By Mel J. Mirliss
Summary: This article intends to provide an objective comparison of diatomaceous earth (DE) filtration with crossflow (membrane) filtration to allow informed decisions on use of these technologies for municipal, commercial and some industrial applications.
Over the past several years, filtration continues to evolve. Diatomaceous earth (DE) and crossflow filtration, also referred to as membrane separation, could perhaps be considered two extremes of that evolution. As membrane technology becomes more accessible, the question of which technology to use continues to be debated. The following discussion includes advantages and disadvantages of both technologies, a review of areas where each is currently in use, and a comparison of capital and operating costs.
Also known as precoat filtration or diatomite filtration, DE filtration has been used in food and beverage applications for over 70 years. Two types of filters exist: 1) pressure filters, which have a pump or high pressure source on the influent side, and 2) vacuum filters (leaf and rotary), which have a pump on the effluent side and are open to the atmosphere. The leaf or “filter element” is a screen or cloth septum on which a precoat is applied. The precoat is a layer of fresh DE that’s applied to the septum by recirculating a dilute DE slurry through the filter leaves. The rotary vacuum filter is a drum rotating on its axis, partially submerged in the liquid to be filtered, and coated with a thick precoat (often 4-inches or more) of DE.
This technology offers plant operators considerable flexibility in tailoring their operation to specific needs, i.e., when there are changes in the quantity, quality and size of the solids entering the system, operators can either switch to another, more appropriate grade of filter aid, or change the rate of DE addition (body feed). The filters are simple to operate and highly effective in removing solid impurities including cysts, algae and asbestos. Moderate changes in pressure or flow rate usually won’t compromise the filtration. These changes may vary depending on system condition. This is why it continues to be a method of choice for many of the world’s clarification needs for foods, beverages and chemicals.
Diatomaceous earth is a sedimentary rock comprised of the skeletal remains of microscopic water plants called diatoms. These single-cell organisms have the unique ability to extract silica from water they live in and construct exoskeletons having a highly intricate, microporous structure. Diatoms range in size from under 5 microns (µm) to over 100 µm and are characterized by a porous structure with openings as small as 0.1 µm in diameter. When the life cycle is completed, the diatom organic matter decomposes and washes away leaving a rigid amorphous silica shell. The siliceous skeletons become an inorganic mineral and form a sedimentary repository capable of filtering waterborne contaminants of very minute sizes.
The use of diatomite in filtration applications is based on this unique microscopic structure that’s able to trap sub-micron particles while maintaining a permeable filter cake. Normally, use of diatomite in filtration is a two-step operation. First, a thin layer of clean filter aid is coated on the filter element (cloth or wire screen) by recirculating a dilute slurry of filter aid through the filter. This is referred to as “precoating” the filter. The precoat serves two purposes; it protects the filter element and also produces almost instant clarity when the filter goes on line. On a rotary vacuum filter, the precoat also provides a disposable and renewable media exposing a fresh surface for filtration. Coagulants generally aren’t used or necessary.
Following precoating operation, a small amount of filter aid (“body feed” or “admix”) is continuously added to the liquid being filtered. As the filter cycle progresses, the body feed produces a fresh new filtering surface, reducing cake resistance and facilitating entrapment of particles. This provides additional microscopic channels through which clarified fluid can flow, thus keeping permeability and porosity high. Diatomite filtration, with body feed, can thus be considered dynamic filtration, i.e., the filtering surface is constantly rejuvenating itself. Figure 1 is a graphic representation of how the precoat and body feed interact to produce exceptionally clear filtrates associated with DE filtration.
DE filtration is often chosen for projects desiring low initial capital, and for emergency or standby capacity to service large seasonal increases in demand. It’s simple, operating parameters are easily developed and a substantial knowledge base already exists. A recent advance of particular note is discussed in a series of papers by Dr. Jerry Ongerth (visiting Fellow at the University of New South Wales, Sydney, Australia) that demonstrate up to 6-log (99.9999 percent) reduction of Cryptosporidium cysts from drinking water under routine operating conditions.1,3 DE filtration is currently one of the U.S. Environmental Protection Agency (USEPA) approved technologies for meeting requirements of the Surface Water Treatment Rule (SWTR) and is most suitable for small communities seeking to comply with the rule.
Membrane filtration utilizes semi-permeable membranes as the filtration media except for reverse osmosis (RO), which is based upon selective and electrochemical ion transfer. Varied manufacturing processes, configurations and materials selection will offer different pore sizes, pressure requirements, chemical tolerance and particulate removal capability. The membrane is a selective barrier that permits separation of particles in a fluid down to ionic constituents and which can sometimes simultaneously accomplish concentration, clarification, fractionation (separate into different fractions) and purification.
