Optimized Mixing for Enhanced Biomass Conversion Using CFD Modeling of Multiphase Flow in Anaerobic Digester

Research objectives
(1) In full-scale anaerobic digesters, the hydrodynamic mechanism of specific key influencing factors (temperature, rheology, mixing mode) affecting mixing will be quantitatively identified and clearly demonstrated by utilizing CFD modeling and experimental validation.
(2) The optimized operational performance (more biogas production, better digestate biological stabilization, and lower energy consumption) will be achieved based on improving the mixing and hydrodynamics in practice.

Project outline
The anaerobic digestion is a complicated multiphase flow system containing  biogas, slurry and various solid particles. So a good mixing between phases is  crucial to achieve an effective operational performance. Nevertheless, it is difficult to reach the expected mixing performance in scaled-up digesters, resulting in lower operational performance in practice.

A good mixing performance usually depends on various factors, including temperature, mixing mode, sludge rheology, configuration, etc. So it is important to investigate the relative contribution of them, the correlations between them, and thereby address the operational optimization. However, enough information can hardly be obtained by means of only experimental studies, thus requiring additional research tools.

Computational fluid dynamics (CFD) is a numerical simulation to explore hydrodynamic characteristics in terms of mass transfer, heat transfer and reactions. Compared to physical experiments or prototype development, it can provide visualized, comprehensive information about the hydrodynamics in a more economical, effective and flexible way. During the last few decades CFD has been developed and become a promising approach to gain insight into the flow behavior in anaerobic digestion tanks (Wu 2013).

Moreover, the mixing effect on operational performance is unclear and the controversial result about biogas production still remains (Lindmark et al. 2014, Stroot et al. 2001). More research is needed to understand the coupling between mixing performance and biogas production. Therefore, this project is coupled to the STOWA project “Community of Practice - mixing sludge digesters”. In this project, the influence of retrofitting sludge digesters on the mixing performance and biogas production will be monitored closely.. A selection of these digesters allows for coverage of the different situations that will be studied, including type of mixing, reactor configuration, temperature and pretreatment. Hence, the interests are mainly in the numerical research on mixing in anaerobic digesters, to shed light on the hydrodynamic mechanism with the ultimate goal to improve mixing in practice.

(1) Object identification, to determine the specific full-scale anaerobic digester(s) for modeling. Till now a digester in Susteren is selected, which utilizes gas mixing and is going to be changed from mesophilic to thermophilic. Therefore, the temperature effect on operational performance will be investigated.
(2) CFD modeling implementation, mainly by software package ANSYS-Fluent. The sludge is assumed as non-Newtonian fluid and turbulent flow is solved by specific turbulence models. The heat transfer is also considered to demonstrate the distribution and variation of specific parameters (temperature, density, pH, etc) under thermophilic condition.
(3) Validation, including  tracer experiments and operational monitoring in full-scale digester, and additional necessary measurement and validation in lab-scale digesters.

Scientific relevance
A clear demonstration of hydrodynamics on how mixing is affected by temperature, rheology, mixing mode and configuration in digester. The development of mixing evaluation by coupling with energy consumption and biokinetic process.

Social relevance
Provide reliable prediction and suggestion on optimized operation and enhanced biomass conversion in practice, as well as the modification and/or design of full-scale digesters, leading to a more sustainable processing of residual organic streams.


  • Wu, B. (2013) Advances in the use of CFD to characterize, design and optimize bioenergy systems. Computers and Electronics in Agriculture 93(0), 195-208.
  • Lindmark, J., Thorin, E., Bel Fdhila, R. and Dahlquist, E. (2014) Effects of mixing on the result of anaerobic digestion: Review. Renewable and Sustainable Energy Reviews 40(0), 1030-1047.
  • Stroot, P.G., McMahon, K.D., Mackie, R.I. and Raskin, L. (2001) Anaerobic codigestion of municipal solid waste and biosolids under various mixing conditions—I. digester performance. Water Research 35(7), 1804-1816.