Biochemical methane potential of food waste: Influence of microbial community composition

Municipal solid waste is a major environmental issue since its production has increased over the years and safe disposal must be assured to reduce the negative impact on the environment. An important proportion of municipal solid waste is food waste, which in Mexico accounts for 52.4 %. On the other hand, there is a worldwide raising interest in producing sustainable energy to meet the increasing energy demand for economic and social development. Anaerobic digestion is a complex biological process that allows food waste treatment and energy provision (i.e., biogas). In addition, other advantages include the reduction of landfill space for waste disposal and a decrease in methane emissions to the atmosphere. Food waste as a feedstock in anaerobic digestion can vary in composition accordingly to the human diet and impact biogas production. Therefore, it is important to determine the feasibility of using a specific food waste mixture. There are a variety of reports about anaerobic digestion of food waste using different waste mixtures and inoculum sources. For instance, samples from wastewater sludge are frequently used as microbial inoculum because high volumes of methane are commonly produced, around 500 mL of CH4 g-1 VS. However, there are other sources of microbial inoculua that can be used in anaerobic digestion of food waste. Particularly, in landfill sites, anaerobic degradation of waste occurs naturally. Interestingly, only leachate and not solid samples have been tested for anaerobic digestion, even though, this inoculum source is hypothesized as possessing a highly active biogas-producing microbial community since methane is produced in landfills under uncontrolled conditions. Moreover, during anaerobic digestion of food waste particular physicochemical conditions have been tested (e.g., carbon to nitrogen ratio, inoculum to substrate ratio, organic loading, among others). Specifically, the inoculum to substrate (I/S) ratio has been tested for different inoculum sources. As expected, some findings are contradictory; in addition, there are no reports for Mexican food waste. In this study, a food waste mixture modeled to the Mexican diet was used to determine its methane potential through the biochemical methane potential assay. Two inoculum sources were used: 1) sludge from an anaerobic wastewater treatment plant and, 2) a solid sample from a methane-producing landfill. Also, a mixture of them (sludge +landfill solid sample 1:1 VS basis) was tested. Different I/S ratios were tested (1 to 3) and each inoculum metagenome was determined. For sludge, maximum methane production was reached at I/S ratio 1, while for landfill solid sample the optimum I/S ratio was 2. Landfill solid sample produced more methane than commonly used sludge (311.5  11.1 and 282.5  18.1 mL CH4 g-1 VS, respectively) but presented a lag phase around 4 days. The combined inoculum at I/S ratio 1 showed a synergistic effect in methane yield (374.5  18.1 mL CH4 g-1 VS) biogas quality (58.0  0.3% CH4) and COD removal (90.0  0.6%) without a lag phase. In addition, the combined inoculum exhibited an enriched microbial population, compared to each inoculum individually. Further studies on population dynamics during digestion would help understand the synergistic effect of the combined inoculum for effective industrial application.Abstract 5 Contents 7 List of figures 9 List of tables 11 Acronyms 13 1. Introduction 14 1.1. Summary of contributions 16 1.2. Thesis outline 18 2. Theorethical Background 19 2.1. Food waste in Mexico and its characteristics 19 2.2. Anaerobic digestion process 20 2.2.1. Hydrolysis 21 2.2.2. Acidogenesis 22 2.2.3. Acetogenesis 23 2.2.4. Methanogenesis 23 2.3. Digestate 24 2.4. Microbiology of the anaerobic digestion reactions 26 2.5. Biochemical potential test 28 2.6. Important parameters in anaerobic digestion 29 2.6.1. pH value 29 2.6.2. Temperature 31 2.6.3. Organic loading 32 2.6.4. Carbon to nitrogen ratio 33 2.6.5. Inoculum source 34 2.6.6. Inoculum to substrate ratio 35 2.7. Characterization of the microbial community in anaerobic digestion by next generation sequencing 37 3. Objectives 40 3.2. Broad objective 40 3.3. Specific objectives 40 4. Experimental strategy 41 5. Materials and methods 43 5.2. Food waste model mixture 43 5.3. Microbial inocula 48 5.4. Biochemical methane potential test 49 5.5. Analytical methods 50 5.5.1. Total solids measurement 51 5.5.2. Volatile solids 52 5.5.3. Chemical oxygen demand 52 5.5.4. pH 53 5.5.5. Ultymate analysis 53 5.5.6. Biogas measurement and composition analysis 54 5.6. Microbial characterization of inocula 55 5.6.1. DNA extraction 55 5.6.2. Sequencing and bioinformatic analysis 56 6. Results and discussion 58 6.2. Characterization of food waste model mixture 58 6.3. Characterization of the inocula 60 6.4. BMP test 63 6.4.1. Biochemical methane potential test using sludge as inoculum source at different inoculum to substrate ratios 63 6.4.2. BMP test using landfill sample as inoculum source 69 6.4.3. BMP test using the mixture (sludge + landfill solid sample) as inoculum source 72 6.5. Microbial community analysis 77 6.5.1. Whole microbial community in each inoculum 78 6.5.2. Bacterial and methanogen community at high taxonomic levels 80 6.5.3. Bacterial and methanogen community at the genus level 83 7. Conclusions 96 8. Future work 100 Publications and congress 102 Congress attendance 102 Publications 102 References 103Doctor of Philosophy in Engineering Science Major in Energy and Environmental System