There are two types of microfiltration media used in laboratories:
More on Depth Filters
- Depth filters are comprised of fibers, or fibrous materials, that trap particles as they pass through the tortuous path between fibers.
- Membrane filters, also called screen filters, have pores of a certain size that allow only smaller particles to pass through.
Depth filters have a random network of pore channels that vary in size and geometry. They are manufactured from a variety of solid materials. Materials of construction include various forms of plastics, cellulose, and glass, either singly or in combination. The processes used to manufacture depth filters do not result in a regular arrangement of the solid matrix. Instead, there is a range of pore sizes within a given structure that includes pores significantly larger and significantly smaller than the pore rating.
The randomness of the structure does not allow the assignment of a definitive upper limit on the size of particles that may pass through the filter. A portion of the particles in the filtrate will exceed the pore rating. Depth filters also can entrap a large percentage of particles smaller than the pore rating. Because depth filters trap particles throughout the structure, they typically exhibit a high particle-handling capacity. This makes them particularly useful in applications where the solution being filtered has a high particle load. Depth filters are not considered sterilizing-grade.
Depth filters are typically used as prefilters because they are an economical way to remove ≥ 98% of suspended solids and protect elements downstream from fouling or clogging. They owe their high capacity to the fact that contaminants are trapped and retained within the whole filter depth.
Depth filters can be made out of the following materials:
- Glass Fiber
Polymeric depth filters are manufactured from plastic fibers of various lengths, morphologies, and diameters. To improve the strength of these filters and reduce the level of fiber shedding, the filter can be calendered, the process of running the material between cylindrical rollers to apply pressure and/or heat. Most polymeric depth filters are inherently hydrophobic. For low pressure aqueous filtration, the filter may require a surface treatment to render it wettable. Polymeric depth filters are normally very strong and easy to handle.
As implied by the name, cellulosic depth filters are made from cellulose fibers. The fibers can be derived from a relatively crude source, such as wood pulp, or a highly purified source, such as cotton. The filters are manufactured by techniques very similar to paper manufacture and are very economical. Although they are generally very easy to handle when dry, they are mechanically very weak when wet. Cellulosic filters are prone to fiber shedding during fabrication into a device and when used in filtration. If required, a membrane filter can be placed downstream to retain any fibers.
As implied by the name, glass fiber depth filters are made from glass fibers. In sheet form the fibers are initially held together only as a consequence of mechanical interaction. To improve the handling characteristics, the filter is sometimes treated with a polymeric binder, such as polyvinyl alcohol, which serves to hold the matrix together. Glass fiber filters are also prone to fiber shedding. If required, a membrane filter can be placed downstream to retain any fibers.