Batch anaerobic digestion of banana waste - energy potential and modelling of methane production kinetics

Abstract

Different fractions of banana (stalk, peel, and flesh) as well as the whole unpeeled banana were studied in a laboratory Biochemical Methane Potential (BMP) assay. After completion of 35-day digestion at 37°C in 2L-reactors, specific methane yields reached 0.256, 0.322, 0.367 and 0.349 m³/kg VS (volatile solids) for stalk, peel, flesh, and unpeeled banana respectively. Considering the country of Uganda, East Africa, the collection of peels and stalks from banana production would yield a theoretical potential of about 60 GWh of electrical energy per year in biogas plants. In order to verify the suitability of banana fractions to the biogas process, their chemical composition was analyzed, and their methane production kinetics was estimated with exponential and logistic models. Banana peel was found to be easily degradable, and well suited for biogas production. Banana flesh had the fastest degradation rate of all banana fractions, and banana stalk had the slowest degradation rate, respectively. Methane production kinetics was fitted with first order and logistic models. The kinetics of methane production from banana flesh correlated well with a logistic model, but did not with exponential models. Alternately, methane production kinetics from banana stalk correlated well with exponential models, but did not with the logistic model. Methane production kinetics from banana peel did not correlate well with any model. Hence, the biochemistry of anaerobic processes may follow different patterns depending on substrate degradability, explaining the difficulty of finding a universal explanatory model of methane production kinetics in batch mode.