Biomethane Potential of Water hyacinth and its pilot scale application using a floating drum digester
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The use of biomass as a renewable energy resource has become popular in recent years because of the reduction on dependence on fossil fuels and the reduction of greenhouse gas emissions to the atmosphere. Water hyacinth (WH) is popular for causing severe environmental degradation and being an economic burden to manage. However, it offers substantial prospects if exploited, especially by rural communities. Tropical climate, eutrophic conditions and the weed’s reproductive capacity promote the proliferation of the plant in regions where it has been introduced rendering it nearly impossible to control and eradicate. Water hyacinth being a lignocellulosic biomass has great potential for biogas production though its mono digestion has been reported to give low biogas yields. This has made its codigestion with other organic waste worth exploring since it is proposed to improve the biogas yields. In this study, water hyacinth and cow manure were characterized for their physicochemical composition using parameters that were relevant to anerobic digestion. The codigestion of WH with cow manure was then evaluated at different Inoculum to Substrate Ratios (ISRs) of 4:1, 3:1, 2:1 and 1:1 with the blank as cow manure using a Biomethane Potential (BMP) setup which comprised of a temperature controlled, shaking water bath maintained at 37±2℃. The reactors for each ISR were set up in duplicates for control of errors and this setup was run for 45 days. The volume of biogas produced and the biogas quality were measured daily using the water displacement method and a digital biogas analyzer respectively. Because of its highest quality and quantity of biogas, the ISR of 4:1 was further piloted in the field using a floating drum digester of 6 cumecs capacity at ambient temperatures and its performance was evaluated and compared to the performance of the ISR 4:1 in the BMP setup. It was operated at an organic loading rate of 1.1kgVS/m3/day and with a hydraulic retention time of 60 days. It was found that cow manure and water hyacinth had Volatile Solids (VS) as a percentage of Total Solids (TS) of 84.03±0.90 and 77.79±0.90 respectively. Their C/N ratios were 15.33 and 19.71 for water hyacinth and cow manure respectively. From the laboratory controlled anaerobic biodegradability test, the control and ISRs 1:1, 2:1, 3:1 and 4:1 produced biogas of 21.3±14.0, 327.7±106.1, 440.8±4.2, 447.2±64.9, 477.4±26.6 mL/ kg Volatile Solids and 12.0, 11.2, 5.8, 12.8 and 12.9 average methane percentage respectively. In the pilot experiment, the cumulative biogas volume amounted to 100 mL/gVS with an average methane composition of 51%. The water hyacinth and cow manure demonstrated attributes such as high organic content that made them suitable for anaerobic digestion. Their codigestion had a complementary effect on the yields of biogas produced from the BMP setup. The performance of the ISR 4:1 with regard to biogas yield for the BMP setup reduced by a factor of 4 in the pilot scale experiment. From the study findings, it was drawn that further investigation on the bioavailability of essential nutrients and the fiber content needed to be done to enable a better understanding of nutrient utilization in the digestion systems. It was also suggested that the feedstocks’ Carbon content is increased through augmentation with Carbon rich compounds such as biochar and the effect explored. The consideration of constant weights in the design of floating drum digesters was also recommended in order to achieve a constant gas pressure for a continuous supply of biogas in case of connections to household appliances.