Methane liquid-gas phase distribution during anaerobic sludge digestion: A thermodynamic approach

Anaerobic digestion is certainly one of the options that can help solve the dilemma of energy demand, waste management and climate crisis mitigation mainly. Under ideal conditions, it is expected that all biomethane will be transferred from the liquid to the gaseous phase, ensuring maximum recovery. However, for concentrated wastewater or complex organic waste blends composed of functional groups with different sizes, the molecular interactions become important since the system is not only biphasic. Since the formation and transfer of a gas is related to the equilibrium condition, a thermodynamic approach could help to estimate the degree of variation of biomethane between the liquid and gaseous phases. Therefore, this investigation aimed to obtain the distribution between liquid and gaseous phases of the biomethane produced in the sewage sludge anaerobic digestion considering the substrate as a non-ideal solution. The nonlinear differential equations of the ADM1 were integrated with Aspen Plus® to verify the equilibrium conditions and the model was calibrated with data obtained through experiments conducted in a lab scale sequence batch reactor (SBR) fed with synthetic substrate (1500 mgCOD.L^-1) and inoculated with flocculent sludge (500 mgSSV.L^-1) from a full-scale UASB reactor. Considering the thermodynamic approach, the percentage of dissolved biomethane remains below the 2.97% percentage estimated by using the ADM1. It is possible to state that the principle of ideality is maintained in the system. On the other hand, CO₂ phase distribution was considerably affected by the species defined in the equilibrium reactions. Therefore, it can be stated that Henry's Law simplification in ADM1 model is valid to represent the phenomenon investigated.