Abstract
<div>Municipalities are facing increasing challenges regarding management and disposal of solid waste. Anaerobic digestion (AD) of municipal biowaste enables waste reduction and biogas production that can be utilized as a renewable source of energy for heat and power generation. Anaerobic co-digestion (AnCoD) enhances the performance of conventional mono-digestion. The mixing ratio of the feedstocks is an important criterion in AnCoD design which is typically determined based on the optimum carbon to nitrogen (C:N) ratio within the range of 25-30 or COD:N ratio in the range of 50-140. However, literature has shown contradictory results for the optimum C:N and COD:N ratios.</div><div>Therefore, the main objective of this study was to primarily investigate the influence of the mixing ratio of the feedstocks including thickened waste activated sludge (TWAS), manure and source separated organics (SSO) on improving biomethane production and introducing a new methodology for optimizing the mixing ratio in AnCoD based on the lipids, proteins, and carbohydrates contents as the three main compounds existing in biowaste. The hydrolysis/acidification performance in AnCoD of manure, TWAS and SSO individually and in different combinations was also investigated. This study has introduced an empirical model to explain the relationship between the biomethane production and lipids: proteins: carbohydrates ratio of the feedstocks in anaerobic co-digestion of TWAS, manure and SSO. Among the binary ad ternary combinations, the ternary co-digestion of TWAS/manure/ SSO at the mixing ratio of 2:4:4 and lipids: proteins: carbohydrates ratio of 1:3:12 resulted in the maximum ultimate methane production. The maximum methane yield of 363 ml CH4/g COD added corresponded to co-digestion of manure/SSO at the mixing ratio of 7:3. The maximum hydrolysis rate corresponded to the co-digestion of TWAS/manure at the ratio of 9:1. Overall, the best performance in both hydrolysis and methanogenesis was achieved by the co-digestion of TWAS with SSO at the ratio of 3:7 as well as TWAS/manure/SSO at the ratio of 2:4:4 compared to other feedstock mixes. It was observed that the proposed second order polynomial model could describe the relationship between biomethane production and lipids, proteins, and carbohydrates content of the feedstock.</div>
Highlights
Waste materials including biowaste are constantly being generated due to inevitable human activities
The total lipids and total carbohydrates contents of Source Separated Organics (SSO) are significantly more than that of thickened waste activated sludge (TWAS) and by increasing the proportion of SSO in the co-digesters the concentrations of lipids and carbohydrates increased. pH was kept at neutral level from 7.0- 7.3 in all the mixtures to satisfy the favorable condition for methanogenesis
The values are the average of each parameter that was measured in triplicate
Summary
Waste materials including biowaste are constantly being generated due to inevitable human activities. Different techniques have been employed to manage and reduce the growing amount of biowaste Such technologies result in secondary environmental impact. Biological processes for the treatment and/or conversion of OFMSW to value-added products has aroused significant attention due to its financial benefits and less environmental impacts compared to the other waste disposal methods such as landfilling, incineration, gasification, etc. A considerable amount of biogas through anaerobic digestion process can be produced by food wastes from all of these sources (Government of Canada, 2013; Kelleher Robins, 2013). Biosolids are the nutrient-rich, organic materials which makes them a useful resource, containing necessary plant nutrients, and organic matter. They can be recycled as a fertilizer and soil improvement for agricultural utilization. The City’s biosoilds management can be classified as the following options (City of Toronto, 2019): Land application for agricultural and other purposes Pelletization for fertilizer production Alkaline stabilization for producing fertilizer, landfill cover, or for the pH adjustment of acidic soil Landfilling Incineration
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