Batch Anaerobic Digestion of Banana Plant Residues for Methane Production

Abstract

Agriculture is one of Kenyans’ main economic activities and among the fruit grown is the banana. Kenya produces about one million tons of bananas yearly, of which the banana plant produces the fruits once in a lifetime and the rest of the banana plant parts are residues. The banana plant residues include leaves, stems and peduncles most of which are agricultural wastes. This study aims to establish the quantity and quality of methane gas produced from batch anaerobic digestion of banana plant residues. The leaf, stem and peduncle were the substrates used due to the ease of collecting them on the farm. The inoculum used was acclimatized sewage sludge from an anaerobic reactor in a wastewater treatment plant. Kinetic modelling was used to obtain the optimal kinetic parameters for the optimization of methane production. An anaerobic batch digestion test was conducted in triplicate batch systems, at a mesophilic temperature of (37°C) for 51 days. The biogas produced from the anaerobic digestion process was then subjected to gas chromatography to know the quality of the gas. The quality of biogas produced was of good quality and had a methane composition of 68%,65% and 69% for the stem, leaves and peduncle respectively and the methane yields were 0.125, 0.132 and 0.062 m3CH4/kg oDM for the stem, peduncle, and leaf, respectively. The digestate pH at the end of the batch digestion was checked and was found to be 7.56,7.58,7.64 and 7.84 for the stem, leaf, peduncle, and seeding sludge respectively. From the results of the batch fermentation and gas chromatography, kinetic modelling was done for the methane yields using three models that is the First Order Kinetics, Logistic and Modified Gompertz models. To determine the model that best describes the degradation of complex substrates containing lignocellulosic materials and optimize the kinetic parameters and design parameters for an anaerobic digester. The Modified Gompertz model predicted data had the best fit with the experimental data as it had high R2 values and low RMSE values for all the substrates. The values for R2 were 0.992,0.974 and 0.994 for stem, leaf and peduncle respectively and the RMSE values were 2.293,2.382 and 2.342 for the stem, leaf and peduncle respectively. The predicted yields by the Modified Gompertz model were 0.123, 0.121 and 0.057 m3 CH4 / kg oDM for the stem, peduncle and leaf, respectively. This study can be replicated on other agricultural and organic wastes to be used as feedstocks in the production of biogas and to promote anaerobic digestion technology as a source of clean and affordable energy.