By Vikas Thusoo
Summary: Since the industry has yet to formulate a standard for rating a filter cartridge, a greater impetus should be placed on consumer awareness as well as validation. With this as a backdrop, the following article looks at some of the factors that should be considered when selecting the proper filter for a particular application.
Cartridge filters are widely used in industrial and domestic applications for removal of suspended solids in drinking water. The variety of cartridges available and confusing methods of ratings, however, makes selection of cartridges difficult for consumers. It becomes important for users, therefore, to understand cartridge filters, how they work and how manufacturers rate them.
Let us begin with filtration itself. Filtration—defined in its most basic sense—is a process of removing unwanted solids from fluids by passing the fluid through a form of sieving material that retains the solids, but allows the fluid to pass through. Filtration efficiency is, therefore, the percentage of solid retention by the sieve. It’s this “sieve” that we refer to as the filter medium, or simply the filter. A contaminant is generally referred to as the material that’s to be removed from the fluid, and the clean fluid is called the filtrate.
In today’s market, manufacturers use three types of ratings to evaluate filters: nominal rating, absolute rating and beta ratio.
The Water Quality Association (WQA), in its Glossary of Terms, defines nominal to mean 85 percent rejection at the stated micron rating at the recommended flow rate. This means that a filter with a nominal rating of 5 microns (µm) will remove 85 percent of contaminants that are 5 µm and above.
Nevertheless, different manufacturers rate their filters at different efficiencies. This means that the same 5-µm nominal rated filter from two different manufacturers could mean different efficiency ratings. One could be 85 percent efficient at 5 µm while the other could be 70 percent. Always ask your supplier at what efficiency they grade their filters for nominal rating.
Nominal rating is generally given to filters to be used for non-critical applications such as pre-treatment for reverse osmosis (RO) or point-of-entry (POE) units and other pre-filtration applications.
Absolute rating is defined as essentially 99.9 percent (3-log) rejection at the specified micron rating, according to the WQA Glossary of Terms. This means the filter removes 99.9 percent of the particles of the stated pore size. It’s difficult to verify the absolute rating of a filter at the user location.
Bear in mind, though, there are no industry standards when it comes to micron rating claims for filter media. Definitions may vary by industry or fluids to which the filters are being applied. Thus, some manufacturers may rate their filters at different efficiencies, so be sure to clarify this with your supplier. It’s best to ask for a third-party validation from the supplier.
Beta ratio is a quantitative method of determining a cartridge filter’s efficiency. It’s defined as the ratio of the number of contaminant particles entering a filter (PE) to the number of particles passing through (PL).
Beta ratio = PEE/PLL
Clearly, the lower the number of particles leaving the filter, the higher the beta ratio and correspondingly, the better the contaminant-holding capacity of the filter.
A beta ratio test is conducted in controlled conditions and involves a test filter being challenged by a specific number of particles of a certain pore size. Typically, AC fine dust may be used. As the fluid enters and leaves the test filter, the number of particles coming out of the filter is measured using an online particle counter. After the inlet and outlet readings are noted, beta ratio is calculated.
Normally, users come across beta ratios of 100, 1,000 and 10,000 for cartridge filters. Keep in mind, a beta ratio of 1,000 (β1000 or b1000) is 10 times more efficient than a filter with a beta ratio of 100 (β100). This is because a β1000 filter allows only 1 out of 1,000 particles to pass while a β100 filter allows 1 out of 100.
Beta ratio tests are usually conducted on production batches on random samples. A higher beta ratio is considered more efficient; however, a higher beta ratio doesn’t necessarily correspond to high filtration efficiency in all conditions. You must remember beta ratio tests are conducted in test conditions with a specific contaminant of a specific structure—or an accepted surrogate. In actual usage, conditions, contaminant size, structure and characteristics may be different. This renders a distinct on-site efficiency to the filter, which only the user can validate for the process.
It’s advisable to run extensive tests with different filters and validate their on-site performance. Remember, the cartridge that works best for an application may not be best for a similar application elsewhere. This is due to the difference in contaminants, process conditions, etc.
Types of cartridge filters
Cartridge filters can be broadly classified into the following categories—depth filters, surface filters and adsorptive filters.
Depth filters can be made from cellulose, ceramic, polypropylene, acrylic fiber, polyester, glass fiber, etc. Sometimes, the filter is designed to reduce porosity from the periphery to the core. This is called a graded density structure where the larger particles are trapped at the surface of the filter and progressively smaller particles are trapped toward the core (see Figure 2).
This construction in depth filters has been known to provide much higher dirt retention capacity than ordinary construction. Grooving is another feature that’s seen in depth filters (see Figure 3). This is essentially to increase the surface area. Grooving isn’t possible on string wound or other types of soft cartridges.
Filtration with surface filters takes place only on the surface of the medium. Sheets of polypropylene, nylon and teflon, etc., are pleated and bonded on ends to provide high filtration area. The contaminants are trapped on the surface of the sheet, forming a cake. This cake can be washed off and the cartridge can be reused.
While pleated filters are occasionally used as coarse pre-filters (when they can be washed), they’re preferable in sub-micron applications as they provide finer and consistent filtration attributes due to their definable pore size and structure.
