CFD modeling of homogenization in a anaerobic digester

Anaerobic digestion is the practice of hamessing consortia of micro-organisms in an oxygen-free environment to breakdown biodegradable materials and produce biogas, a mixture of mainly carbondioxide and methane. This practice is used as a waste-treatment and applied to various industrial streams like slaughter waste and sewage water, and also for agricultural streams like roadside grass and manure.

Mixing is acknowledged to play an important role in the performance of anaerobic digesters. Too high shear rates lead to damage of so-called anaerobic granules, structures in which various species of bacteria live in close proximity. Too little mixing leads to the formation of dead zones, in which bacteria die because toxic waste products are not discharged and new feed is not supplied.

During this study the flow and mixing in anaerobic digester is studied by performing CFD analyses in Ansys Fluent. For this reason steady-state computations for flow field analysis supplemented by transient computations for species transport are carried out. Rotating parts were included using the Muliple Reference Frames (MRF) approach.

The performance of turbulence models for the purpose of mixing in non-Newtonian liquids is examined through simulating various validation-cases. Although good agreement with literature data was obtained, the applicability of the MRF method to full-scale anaerobic is questionable, due to difficulties in obtaining stable steady-state solutions. Large scale differences within the geometry were identified as a reason for lack of convergence and stability.

As an alternative for the MRF method, a momentum source model is proposed and developed. The applicability of representing rotors using a source term for the momentum equations instead of application of MRF was studied and successfully implemented.