Biogas Production from Lignocellulosic Biomass : Impact of pre-treatment, co-digestion, harvest time and inoculation

Abstract: Biogas or methane production through anaerobic digestion (AD) is gaining increasing attention worldwide due to concerns over global warming, energy security and the need for sustainable waste management. AD of lignocellulosic biomass is one facet that is highly appreciated since the conflict over biomass for food/feed or energy can be avoided. As a result the need for non-food based lignocellulosic biomass feedstock has emerged as (co-) feedstock of choice for the AD process. Despite these advantages, lignocellulosic biomasses are generally viewed as recalcitrant to hydrolysis, laden with insoluble lignin, poor in essential nutrients, and unmanageable in conventional wet/liquid AD processes i.e. may float in continuous stirred tank reactors.This thesis presents the feasibility of using lignocellulosic biomasses (Miscanthus sp. and seagrass) as feedstock for biogas production via AD. Various operations (pre-treatment, co-digestion, harvest time management and inoculation optimization) were adopted and investigated as means to accumulate both experience and knowledge for evaluation and suitable utilization of the abundant lignocellulosic biomass. Furthermore, the use of solid-state anaerobic digestion (SS-AD) was investigated by means of biogas production from lignocellulosic biomass. Specifically, a sequential aerobic (pre-treatment) and SS-AD process together with liquor supplementation was studied as means to improve performance and recovery of inhibited/failed SS-AD processes.Results of the experiment showed that cellulose degradability in different species of Miscathus decreased with longer cultivation period or age (later cut). This was inversely proportion to the methane yields at early harvest (2 months) ranged from 247 to 266 ml CH4/g VS which were noted to decrease with as much as 35% when the late harvest was carried out. It was demonstrated that when using Miscanthus as feedstock, pre-treatment, especially steam explosion, could lead to approximately 50% increase in methane yield. In addition, aerobic pre-treatment of rice straw at 35°C for 2 days was shown to be a viable method to improve hydrolysis and decomposition of lignocellulosic structure and therefore resulted in highest biochemical methane potential (BMP) (355.3 ± 18.7 ml CH4/gVS). Co-digestion of Miscathus and manure, and sea grass and manure was demonstrated to ameliorate the nutrient content, especially the carbon to nitrogen ratio, in the AD process. The co-digestion of Miscanthus or seagrass with manure demonstrated to show a synergistic effect with 11 to 34% higher methane yields as compared to values obtained from the mono-digestion.The SS-AD proved to be feasible in the treatment of rice straw as biomethane production reached over 70% of the biochemical methane potential especially after supplementation (of the inoculum) with recycled water. In fact, the addition of recycled water could improve the buffering capacity and mitigate the accumulation of toxic intermediates such as volatile fatty acids which could have led to an improved process performance and greater stability. In a related mesophilic and thermophilic SS-AD study, liquor (recycled water and sludge supernatant) was demonstrated to alleviate process inhibition or failure. The liquor supplementation minimized inhibition and boosts the SS-AD process performance as evident by the doubling of the methane yield under the mesophilic conditions. Making the lignocellulosic biomass more accessible to anaerobic consortium (appropriate harvest time, pre-treatment and co-digestion), treatment of the inoculum in the SS-AD process and the sequential aerobic and batch SS-AD process were therefore demonstrated as viable means to improve methane production from lignocellulosic biomass.