Reverse Osmosis. Microfiltration. Distillation. Deionization. With so many types of filtration and purification systems on the market today it is difficult to determine which type of technology is best for your needs. The filtration of liquids is performed at many levels. To understand how to maximize your filtration needs, one needs to consider the size range of the particles. Once this is understood, determining which filtration system is best for you is easy.
IN FIGURE 1
The filtration spectrum is a handy way to determine what type of filtration products you need. The filtration spectrum is a chart, which shows the size range of particles, and what types of filtration are used for each range. The spectrum can be broken down into five major areas: macro, micro, macro molecular, molecular, and ionic particles.
Macro particles are visible to the naked eye and range in size from 50 m to 1000 m. Examples of particles in this size range include: beach sand, granular activated carbon, human hair, mist, pollen and milled flour. To filter constituents of this size, particle filtration is used.
Filters that are designed for particle filtration are usually pre-coat filters, screen, sand, bag, plate and frame, activated carbon, and depth filters. Precoat filters use a filtration media precoated with diatomaceous earth that will remove very small particulate matter including some bacteria. They are only practical for limited volume applications. Screen filters use a coarse screen to filter out large particles at the intake point. This type is prone to blinding. Sand filters are able to process large volumes rather inexpensively. The location of fine sand on top of the coarse sand causes the filter to clog quite quickly. The coarseness of sand and lack of uniform packing allows many smaller impurities to pass through. Bag filters are constructed of non-woven media such as polypropylene in the shape of a bag. Fluid is placed in the bag and filtered through the bottom by gravity or pressure while the impurities are left behind. The bag eventually fills up with impurities and is discarded. Plate and frame filters consist of thick woven materials such as cotton or polypropylene mounted on a frame. Rows of the filters are lined up and fluid is pushed through them. The filters tend to drip and leak and are not effective for high precision filtration. Active carbon (AC) filters remove large particles and adsorb low molecular weight organics and chlorine but must be backwashed frequently and changed periodically to avoid bacterial growth and maintain efficiency.
Figure 1
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Depth filters are the most common type of particle filter and the most suitable for the majority of applications. The water flows through the thick wall of the filter media where the particles are trapped throughout the complex openings in the media. The filter may be constructed of cotton, cellulose, synthetic yarns or "blown" microfibers such as polypropylene. The most important factor in determining the effectiveness of depth filters is the media density throughout the thick wall. The best depth filters have lower density on the outside and progressively higher density toward the inside wall. The effect of this "graded density" (see Figure 2) is to trap coarser particles toward the outside of the wall and the finer particles toward the inner wall. Graded-density filters have a higher dirt-holding capacity and longer effective filter life than depth filters with single-density construction.
FIGURE 2
Generally, depth filters are not an absolute method of purification since a small amount of particles within the rated micron range may pass into the filtrate. However, there are an increasing number of depth filters in the marketplace that feature absolute retention ratings.
Industry uses depth cartridges in several ways including: bulk particle removal, prefiltration, waste treatment and incoming water treatment. Depth filters may be used to filter chemicals as long as all the materials that are used in the filter are compatible with the chemical. Depth cartridge filters are cost-effective and the filter materials are disposable.
MICRO PARTICLE RANGE
Micro particles are not visible to the naked eye; an optical microscope is needed. These particles range in size from about 0.05 m to over 2.0 m.
Examples of particles in this size range include: giardia cyst, yeast cells, some bacteria, coal dust, red blood cells, blue indigo dye, A.C. fine test dust, milled flour, and latex/emulsion. Microfiltration is used to filter particles of this size.
Surface filters are used to filter particles in the micro particle range although occasionally a high-grade depth filter is used. Surface filters tend to be very precise, providing a barrier, which only certain size particles may pass through. Since there is a definite barrier, the filter is prone to blinding and the service life is rather short. To overcome this problem, many surface filters have been designed with as much surface area as possible, in the form of pleats, folds or as a spiral. Pleated cartridge filters typically act as an absolute particle filter, using a flat sheet media, either a membrane or specially treated non-woven material, to trap particles. The media is pleated to increase usable surface area. When used to trap larger particles of more than a single micron size, pleated filters are usually not cost-effective for bulk water filtration without the use of prefilters. However, pleated membrane filters serve well as sub-micron particle or bacteria filters in the 0.1 to 1.0 micron range and are often used to polish liquids in critical applications. The submicron pleated filters are constructed with a polymer membrane which is disposable. Some cartridges have been designed to perform in the ultra-filtration range: 0.005 to 0.15 micron.
Industry uses depth or surface cartridge filters in several ways including: final filtration, post-treatment, point-of-use, and precise particle removal.
MOLECULAR RANGE
Submicron particles, 0.00005 m to 0.05 m, are only visible with a scanning electron microscope (SEM) or an ST microscope. Particles of this size are filtered using crossflow membrane technology - reverse osmosis (RO) nanofiltration (NF) and ultrafiltration (UF).
Figure 2
IN FIGURE 3
RO, NF, and UF are composed of semipermeable membranes in various configurations, namely tubular, hollow fiber, plate and frame or spiral-wound. The spiral-wound configuration is the most common configuration for the crossflow or tangential filtration. These membranes are created by coating a thin layer of a very porous polymer, or plastic, onto a backing material. A pressurized flow of feedwater is sent across the surface of the membrane. A portion of the feed permeates through pores in the membrane becoming the permeate, and the balance of the feed is passed across the outer surface of the membrane becoming the concentrate. The flow of water across the membrane forms a turbulent cleaning action, which keeps the membrane from fouling.