The openings in membrane material (pores) are usually so small that a significant fluid pressure is required to drive the liquid through them; the pressure required varies inversely with the size of the pores (classic orifice theory). As a result, membrane systems can be prone to fouling by debris and can plug and blind off almost instantly, unless they’re operated in a “crossflow” mode. Hence, the term crossflow.
In crossflow filtration, a feed stream flows parallel to the membrane surface at high velocity, instead of perpendicular flow employed in DE filtration or conventional membrane filtration (see Figure 2). Crossflow filtration minimizes solids build-up on the membrane surface, delivers a higher permeation rate and maintains stable throughput rates thus reducing need for filter replacement for many commercial/industrial and municipal applications. Extensive site-specific testing is usually necessary to determine the most appropriate membrane.
The membrane splits the feed stream into two streams: one stream is the permeate, which includes components small enough to pass through the membrane pores; the other stream is the concentrate (retentate), which includes components large enough to be retained by the membrane. The retentate stream may be recirculated through the membrane module for greater removal of contaminants or simply water reuse. The process is very sensitive to pressure, which must be developed for each specific application. Excessive pressure may aggravate membrane fouling. Low pressure may reduce permeate flow rate too much.
Membranes are available in several different configurations—tubular, hollow-fiber, plate-and-frame, and spiral-wound. Some of these designs may work better than others for a particular application, depending on such factors as viscosity, concentration of suspended solids, particle size and temperature.
There are now four commonly accepted categories or “classes” of membrane, defined based on size of solid material that must be removed. From the smallest to largest pore size, these are RO, nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF).
Reverse osmosis affects separation of the smallest solutes, including ionic and molecular salts only a few angstroms (Å) across. The mechanism of rejection is more complex than in the other types and isn’t based solely on a sieving mechanism, but on electrochemical interactions as well. Theories of the exact mechanism are debated.
NF, also referred to as membrane softening or low pressure RO, has pores close to one nanometer (nm) in diameter, which equals 10 angstroms (Å) or 0.001 µm, and can accomplish partial salt rejection. Nanofiltration membranes span the gap between RO and UF classes.
UF membranes can separate medium to large size dissolved molecules from the solvent, due largely to a simple sieving mechanism. This membrane class is usually meant to include only membranes with pores too large to reject or remove salt ions, but small enough to reject larger dissolved and colloidal species. Pores are generally accepted as ranging in size from 20 to 500 Å diameter. The overwhelming majority of UF membranes in commercial use are polymeric, although new ceramic and metal oxide aggregate UF membranes are emerging.
MF is well understood as the “fine” end of particle filtration, with pores of from 0.1 µm to 1 µm diameter (perhaps up to 3 µm or even 10 µm). As such, MF membranes have pores two to five orders of magnitude larger than the other classes. What isn’t as well known is, when the MF media is a membrane, it also can be run in crossflow as well as normal-flow mode. This may provide lower cost operation and much longer media life.
All of the above categories of membrane processes are listed as approved technologies by the USEPA for its SWTR Small System Compliance Technology List. It should be noted, though, the agency stresses the need for disinfection in conjunction with these processes.
DE filtration is simple and inexpensive to evaluate, install and operate. Its greatest advantage is its successful history in a wide variety of applications filtering out particles from 1 µm and above.
Crossflow filtration, by comparison, has a limited and somewhat stormy history with respect to use in municipal, beverage (juices, wine, beer), and some industrial applications. In many instances, it’s still experimental and requires high capital investments. When absolute filtration is necessary, cost isn’t a significant issue. And when particles below 1 µm (sub micron) must be removed, crossflow filtration can be most effective.
- Ongerth, J.E., and P.E. Hutton, “DE filtration to remove Cryptosporidium,” Journal of the American Water Works Association (JAWA), December 1997.
- Bhardwaj, V., and M.J. Mirliss, “Diatomaceous Earth Filtration for Drinking Water,” Tech Brief, On Tap, Volume Issue 2, Summer 2001.
- Ongerth, J.E., and P.E. Hutton, “Testing of Diatomaceous Earth Filtration for removal of Cryptosporidium Oocysts,“ JAWA, December 2001.
About the author
Mel Mirliss has been associated with the diatomaceous earth industry for the past 29 years. For 10 years, he has served as executive director of the International Diatomite Producers Association (IDPA), where he has been a director since its founding in 1987. Mirliss holds a bachelor’s degree in chemistry from the University of California. He can be reached at (562) 598-8109, (562) 598-8109 (fax) or email: firstname.lastname@example.org