Optimal operating conditions for maximum biogas production in anaerobic bioreactors

•We introduce a general transient mathematical model for anaerobic biodigesters.•The model was experimentally validated.•A simulation and optimization study was conducted with the model.•The existence of optimal hydraulic retention time was investigated.•The model could be a tool for simulation, design, control and optimization of biodigesters.

The objective of this paper is to demonstrate the existence of optimal residence time and substrate inlet mass flow rate for maximum methane production through numerical simulations performed with a general transient mathematical model of an anaerobic biodigester introduced in this study. It is herein suggested a simplified model with only the most important reaction steps which are carried out by a single type of microorganisms following Monod kinetics. The mathematical model was developed for a well mixed reactor (CSTR – Continuous Stirred-Tank Reactor), considering three main reaction steps: acidogenesis, with a μmax of 8.64 day−1 and a KS of 250 mg/L, acetogenesis, with a μmax of 2.64 day−1 and a KS of 32 mg/L, and methanogenesis, with a μmax of 1.392 day−1 and a KS of 100 mg/L. The yield coefficients were 0.1-g-dry-cells/g-pollymeric compound for acidogenesis, 0.1-g-dry-cells/g-propionic acid and 0.1-g-dry-cells/g-butyric acid for acetogenesis and 0.1 g-dry-cells/g-acetic acid for methanogenesis. The model describes both the transient and the steady-state regime for several different biodigester design and operating conditions. After model experimental validation, a parametric analysis was performed. It was found that biogas production is strongly dependent on the input polymeric substrate and fermentable monomer concentrations, but fairly independent of the input propionic, acetic and butyric acid concentrations. An optimisation study was then conducted and optimal residence time and substrate inlet mass flow rate were found for maximum methane production. The optima found were very sharp, showing a sudden drop of methane mass flow rate variation from the observed maximum to zero, within a 20% range around the optimal operating parameters, which stresses the importance of their identification, no matter how complex the actual bioreactor design may be. The model is therefore expected to be a useful tool for simulation, design, control and optimisation of anaerobic biodigesters.