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Abstract Nowadays, the world is looking for renewable energy sources that are both cost-effective and environmentally friendly. The purified biogas is an important source of renewable energy that can be used in place of fossil fuels. Anaerobic digestion is a biochemical method of producing biogas that the organic compounds can be converted into a renewable energy source. Anaerobic co-digestion, or AcoD, is regarded as a practical method for resolving substrate property and system optimization issues in single-substrate digestion processes. Improving the mesophilic anaerobic co-digestion process in the high solid state to improve the biodegradation process and improve the production of biogas is the main aim of the research work of this thesis. Anaerobic digestion is difficult for sewage sludge. The contents of the cells are biodegradable, but they are protected by tough cell walls. Biomass also retains water, making dewatering difficult. So, the conventional mesophilic anaerobic digestion process degrades VS of sludge by about 40% at 30 and 40 days. Food waste is one of the best available feedstocks for AcoD all over the world. It consists of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and Sulphur (S) which indicates an important quantity of carbon is available in FW, which makes it highly biodegradable. Generally, FW prevails with a high C/N ratio (11.1–36.4), while the SS revealed a low C/N ratio (6–9), which can improve in the range of 20–30 by proper mixing ratios. In this research, the biological methane potential test is a preferred technique for measuring the biodegradability and decomposition rate of organic substances. Sixteen digesters models had been constructed to investigate the effect of thermal pre-treatment on the physical characteristics, the performance of the biological degradation process, and the gas production of the anaerobic co-digestion process in the high solid state. An electric oven is used for pretreatment of the well-mixed FW and SS individually at different temperatures of 100°C, 120°C, 140°C, 160°C, and 180°C. The food waste (FW) to sewage sludge (SS) ratio used in this research is 1:2 (VS-based) to form a final total solids concentration of 11.20%. The inoculum to substrate ratio was set at 1:1 (Volume-based). The biomethane potential test reactors had been duplicated. The food waste and sewage sludge were covered before putting it in the pretreatment oven to prevent the evaporation of the water from the samples. After pre-treatment the samples were cooled to the room temperature before mixing in order not to affect the methanogenic microorganisms in the inoculum sludge inversely. The results show that thermal pre-treatment has changed the physical characteristics of the food waste and sewage sludge mixture. As the pre-treatment temperature increased, the viscosity of the food waste and sewage sludge reduced so it was improved noticeably, which led to the improvement of the mixing process. One of the main challenges facing high solid AcoD of the waste and sludge - in this research the total solids was 11.2% - is the mixing process but the thermal pre-treatment might help to improve the mixing process because TP reduce the viscosity and this lead to use low energy and not specialized mixers. After the biomethane potential test, the final pH values were ranging between 7.23 and 7.72 which was appropriate for anaerobic co-digestion because methanogenic bacteria are incredibly affected by pH variations and require a pH of around 7.0 for optimal growth. The minimum achieved volatile solids removal ratio is 44.77% in R1(120) and the maximum is 64.58% in R3(180). These values are within the optimal values achieved by most of the studies that reported the optimal temperatures for thermal pre-treatment range from 160 to 180°C, and that treatment durations range from 30 to 60 minutes. The achieved TS removal Ratio ranges between 15.56 to 49.29% and it is higher if compared with the untreated reactor. As the pre-treatment temperature increased, the removal ratio increased. COD removal ratio of the blank digester was 39.54% and it was lower than the removal ratio of all other digesters. The achieved removal ratio ranged between 42.59 and 53.33% in all reactors. the removal ratio increases along with pre-treatment temperature for R3(T) but it increases up to 160 and 140 °C, and then decreases for R1(T) and R2(T), respectively. The best reactor in COD removal is R1(160). The thermal pre-treatment has an effect on the TOC removal. The TOC removal ratio increased as the TP increased till the treatment temperature of 140℃ and the still approximately the same as the temperature of TP increased till 180℃. The best case in removing the TOC was R2(140). Finally, the results show that the pre-treatment increased the biogas production from 4385 ml for the untreated reactor (RB) to 5685 for the reactor R2(140) at 140°C and the improvement in biogas production reaches 29.65% in the reactor R2(140) and the removal of volatile solids was 58.90%. Therefore, after the biomethane potential test, the temperature of 140°C was found to be optimal to produce biogas. The optimal condition is to use a mixture of pre-treated SS at the temperature of 140°C and untreated FW, so TP is recommended to be used in anaerobic digestion of the mixture of food waste and sewage sludge. |