Filtration ratings can be anywhere between 10 µm down to 0.04 µm absolute, which is generally used in semiconductor or pharmaceutical applications. Compared to a depth filter, a surface (pleated) filter will have a much higher filtration area to compensate for absence of depth. Filtration areas can go as high as 11 square feet in a regular 10-inch cartridge, or even higher.
Adsorptive filters can be either depth or surface filters. In these filters, the contaminants adhere to the cartridge through electrokinetic adsorption. Colloids and microorganisms, which are generally negatively charged, are attracted to positively charged sites; otherwise, they would pass through. This results in higher filtration efficiency.
Charged filters are finding extensive usage in pharmaceutical plants, wineries and breweries, and food plants as they greatly reduce the contaminant burden on the final filters, thereby lowering replacement costs and improving system efficiency. Of course, there are factors that affect filter performance.
Let us assume that the contaminant retention capacity of 1 square centimeter (cm2) of a given sheet of filter medium is 1 milligram (mg). This would mean that 10 cm2 will take 10 milligrams (mg) of contaminant load. Now assume that this 10 mg load is distributed in 100 liters of water. We can, in other words, say that the media (10 cm²) will be able to filter 100 liters of water after which it will choke (i.e., plug, or meaning that no further fluid can pass at the set pressure). If we double the area of the media to 20 cm2, the filter will bear a load of 20 mg and it will filter 200 liters of water. The observation, therefore, is—as we increase the surface area of a medium, it filters more liters/gallons of water. In short, it has more life. Thus, filter manufacturers devise various methods to increase the filtration area so that the user gets more life out of the filter.
The increase in surface area should have minimal effect on the differential pressure or flow rate. When discussing the life of a filter, we refer to the service capacity in terms of fluid liters. For example, when a filter has a life of 10,000 liters of water, it’s scheduled to plug after that many liters. During system sizing, a user that designs the system for lesser number of housings may reduce capital costs, but it will also reduce the filter’s life. As a result, replacement costs will increase. This is because the contaminant load—instead of being shared by a greater number of filters—is taken by a fewer number, resulting in faster plugging.
Lower flow rate generally corresponds to better filtration attributes. At higher flow rates, the trapped particles are forced to push their way through the filter, thereby resulting in reduced efficiency of the filter. It’s always advisable to run the filtration system at or even below the recommended flow rates specified by the manufacturer.
Rigidity is generally an overlooked criterion for cartridge selection. Rigidity is of equal importance in both surface and depth filters. Filters must be rigid enough to take the forward fluid pressure. Some improperly designed filters deform solely on the forward pressure; however, this is rare. What’s more likely is a pressure surge in the line, which lasts less than a second and is difficult to identify on regular pressure gauges. During a pressure surge, there’s a very high differential pressure created in the filter system, which squeezes the filter and releases most of the dirt already captured into the product line. This can lead to huge losses. Moreover, if this goes undetected, the filter may seem to have an endless life as it unloads regularly.
In surface (pleated) filters, manufacturers provide the cartridge with inner cores and outer cages to render strength to the cartridge and protect it from forward and differential pressure. Wherever depth filters are used, they should be checked for rigidity prior to approval. Physical examination of cartridge rigidity is useful in depth filters. For pleated filters, it’s advisable to verify pressure/rigidity parameters form the manufacturer.
A low initial differential pressure across the filter means that the cartridge is offering low resistance to the fluid flow. As the cartridge begins to choke, however, the differential pressure begins to build slowly and spikes up at a critical point, indicating the end of the cartridge filter’s life. The life vs. differential pressure curve of a filter shows that very little additional life can be expected out of the filter once the critical stage has been reached.
Some filters can be backwashed to clean off the dirt and be re-used, but the initial differential pressure after the backwash is usually higher than when the cartridge is new. Over a period of several backwashes, the cartridge finally acquiesces and chokes.
Backwashing isn’t a good idea on depth filters because the contaminants are trapped deep inside the depth of the filter, and even a reverse flow cannot clear them out of the filter. Scrubbing the filter also wouldn’t help much in rejuvenating the filter.
A filter cartridge will consist of one or more of the following—the filter media, core and cage, and end fittings. The media and core and cage were discussed earlier. The end fittings are necessary to seal the cartridge into the housing. Some of the popular end connections are double-open end, bayonet lock, single-open end with O-ring seal, etc.
Filter vs. filtration cost
With filter cartridges being con-sumables, it becomes necessary to calculate the filtration cost per liter of the filtrate over a specific period or flow. It’s possible that a relatively expensive cartridge lasts much longer, thereby reducing the replacement, downtime and labor costs. Such a filter could eventually provide much lower filtration cost. In very coarse applications, though, a low-cost filter could prove less costly as efficiency isn’t as important. It’s therefore advised that every user must assess his needs and choose a filter based on personal requirements and needs.
Cartridge filtration is dependent on varied conditions that affect the system performance. Its performance, therefore, should be field validated where efficiency is critical. With the market flooded with various types of filter cartridges and in the absence of a common industry standard for rating a filter cartridge, there’s a greater need for consumer awareness of the various facets of cartridge filtration. It’s recommended that a user carefully select the best-suited cartridge for the application through production trials and validate a product for that process.
About the author
Vikas Thusoo is a filtration expert with more than seven years of experience. Thusoo previously served as manager for CUNO Asia in India and business manager with EA Water. He is based in Toronto, Canada, and can be reached at (416) 439-2323 or email: email@example.com