DEPTH FILTRATION

Depth filters remove particles from a liquid. In other words, the liquids can be clear, fine or sterile filtered. The “thick” filter media for depth filtration are between 2.5 – 4.5 mm thick. The particles are retained using two filtration principles:

firstly:  surface filtration and
secondly: depth filtration

The liquid flows through a three-dimensional, asymmetrical fiber network in the depth filter. Mechanical and electrokinetic effects retain the solid components. This significantly increases the intake and adsorbing capacity.

The purpose of a filtration process is either to purify a liquid (filtrate) or to retain solids (retentate). Depth filtration concentrate mainly on the production of liquid filtrate.

Filter sheets

Filter sheets consist of a combination of especially receptive fibers like  cellulose and powdery, anorganic filter aids such as kieselguhr and/or perlite. You can specify the  retention rate by the grinding method and the volume and type of base material used as the filter aid. Small added quantities of polymer resin act as a wet-strength agent. Depending on the type of polymer resin, a lesser or greater positive charge – also called the “zeta potential” – is produced when passing through the filter sheet. The positive charge improves the adsorption of small, negatively-charged particles or micro-organisms.

The interior absorption volume of a typical depth filter sheet is up to 4 L/m² of filter surface.  A sheet filter contains during the filtration process multiple depth filter sheets, lined one after the other. This way a relatively small space can house a large filter area.

Another method of using depth filter sheets in a less time-consuming and more effortless way is to use depth filter modules. This method consists of a filter housing with a pre-loaded filter pack. The depth filter modules are exhausted when the inner matrix is filled with slurry particles. One indication of this is the increasing difference in pressure between the inlet (unfiltrate) and the outlet (filtrate). A lower flow rate of the liquid (1.5 bar for sterile filtration; 2.5 – 3 bar for clarifying filtration) is also an indication of exhaustion. Under certain circumstances the regeneration of the filter is possible.

Filter aids

With high particle loads (approx. 1.5%), standard depth filters can blind fast. By using a filter aid such as kieselguhr (alluvial filtration process), the capacity can be increased. With alluvial filtration, filter aids are suspended in a liquid and then floated onto a supporting sheet in a plate & frame filter to form a filter cake. When doing this, the supporting sheets themselves function not as filters, but merely as a support for the filter cakes, which is where the actual slurry removal takes place.

The separation of particles in this process uses a mechanism similar to that with filter sheets. Likewise, as with sheet filtration an increase in the differential pressure indicates the exhaustion of the filter configuration.

As the filter cake is usually thicker than the filter sheets, alluvial filters have a longer lifetime due to their greater slurry absorption capacity. Depending on the nature of the slurry, the removal and disposal of the filter cake is possible.