CFD simulation of anaerobic digestier to investigate sludge rheology and biogas production

Nowadays, the use of anaerobic digester is a good option for renewable energy and the replacement of fossil fuels. Mixing speed in digester is one of the most important factors affecting the biogas production and operating costs of biogas plants. Thus, optimization of mixing speed is necessary to investigate sludge behavior and biogas production. In this study, simulation of mechanical mixing in anaerobic digestion of laboratory scale at mixing speed of 50, 100, and 200 rpm was performed to investigate flow patterns, digestion stability, microbiological communities, and gas production. Investigation of the velocity contours, the turbulence kinetic energy, the turbulence intensity, and the velocity gradient showed that increasing the mixing speed leads to higher levels of turbulence and in this digester, increasing the mixing speed reduces the stability of methane production process. Accordingly, it has detrimentally affecting on biogas production. According to the velocity gradient in digester, which is considered as a substitute for turbulence, it is determined by the optimal range 6–8 s−1. Increasing the mixing speed until the velocity gradient is close to this range increases the biogas production, but increasing the mixing speed above this range is detrimentally affecting to both digestibility and biogas production.