Novel Simulation of Anaerobic Digestion Using Computational Fluid Dynamics

Abstract

In an effort to optimize the economy and performance of covered anaerobic reactor systems, a comprehensive dynamic and mechanistic model was created to simulate the constituent processes of full-scale anaerobic digestion. These processes included the following: bulk fluid motion, sedimentation, bubble mixing, bubble entrainment, buoyant mixing, advection, biological reactions, internal heat transfer, and heat exchange with the environment. This model contrasted with conventional models that assumed uniform concentrations and temperature throughout the reacting medium. Novel numerical simulation techniques were developed to simulate the heat and mass transfer resulting from two phase gas-liquid flow and unsteady buoyancy driven flow. The complete model was implemented in a computer code called LagoonSim3D.

Three years of performance data from a full-scale covered anaerobic digestion system in central North Carolina were used to quantify unknown parameters as well as validate the LagoonSim3D software. The LagoonSim3D software predicted the temperature of the covered lagoon within 5.7% and the dynamic monthly gas production within 11%. The external convective heat transfer coefficient was found to be a linear function of wind speed. The convective heat transfer coefficient of the gas gap between the cover and the slurry was found to be 10 W/m²K. The average particle settling velocity was found to be 0.02 cm/s. These previously unknown parameters were important for the design of future anaerobic digestion systems.

The validated LagoonSim3D model was used to determine the effect of design changes on reactor performance. In part, it was found that the case study system had at least twice the optimal volume, and a nearly optimal depth. It was also found that the performance of the case study system could be improved by cutting the flush water volume in half. It was concluded that the LagoonSim3D software enabled a flexible and general evaluation of covered anaerobic lagoon designs that was not possible with previously available steady state and complete-mix models.