RO, NF, and UF are the finest form of filtration presently known, with reverse osmosis permitting only pure water through the membrane, nanofiltration allowing some salts through the membrane and ultrafiltration allowing passage of all salts and larger molecular weight particles. The actual size of the pores in any of these polymeric type membranes is measured in terms of Angstroms, which is one 10 billionth of a meter.
To put this in perspective: consider if one square foot of membrane were the size of the entire Pacific Ocean, a reverse osmosis pore would be roughly the size of a dime. Nanofiltration would be about the size of a half-dollar and ultrafiltration would be similar in size to an old silver dollar (Figure 4).
Figure 3
A Scanning Electron Microscope (SEM) at 8000x magnification shows that the pores of an RO membrane would be undetectable, (Figure 5). while the pores of a submicron (0.2m) filter or a pleated filter are very distinguishable (Figure 6).
Figure 4
Each membrane performs a very different function. The larger the pore size, the larger the particles that pass through. At a point where all the salt passes, the system is then referred to as ultrafiltration (UF). At that point only organics of greater than 1000 mw are rejected.
Figure 5
Figure 6
Because UF has larger pores, it is the least costly to operate, requiring fewer membrane elements and lower operating pressure (and thus less horsepower) than the other two processes. As the pores get smaller, as they do for NF and RO (in that order), the cost of operation increases somewhat. The capital costs tend to be relatively similar since a large portion of these costs are associated with piping, frames, controls, etc.; all of which are related to the quantity of water to be processed - not the extent of filtration.
Generally, UF is used when the only issues are fine filtration and removal of most organic contaminants. Examples of particles in this size range include: paint or ink pigment, asbestos, tobacco smoke and larger particles of carbon black viruses, gelatin, colloidal silica, tannins and lignins (color components found in surface waters) and albumin protein. Occasionally nanofiltration (NF) is used if particle sizes are exceptionally small yet still beyond the ionic range. Different membrane media is used for different size particles. Ultrafiltration uses cellulosic (1000 - 50,000 MWCO), fluoropolymer (700 - 20,000 MWCO), or polysulfone membranes (800 - 100,000 MWCO). Each membrane has a different chemical make up and is compatible with different chemical environments.
Industry generally uses ultrafiltration for many different applications including: pretreatment for other purification systems where organics are not removed (such as ion exchange systems), gelatin and protein concentration in the pharmaceutical industry, sugar clarification in the food and beverage industry, cheese whey concentration, oily waste concentration in heavy industrial applications, and electronic deposition for paint applications as well as many others.
Nanofiltration is used for particles in the molecular range (0.05 m to 0.005 m). These size particles are only visible with a scanning electron microscope. Sugar, synthetic dye, endotoxins and pyrogens are particles rejected within this range as well as smaller particles of gelatin, colloidal silica, viruses and larger charged ions such as hardness. To filter particles of this size, nanofiltration is used. It covers an area in-between with organic rejections of 300-1000 mw and salt passages of 15 - 90%. Several different types of nanofiltration cover the entire spectrum of the area between RO and UF.
Nanofiltration is used by industry in a variety of ways including: pyrogen removal, sugar concentration - for food and beverage applications, dye desalting, water softening and color removal in water.
Other methods used for these types of applications would include distillation, evaporation and preferential adsorption using ion exchange. These systems are becoming less and less common because they are expensive from both a capital and operating cost standpoint and also inefficient.
The ionic range consists of particles that are only visible with a ST microscope. These particles range in size from 0.002 m and below. Particles in this size range include aqueous salt, metal ion, and smaller particles of sugars, synthetic dye and endotoxins/pyrogens. To filter particles of this size or to reduce inorganic salts (e.g. sodium, alkalinity), reverse osmosis systems are used.
Reverse osmosis "filters" most inorganic salts from water by allowing only the water to pass through the pores. The membrane will also reject some forms of non-ionic organic compounds such as a fructose molecule (molecular weight = 180). It also will pass smaller organics such as ethyl alcohol (mw = 46).
Applications that use reverse osmosis include: boiler feed water, potable water, car wash rinse water, glass rinsing, electronics rinsing, plating rinse makeup, pure water for dialysis, beverage makeup, pharmaceutical water and maple syrup concentration.
Distillation and deionization are other means of removing impurities at the ionic level. Deionization or ion exchange systems consist of a tank containing small beads of synthetic resin. The beads are treated to selectively adsorb either cations or anions and exchange certain ions based on their relative activity compared to the resin. This process of ion exchange will continue until all available exchange sites are filled, at which point the resin is exhausted and must be regenerated by suitable chemicals.
Distillation is the collection of condensed steam produced by boiling water. Most inorganic contaminants do not vaporize and, therefore, do not pass to the condensate or distillate.
Deionization and distillation are becoming less common because they require high amounts of chemicals and energy which respectively leads to high costs. Often times deionization is used after reverse osmosis to remove the remainder of the impurities. By using RO as a primary filter and deionization as a polishing filter, the costs are kept down while particle removal is maximized.
Whether you need to filter metal filings from wastewater or submicron size particles of aqueous salts from high grade electronic rinse water, a firm understanding of the filtration spectrum can offer tremendous insight into the selection of the right filtration system for your needs.
