Design of a co-digestion biogas plant to curb deforestation: Case study of Phalombe boarding secondary school in Malawi

Abstract

Biogas is one of the renewable sources that are used for cooking, lighting and heating our homes. The raw materials used to produce biogas are human waste, crop residues, food waste and many more other wastes which are biodegradable. Through anaerobic process, biodigesters are used to convert these wastes into biogas which is rich in methane. A co-digestion biogas plant that uses human, animal, agriculture and canteen food waste has been designed to produce biogas that can be used for heating, cooking and lighting at Phalombe Boarding Secondary School in Malawi. This thesis is about the design of a co-digestion biogas plant that will be constructed at the school which is at present solely dependent on firewood as a source of energy for cooking and heating. Various studies have shown that co-digestion of human waste with agricultural or canteen food wastes improves biogas production efficiency, C/N ratio and stability of the digestion process. Phalombe Boarding Secondary school has a population of 562 students and 195 members of staff including their dependants. It uses approximately 52 tons of firewood in a school term; 156 tons in one academic year of three terms. One ton costs US$19.00. Therefore, 156 tons cost the school approximately US$3,000.00 in one academic year. The school also spends US$344.00 per month on electricity for lighting, translating into US$1,032.00 in one academic term of approximately 4 months. The amount of money spent on electricity in an academic year of three terms is approximately US$3,096.00. Thus in total, the school spends approximately US$6,096.00 on firewood and electricity for cooking and lighting. A field survey at the school was carried out to appreciate the problem the school is facing so that a solution could be found. It was concluded that construction of a biogas plant at the school was the best option to save the money the school is currently spending on lighting and cooking and also to curb deforestation which is rampant since the school and surrounding communities get their firewood from the natural forests surrounding them. Biogas is a cheap, clean, green and environmental friendly source of energy to replace firewood. Common biodigesters in use worldwide have been discussed in this thesis. Based on factors such as energy demand at the school, availability of feedstock, size of the digester, biogas yield, life span of the biodigester and availability of construction materials, the type of biogas plant suitable for this purpose has been selected and designed. These design parameters were arrived at through a baseline survey and literature reviews. Through a questionnaire, a detailed energy demand analysis was carried out from whose results a fixed dome biogas plant of digester size 62 m3, gazometer of size 19 m3 and digestate collection tank size of 61 m3 has been designed. A cost estimate of the design has been carried out to appreciate the economic viability of the biogas technology and is estimated at US$5,277.00 An OBA simulation of gas production is run using the calculated/estimated substrate and the results show that with this design, methane gas production in access of 60% of the total biogas produced is achievable. The cost of constructing a biogas plant at the school being less than what the school is spending currently on firewood and electricity, a recommendation has been made to adopt the technology in order to reduce the financial burden the school is facing.