Feasibility study of an Institutional biogas plant for central sewage system at KNUST

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Biogas generation is one of the most promising renewable energy sources in Ghana. Apart from its energy source, anaerobic digestion is a reliable method for waste treatment and the digested sludge can be used to enhance the fertility of the soil. This thesis looks at the possibility of constructing a biogas plant at the KNUST sewage treatment plant tapping its feedstock from the Primary Sedimentation Tank. A laboratory experiment was done to determine the faecal sludge quality at the Primary Sedimentation Tank. The flowrate of the sludge was estimated based on the number of times the penstocks (valves) are operated to desludge the sewage which depends on whether the university is on vacation (35.72m"Vday) or in session (71.44m3/day). These parameters were used to determine the biogas potential of the sewage using 10, 20 and 30 days retention time for plant sizes of 800m3, 1600m3 and 2400m3 respectively. The Puxin biogas digester was selected for this design. It was estimated that 152344, 304689 and 358009 m3 of biogas can be produced in a year and the power production was estimated to be 40, 80 and 100 kW for 800m3, 1600m3 and 2400m3 plant sizes respectively. The annual greenhouse gas emission reductions were estimated at 1373, 2751 and 3234 tCC^-e for the 800m3, 1600m3 and 2400m3 plant sizes respectively. A financial analysis was then conducted using UNIDO's COMFAR software to ascertain which one of the three designs was financially viable having environmental impact in mind the possible sale of the digestate as fertilizer from the biogas plant. The estimated total investment cost for the three designs were GH£430,397.40, GH0799,288.90 and GH01,093,262.78 for the plant sizes of 800m3, 1600m3 and 2400m3 respectively. The financial results showed that the NPV for all the plants were negative which implied that none of the project was financially viable.  
A thesis submitted to the Department of Mechanical Engineering, Kwame Nkrumah University of Science and Technology in partial fulfilment of the requirements for the degree of Master of Science in Mechanical Engineering.