Modeling of the biomethane production from ultrasonic pretreated fruit and vegetable waste via anaerobic digestion

Abstract

The global dependency on the depleted fossil fuels has led to the quest for acquiring alternative energy sources. Different types of waste material are generated at a high rate and tapping into their use for greener, alternative energy production is an option. The mesophilic anaerobic co-digestion of fruit and vegetable waste and wastewater treatment plant sewage sludge experiments were conducted using ultrasonic pretreated substrates. Sonication exposure times from 0 to 45 min were selected for the experiments. An automatic methane potential test system (BMP) was used to determine the production rate of biomethane of the fruit and vegetables waste containing 60% fruit and 40% vegetables. The highest cumulative methane production of 238 mL g−1 VS was achieved at sonication time exposure of 45 min. It was observed that an increase in ultrasonic sonication exposure time, improved methane yield. The resulting experimental data was fitted with the modified Gompertz, co-digestion modified Gompertz, original Richards, modified Richards and co-digestion modified Richards models. IBM SPSS Statistics software was used for curve fitting and the estimation of the models’ kinetic parameters. The modified Gompertz and Richards models showed higher goodness fit, both with R 2 of 0.93 and modified Richards models did not produce a good fit for the data, with R 2 of 0.7. The developed co-digestion models considered a combination of substrates that were easily digested as well as complex substrates that required multiple steps of digestion. The results show that the co-digestion modified Gompertz model had a goodness of fit of 0.98. Co-digestion modified Richard’s model perfectly fit the experimental data, with R 2 of 1. Both the co-digestion modified models are recommended due to their fitting performance. Fruit and vegetable waste comprise multiple substrates including simple sugars that digest readily and much more complex cellulose substrates that require more steps to digest and requiring the second step of digestion after undergoing hydrolysis. Both models took that into account. The aim of this study was to evaluate the suitability of the Gompertz and Richards model in the co-digestion of fruit and vegetables waste with sludge, as well as to develop co-digestion models for the substrates at hand.