(4d) Influence of Filter Cake Thickness during Backwash Regeneration

In industrial solid/liquid separation, the removal of fine particles (≤ 10 μm) in a suspension with a low solids content (0.01-1 v/v %) is a challenging and demanding task. This separation occurs in a wide array of industrial production applications such as metal processing and treatment industries. In order to achieve the required purity, different types of filters, such as candle and leaf filters, can be used. The selected filter will be used (semi-)discontinuously and the actual "filtration" process step is just as important as the subsequent "regeneration" of the filter cloth. In case of an improperly regenerated filter, the next filtration process becomes shorter until a subsequent regeneration is required. This behaviour is presented in the well-known filtrate flow – time – plot in figure 1. In the case of a properly regenerated filter (a), the filtrate flow at the beginning of each step is still equal whereas the initial filtrate flow decreases in the case of an improperly regenerated filter (b).

With respect to the field of applications, the removal procedure has revealed that there is potential for improvement. A properly regenerated filter cloth highly depends on the interaction between the particle system and the filter cloth (weave type, thread thickness, material). In order to guarantee the removal of the filter cake, the applied removal force has to be larger than the adhesive forces. Furthermore, to remove the filter cake in large fragments, the removal force has to be lower than the cohesive force. These two effects also depend on the porosity of the filter, pH-value, and further properties.

Currently, the boundary conditions for a properly regenerated filter cloth has not been investigated effectively. Hence, further research must be carried out to improve the backwash treatment with respect to the process conditions in an economic and process-technical context.

This presentation shows the influence of cake thickness on the discharge during backwash filtration. For this, different types of cake discharge will be described. Furthermore, the project and experimental facility will be presented.

Figure 1: Filtrate-Time-Diagrams and schematic representation of process management with properly regenerated filter cloth (a) and improperly regenerated filter cloth (b). In case of the properly regenerated filter, the filtrate flow at the beginning of each filtration step is equal. In case of an improperly regenerated filter, the filtrate flow decreases as a result of the remaining filter cake fragments on the filter cloth surface.