Optimization of Biomethane Production from Vegetable Waste Collected in Ouagadougou Markets and “Yaars” Using the Response Surface Method (RSM)

Large quantity of vegetable waste is produced in Ouagadougou’ markets and “yaars”, causing damage to the environment and, consequently the risks of to some diseases. Reusing methane in the production system could constitute one of the best options for the management of this waste. The objective of this study is to contribute to the energy recovery of vegetable wastes produced in Burkina Faso cities. Thus, a sampling of vegetable wastes was carried out at vegetable sales points in Ouagadougou. The physicochemical characterization of vegetable waste samples was investigated using standard methods. The response surface method through an experimental design implemented by the Expert Design software was used to determine the optimal production conditions of biogas in codigestion with cattle dung. A pilot scale production was carried out in a digester of 5 liters based on the optimal parameters obtained by the response surface method. The biogas was estimated through the volume of the torus and its composition determined by a biogas analyzer. The physicochemical parameters showed that the vegetable wastes contained 84.84% of dry matter (DM), 88.28% of volatile dry matter (VDM), 11.70% of ash, 1.5% of total nitrogen (TN) and 50.73% carbon content. The carbon-to-nitrogen (C/N) ratio was 33.82. These data show that vegetable wastes are potential substrates for anaerobic digestion however, they can be co-digested with animal manures to balance the low nitrogen content. The pilot production tests in the laboratory, based on the optimized model, produced an average volume of biogas equal to 30525.326 cm3 with 57.61% as the proportion of methane. The production yield was 3540 L CH4/kg VDM. These data obtained show that the codigestion of cattle dung with vegetable waste would have an increasing effect on biogas production. Also, the experimental production yield, higher than theoretical yield generated by the optimization equation, allows us to admit that this study has given satisfactory results.