By Rick Andrew

When manufacturers of water filters think of NSF/ANSI Standard 42 and 53 requirements, they typically think about four categories:

  • Materials (Section 4)
  • Structural performance (Section 5)
  • Mandatory testing for elective claims (Section 7)
  • Instruction and information (Section 8)

These are the “big ticket” requirements—the ones that product design engineers and other product development specialists focus on when considering how proposed new filters will conform to these Standards. Materials of construction are carefully chosen so the product can conform to the strict material safety requirements. Plumbed-in systems are designed to withstand the rigors of hydrostatic and cyclic structural integrity testing. The specifics of influent challenges, rated capacities and flow rates are carefully considered with respect to contaminant reduction performance. And great care is taken to ensure that all of the various required statements are included in product manuals and performance data sheets, on product data plates and on replacement element packaging.

Minimum performance requirements
However, in all of this careful planning and consideration, there is one section of the Standards that can sometimes be overlooked or taken for granted:

  • Minimum performance requirements (Section 6)

This section is a bit of a catch-all that includes various requirements that do not fall into other categories, many of which have no specific test procedure. A description of minimum performance requirements can be found in Table 1.

These requirements are usually handled as opportunities present themselves during the Certification process. For example, performance indication device testing can often be combined with a contaminant reduction test. The same is true of the filter media requirement and the active agent evaluation. Some of the requirements can be addressed through physical examination of the system, such as those regarding hazards, product water dispensing outlets, waste connections and elements.

Minimum service flow—simple yet subtle
Minimum service flow is another example of an evaluation that can often be combined with a contaminant reduction test. It is a relatively simple matter to install and flush a product in accordance with the manufacturer’s instructions and then adjust the influent pressure to 30 psig and measure the flow rate prior to beginning a contaminant reduction test. The flow rate is then evaluated against the requirement for the specific type of system, as described in Table 2.

There are some nuances in this evaluation, however. The first one to note, per Table 2, is that refrigerator filters have no minimum service flow requirement, similar to batch systems. While this is more obvious for batch systems as they are not connected to plumbing, it is a point that can be missed with respect to refrigerator filters if the Standard is not carefully read. The logic here is that refrigerator filter systems often incorporate lengths of coiled tubing used to cool the water prior to filtration, which causes significant pressure loss. Because of this design constraint and the fact that these systems are single outlet, point-of-use (POU) systems, the minimum service flow requirement has been waived.

A second nuance is with faucet mounted systems. If these systems include a means to bypass the filter for dishwashing or other general water usage, the minimum service flow requirement is 0.2 gpm. If there is no way to bypass the filter, the minimum service flow requirement is 0.5 gpm, as 0.2 gpm would not be acceptable to most consumers for general purpose use.

A third subtlety to minimum service flow requirements relates to POE systems. Table 2 indicates that POE systems must achieve a 4 gpm minimum service flow. However, there is a second minimum performance requirement for POE systems that must be considered. The rated pressure drop requirement specifies that POE systems without a built-in flow control shall have no more that 15 psig pressure drop at the rated service flow with an inlet pressure of 30 psig. This requirement supercedes that of minimum service flow for those systems that do not have built-in flow control. It is only those POE systems that do not have built-in flow control that must meet the 4 gpm minimum service flow at 30 psig inlet pressure.

POE is defined in Standards 42 and 53 as a system that treats all or part of the water at the inlet to a residential facility. POU is defined as a system that treats the water at single or multiple taps, but not the majority of the facility. As seen from these definitions, there is a fine line in distinguishing POE from POU. Some manufacturers may ultimately wait to categorize certain systems as either POE or POU, plumbed-in, until they have been able to evaluate minimum service flow, as POU systems must achieve only a 0.5 gpm minimum service flow.

Conformance to Standards 42 & 53—attention to detail is key
Product designers have a myriad of requirements to consider as they design new residential water treatment systems to conform to NSF/ANSI Standards 42 and 53. Their primary focus is typically on major issues such as contaminant reduction performance, because of their target market and their competitive environment. They must also consider material safety requirements when choosing materials of construction. They must also design the systems with enough robustness to conform to the brutal structural integrity test requirements.

With all of these issues to consider, it is easy for them to overlook some of the mundane minimum performance requirements. However, these specifications must be considered as well. It is incredibly frustrating to have a product certification held up because a sharp edge on a system housing is considered a hazard, or worse yet, because the system does not achieve a high enough minimum service flow. The minimum service flow issue can lead to re-categorization of the product from POE to POU (along with potential changes to product literature regarding installation), or it could require some re-design to achieve higher flow rates.

However, these minimum performance requirements are important. What manufacturer would want reports from the field that installers or consumers are cutting themselves on sharp edges of a product? Who would want to deal with complaints from users that it takes forever to fill a glass with filtered water from their POU system? Or who would want to hear that consumers have no water pressure upstairs after installing a POE system?

So, for those of you who are designers, please note this article as a reminder to remember the minimum performance requirements. For those of you who are dealers or other interested parties, please note this article as a reminder that designing new products to conform to the NSF/ANSI DWTU Standards is no picnic. Yet also keep in mind that once conformance to the Standards has been certified, you can have faith that the product has been evaluated from many different points of view to ensure that it meets consumer expectations in a safe and effective manner.

About the author
Rick Andrew has been with NSF International for over five years, working with certification of residential drinking water products. He has been the Technical Manager of the Drinking Water Treatment Units Program for over two years. His previous experience was in the area of analytical and environmental chemistry consulting. Andrew has a bachelor’s degree in chemistry and an MBA from the University of Michigan. He can be reached at 1-800-NSF-MARK or email: [email protected]


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