Filter Pores
Filtration is a mechanical or physical operation widely used for separating substances based on relative particle size. The particle or molecule of interest may either be part of what passes through the filter or part of what is retained by the filter. A successful separation may be completed in a single filtration step or may require many steps. The sample may be a liquid, gas, or supercritical fluid.
Filtration can be driven by gravity, vacuum or pressure. Gravity is generally used at a small scale in laboratory applications where filter paper is used to remove particles from a liquid. Gravity filtration is slow and requires filters with fairly large pores. In the majority of applications, a sample is applied to the filter and forced through it under pressure or sucked through it by vacuum. Obviously, filtration requires housing so that the sample can be isolated to one side of the filter and kept separate from the filtrate.
Types of Filtration:
The types of filtration used in biotechnology include:
Microfiltration and ultrafiltration are the most widely used methods and differ according to the size of what is being separated. Microfiltration is used to separate particles ranging from about 0.05 to 10 µm. Ultrafiltration is used to fractionate or concentrate molecules ranging from about 1000 to 1,000,000 Daltons. Reverse osmosis, used largely for water purification, separates very low molecular weight molecules. All three filtration types use membranes as the filtering material.
Types of Filtration
Microfiltration:
Microfiltration (MF) is the physical retention of particles behind a filter medium while the liquid they were suspended in passes through the filter. Particles are retained because they are larger than the pores in the filter. Other factors affecting retention are fluid viscosity and chemical interactions between the membrane and the particles in the solution. Microfiltration removes particles with a pore size of .05 and 5.0 µm.
Ultrafiltration:
Ultrafiltration (UF) works basically that same way as microfiltration, except that the pore sizes are considerably smaller. Solutes are retained behind the filter on the basis of molecular size while the bulk of the liquid and dissolved salts pass through. A pressure gradient across the membrane, known as transmembrane pressure, drives the filtration process. Ultrafiltration membranes are designed for the concentration and separation of complex protein mixtures.
Reverse Osmosis (RO) and Nanofiltration (NF):
Reverse osmosis (RO) and nanofiltration (NF) are the processes of separating very low molecular weight molecules (typically <1500 Daltons) from solvents, most often water. The primary basis for separation is rejection of solutes by the membrane on the basis of size and charge. Unlike UF membranes, RO and NF membranes retain most salts, as well as uncharged solutes. NF membranes are a class of RO membranes which allow passage of monovalent salts but retain polyvalent salts and uncharged solutes > ~400 Daltons. Reverse osmosis membranes (RO) have very small pore sizes and are designed to separate ions from each other.
Membrane filtration permits complete removal of particles and microorganisms above a certain size as qualified by pre-established specifications and testing regimens.