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Filter Aids and Filter Media

Pretreatment of filtration can be conducted using filter aids, which are inert materials.

Filter Aids

Pretreatment of filtration can be conducted using filter aids, which are inert materials. A layer of solids can accumulate on the filter medium and block its pores, resulting in a solid cake. The resistance to flow will increase when this layer of solids builds up. By using the filter aid, the medium can be avoided becoming blocked, and a porous cake can be formed that reduces resistance to the flow of the filtrate.

Adding filter aids serves two purposes. It is one way to protect the basic medium of the system by forming a layer of the second medium. Precoat is a term commonly used for this. In addition, filter aids reduce cake compressibility and increase cake permeability to improve flow rate. This is referred to as "admixing" or "body feeding". The figure below depicts filters without additives, with additives, and with additives and body feeds. There must also be inert particles, that are soluble, incompressible, and irregular in shape. There are several common filter aids such as diatomaceous earth (Kieselguhr, DE), perlite, and cellulose.


Perlite

Mineral filters can also be aided by perlite. Glassy volcanic rock with splintery breakage planes, characterized by onion-like, naturally occurring breakage planes. This rock explodes in a ten-fold expansion following crushing and heating. Minerals made up of silica are diatomaceous earth and perlite.

Cellulose and others

Among other materials used for filtering, there is asbestos, cellulose, agricultural fibers, sawdust, rice hull ash, and paper fibers. Silica cannot be tolerated by cellulose-based filtration systems. However, cellulose can be incinerated and provides greater cake integrity than DE or perlite in spite of its poor filterability. Filter aids are relatively new, including calcined rice hull ash and recycled paper fibers. Using them to dewater wastewater is relatively new.

Filter Media

It is important to consider a number of factors when choosing a filter type. There are a few different filters available for use in the pharmaceutical industry. Before choosing a filter, it is important to understand how each type of filter works and the advantages and disadvantages of each.

Magnetic Filters

Magnet filters are commonly found in liquid pharmaceutical production systems when a high level of purity in the medium is needed. This is most commonly accomplished by removing iron particles from a liquid culture media. It is possible to remove, clean and reuse magnetic filters once they have reached their maximum capacity.

A few reasons make magnetic filtration advantageous. One of them is its environmental friendliness. All metals can be recycled, so there is less waste to dispose of. Manual cleaning can be done when the filter becomes full, reducing operating costs. Magnetic filters are not affected by flow rates, so pressure drops within the system won't occur even if the filter is full. Filters made of magnetic material can remove contaminants smaller than one micron.

Bag Filters

Filters in bags remove contaminants from liquid materials using a method of microfiltration. Filters contain small pores that allow medium to pass through while debris is caught inside bags that fill with debris. With bag filters, the size of the pores can vary widely, so it is crucial to know what size of filter you need. Bag filtration is very effective in smaller systems since bags will not fill up with contaminants as rapidly, which lowers the cost of bag replacement.

Self-cleaning Filters

Self-cleaning filters are often used in large-scale, high-demand filtration systems. Self-cleaning filters come with the advantage of being able to perform their own maintenance. Using other types of filters requires periodic shutting down of the system as a whole. Backwashing, which is common on self-cleaning filters, pushes debris away from the sides of the filter and toward the bottom of its housing. As debris leaves the system, the filter remains in place, allowing the debris to exit.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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