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Principles of Filtration: How do Filters Filter Anyway?

By Larry Henke

*Editor’s Note In memoriam, WC&P International presents one of Larry Henke’s detailed and informative articles on filtration. A founding member of the Technical Review Committee, Henke will be deeply missed. See the People department in this issue for his obituary.

Summary

Filtration has long been an important technique in water treatment. It’s used to remove iron deposits from water, to clear up waters turbid from silt, inorganic sediments and biological materials, both living and decomposing. It’s a primary method of separating fluids from solids—and is often not well
understood.

The most common perception of filtration is one of straining, or sieving, as if the suspension is passed through a series of successively smaller screens or colanders with the particles being contained according to size. While this is a convenient visualization of filtration, it’s a misperception that leads to faulty filter design. Straining is only one of several mechanisms operating during the filtration process. A better understanding of these mechanisms is important to the design and operation of filters. So how do filters work?

In this discussion, we’ll focus on the mechanisms of capture in granulated media filters, and allow for the likelihood of similar mechanisms in case of fiber filters. We’ll also focus on capture of particulate, not ions in solution whose capture can be better described as by adsorption. Adsorptive media include granular activated carbon (GAC), activated alumina (AA) and zeolite, all of which are better referred to as “contactors,” not filters, although they do have limited filtration capabilities. Moreover, we’ll limit our attention to water filtration, although the principles apply to particles in air, wastewater and other fluids as well.

Match the filter to the task

First, let’s establish that there are different types of filters with different filtration applications. Fabric filters, such as bags or cartridges, are composed of fiber that allow for capture into a fixed matrix that is replaced when filled. Membranes exclude particles by establishing pore sizes smaller than the particles (here, straining is the primary mechanism). In the case of granulated media, some capture is accomplished by straining, but most is by other means of attachment to the collector grains.

Filters composed of granulated media can capture material throughout the depth of the bed, and can be cleaned by backwashing. Some membrane filters use a crossflow pattern of water flow so that the rejected debris is continuously swept away. Bag and cartridge filters made of porous fibers, ceramics or a combination of materials generally must be replaced when dirty, although they can sometimes be chemically cleaned.

Pre-coat filters with diatomaceous earth, or slow sand filters that rely on formation of a cake or schmutzdecke (literally, “dirty layer,” a layer of biological growth, exopolysaccerides, or slime, and captured debris), are cleaned by scraping or replacing the media. Membranes can also filter by forming a cake. A cake is a layer formed by the removed particles themselves. Membranes can also be designed to have a cleaning cycle but, when heavily fouled, they too must be chemically cleaned or replaced.

It’s also important to note that nature seldom presents particles in a uniform manner. Filters are usually faced with a large variation in the nature, composition, and shape and size of the material being collected. From a spectrum of sizes between 0.1 micron (μm) to 20 μm or more, a range of compositions consisting of organic and inorganic flocs—microorganisms and particles that drift in and out of solution depending on conditions—filters face a wide array of materials.

Transport and attachment

Filtration physics is commonly divided into two steps—transport and attachment. Transport is the means by which a particle is moved by the fluid through the filter and into proximity to the granule, fiber or membrane. Attachment is the means through which the particle is captured. In the simple case of sieving, capture occurs because the particle is too large to pass through the pore between the granules or grains, the fibers or the semi-permeable membrane surface. As particles are captured in the pores, the pores themselves become smaller and efficiency improves. As efficiency improves, the headloss—or pressure across the filter—increases until the filter is fully clogged and cleaning is necessary. If this is the only mechanism, however, particle capture could be easily quantified by measuring the sizes or pores. That’s not the case, though, as particles much smaller
than the pores are captured.

