It is widely accepted that flocculation improves filtration performance by increasing cake permeability. This principle is important in submerged membrane filtration for drinking water applications where the feed material can potentially contain fouling components which prohibit the extended operation of the filter. Less well understood is the impact of floc properties on the hydraulic properties of the fouling layer formed on the membrane or the impact of hydrodynamic conditions during treatment on the floc-fouling layer relationship. In order to advance knowledge of this area, a set of tools were developed to characterise the cake formed during constant pressure filtration in terms of the compressive yield stress and permeability as a function of solid volume fraction. Using an iterative procedure, the optimal parameters for these models are calculated as are pressure and solid fraction distribution profiles. Input parameters to the numerical analysis are flux and final cake height data obtained from batch filtration experiments which are driven to steady state. The calculated material properties are compared against piston and centrifuge data with good agreement. Application of the material properties to constant flux filtration involved development of a numerical model for simultaneous consolidation and cake formation. Flocculated yeast was used as the test system with the predicted transmembrane pressure rise as a function of time under constant flux conditions compared with experimental data. Good agreement is observed between model and experimental trends. The close correspondence between experimental and predicted results also suggests that it may be possible to predict trans-membrane pressure rise during constant flux filtration on the basis of material properties determined through simple constant pressure steady state experiments. A good account of the data was also achieved through extension of the general equation to include an empirical model for the consolidation time constant. These new tools were applied to characterise the cakes formed under well controlled shear conditions. To avoid complications with modeling the sheared filtration system, the filtration was performed below the critical shear rate for particle rejection. This was verified by in-situ particle counts and size measurement. The material properties were determined for flocculated yeast filtered in a coni-cylindrical Couette at several shear rates below the critical shear. Comparison of the compressive yield stress showed that cakes subjected to shear required less compressive stress to collapse. It is shown that the general equation for constant flux could be modified to encompass this effect through inclusion of an empirical shear parameter. The transmembrane pressure rise is able to be described well by this model. DEM particle simulation was performed to investigate the effect of floc size and structure on cake permeability. Flocs of known size and structure were placed in a virtual suspension and the process of consolidation simulated by application of a compressive force. The permeability of the cake was calculated by computational fluid dynamics at various stages of the consolidation showing that the larger compact floc showed the highest permeability despite the highly compact structures formed. Comparison of pore size distribution also confirmed that several larger pores remained after consolidation of the larger compact flocs. Further work needs to be undertaken to pin point the microstructural mechanism governing this behaviour and whether the presence of fluid passing through these pores under normal filtration flows affects the retention of permeability of cakes under compression. Furthermore, the shear environment required to minimise the detrimental effects caused by shear enhanced cake collapse and also to form flocs of compact structure and large size needs to investigated.
| Identifer | oai:union.ndltd.org:ADTP/258379 |
| Date | January 2008 |
| Creators | Kovalsky, Peter, Chemical Sciences & Engineering, Faculty of Engineering, UNSW |
| Publisher | Publisher:University of New South Wales. Chemical Sciences & Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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