By Peter S. Cartwright, P.E., CWS-VI
Probably the single greatest cause of water treatment technology performance problems is fouling—the accumulation of insoluble materials inside or on the surface of the filtration medium, deionization resin, activated carbon medium or membrane. There are several mechanisms that cause “fouling,” and these will be identified and described here.
To further examine the fouling issue, it’s important to differentiate between batch and continuous processes. Media filters, cartridge filters and deionization resins in softening or ion exchange applications are all examples of batch processes. That is, when they’ve accumulated a certain quantity of suspended material—or adsorbed a sufficient quantity of dissolved contaminants—they must be taken out of service and cleaned, regenerated or replaced.
Regeneration can be as simple as running filtered water backwards through the bed to remove suspended solids (backwashing) or as complicated as adding chemicals to restore the adsorptive properties of the media following backwashing. Bear in mind that for some of these technologies (such as cartridge and media filtration), their primary function is to remove suspended solids, and some technologies (coagulation/filtration, for example) are designed to generate insoluble materials so that they can be removed by filtration.
The continuous processes—the most widely known of which are the crossflow pressure driven separation technologies of reverse osmosis, nanofiltration, ultrafiltration and microfiltration—use crossflow operation to continuously remove suspended materials from the membrane surface to minimize fouling. They aren’t designed to accumulate suspended solids.
Types of foulants
To adequately address types of foulants, it’s necessary to introduce the categories of contaminants found in water supplies. These are:
- Suspended solids,
- Dissolved organics,
- Dissolved ionics, and
- Dissolved gases.
In other words, suspended solids can result from dissolved contaminants (organic or inorganic) chemically reacting to form an insoluble material; certain dissolved gases may chemically react with other contaminants to produce insoluble materials; inorganic salts may be concentrated beyond their solubility limit to form insoluble material—scale.
Scalants are a particular type of foulant, and are almost always inorganic salts in a crystalline structure, which tend to build up in layers on the surface of the membrane or media. Table 1 lists typical foulants by category and provides examples.
Some people consider colloidal fouling a separate category; however, colloids are simply very small suspended solids, which—because of their size and often negative charges—require considerably finer filtration for effective removal. Typical colloidal foulants include hydroxides and oxides of iron, aluminum and silica.
A discussion of the many mechanisms of suspended solids capture, which result in fouling is beyond the scope of this article; however, suffice it to say that simple filtration is only one. Surface charges on resins and membranes can also play a significant role in that many particles as well as microorganisms are also charged.
The most important consideration in fouling is that, in virtually all cases, it involves several, if not many, of the contaminants described above. Rarely is it limited to just one contaminant, for example. As water supplies contain a multiplicity of contaminants, so do the mass of solids that comprise the material that we call foulants.
Cartridge filters: These are only designed to remove suspended solids, according to their rating classification. At a given rating, if the percentage removal is close to 100 percent, the cartridge is said to have an absolute rating. The rating is said to be nominal if the percentage removal at that rating is lower—approximately 90 percent, but this varies from one manufacturer to another. The WQA Glossary of Terms defines a nominal filter rating as meaning that 85 percent of the particles of the size equal to the rating will be retained by the filter. Submicron filter cartridges are those with ratings below one micron, and most are made with absolute ratings. In almost all cases, cartridges are discarded when they’ve removed as much suspended solids as they can practically hold—usually based on a pressure drop through the cartridge exceeding 10 pounds per square inch gauge (psig).
Bed filters: These filters typically consist of a quantity of granular material, either of one kind (a medium) or two or more kinds (media) mixed together or in layers. A filter bed normally consists of several layers of sand-like media layered by size to efficiently remove suspended solids, differentiated by size.
Resin beds: Whereas activated carbon and deionization resin beds aren’t intended to be used as filters, they typically accumulate suspended solids. For carbon, in the event the pressure drop through these beds exceeds a certain figure—usually 10 psig—backwashing is typically performed.
In ion exchange, anything that attaches itself to the resin obstructs the exchange sites and that does not readily regenerate off is said to be a foulant. Foulants can be scalants that are mineral in nature such as precipitated metals, calcium scale, oxidized iron or polymerized silica; particulate such as silt and clay, or organics such as humic and fulvic acids, natural tannins and oils. Flocculants often used in water pretreatment that carry over can also be foulants.
Softeners typically regenerated with salt can foul in a number of ways. Iron and manganese, which are easily adsorbed by the resin, can oxidize and aren’t readily removed by salt regeneration. Aluminum and lead also are readily adsorbed by resin in service and can be very difficult to remove with normal salt levels, accumulating and thus reducing capacity and performance of the softener.
In deionization, cation resins can foul with calcium and barium sulfate due to improper regeneration with sulfuric acid. Anion resins are scavengers for organics, which require special regeneration techniques to remove. Under certain conditions, silica can polymerize on anion resin, causing permanent capacity losses.
Membrane technology: As described above, the membrane technologies most commonly used in water treatment today utilize crossflow current in an effort to minimize fouling (see Figure 1). Certainly, as a result of this approach, fouling is minimized, but definitely not eliminated. Fouling virtually always occurs on the surface and, because of the concentration effects inherent in membrane processes, much of this is from inorganic salts precipitation resulting from having exceeded solubility limits—scaling. In certain instances with microporous membranes, suspended materials and/or scale may actually penetrate into the pores, causing what is known as plugging.
Figure 2 illustrates a membrane with biofilm attached to the surface. The extracellular polysaccharide (glycocalyx) matrix—biofilm—surrounding the bacterium captures suspended solids as well as solutes that are rejected by (or adsorbed onto) the membrane surface. Not only can these biofilms build up along with collected contaminants blocked by them, but they can facilitate the grow-through of bacteria onto the permeate side of a membrane if proper care isn’t taken in terms of operation and maintenance of the system.
In addition to fouling membrane surfaces (above), foulants will also accumulate on the surfaces of other water treatment technologies such as ultraviolet light tubes as well as the interior surfaces of piping, tubing and tankage. In most cases, the fouling is initiated by bacterial growth. Bacteria readily adhere to surfaces, and immediately start forming biofilms, which serve as tiny porous structures that capture suspended solids and microorganisms from the flowing water stream, building up thicker and thicker layers and, ultimately, negatively affecting performance of the water treatment system over time. Care, which may vary based on the particular technology and type of fouling encountered, should be taken in each case to minimize the potential for such events.
Regardless of the names associated with the fouling phenomena, the net result is the same—the performance of the water treatment technology is deleteriously affected. Although the subject is extremely complex and no less complicated than other aspects of water treatment, this article has hopefully defined some of the terms associated with fouling and provided clarification of some of the issues.
The author would like to thank C.F. Michaud, of Systematix, Buena Park, Calif., for assisting on the ion exchange section of this article and ResinTech, West Berlin, N.J., for providing the images of fouled resins used to illustrate the section.
About the author
Peter S. Cartwright, president of Cartwright Consulting Co., Minneapolis, is a registered professional engineer in several states. With more than 25 years in the water treatment industry, Cartwright has been chairman of several Water Quality Association committees and task forces. He’s a past recipient of the WQA’s Award of Merit and holds its highest designation, Certified Water Specialist, Level 6. He also has been a member of the WC&P Technical Review Committee since 1996. He can be reached at (952) 854-4911, (952) 854-6964 (fax) or email: CartwrightConsul@cs.com