Key considerations when implementing microbial filtration technology
Point-of-use (POU) and point-of-entry (POE) water treatment for microbial contaminants requires the end user and sales and service personnel to select their solutions wisely. The wrong choice could result in illness or even death from waterborne pathogens. While many professionals are well aware of water treatment choices, they may not be aware of the precautions necessary for micro- bial reduction applications.
Membrane technology—reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF)—and absolute-rated particulate filters have one thing in common: They all rely, to some degree, on media pore size. In cases in which the pores within the media structure develop an integrity breach, or if a breach of the vessel seal takes place, the results can include high levels of microbial contaminants bypassing the filter material and the release of contaminants that could enter the clean water system. These high levels of microbes can be catastrophic.
The marketplace can be confusing, with many RO, NF, UF and MF manufacturers making microbial claims for bacteria, virus and cyst reduction. These normally are expressed as a percentage or log reduction value (LRV) for the membrane (spiral, hollow-fiber or pleated designs). Pleated particulate filters with an absolute rating of 0.1 to 1 μ also may make claims for microbial reduction. It is important to carefully investigate filter choices and performance claims, as performance changes will occur depending on the seal integrity within the filter vessel.
Double-open-end (DOE) or single-open-end (SOE) gasket cartridges from one manufacturer may have high-efficiency capability, but if they rely on a knife seal or are used in another manufacturer’s vessel, they may not maintain a seal sufficient to filter out microbial contaminants due to pressure shifting or pulsation changes from flow rates breaking the seal. Without a proper seal downstream, microbial contamination is a reality from the start, and it only goes downhill from there. Simply put, one must ensure that the elements or cartridges are designed for the vessel they are put into, and know that a plastic- or stainless-style vessel using a knife seal does not have the tolerances sufficient for microbial claims.
SOE filter elements with a double O-ring seal, such as a 222 or 226 (or similar design), that fit into a receiver within the element vessel and that provide the O-ring compression necessary to prevent bypass should be used. This compression is typically 10% or more. Proper vessel design is essential for the elements to be able to perform as designed.
Integrity of Elements & Cartridges
The second piece of the membrane puzzle is the integrity of the membrane or particulate filter elements or cartridges. Viruses can be less than 0.02 μ in size, and if the membrane or filter element is not integral, all is for naught, as microorganism bypass can result. The supplier should be able to provide a Certificate of Conformance of Integrity for this style of membrane or cartridge. NSF/ANSI Standards 42 and 61 cover material safety and structural integrity, but do not verify that the membrane element or cartridge is integral. These standards assure that the product material is not imparting anything into the water, and is approved for drinking water treatment units. They are for filtration efficiency issues.
NSF Standards P231: Microbiological Water Purifiers and P248: Military Operations Microbiological Water Purifiers, which test for microbial efficiency, do not test the integrity of the elements. NSF Intl. is now promoting its latest standard for municipal drinking water treatment technology, NSF/ ANSI Standard 419: Public Drinking Water Equipment Performance – Filtration. This is an efficiency-driven standard for microbial LRV, but not integrity verification of the elements or cartridges.
Public water supplies will not give virus LRV credits to membranes without requiring a continuous integrity verification of the membranes or cartridges for every module, typically every 15 minutes. Because this technology does not kill the organisms—it merely filters them—regulatory agencies are well aware that if any breach of integrity occurs, public water systems risk their customers’ health and well-being. Death could even result for people with compromised immune systems who receive a high dose of microbes from a possible breach in the element, cartridge or vessel design.
Any membrane can develop a breach after it has been installed or immediately after an online integrity test. Nephelometric turbidity unit and conductivity meters are not capable of detecting the breach until it is large enough to register on them. When used as post-treatment for microbial issues with ultrapure water, these meters may never detect a breach.
Disinfection Technology Preferences
Chorine, chloramine, chloride dioxide and high-dose ultraviolet (UV) at 186 mJ are the preferred methods of disinfection in municipal applications. UV and ozone are routinely used in the bottled water and food and beverage industries. Disinfection per U.S. Environmental Protection Agency and state regulations is defined as one or multiple treatment processes combined to provide a validated greater-than-4-LRV reduction of bacteria, viruses and protozoan cysts.
Therefore, anywhere water is used for drinking, misting, cooling, washing vegetables or other potable applications, if the water is not properly filtered, microbial contamination can cause food spoilage, illness and liability for those selling, installing and using these devices. Thus, water professionals must be certain when selling any filtration device that the end user has technology capable of providing the LRV appropriate for the application.
Advancements in Technology
Electroadsorptive technology that does not rely on pore size exclusion for high levels of microbial reduction and does not release the retained contaminants in the event of an integrity breach now is commercially available. The electropositive charge captures and retains contaminants throughout the depth of the pore structure. This method reduces microbes via adsorption and mechanical filtration, not simply pore size exclusion.
The electroadsorptive charge provides a net adsorption efficiency of approximately 0.002 μ, and mechanically, with a mean pore size of approximately 1.2 μ. The charge field removes the negatively charged submicron particles, while larger particles are captured within the fiber structure of the media. This method reduces viruses by greater than 4 LRV with a mechanical mean pore size of approximately 1.2 μ. (The validation test process for state and federal drinking water regulations often results in a lower overall log reduction credit than the LRV indicated in manufacturer reduction testing. Regulatory agencies do not assign virus disinfection credits for standalone cartridges for membranes.)
Electroadsorption provides retention of the organisms through these charged structures within the media, and small integrity breaches, which may occur, do not produce a catastrophic release of organisms. These filter media have more than 400 layers of pore depth created by a wet-laid non-woven manufacturing process. This results in media with a tortuous flow pattern and high flow rates at low pressure drop. The charge field results in a high initial removal efficiency as well as high loading capacity.
These media have been sold commercially for years with positive results. When used with appropriate and proper prefiltration for particulates, they provide sufficient lifecycles between cartridge changes. A cost comparable with MF and less than RO, NF or UF membranes makes them competitive in the marketplace.
While the media performance is excellent, they still require good manufacturing practices, and the integrity of the finished product is just as critical as it is with membranes. These media require manufacturing knowledge and experience, as they can be difficult to pleat due to their 1-mm thickness and composite structure. A quality control procedure with a 100% non-destructive performance test (NDPT) or reverse bubble testing in alcohol solution is required at the production facility for every cartridge to ensure its integrity.
Spent elements and cartridges that remove microbes become biohazards, and without an antimicrobial additive to control the microbes, they cannot be disposed of as normal waste. This issue often is overlooked in the marketplace, and elements and cartridges typically are disposed of in the trash. Laboratories conducting microbial testing must incinerate their used plates or autoclave the tubes to ensure that microbial contamination is controlled. This is an issue that should be considered when selecting an element or cartridge for microbial reduction. With new government regulations being introduced daily, it is only a matter of time before this issue will need to be addressed by the industry.
Microorganisms may be difficult to identify and test, but have a high affinity to be present in the water source that is critical in many applications. Cost is the secondary factor in the end selling process, whether one is concerned about microbes in drinking or process water or the aero- sols from misting or spraying applications.
So, while your supplier may offer you a solution for microbial reduction, it is ultimately each professional’s responsibility to be educated on the pros and cons of mismatching elements and vessels that do not provide a sealing system appropriate for their end use. It is important to ensure that new elements or cartridges have undergone NDPT for microbial performance requirements prior to use and that proper dis- posal procedures are followed.