Optimization of biogas production from anaerobic co-digestion of fish waste and water hyacinth

Abstract

Fish waste (FW) and water hyacinth (WH) are biodegradable wastes that remain underutilized and unexploited and cause a problem to the environment since the existing disposal techniques result in environmental pollution and health risks. Anaerobic Co-digestion of FW and WH can be used to improve biogas generation as a source of energy to replace fossil fuel consumption. The conversion of wastes to energy can provide an answer to environmental pollution, waste treatment and management, and rising energy costs. The main objective of this study was the optimization of biogas production from anaerobic co-digestion of FW and WH. The specific objectives were to characterize the substrate, evaluate operating conditions to maximize the biogas production, and determine the biogas yield model equation. The WH was collected from Lake Victoria and FW from fish point Eldoret, Kenya, and the inoculum was collected from the Moi University biogas plant. Laboratory scale experiments were carried out in Moi University laboratories (Chemical and Process Engineering Laboratory and Chemistry Laboratory) under mesophilic temperature (37 o C). The physiochemical characteristics (Total solids, Moisture content, Volatile solids, and Carbon to nitrogen ratio) of the substrate were tested using standard methods. Design- Expert 13 was used for optimization and results analysis. RSM (Response surface methodology) was used to examine the effects of operating parameters and identify optimum values for biogas yield. Experimental variable levels for biogas were substrate ratio (WH: FW, 25-75g), inoculum concentration (IC, 5-15g), and dilution (85-95mL). The total weight of the substrate was 100 g. The total volume of the biodigesters was made between 190-210 mL. The quantity of biogas produced was measured by the water displacement method on daily basis (20 days). The initial analysis of FW was 61.78, 99.48, and 38.21% for MC, VS, and TS respectively while for WH was 94.4, 83.3, and 5.6 % respectively. The C/N ratio of FW (5.89) was out of range for the accepted C/N ratio (20-30:1). However, the C/N ratio of WH (21.35) and inoculum (23.47) was in a suitable range. Optimum values for maximum biogas yield of 690mL with the highest methane yield of 68.15% were found to be WH: FW ratio, 25:75g, 15g of IC, and 95 mL for dilution. The yield was 16.1% and 32.4% greater than FW and WH mono-digestion, respectively. The biogas yield was expressed as function of operating variables.The model was significant (P<0.05). All factors had significant linear and quadratic effects on biogas while only the interaction effects of the two factors were significant. The coefficient of determination(R 2 ) of 99.9% confirms the good fit of the model with experimental variables. In conclusion, FW and WH were potential feedstock for biogas production. AnCo-digestion of FW and WH feedstock has been shown to enhance biomethane yield. Optimum values for RSM were within the range of experimental results. Biogas yield decreased as substrate ratio increased. FW had a lower C/N ratio, further study needs to consider co-digestion with other higher C/N ratio substrates.