By Gary Battenberg
In the final part of this series, we are going to look at the diverse types of filters for clarifying water using bag and cartridge type filters. Up to this point, we focused on removing large solids, particulates, turbidity, odor and color removal with treatment methods commonly used by water treatment dealers and the do-it-yourself (DIY) consumer.
Bag filters
Hippocrates, the father of medicine, invented the first bag filter around 500 BC and it came to be known as the Hippocratic Sleeve. He believed clean water had healing properties and reduced the possibility of infection and disease. After boiling the water to inactivate microbiological organisms, he would pour the water through his bag filter to remove sediments and related tastes and odors. Since that time, bag filter design has evolved and used in many different markets including:
- Municipal and industrial water treatment
- Pulp and paper
- Lubricants for metalworking fluids
- Paint, inks, resins, solvents, coatings and varnishes
- Petrochemicals and chemical processing
- Automotive and electronics
- Pharmaceutical
- Food and beverage, beer, wine and edible oils
Micron ratings for typical bag filters range from nominal 1 to 100 microns. Nominal filter ratings indicate the approximate particle size of which 85-percent of the particle micron rating will be captured within or on the filter. These filters are constructed of polypropylene or polyester needle felt of varying thickness. The felt seams are sewn to ensure a consistent sediment capture. It is important to note that it is advisable to use a bag filter insertion tool to ensure correct alignment in the restrainer basket of the filter housing for optimal sealing against filtration bypass.
Multi-layer sewn bag filters offer greater particle retention because of their advanced design, which includes a polypropylene felt pre-filter layer, melt blown polypropylene microfiber layer and a polypropylene external migration barrier. Welded bag filters are the premium product and have particle capture from 1 to 200 microns. The advantage with welded construction is that nothing bypasses the filter media (that sometimes occurs with sewn bags) where a hole may develop.
Pleated bag filters feature a greater surface area, which may increase dirt holding capacity as much as 10 times that of standard bag filters and up to three times that of multi-layer bag filters. Another advantage of the pleated bag filter is the reduced maintenance cost due to typically longer service life. Pleated bag filters are rated for particle capture from 1-50 microns in size. Where finer filtration is required, a pleated filter rated for one micron is used as a safety filter downstream of a standard bag filter for optimal sediment capture.
Absolute melt blown bag filters feature progressive density filter media and may provide up to 99.9-percent particle retention for particulates as small as 0.2 microns. Absolute filter ratings indicate that all particles larger than a specified micron rating will be captured within or on the filter. These bag filters are typically used where FDA compliance is required for food and beverage service and are limited to polypropylene construction.
Cartridge depth filters
Depth filters are available in different configurations, including string wound, using cotton or spun polypropylene cord (fibrillated) materials and pleated cartridges. Among the most common types of cartridge depth filters are the spun polypropylene and melt blown varieties commonly used for prefiltration to RO, as well as post filtration to capture granular activated carbon (GAC) fines. The spun and melt blown 2.5-inch diameter filters range in lengths from 5 to 40 inches and are available in nominal ratings of 1, 3, 5, 10, 20, 30, 50 and 75 microns. These are the most economical solution to prefiltration for general, rural, residential, commercial and specialty applications.
The 4.5-inch diameter melt blown depth filters are produced using polypropylene microfibers, which allows for dual gradient density (DGD) filtration capability by combining two separate layers into one filter. This dual gradient provides exceptional particulate capture efficiency and significantly higher dirt holding capacity than typical spun polypropylene or string wound cartridges. Filtration densities range from [pre/post] 75/25, 50/5 and 25/1 microns. There are also multi-gradient filters combining four separate layers into one filter.
The graded densities provide sediment capture through the entire cross section (layer-by-layer) of the filter, which results in lower pressure drop, reduced surface blinding and longer service intervals. An example of a quad-layer depth combines densities of 50/25/10/5 microns, respectively.
The polypropylene filters are the most economical solution to prefiltration for general and specialty applications. Certification to NSF 42/FDA for material requirements makes these filters ideal for many different applications which include:
- Potable water filtration
- RO and DI water filtration
- Ice machines
- Beverage, coffee, wineries
- Analytical
- Chemical filtration
- Electroplating baths, etching
- Film processing
- Prefiltration for gray water recycling
- Amine filtration
Pleated cartridge filters
Pleated cartridges are made using varied materials including cellulose, polyester, polypropylene and non-woven electropositive fibrous media. Construction includes polypropylene end caps and cores, and EPDM gaskets. The polypropylene core supports the pleats for maximum radial wet strength to prevent cartridge collapse and particulate migration at higher flow and pressure drops. The filter media is cut to length (slit width), pleated and the long seam is sonically welded to ensure no raw water passage.
