Filtering Out Confusion

Filtration Mechanisms

From largest to smallest particles, liquid filtration is divided into the following classes: Macro-filtration, micro-filtration, ultra-filtration, nano-filtration and reverse osmosis (RO).

Macro-filtration typically involves particles of 10 µm or more in size, as in metalworking, enzyme processing and sediment filtration applications. Due to the large particle sizes, a significant mass of particles may be removed from a fluid stream over time. For this reason, a variety of filtration techniques may be employed.

Very large particles may settle out of the stream in a settling tank. Particles between 100 and 500 µm may be removed by a washable screen or belt filter. Particles under 100 µm will typically require some type of media-based filtration. Various fluid chemistries make it crucial to check with filter manufacturers on the chemical resistance of their filters in your application to make sure that contaminants, binder chemistries or shed fibers are not leached out into the process stream.

Macro-filtration filters can take the form of filter disks, bags or cartridges. Bag filters are usually depth filters and fall into two broad classes: Nominal efficiency felt bags made of polypropylene or polyester with fiber diameters in the range of 20 to 40 µm, and much higher efficiency bags, often called absolute filter bags, made of a gradient density structure of meltblown polypropylene or polyester media with fiber diameters in the range of 1 to 5 µm.

Likewise, many types of cartridge filters are available from low-cost, string-wound and meltblown cartridges, followed by a wide variety of pleated cartridges (made of cellulose, cellulose/polyester, polypropylene or polyester media) and finally particulate-loaded cartridges, made with powder-activated carbon, phosphates and other types of media.

Cartridges can be custom-built to remove specific contaminants from a fluid stream. A wide variety of these filters are available at a relatively low cost. Even when finer filtration is required, macro-filters will often be employed upstream of the final filter to remove the large particle loads and extend life on more expensive finer filters downstream.

Micro-filtration generally involves particles ranging in size from 10 µm or below down to 0.05 µm. Examples of contaminants in this range include flour, giardia cysts, paint pigments, EDM process particles and coal dust. To remove these particles, very high-quality depth filters or fine-pore-surface filters are required. These filters are generally made of resin-bonded micro-fiberglass, cellulose or cellulose/synthetic blends, or ceramics (especially in high-temperature applications) and are more resistant to fluid flow, making it important to consider the quantity of fluid being filtered when sizing the system. Keep in mind that lower flow rates improve filter performance, because particles are more likely to be trapped by slower-moving fluid streams.

Ultra- and nano-filtration involves extremely small particles below 0.05 µm. Contaminants in this range include latex emulsions, carbon black and viruses. These particles must be removed by membrane filters, which can be made of cellulose acetone, polycelphon, PTFE or PVDF, among other materials. These membranes are semi-permeable and are constructed in cartridges of various forms, including tubular and spiral-wound membranes. An important consideration in employing membrane filters is membrane fouling, in which the small pores in surface-loading membranes become plugged, impeding the flow of the liquid through the filter. If a wide particle size distribution is present, a pre-filter may be advised.

The finest level of conventional liquid filtration today is RO that employs a non-porous membrane in a cartridge form. These filters are typically cross-fed under high pressure and rely on osmotic forces to move water molecules across the membrane. High pressure applied to the filtrate causes natural osmosis to be reversed; water molecules will move from an area of higher concentration to lower concentration. The cross flow helps prevent filter fouling by carrying away concentrated filtrate with the flow. In addition to their natural filtration properties, crossflow membrane filters can be employed to concentrate solutions by removing water from the feed.