Step-by-step Simulation of Gas-lift Anaerobic Digester of Municipal Wastewater Sludge

The purpose of this study was to present a strategy for designing and simulating an anaerobic digester for the digestion of raw municipal sewage sludge, based on the flow patterns within the digester, and to investigate the sediment deposition conditions in this anaerobic digester. For this purpose, the simulations were performed based on a step-by-step strategy. First, the inlet gas velocity to the digester was determined, then in the next steps, the draft tube and the conical hanging baffle were added to the digester design, respectively, and simulations were performed with them and determined their characteristics. Simulations were performed for a pilot-scale cylindrical digester. A transparent cylindrical digester was built based on the computational fluid dynamics (CFD) simulations results. Sludge analysis was performed, and determine the sludge sedimentation velocity using the image processing process, so the sludge particle size and each percentage of them based on the total solids (TS) of sludge was performed. According to the results, only in a small area of the floor attached to the digester wall, the velocity of the fluid is less than the sedimentation velocity of the largest particles in the sludge (4.71 × 10-5 m / s) so there was a sedimentation possibility of a small amount of these particles in this area, which confirmed the success of digester design based on step-by-step simulation and flow patterns for mixing in a gas-lift anaerobic digester and showed a step-by-step simulation strategy is suitable and can be used to simulate gas-lift anaerobic digesters successfully.