The pleated media is fused to the end caps and core with molten polypropylene to embed and seal both ends, thereby creating a [unitized] water-tight seal to ensure no bypassing at the seam or under the caps. The EPDM gaskets interface with knife edge seats of the filter housing cap and bowl to further ensure no raw water bypass at those sealing interfaces. Finally, the cartridge is encapsulated in a polyethylene outer sheath for added strength to resist operational system pulsation. Polyester and polypropylene pleated filters may be washed or flushed for reuse due to their inherent bacterial and chemical resistance. Cellulosic filter media, however, may provide a food source for bacteria and is therefore not cleanable or flushable for reuse.
Non-woven electropositive fibrous media is not cleanable or flushable for reuse and considered a dead-end or final barrier filter media. Typical filtration separate particles by the mechanisms of sieving, inertial impaction, interception, and diffusion. Electropositive filters principally use adsorption (electro-adhesion) as the filtering mechanism. Electro-adhesion is that phenomenon that utilizes the difference in charge that may exist between a surface (or fiber) and a particle in an aqueous solution, where a charge is built up by the double layer effect. Much like a typical pleated cartridge filter in appearance, the electropositive fibrous media mechanically sieves particulates larger than its average pore size.
Most bacteria and other particulates in an aqueous solution are electro-negatively charged, making the electropositive fibrous media a ‘broad spectrum particle magnet” capable of capturing sub-micron particulates down to 0.01 micron. In critical applications, removing sub-micron particles is vital. They are responsible for much of the fouling of RO membranes and will degrade the efficiency of UV and ozone (O₃) disinfection systems.
In comparison with ultrafilter membranes, fines and colloidal matter can also affect chemical processes and impact the quality of surfaces of precision instruments and products. Membranes, like ultrafilters, are surface filters with little dirt holding capacity (DHC), typically about 1.5 milligrams per square inch (mg/in²). Consequently, while their efficiency may be high, their capacity for holding dirt will often lead to premature fouling resulting in frequent cleaning cycles and increased operational costs. Conversely, electropositive fibrous media has a DHC of 572 mg/in² and will remove virtually all the sub-micron particles that pass through a conventional filter.
Carbon cartridge filters
Granular activated carbon, carbon block and powdered activated carbon are three major players in water treatment. Each has a place in the industry and all three are exceptionally good at removing/reducing chlorine, taste and odor (CTO), and certain VOCs and pesticides, as well as virtually all organic contaminants. Advancements in carbon block technologies are emerging at a steady pace considering the emerging contaminants of concern (ECOC) such as PFOA and PFAS (PFASs). Sorbents for lead and arsenic are commonplace in carbon block filter production with more advancements in research and development in the works.
Granular activated carbon filters are available in axial and radial flow varieties. These types typically yield the lowest pressure drops because of the larger size of the carbon granules. They are typically used as a post filter for RO systems to control taste and odor in the dispensed water. Carbon block filters are commonly used in single or multiple cartridge drinking water systems and in RO systems where low molecular weight organics are present in the water and require longer residence time in the carbon to effectively remove/reduce those contaminants.
Advanced carbon block technologies continue to evolve for removal of Cryptosporidium cysts, Giardia, Legionella, viruses or bacteria at greater than 99.99-percent efficiency and up to 8-log reduction value (LRV). LRV is the measure of how thorough a process reduces the concentration of living microbial contamination. Example: 1 log = 90-percent reduction, 2 log = 99-percent reduction, 3 log = 99.9-percent reduction, 4 log = 99.99-percent reduction, etc.
Powdered activated carbon is used in pleated cartridges where it is embedded in the matrix of the media to provide clarification, and CTO removal. Because of the extremely small particle size of this carbon, the kinetics are much faster than that of GAC.
Final note
Before selecting any filtration cartridge for as specific application or task, it is vitally important to understand how it works, how it is rated and what its capabilities and limitations are relative to flowrate and loading capacities. Take the time to study the application bulletin(s) for the filters used in your business. When faced with a question or need for clarification, do not hesitate to contact the manufacturer or supplier for accurate answers and application assistance. Solving complex filtration issues is a rewarding endeavor especially where severe water problems are present.
Gary Battenberg is a Business Development Manager-Senior for Argonide Corporation. Previously, he was Technical Manager, Water Treatment Department of Dan Wood Company. Prior to that, Battenberg was Technical Support and Systems Design Specialist with Parker Hannifin Corporation. His nearly four decades of experience in the water industry include a proven, successful track record in areas of sales, service, design and manufacturing of water treatment systems. Battenberg’s technology base covers mechanical and adsorptive filtration, ion exchange, UV sterilization, RO and ozone technologies. He has worked in the domestic, commercial, industrial, high-purity and sterile water treatment arenas. A contributing author to WC&P International magazine and a member of its Technical Review Committee since 2008, Battenberg was voted one of the magazine’s Top 50 most influential people in the water treatment industry in 2009. He can be reached by email at [email protected] or by phone (407) 488-7203.