Filtration FAQs

Scott Laboratories offers filter sheets and modules for depth filtration and cartridge filters for depth and absolute filtrations. We also offer filter equipment such as pressure leaf filters, cartridge housings, plate and frame filters and more.

There are two main categories of filtration, depth filtration and surface filtration.

1. Depth filtration removes particles from your product within the depth structure of the filter medium itself. The filter media in depth filtration will be many times thicker than the particle sizes that are being removed. Depth filtration is required when high solids removal is required. Filtrations wil occur within an established micron retention range. Tighter filtration will occur at lower flow rates, while looser filtration will occur at greater flow rates.
2. Surface filtration can be either absolute or nominal with a minimal depth capacity. Surface filtrations consist of a thin membrane or a thin membrane covered with polypropylene or polyethersulfone. Surface filtration relies on liquid pressing through an array of small gaps of a certain absolute maximum size in the surface of a membrane. Prior filtration with a depth filter is usually required to prevent clogging the surface of a membrane filter. Absolute filtration is useful for sterile filtration at the bottling line and at times when you require precise accuracy of filtration to limit risk.

Scott Labs stocks depth filter sheets from 55 microns down to .2 micron. Depth filter cartridges are available from .8 microns to greater than 10 microns.

A micron is one one-millionth (0.000001) of one meter. Bacteria are typically less than one micron in their largest dimension. Human hair is 60 to 80 microns and the smallest visible object is about 40 microns across. "Sterile" filtrations generally involve depth filtration media (normally a depth filtration sheet and possibly a depth cartridge) followed by an absolute membrane. Depth filtration at sterile micron retentions may be desirable prior to sterile membrane filtration in order to protect the membrane cartridge. The membrane filter then becomes your insurance policy to ensure near absolute exclusion of particles smaller than the membrane rating.

Differential pressure is the difference between the pressure in the system before fluid reaches the filter and the system pressure after the fluid flows through the filter. As the filter clogs, differential pressure increases.

Like Theseus and the Minotaur, particles moving through a depth filter are caught within a labyrinth of channels, destined to never escape. Whether constructed of diatomaceous earth, polymers or even glass, depth filtration is based on trapping particles larger than the filter's porosity rating within a matrix of nearly infinite passageways. Certain filter media, like some pad filters, also exhibit a positive ZETA potential that attracts and holds particles exhibiting a certain charge. These two attributes allow depth filtration to retain a large number of solids for effective and economic filtrations. However, when solids are trapped in the depth of a filter, this also makes back flushing a moot point.

Filter life is expressed in terms of time or volume filtered between changes of filter media or filter elements. A filter's life span will depend upon the solids load and conditions of filtration (i.e. flow rate, density and temperature). Laboratory numbers will not reflect real life numbers. Laboratory numbers might be useful for comparison, but actual experience with the product under real life operation conditions will provide the best determination of filter media life.

No, each drop of liquid passes across only one sheet of filter media and into the stream of filtered product headed to the receiving vessel. Put differently, more filter sheets means more capacity.

The code of a filter is related to the filter housing and the receptacle in which the filter element will fit. They must match! It also refers to the configuration at the closed end of the filter element. For example, a code 7 filter element will have a bayonet (insert and turn to lock) fitting on one end and a spear point on the other end. Please call for technical assistance if you are not sure which code you need.