Enhanced Biogas Production from Co-digestion of Lignocellulosic Biomass with Food Waste: An Experimental Study with Coffee Husk and Water Hyacinth

Abstract

Waste-to-bioenergy conversion from lignocellulosic materials like water hyacinth (WH) and coffee husk (CH) can lead to multiple benefits such as freshwater body protection, eutrophication, and renewable energy generation besides environmental protection. Water hyacinth is causing significant damage to freshwater ecosystems disrupting ecosystem processes while coffee husks are openly burnt, thus causing loss of energy, resources, and environmental harm contributing to various diseases. However, determining how to convert it into biogas requires understanding the material's chemical components, which are mostly complicated lignocellulosic materials. The production of biogas can be restricted and its degradability is impacted by the intricate structure of lignocellulosic materials. In addition, WH and CH have lower nitrogen content and moisture levels compared to food waste. These factors can hinder microbial activity and slow down the anaerobic digestion process. However, by introducing easily degradable waste such as food waste into the co-digestion mixture, these limitations can be compensated. Moreover, food waste also helps in balancing the carbon-to-nitrogen ratio, promoting faster hydrolysis rates, and improving overall digestion efficiency. Furthermore, WH and CH were ground using a coffee grinder to decrease cellulose crystallinity and reduce particle size in order to enhance the substrate's surface area and improve enzymatic accessibility, thereby facilitating the efficient conversion of these materials into biogas. Thus, this study examined the impacts of substrate mix proportions, inoculum-substrate ratios (ISR), and initial pH levels on biodegradability and biogas production for lignocellulosic materials digested with Food Waste (FW) under mono-and co-digestion modes. To assess the potential of organic materials as biogas resources, an investigation was conducted to characterize food waste, water hyacinth, and coffee husk in terms of their volatile matter, moisture content, ash content, fiber content, carbon content, sulfur content, and nitrogen content, and then three sets of batch experiments were performed under mesophilic conditions. The first group included eight digesters that treated the FW/WH at varying mixing ratios (i.e., 100:0, 50:50, 40:60, 0:100) and varying ISR (0.5, 0.75, 1, and 2) at FW/WH of 50:50. Second, the effect of FW/CH ratios (100:0, 60:40, 50:50, 40:60, 0:100) and initial pH levels (5, 6, 6.5, and 7.5) with a FW/CH ratio of 60:40 were evaluated to get the initial optimal conditions that allows the maximum biogas production. The third set involved seven digesters that tested the CH/WH/FW at different blend proportions (i.e., 100:0:0, 0:100:0, 35:35:30, 30:30:40, 25:25:50, 20:20:60 and 0:0:100). The modified logistic, modified Gompertz, and first-order kinetic models were used to simulate the experimental results to portray the kinetics of the co-digestion. The volatile matter content in food waste, water hyacinth, and coffee husk was found to be 90.25, 87.24, and 89.82% respectively. The concentration for cellulose, hemicellulose, and lignin in WH were 41.39, 19.33, and 8.31%, respectively compared to 24.88, 28.96, and 23.16% for CH. The characteristics of each material indicated their potential for bioenergy recovery through anaerobic digestion. In addition, the carbohydrate component of FW was higher than that of CH and WH, indicating the higher biodegradability of FW, which improves the VFA quantities in the biodigester and facilitates the microbial activities to degrade lignocellulose. In this study, the C/N ratio of WH is about 25.5, which is within the optimum range for the methanation process. The C/N ratio of CH was 34.46, which is comparatively high and indicates some nitrogen deficiency while that for FW was 19.9, which signifies some carbon deficiency. The results specified the potential for nutrients complementing in co-digestion of lignocellulosic materials with food waste, for better biogas production. For the first test, maximum biogas production of 495.45 ml/gVS was observed at WH/FW of 40:60% with a maximum biodegradability (BDfpc) of 89.3%. The modified Gompertz equation fits accurately with experimental data. For the second set, peak biogas yield was 540.78 xv ml/gVS with synergy of 1.46, as well as the highest BDfpc and organic biodegradation degree (ηBD) of 85.64 and 56.80%, respectively, for FW/CH ratio of 60:40 and pH 7, representing 164% increase over mono digestion of CH. The theoretical biogas production potential of the CH, WH, and FW were 438.74 ml/gVS, 608.46 ml/gVS, and 759.92 ml/gVS, respectively. The findings showed that without pretreatment, the mono-digestion of CH is not viable for biogas production due to its high lignin content. Finally, for the third set, the maximum biogas yield of 572.60 ml/gVS, the highest BDfpc of 89.22%, and ηBD of 57.82% was obtained at a blend ratio of 25:25:50, leading to 179.71% increment compared to CH mono-digestion. The positive synergy reached in this experiment may be due to multiple-factor couplings, including increased buffering capacity and improved nutrient balance. Three substrates co-digestion comparatively enhanced biogas yield and its quality over two substrates co-digestion. For the second and third sets, a modified logistic model outperformed others with the best fit, and high correlation suggesting that it might well describe the kinetics of co-digestion at different initial conditions. This study provided evidence that the biodegradability and subsequent biogas yield can be positively influenced by the co-digestion of multiple substrates and the introduction of inoculum. In order to enhance the biodegradation rate and biogas production from lignocellulosic materials, the current study findings highlight the significance of increasing easily biodegradable waste fractions in the co-digestion and inoculum-substrate ratios under controlled initial conditions. Pretreating these substrates with alkali-assisted ultrasonication, can significantly improve the accessibility of cellulose and hemicellulose, remove lignin barriers, and reduce glucose crystallinity, eventually improving the overall performance of bioconversion processes. Thus, it is recommended that future studies apply this pretreatment technology to improve biogas production. Additionally, further research is necessary to examine the chemical oxygen demand and biochemical oxygen demand of substrates, as well as the microbial community, and understand the intermediate reactions occurring during co-digestion.