Filtration with filter sheets

Cellulose fibers form the basic structure of the filter layers. By the fibrillation of the cellulose fibers and the integration of highly porous diatoms, extremely filtration-active structures are built up which, if necessary, are loosened up by perlite or coarser diatomaceous earth.

The composition and structure of deep-bed filters are therefore comparable to an extremely close-meshed, three-dimensional sieve with countless intersections. Particles and microorganisms are mechanically held back in the ever-narrowing pore labyrinth. As a result, the particles are deposited in the pores of the filter medium.

1 Surface filtration, sieve effect
2 Deep-bed filtration, mechanical absorption
3 Deep-bed filtration, adsorption, accumulation
4 Drain side, wet-solidified

The deep-bed filtration effect can also be effective for particles that are smaller than the pores of the filter layer. That is why the filtration effect of filter layers is not indicated by specifying pore diameters, as is the case for membranes. Instead, the filtration capacity is characterized by specifying the water permeability at defined conditions.

Three parameters are thus responsible for the retention capacity of filter layers:

• Surface filtration or the mechanical sieve effect
• Deep-bed filtration, mechanical absorption
• The adsorption effect

An ideal selection of the filter sheet means obtaining both the highest possible flow rate and also a sufficient filtration capacity. Other important criteria are pressure differences and the flow velocity. In order to avoid turbidity, certain filtration speeds must therefore not be exceeded, depending on the type of filtration and the medium.