Abstract

Food waste (FW) has become a significant concern because it can cause serious environmental pollution and greenhouse gas emissions, and hence investigating advanced methods to recycle
food waste into energy and materials is of immense importance. Food waste has a high energy potential and excellent biodegradability, and high-water content. As a result, food waste can be treated by anaerobic technology that allows efficient energy recovery and reduces the environment's carbon footprint. However, in cold regions like Canada, mesophilic anaerobic treatment (30–35°C) can be energy intensive. Therefore, in this study, a novel approach has been established to treat food waste at a lower operating temperature. The objective of the study is to develop a sustainable system to treat food waste at a lower ambient
temperature since climatic conditions in Canada are more suited for psychrophilic (1-20°C) temperatures rather than mesophilic (30–35°C) anaerobic treatment. Low temperature has a negative impact on cellular processes during anaerobic treatment, making
substrates unavailable to microbes. Hence, this study introduced a novel biogas recirculation strategy to overcome the thermodynamic constraints at lower temperatures. Food waste was collected exclusively from the residential area of downtown Montreal near the
Concordia campus for three months. The collected food waste sample was tested under anaerobic treatment conditions over a time period of 30 days to determine the biomethane generation
potential of food waste at mesophilic (30–35ºC) and psychrophilic (<20ºC) temperatures. In order to investigate the feasibility of the anaerobic treatment potential of food waste under psychrophilic
temperature and evaluate the efficiency of the biogas recirculation technique for enhanced biomethane production and system stability, batch tests were carried out on food waste in different trials. The anaerobic treatment potential of food waste under mesophilic conditions was examined for comparison. The impact of different substrate-to-inoculum ratios (0.5, 0.75, 1.0, 1.5, 2.0, and 4.0) and six different total solid percentages (5%, 10%, 12%, 15%, 18%, 20%) under mesophilic (30–35ºC) and psychrophilic (1-20ºC) temperatures were investigated for evaluating the optimum conditions for anaerobic treatment of food waste. Additionally, to evaluate the effect of particle size distribution on methane yield, food waste samples were generated using two different particle size
distributions: (1) majority of particle sizes less than 2mm and (2) majority of particle sizes larger than 2mm.
Methane concentrations in the produced biogas were found to be 69%-94% under mesophilic anaerobic treatment, and methane concentration was observed between 68%-93% under
psychrophilic anaerobic treatment with methane recirculation which was close to mesophilic treatment. The COD removal efficiency was found to be between 40% and 79% at mesophilic anaerobic treatment, while the removal of COD ranged from 34% to 82% for psychrophilic anaerobic treatment with biogas recirculation. A maximum reduction of 91% in volatile solids(VS) was observed under mesophilic anaerobic treatment, whereas the highest VS reduction was found to be 92% after psychrophilic anaerobic treatment with biogas recirculation. Furthermore, a larger particle diameter has no major impact on methane production. Thus, the biogas recirculation approach eliminates the need for energy-intensive mechanical particle size reduction. In conclusion, the biogas recirculation technique at psychrophilic conditions has demonstrated improved methane generation and resulted in higher VS reduction and COD removal efficiency comparable to the results obtained from the mesophilic anaerobic treatment. It is evident that applying anaerobic digestion systems with biogas recirculation under psychrophilic conditions could have significant potential in providing economically feasible and energy-efficient options for treating food waste in cold regions. However, further study will be required to develop a comprehensive guideline in this field.