Microfiltration uses membranes with microscopic pores to remove particles larger than 1 micrometer from liquids, without using heat. It is used for cold sterilization, treating wastewater, separating oil and water, and as a pre-treatment for other processes. Cross-flow microfiltration allows for increased inflow without adding new equipment. Pre-filtration is necessary to prevent damage to the microfilters.
Microfiltration is a type of filtration that uses special membranes with microscopic pores ranging in size from 3.9 millionths of an inch (about 1 micrometer) 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 larger than 39 millionths of an inch (about 1 micrometer) from a solution.
Heat sterilization is generally not desirable for use with all liquids. Some beverages, such as juice, wine, and beer, lose flavor when heated; likewise, some pharmaceuticals lose their effectiveness when heated. One of the main uses of microfiltration is in cold sterilization. Bacteria are larger than a micrometer, 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 returned safely to the environment. It is used to separate oil and water, and can be used to separate fat from food products leaving water and protein behind. Furthermore, it can be used as a water pre-treatment for other processes, such as reverse osmosis.
Generally, all filtration works in part by the diffusion process. Each molecule of a solution has a certain kinetic energy and moves, bouncing off other molecules and off the walls of the container. Those molecules that hit the filter pass through it and are filtered by it.
A microfiltration system is generally composed of a clean liquid and a solution separated by a microfilter. A low pressure is applied to the solution side forcing it through the microfilter at a faster rate than diffusion alone will allow. The filter removes all large particles and the liquid that emerges through the other end is purified of anything larger than the pores of the filter.
There is a common variant of microfiltration called cross-flow microfiltration (CFMF). In this version the liquid is pumped tangentially over the surface of the microfilter. The purified liquid falls through the filter and is collected underneath it. The benefit of cross-flow microfiltration is that it allows current waste management systems to increase their inflow without adding new digesters to pump liquids through the microfilters. With CFMF, liquids simply need to flow over additional filters, thus reducing the need for new equipment.
Hard or sharp particles in the inflow can damage the microfilters. For this reason, it is usually important that all liquids are pre-filtered before microfiltration. This allows larger and more dangerous particles to be removed before they can damage the delicate micro-filters.
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