By Rick Andrew

Rated capacity is a fundamental aspect of active media water filters that rely on adsorption or absorption of chemical contam­inants as a mechanism of action. At some point, the active media will begin to reach equilibrium with the water being treated, in terms of the concentration of chemical contaminants in each and the effectiveness of that media will decline. This characteristic of active media filtration systems is recognized within the NSF/ANSI Drinking Water Treatment Unit (DWTU) standards, such that chemical reduction claims are tested based on the manufacturer’s rated capacity. The systems must perform according to requirements either to the rated capacity in the case of aesthetic contaminant reduction claims, or beyond it for health claims.

NSF DWTU standards and rated capacity
It is recognized that an active media water filter may be more effective in treating certain contaminants compared to others. Chemical contaminants vary in structure and properties, which impacts their solubility in water, their affinity for various treatment media and ultimately, the effectiveness of the media in treating them. Also, a review of contaminant occurrence in source water and drinking water informs us that different contaminants tend to be present in different concentrations, as well. Typically, an active media filter will have a higher capacity for a given contaminant when the concentration of the contaminant is lower.

Given these variables related to capacity, it is understood that rated capacity for filtration systems with multiple chemical reduction claims must be specifically addressed within the requirements of the NSF/ANSI DWTU Standards. For example, one approach could be to establish individual and potentially different rated capacities for each chemical contaminant evaluated for reduction. The NSF Joint Committee on Drinking Water Treatment Units has considered this issue and has determined that for the sake of simplicity and clarity for end users, there must be only one rated capacity for each active media filter system. In effect, this means that the lowest evaluated capacity for the various chemical contaminants evaluated and claimed becomes the rated capacity for the system. The result is that filter systems with multiple chemical reduction claims may have a safety margin with the rated capacity when it comes to certain contaminants evaluated under the NSF DWTU standards.

Commercial modular systems and capacity
Commercial modular systems are specifically designed and manufactured for commercial applications, such as treatment of water for fountain beverage dispensers used in restaurants. These systems typically consist of a manifold and various filtration elements that can be employed for different purposes, depend­ing on the nature of the application, such as the source and composition of the water being filtered. Professional installers use their water treatment and product application training and knowledge to assure that the filter elements put into service are appropriate for the specific application.

The approach to rated capacity for commercial modular systems has been to establish it per treatment element, as opposed to per each possible permutation of the treatment system. This approach makes it simpler for manufacturers to describe capa­bilities for their products and makes it clear for installers how frequently the elements must be serviced given the usage level and the configuration.

Consistent with the overall approach, the NSF DWTU Standards have required establishing one treatment capacity per modular element based on the lowest capacity for any chemical contam­inant reduction claim, for those elements with more than one such claim.

Recent developments
Recently, this approach of one rated capacity per modular element was reconsidered with respect to the situation with chlorine reduction versus chloramine reduction. Typically, active media filter systems with the ability to treat chloramine will also have the ability to treat chlorine and with a significantly higher capacity for treatment of chlorine.

Considering that commercial modular systems are installed and serviced by trained professionals, this approach has been revised. Service professionals have knowledge regarding the disinfection approach being taken by the various water systems in which they are working and whether they may be using chlorine or chloramine. Ultimately, these professionals have the capability to allow them to understand and manage filter elements with claims of chlorine reduction and chloramine reduction that vary in rated capacity depending on which type of disinfection is used on the water being treated.

With this point of view in mind, NSF/ANSI 42 has recently been updated to indicate that for modular elements that have contaminant reduction claims of chlorine reduction and chloramine reduction, and no other claims with a rated capacity, then different rated capacities may be stated for chlorine reduction and chloramine reduction. See Figure 1 for the exact wording from the standard.

Claims without rated capacity

Under the NSF/ANSI DWTU Standards, mechanical filtration claims do not have rated capacities associated with them. The testing for these claims does not have an end point defined by volume, like chemical reduction claim testing does. Instead, the testing for mechanical filtration claims involves introducing test particles into the filters and ending the test based on a reduction in flowrate based on clogging of the filter. Because the end point of the test is based on flow reduction, there is no tie to a specific volume of water and therefore, no rated capacity established by the test.
Therefore, a commercial modular element with claims of chlorine reduction, chloramine reduction and one or more mechanical filtration claims can have different rated capacities for chlorine reduction and chloramine reduction. See Figure 2 for a list of mechanical filtration claims that do not have rated capacities associated with them.

More flexibility

This recent update to NSF/ANSI 42 provides flexibility regarding the rated capacity of certain modular elements of commercial modular systems, allowing water treatment professionals to best serve end users by tailoring the replacement frequency of these elements to the type of disinfection used on the source water for the commercial facility. End users are assured a product that will function as claimed by the manufacturer and maintenance costs can be controlled more effectively by leveraging product capabilities and the advanced training and knowledge of these professionals.

About the author
Rick Andrew is NSF’s Director of Global Business Development – Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email:


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