Microfiltration is a type of filtration that uses special membranes with microscopic pores that range in size from 3.9 millionths of an inch (about .1 micrometers) to 390 millionths of an inch (about 10 micrometers). The size of the pores is what gives microfiltration its name. It is used to remove all particles that are larger than 39 millionths of an inch (about 1 micrometer) from a solution.
Heat sterilization typically is not desirable for use with all liquids. Some beverages, such as juice, wine, and beer, lose flavor when heated up; likewise, some pharmaceuticals lose their effectiveness when heated. One of the chief uses of microfiltration is in cold sterilization. Bacteria are larger than a micrometer in size, so these micro membranes are used to remove bacteria from liquids without using heat.
Microfiltration is also used to treat wastewater so that the water can be safely returned to the environment. It is used to separate oil and water, and can be used to separate fat out of food products while leaving the water and protein behind. Also, it can be used as a pre-treatment of water for other processes, such as reverse osmosis.
Generally, all filtration works in part by the process of diffusion. Every molecule of a solution has some kinetic energy and moves around, bouncing off other molecules and the walls of the container. Those molecules that hit the filter flow through it and are filtered by it.
A microfiltration system is generally composed of a clean liquid and a solution separated by a microfilter. Low pressure is applied to the solution side that forces it through the microfilter at a higher rate than diffusion alone would allow. The filter removes all large particles and the liquid emerging on the other side is purified of everything that is larger than the pores in the filter.
There is a common variation of microfiltration called cross flow microfiltration (CFMF). In this version, the liquid is pumped tangentially across the surface of the microfilter. Purified liquid falls through the filter and is collected below it. The advantage of cross flow microfiltration is that it allows current waste-management systems to increase their inflow without adding new digestors to pump liquids through the microfilters. With CFMF, liquids merely have to flow over additional filters, thereby reducing the need for new equipment.
Hard or sharp particles in the inflow can damage microfilters. For this reason, it usually is important for all liquids to be prefiltered before microfiltering. This allows the largest, most dangerous particles to be removed before they have a chance of damaging the delicate microfilters.