Intermediate ozonation to enhance biogas production in batch and continuous systems using animal dung and agricultural waste

Abstract

Agricultural waste and animal manure (dung) pose an environmental threat in developing countries. This investigation focused on the possible use of such waste as an energy source in the form of biogas produced via anaerobic digestion (AD). The impact of single and mixed substrates on methane production under controlled batch and continuous experimental setups was considered. The study was extended to investigate the effect of substrate size and the impact of an intermediate ozonation process on enhancing the production of biogas from single and mixed substrates. Cumulative methane production (CMP), ultimate methane yield (UMY), methane production potential (MPP), methane production rate (MPR), and maximum methane production rate (MPRmax) were used as performance indicators of the effectiveness of the anaerobic digestion process. CMP and MPP from mixed substrates were found to be higher than values obtained from a single substrate feeds, which may be attributed to a more balanced nutrient and organic matter found in mixed substrates. The large surface area of fine substrate influenced MPR and MPRmax values in the first 30 days of digestion. In later AD stages, the effect of substrate size was negligible. The MPRmax for fine substrates was 12.3 ± 0.3 LkgVS−1 compared to 8.8 ± 0.2 LkgVS−1 obtained for coarse substrate. Continuous AD with organic loading rate (OLR) of 4 kgVSm−1d−1 showed a %ADefficiency of 62%, an average specific methane production in the range of 98–230 LkgVS−1 and a volumetric methane production rate in the range of 1.94–2.35 m3 m−3d−1. Increasing the OLR increased the accumulation of volatile fatty acids in the system and resulted in decreased methane production. Two-stage AD with an intermediate ozonation process showed a significant increase in CMP and %ADefficiency compared with single-stage AD. The %ADefficiency for two-stage AD ranged from 63% to 83%, and for single-stage AD, it was in the range of 42.2%–64.3%. Anaerobic digestion of mixed agricultural waste improved the filtration, dewaterability, and settling ability of the final substrate, making it suitable for use as a soil fertilizer.