Bioenergy recovery from Southern Tunisia’s organic wastes: analysis and kinetic modeling study of biomethane production

Tunisia, like many other countries in the world, suffers from a shortage of conventional energy resources. Further, it exhibits large and increasing volumes of organic wastes that cause environmental and health problems. In this study, the considered wastes consist of the organic fraction of municipal solid wastes (OFMSW), the olive mill wastewater (OMWW), and the chicken wastes (CW). These bio-wastes can therefore be valorized by a biological process such as the anaerobic digestion to satisfy on the one hand the energy demands and on the other hand, to reduce the environmental problems. Accordingly, the first aim of this work is to investigate the biomethane potential of the mentioned bio-wastes and to assess the energetic content of the obtained biomethane production. Secondly, a theoretical analysis is conducted by using the Buswell equation to predict the biomethane generation and to determine the biodegradability of the studied substrates mixture. Finally, the experimental findings are used to carry out a kinetic study, by using four kinetic models to assess their suitability to fit the present anaerobic digestion process. To do this, the biomethane potential estimation of these biomasses, under mesophilic conditions, is carried out at a laboratory scale by using the biochemical methane potential (BMP) tests. These testes are performed under an optimal substrate-to-inoculums ratio and a controlled temperature during the testing period at a laboratory scale. The experimental results show that the net yield of the biomethane from the studied bio-wastes mixture is equal to 0.384 Nm3/kg VS, which is equivalent to 130.29 Nm3 of biomethane, extracted from one ton of these bio-wastes mixture. The theoretical and the experimental results of the net produced biomethane show that the substrates mixture biodegradability is found equal to 66.89%. The studied kinetic models show good agreements with the experimental results, with a correlation coefficient R2 ≥ 95. It is found that the Gompertz model is the best one for predicting the anaerobic digestion process, due to the lowest root mean square prediction estimate (RMSPE) coefficient, as compared with the other studied kinetic models.