Evaluating anaerobic and aerobic digestion strategies for degradation of pretreated pine needle litter

Abstract

The present study evaluated the effect of different pretreatments on unlocking the lignocellulosic composition of pine needle litter biomass followed by performance under aerobic/anaerobic digestion. The pretreatment strategies employed were milling, steam explosion, and chemical treatment with acid–base–acid. The results revealed that the most effective method of pretreatment of pine needle litter was the physico-chemical treatment (milling + steam treatment + acid–base–acid hydrolysis), which resulted in 65.92% increase in holocellulose content with a corresponding 21.34% decrease in total lignin content as compared to untreated control. The effect of pretreatments on anaerobic digestion using digested sludge as inoculums was evaluated through biochemical methane generation. During the liquid-phase anaerobic digestion process of different pretreated pine needle litter, the mechanically milled pine litter yielded a methane level of 5.53 ± 0.11 mmol CH4/g VSin in 40 days, which was 40.7% higher than during anaerobic digestion of untreated pine needle litter. The physico-chemical pretreatment resulted in 21.4% higher methane levels as compared to untreated pine litter. Additionally, the aerobic digestion of pretreated pine needle litter with white rot fungal strain, Cotylidia pannosa for 60 days at 30 °C, resulted in 95.5% decrease in holocellulose content and 52.3% decrease in total lignin content. Findings in the present study suggest that in spite of significant reduction in lignin content during the pretreatment, the addition of pine litter as a substrate inhibits anaerobic digestion process.