Getting the particle there

Transport is usually further divided into diffusion, straining, interception, inertia, sedimentation and hydrodynamic action. Transport mechanisms describe the path of a particle through the filter. As a particle approaches a collector grain, the fluid is directed around the grain and through pores between grains. Although water is normally moving slowly (laminar flow), the particle will move in response to the flow stream, the effect of collisions with other particles and water molecules, and rotational forces on the particle itself (which depend additionally on the particle’s shape). As this would suggest, a precise mathematical description is elusive for any filtration application and is outside the scope of this article; however, filter scientists have generated a number of models to illustrate the various mechanisms.

In the instance of cake filtration and as the cake is formed, you would expect the pores to become smaller and smaller. This is the case, which explains why cake filtration improves in quality just before the filter fails. Moreover, the compression of the filter cake plays a role in filtration. An example would be applications involving slow sand or biological filtration beds where a layer of collected contaminants is formed at the top that acts as an additional filter.

In granulated filters, though, the filter grains withstand pressure applied by the flow, and attachment mechanisms are such that particles many times smaller than the filter grain can be captured. The pore between the filter grains is, of course, dependent on the size and shape of the grain. In the ideal case of spherical grains, minimum pore size can be mathematically calculated to be 15.47 percent of the size of the sphere.

Particles are moved through the bed in the fluid subject to inertial forces—their own tumbling movement through the fluid— and gravitational forces acting on them. Gravitational forces are those apart from the force of the moving water itself. In addition, molecules of water impinge on the particles to move them about randomly, also known as Brownian motion. Some particles will impact on the top surface of a collector grain and stay there. Others will impact on or around them, and will attach to the top as well, building a “dome.”

Some particles will move through the pores and will hit the sides of the grain where they may attach to the side. A particle in this position is subject to subsequent detachment through inter- action with other particles or through water scour, or scrubbing force of the water.

Brownian motion can send a particle into the bottom of a collector grain as well. Thus, a collector grain isn’t restricted to capture only on its top or in the pores. It’s helpful to remember that water molecules are several magnitudes smaller than the particles and many magnitudes smaller than the collector grains. A water molecule has an approximate spherical radius of 3.15 angstroms (Å)—1 Å = 0.1 nanometers (nm) = 0.0001 μm = 0.0000001 millimeters (mm)—so a 3-μm particle, which is the size of a Cryptosporidium oocyst, is 10,000 times larger. To put this in perspective, if the water molecule were the size of a grain of filter sand (0.500 mm), the particle would be five meters (16 feet) in diameter, and the collector grain would be about half a mile in diameter.

Following collision with a collector grain, attachment (and detachment) forces enter into play, since most particles in water are slightly electronegative. That is, they’re slightly out of charge balance because of additional electrons. As a result, they repel each other and are in turn repelled by the collector grains, which are also electronegative. While some media are positive at some pH levels, attachment is inhibited by electrical repelling forces. Since pH is a measurement of the concentration of hydrogen ions, which are positive (protons), the pH of the water is an important factor in attachment. So what does allow the particle to stick to the grain?

Again, as there are a number of transport descriptions, there are a number of attachment mechanisms and one or more may be involved in any single filter circumstance. It may be that in a single filter run, several possible mechanisms are employed. Opposite electrical charge will cause attraction for some particles, not only to the collector grain but to each other, thereby changing the size, shape, weight and inertia of the particle. But the “stickiness” is a function of several forces that occur between and among particles and collector grains, double layer forces and van der Waals forces (see EXTRA), among others.

Finally, it can be said that different contaminants require different mechanisms for a filter to work. Nature seldom offers a single constituent in a water suspension, i.e., a particular solution. Often, different mechanisms are required for the filter to perform effectively on the various contaminants and combination of compounds that may be present.

For example, positively charged iron flocs may attach by means of the “opposites attract” factor. Particles of calcium carbonate (calcite) or biological particles (electronegative) may collide with the filter collector, drive through an electric barrier and attach through van der Waals force. Electrically neutral particles of aluminum or silica may also attach through the weak double layer force, while ions of calcium surrounded by water molecules—in solution—will pass through entirely only to be later captured by a softener.

