved lignin was extracted from a lignin-rich aquatic macrophyte, and anaerobic toxicity tests were performed in batch mode at dissolved lignin concentrations of 0, 0.5, 1.0, 2.5, and 5.0 g L−1 under mesophilic conditions. Minor reductions of up to 15% and 10% against the control were observed in methanogenic and acidogenic activities, respectively, at a dissolved lignin concentration of 5.0 g L−1. By contrast, hydrolysis efficiency dropped by 25% against the control at a dissolved lignin concentration of 1.0 g L−1 and dropped by 35% against the control at a dissolved lignin concentration of 5.0 g L−1. These results suggest that enzymatic hydrolysis of cellulose is the step that is most susceptible to inhibition by dissolved lignin in the anaerobic digestion process.