Anaerobic digestion of lignocellulosic biomass and potential use of low-cost biomass residues for the AD process optimisation

Abstract

This research investigated anaerobic digestion (AD) with various types of lignocellulosic biomass feedstocks from Nigeria, and the potential use of low-cost ash-extracts produced from agricultural biomass, such as empty palm bunch, cocoa pod, banana peel, for the enhancement of AD biogas production. Experiments investigated co-digestion mixtures, comprising several tropical grass silages and cassava processing wastes in CSTR under mesophilic and thermophilic temperatures, at an organic loading rate of 2.0 gVS.L-1.d-1 and 20 d HRT. Under each temperature regime, two pairs of 1 L CSTR were used – one pair as control, and the other pair supplemented with biomass ash-extracts. The experiment was run for three HRT, all reactors achieving steady-state operation during the third HRT cycle. At steady-state, the specific methane production (SMP) recorded in the thermophilic reactors, was 299.9 and 338.2 N mL CH4.g-1VS added. d-1 for the control and the supplemented pairs of reactors, respectively, indicating that methane production had increased by 13% due to ash-extract supplementation. Similarly, for the mesophilic reactors, the SMP was 297.9 and 330 N mL CH4.g-1VS added. d-1 for the control and ash-supplemented reactors respectively, signifying that ash-extracts improved mesophilic CH4 production by 11%. Statistical comparison of CH4 production in control and ash-supplemented reactors showed p-values ≤ 0.05 for both temperatures, which confirms that ash-extracts improved biomethane production significantly. Supplementing different feedstocks, such as gamba grass, guinea grass, elephant grass, rice straw and cassava processing wastes, with ash-extracts in both BMP batch assays and CSTR, also gave enhanced methane production. From the chemical analysis data and literature, it was possible to conclude that biomass ash-extracts appear to provide both alkalinity and/or trace metals for the enhancement of methane production when digesting agricultural wastes as both mono-substrates and co-digestion mixtures. The low cost of biomass ash-extracts compared to commercially available chemical additives, i.e. alkali and trace elements, make them economically feasible AD supplements for improving methane production from a wide range of grass biomass and other agricultural wastes in developed and developing countries.