Design of a Co-Digestion Biogas Plant to Curb Deforestation-Case Study of Phalombe Secondary School in Malawi

Biogas technology is one of the renewable technologies that use biodegradable waste such as human waste (HW), agricultural waste, animal and food waste. Over 90% of the population in Malawi is heavily reliant on firewood as their primary source of energy for cooking. This results in deforestation, pollution of the environment, and great monetary expenditure to buy firewood, more especially by boarding schools. A co-digestion biogas plant that uses human, animal, agriculture, and canteen food waste has been designed. This study design was based on the use of HW and canteen food wastes (CFW) as the substrate for the biodigester to produce methane (CH4) gas that could be used for cooking and lighting at Phalombe Secondary School in Malawi to replace firewood. With a school population of 757 people, design calculations/stimations were performed to find out the amount of HW and CFW required per day. A field survey at the school was carried out to appreciate the problem the school is facing so that a solution could be found. 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. A computer-aided design (Auto CAD) software was used for the drawing. These design parameters were arrived at through a baseline survey, observation methods, 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, gasometer of size 19 m3, and digestate collection tank size of 61 m3 has been designed. The design came up with an amount of HW and CFW of 286 and 60 kg per day respectively making total organic raw materials of 346 kg per day. The macromolecular composition of the HW, CFW, and mixture of HW and CFW in terms of dry matter (DM) was 11%, 45% and 56% of carbohydrate, 3%, 15%, and 18% of protein, 15%, 40%, and 30% lipids, and 15%, 0%, and 15% of ash respectively. The substrate showed a high degradability of 90%. The simulation analysis showed that HW produced 185 m3 per kg of biogas which represented 64% and 35.9% CH4 and carbon dioxide (CO2), CFW produced 58.9 m3 per kg that represented 61.1% and 38.4% of CH4 ad CO2, and mixture produced 265 m3 per kg contained 59% and 41% of CH4 and CO2 in 40 days respectively. A cost estimate of the design has been carried out to appreciate the economic viability of the biogas technology and is estimated at the US$5277. The cost of constructing a biogas plant at the school is less than what the school is spending currently on firewood and electricity, a recommendation has been made to adopt the technology to reduce the financial burden the school is facing.