The effects of solid lignin on the anaerobic digestion of microcrystalline cellulose and application of smoothing splines for extended data analysis of its inhibitory effects.

Lignocellulose is an abundant substrate for biogas production; however, for efficient utilization, proper pre-treatment is required to enhance the biomethane yield and hydrolysis rate significantly. Phenolic compounds from dissolved lignin, produced during alkali pre-treatment, have inhibitory effects on the anaerobic digestion; however, the possible inhibitory effects of solid lignin have not gathered enough interest. Especially, the effect of solid lignin on methanogenesis remains a knowledge gap. In this study, kraft lignin was used as a model solid lignin substrate for its co-digestion with microcrystalline cellulose. A new approach of modelling biomethane production curves using smoothing splines was developed to describe the long-term inhibitory effects of solid lignin on hydrolysis and methanogenesis. The method gives possibility to describe long-term inhibitory effects by using batch instead of continuous test data. Results revealed that kraft lignin showed mild inhibitory effects on methanogens. However lignin impact combined with volatile fatty accumulation can prolong hydrolysis and reactor recovery start-up by 47.3% and 75.3%, respectively. For small dosages of solid lignin adaptation of methanogens is possible.