Bleeding filter media

Poor support of the filter medium either in the absence of a porous backing cloth or in the selection of incorrect drainage channels on the surface of the filter plate will lead to processing difficulties. In the particular case of pressure filters, undersizing of the drainage ports fi om the fiber will produce the same effect. A principal effea of the latter is to restrict the escape of ultrafine sohds which bleed through the filter in the first moments of pressing. 

Addition of another powdered solid, termed a filter aid, to the system can produce a significant improvemaat to a filtration operation. The filter aid may be used in two ways, either separately or in conjunction. The first of these methods is to precoat the filter medium with a layer of fiher ail cake. Precoat aids must filter quickfy without bleeding or penetrating throu the cloth and must give a uniform thickness with a reproducible filtering surfiice. The susp ntion is then fihered onto the precoat by surface and depth filtration mechanisms. Thus surface properties are inq>ortant in the choice of aid. 

Routine steam sterilization is a requirement for any system of sterile filtration, and is often carried out after each batch, according to laid down procedures, although this is not absolutely necessary, provided that the filter is kept pressurized, the relative humidity is kept below recommended levels, and a small bleed of air is allowed to flow continuously. It is important to note that the sterilizing steam must be saturated and free from any additives. Dirt in the system can cause filter contamination and hence increased pressure drop, and additives also have the same effect by crystallizing on the filter medium, which may also concentrate any possible chemical effects. 

Since it is virtoalfy in ossible to deal with aD the cloth problems attadiing to a pardcnlar s aration, the con romise referred to above is adopted, between fltrate clarity and blinding of the medium by the smaller particles in the feed. Early fore-mmiings can be recycled, since the first bleeds will then be blended with the feed and will filter on the initial cake layers. 

When the process conditions are such that bridging of the surface pores of the medium is iiqiossible, particles may be deposited within the interstices of the cloth or, particularly for monofllament, bleed through the pores. In multifilament or nonwoven media, internal deposition in the cloth may prove to be permanent, de ite attempts to clean the filter by laundering, back-flushing, etc. 

From the above, it is clear that the operator has many cloth selection factors to consider in order to obtain optimum performance. Often the only way is by trial and error, guided by broad general principles. Once the optimum cloth is found, the following benefits are obtained a clean filtrate with no loss of solids by bleeding, an economic filtration time (production rate), an easily discharged filter cake, no deterioration of the medium by sudden or gradual blinding and an adequate cloth life. 

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