Precoat Filter

General Facts

Precoat filters are used to treat solutions having traces of insoluble contaminations. They embody a rigid, semi-flexible or flexible screen on which a filter is deposited medium. During the filtration process the filter medium and the filtrated solids form a filter bed, which works as an additional strainer element to collect much finer contaminants. The “bed” medium may filter by adsorption and by mechanical means. The filter vessel is filled with the suspension under pressure and it passes through the filter bed, leaving the solids in the filter bed. The purified water is then collected after the filter element.

The name “precoat filter” is sometimes given to other filter elements, which comprise a coating material permanently bonded to a mechanical screen.

Filter medium

Filter media are to a large extent chemically inert materials of fine fibrous or granular structure, which develop filter layers of high permeability. The filter bed consists mostly of diatomaceous earth (diatomite or kieselguhr), which is manufactured from fossil remains of diatoms, a type of algae having a silica skeleton. A number of other media may be employed for precoated filters. Chief of these is perlite, which is processed from a vitreous rock of organic origin. Other precoat media include powered organic rock, activated carbon, asbestos and cellulose.

Operational mode

Before start-up a primary filter medium layer has to be deposited on the basic screen. The correct amount of filter medium is mixed with clean liquid and then pumped into the filter body, where it is deposited uniformly over the filter elements. The precoat layer is formed so evenly because, while it is formed, a slight pressure differential is produced and the flow of liquid always takes the path of least resistance. To obtain extremely fine filtration, the process may be repeated with a further dose of finer medium to superimpose a second layer of this medium on the first. If suspensions with predominantly colloidal, compressible or very fine solid particles are to be filtered, then the filter achievement will decrease fast by the firm filter bed. This problem can be avoided, if the suspension filter mediums are added, which produce a loose and more permeable filter bed.

Principle of a Precoatfilter

Common Applications

Precoat filters can be used for pretreatment to enhance specific properties of the basic media. Typical examples are:

• Incorporation of a de-chlorinating agent to remove excess chlorine in the production of sterile or chlorine-free water.

• Surface treatment (e.g. silver coating) to inhibit the growth of bacteria on the filter membrane.

For decolorisation and taste removal the filter medium activated carbon is particularly suitable and special carbons are also highly suitable for the filtration of strongly alkaline solutions. Cellulose media is employed for the removal of colloidal and compressible solids (e.g. Condensate treatment in power stations, removal of solid particles, metallic impurities and oil).

Other common applications for precoat filters are shown below

• Filtration of liquid sugar and molasses

• Processing and purification of circulating systems for different oils, grinding agents and coolants

• Clarification of fruit juices in food processing

• Purifications of herbicides and insecticides

• Recovery of liquids from brewery wastes

• Polishing applications in metallurgical processing