Conclusion

Filters are a complex of chemical and physical forces acting on particles to allow capture and retention. Knowing the composition of the water contaminants is essential to proper filter design, and an understanding of the methods of collection for different substances will help the water treatment specialist apply the appropriate filtration method to a water treatment problem.

Know Your Filter Terms

The following has been adapted from the WQA Glossary of Terms:

absolute filter rating: Generally, it’s under- stood to mean that 99.9 percent (or essen- tially all) of particles larger than a specified micron rating will be trapped on or within the filter. Conversely, a nominal filter rat- ing is understood to mean that 85 percent of particles of the size equal to the rating will be retained by the filter. Again, both are subject to interpretation.

absorption: To collect, as in a sponge. It’s the process of one substance actually penetrating into the structure of another substance. This is different from adsorption in which one substance adheres to the surface of another.

adsorption: An attachment mechanism generally used to refer to molecular or ionic capture, as in carbon, alumina or zeolite; the physical process occurring when liquids, gases or suspended matter adhere to the surfaces of, or in the pores of, an adsorbent medium; occurs without chemical reaction.

coagulation: The agglomeration of finely divided particles into larger particles that can then be removed more effectively by settling and/or filtration. Alum (aluminum sulfide) and ferric chloride are commonly used as inorganic coagulant aids. By de- stabilizing colloidal particles, the particles can grow.

conventional filtration: In municipal water treatment, a method of treating water to re- move particulates that may consist of the addition of coagulant chemicals, mixing, coagulation-flocculation, sedimentation and filtration.

crossflow filtration: A type of filtration that uses the shear force of tangential flow across a membrane surface (during suspension recirculation) to keep the particle buildup to a minimum.

diatomaceous earth (DE) filtration: A filtration method resulting in substantial particulate removal using a process where a “pre- coat” cake of DE filter media is deposited on a support membrane (septum), and while the water is filtered by passing through the cake or septum. Additional filter media, known as “body feed,” is continuously add- ed to the feedwater to maintain permeability of the filter cake.

direct filtration: A filtration method of treating water that consists of addition of coagulant chemicals, flash mixing, mini- mal coagulation, flocculation and filtration. Physical/chemical reactions are limited. The sedimentation process is omitted.

filter aid: An agent that improves filtering effectiveness in some way, at times by en- hancing the particle attachment or increas- ing permeability of the filter to water flow. A filter aid is either added to the suspension to be filtered or placed on the filter as a layer through which the liquid must pass.

filter area: The effective area, expressed in square feet, through which water approaches the filter media. Also called surface area.

filter cake: 1) Solids deposited on top of a filter media bed, often enhanced by feeding a coagulant or filter aid; or 2) the dewatered residue from a filter, centrifuge or other de- watering device.

filter media: The selected materials in a filter that form the barrier to the passage of filterable suspended solids or dissolved molecules. Filter media are used to remove undesirable materials, tastes, and odors from a water supply. Filter designs include:
1) granulated media filters with grains, res- in, or other particles lying in beds or loosely packed in column-form in tank-type filters; or
2) cartridge-type filters that may contain membranes, fabric, fiber, bonded-ceramic, precoat, or cast solid-block filter media. The media used in some filters are chemically inert, such as sand, which performs only a mechanical filtration. Other media are chemically reactive such as calcite, activated carbon, magnesia, manganese dioxide and manganese greensand.

filtrate: 1) Effluent liquid from a filter system, i.e., the part of the feed stream that has passed through the filter; or 2) liquid remaining after removal of solids, as the liquid extracted in the formation of a filter cake.

filtration: The process of separating solids from a liquid by means of a porous sub- stance such as a permeable fabric or membrane or layers of inert media.

flocculation: The process of bringing together destabilized or coagulated particles to form larger masses or flocs that can be settled and/or more readily filtered.

sedimentation: The process of suspended solid particles settling out (dropping to the bottom of a vessel) of water that has little or no movement. Sometimes it only occurs after particles have begun to coagulate (either naturally or because of use of a coagulant aid such as alum) into larger and heavier flocs.

