Abstract
In this paper, a finite volume based computational fluid dynamics (CFD) model has been developed for investigating the mixing of non-Newtonian flows and operating conditions of an anaerobic digester. A CFD model using the multiple reference frame has been implemented in order to model the mixing in an anaerobic digester. Two different agitator designs have been implemented: a design currently used in a full-scale anaerobic mixing device, SCABA, and an alternative helical ribbon design. Lab-scale experiments have been conducted with these two mixing device designs using a water-glycerol mixture to replicate a slurry with total solids concentration of 7.5%, which have been used to validate the CFD model. The CFD model has then been scaled up in order to replicate a full-scale anaerobic digester under real operating parameters that is mechanically stirred with the SCABA design. The influence of the non-Newtonian behaviour has been investigated and found to be important for the power demand calculation. Furthermore, the other helical mixing device has been implemented at full scale and a case study comparing the two agitators has been performed; assessing the mixing capabilities and power consumption of the two designs. It was found that, for a total solids concentrations of 7.5%, the helical design could produce similar mixing capabilities as the SCABA design at a lower power consumption. Finally, the potential power savings of the more energy efficient helical design has been estimated if implemented across the whole of the United Kingdom (UK)/Austria.
Highlights
Wastewater treatment plants aim to reduce the contaminate level of the water to safe levels, so that the water can be discharged back into the water cycle
An inflation layer was applied along the mesh surfaces to resolve near wall hydrodynamic features, see Figure 3
The average and weighted average plane velocities at every 0.01 m height increment was computed for the experiments and computational fluid dynamics (CFD) simulations, respectively, for both agitator devices at two different glycerol-water mixtures, these results are illustrated in Figures 4 and 5
Summary
Wastewater treatment plants aim to reduce the contaminate level of the water to safe levels, so that the water can be discharged back into the water cycle. Ameur et al [13] characterised the mixing and power performance of a range of helical-type agitators and Amiraftabi et al [15] assessed the performance of a dual helical ribbon agitator for a two-phase stirred tank reactor filled with a shear thinning polymer and investigated the relation between the Reynolds number and power consumption These studies have shown that the helical ribbon blade is more suitable for stirring fluids with high viscosity and solid content, due to the designs lower rotational speed and shear rates over normal conventional agitator designs, which is shown to result in improved power performance. The final section will include: a mesh independence study, an assessment of the MRF method, a comparison of experimental and simulation data to validate the CFD model, and a case study assessing the influence of the non-Newtonian behaviour and comparing the mixing capabilities and power consumption of the two agitator designs at full scale
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