EXTRA–Intermolecular Forces

Most of us are familiar with electrical forces in and between atoms: we know there are two charges—positive and negative—and that like charges repel and unlike charges attract. We also have been taught negatively charged electrons form a “cloud” around the positively charged nucleus of an atom and that these electron clouds are responsible for the atom’s ability to combine with others to form chemical compounds. We’ve also heard about hydrogen bonds. And we’ve learned molecules become stable when charges are balanced so their net charge is neither positive nor negative, but zero. These are true statements as far as they go but, as usual in physics, it’s more complicated than that.

Some particles will be attracted to those of the opposite charge. Yet, when they’re very close, some are attracted to the same charge. This is due largely to exotic, intermolecular attractive and repulsive forces, usually called van der Waals force. Although originally pertaining primarily to molecules in a gas, the same force can be applied to any combination of dipolar molecules. By dipolar (two poles), we mean a charge neutral molecule that divides to a positive and negative “side,” even though the molecule is charge balanced. Water is such a molecule, with the oxygen atom aligning with hydrogen atoms to form a positive side and a negative side. This dipolarity also gives rise to the hydrogen bond that’s so important to water’s properties.

Van der Waals forces are attractive and are very local, meaning they can occur only at very close proximities. They’re proportionate to a constant called the Hamaker constant, which describes dispersion forces, distances between the particles and the radii of the particles.

There are three ways of describing van der Waals forces—orientation, inductive and dispersive. Orientative forces are those between polar molecules (like water). They’re described by formula, but generally decrease with increasing temperature and diminish with increasing energy. In inductive forces, the electrons are “spinning” (or oscillating) and this inner motion induces forces other than the simple electrical force. A spinning electric particle induces a field response in neighboring molecules. This force, although weak, is very real and accounts for any number of van der Waals effects. Dispersive effects are likely the most powerful and are a function of quantum forces, where the energy packets move at near the speed of light.

As the forces attract or repel, molecules are aligned in response. There are electrical forces then between like and dissimilar particles in suspension, between the molecules in the solvent fluid, and be- tween the particles, the fluid molecules, and the surface of a collector grain or vessel wall as well. At the surface of a collector grain, and at the surface of the particle, molecules are attempting to go into—or out of—solution, depending on many other factors such as temperature, pressure and pH.

Double layer forces are similar and occur when a dipolar particle in a fluid attracts the opposing charges within a surface, which then influences the charges in a particle within the fluid. Keeping track of molecular forces is often quite challenging and scientists must resort to approximations and mathematical models. Most of the forces are confined to distances measured in nanometers, however, and be- come very weak with increasing distance.

It’s important, however, to note that van der Waals, London (which is a special way of describing van der Waals) and hydration forces typically apply to molecules and are increasingly limited when molecules aggregate and become particles. Since each substance has its own molecular characteristic, specific descriptions of forces involved in attachment are difficult to establish. For the most part, though, these very uniform forces will apply to most particles, no matter how small or what the composition.

References

1. Cleasby, John. Chapter 8: Filtration in Water Quality and Treatment, Fourth Edition, McGraw-Hill, New York.
2. Farina, C.; Santos, F.C. and Tort, A.C. A simple way of understanding the nonadditivity of van der Waals dispersion forces, American Journal of Physics, 67 (4), p.344-349, April 1999.
3. Trussel, R. Rhodes and Melissa Chang. Review of Flow through Porous Media as Applied to Head Loss in Water Filtration, Journal of Environmental Engineering, 125 (11) p. 998-1006.
4. Holstein, Barry R., The van der Waals interaction, American Journal of Physics, 69 (4), p. 441-449, April 2001.

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