By Emily Bolda, CWS VI


There are many choices available to consumers who are looking to purchase a residential water filter. Several types of certified filters are available: faucet-mount-style filters that can be attached to a kitchen or bathroom sink faucet, pour-through batch-type systems (or pitcher filters) and plumbed-in POU or POE filters. Products undergo rigorous testing by an independent third party to demonstrate that they will perform as the certified claims indicate. Do you wonder how this is determined, and what testing is involved in order to demonstrate these products perform as claimed?

Testing protocols defined

Residential drinking water filters are covered by two industry standards; NSF/ANSI 42 and NSF/ANSI 53. Filters are tested to one or both of these standards based on whether the performance claim is an aesthetic (42) or health related (53). In the laboratory, the products are installed according to the manufacturer manual, in order to mimic how they would be installed in a consumer’s home, and prepared for use. There are three main categories of testing that must be done in order to support certification of a filtration system: materials safety, structural integrity and performance. This rigorous testing is done using worst-case scenarios to demonstrate compliance to the NSF/ANSI filter standards.

Materials safety testing, also called extraction testing, is performed to evaluate contaminants that leach out of wetted parts of systems or components (anything that touches water that is to be consumed) to ensure concentrations being contributed to the product water do not exceed allowable concentration levels. To summarize the protocol, the products are installed and treated according to manufacturer instructions, followed by a protocol with a pattern of filling the product with special test water, allowing it to soak for a specified period, then followed by collection and analysis. Analytical results are compared to established allowable concentration levels to determine compliance.

Structural integrity testing is performed to ensure the products won’t crack, leak or burst under use. High-pressure water is cycled through the product thousands of times and the product is monitored for any signs of structural failure (a cycle test). The product is also held under very high pressure for a short amount of time and monitored for signs of structural failure (the hydrostatic pressure test). Not all filtration systems are required to undergo this testing; only those that are installed into a pressurized water supply must comply with this requirement. Batch-type systems, which include pitcher filters, are not installed to a pressurized water supply and are exempt from this testing requirement.

Structural integrity and materials safety testing protocols are identical between the two standards but a little different for performance testing. Performance testing is performed to validate the contaminant reduction claim made by the manufacturer. The contaminant reduction performance of the product is challenged in the laboratory. For each contaminant there is a specific test protocol to be followed in the laboratory. A set concentration of the contaminant is used to challenge the product. For example, if a product is making a claim for copper reduction, the laboratory prepares test water with a copper concentration of 1,300 ug/L and the water is passed through the product against a set test protocol.

NSF/ANSI Standard 42 is for filters making aesthetic claims. Commonly tested claims in the WQA Lab under this Standard are chlorine, chloramine, zinc, hydrogen sulfide, manganese, iron and pH neutralization. NSF/ANSI Standard 53 is for filters making health claims. There are a very large number of claims in this Standard, but commonly tested ones are heavy metals and VOCs, and other inorganics like fluoride and nitrate/nitrite. In both standards, filters are required to reduce the contaminant to the level prescribed in the appropriate standard, or to a certain percent reduction. Generally a minimum of one performance claim is required to certify a filter system in addition to the materials safety and structural integrity (if applicable) requirements. There is a multitude of claims to choose from in each Standard; manufacturers can claim any number of these.

Testing protocols

The test protocols in the two standards require that two systems be tested for each performance claim, with the exception of some contaminants and POE filters in NSF/ANSI 42. Filters subject to line pressure in the field (plumbed-in) are installed and operated at a 60-psi dynamic (flowing) inlet pressure. (This does not apply to pitcher systems, batch systems and sports bottle types, which are simply assembled and used as they would be in the consumer’s home.) The flowrate used for performance testing in NSF/ANSI 42 is based on a 60-psi dynamic (flowing) inlet pressure and set at the manufacturer’s rated service flow. The flowrate used for performance testing in NSF/ANSI 53 is based on maximum flowrate at a 60-psi dynamic inlet pressure.

Performance testing is carried out using a 16-hour test period, with an eight-hour rest period for each 24-hour period. Manufacturers have the choice of test cycle, or the ’on/off’ time during each test day. Depending on their intended frequency of use, plumbed-in POU systems can be tested using either a 50- percent on / 50- percent off cycle or a 10 percent on / 90 percent off cycle, the exception being POE, or whole-house type filters, which are run continuously during the test period with no on/off time. Batch-type systems are tested using the manufacturer’s use pattern, generally given in gallons or liters per day.

For NSF/ANSI 42 testing, filters are run until the capacity (or expected life, usually expressed in gallons) claimed by the manufacturer is reached; exceptions exist for iron and manganese, which are run to 120 percent of the claimed capacity. For health claims tested in NSF/ANSI 53, filters are always run beyond their rated capacity; this provides a safety margin to the consumer. If the filter is not changed right at its rated capacity, there is a measure of assurance that it is still offering protection against the particular health contaminant, since it has been tested beyond its rated capacity. Filters are run to 200 percent of the claimed capacity, unless the filter has a performance indication device (PID). In this case, the filters are run to 120 percent of the claimed capacity. A PID is a built-in device that informs the user when capacity has been reached. A PID can come in many forms; commonly seen types include a flashing light, a light with color indicator, devices with an alarm or those with the ability to reduce or stop the flow of water through the system.

Testing continues until the determined endpoint is reached. Each test protocol prescribes all the points at which samples must be collected during this time. The challenge water (influent) and the product water samples (effluents) are collected at these points and analyzed by the appropriate method. Results are used to determine whether or not the product meets the criteria of the Standard and to calculate percent reduction, or the amount of the contaminant that the product has demonstrated it removed.


Structural integrity, materials safety and performance testing must all meet the requirements of whichever filter standard the product and claims fall under. This rigorous testing is performed as an integral part of the work done to support a product certification, and to ensure customers that the products they are purchasing have been tested to confirm they will perform as claimed.

About the author

Emily Bolda, CWS-VI, is Laboratory Supervisor for the Water Quality Association’s Gold Seal Certification Program at WQA’s International Headquarters and Laboratory. She has been with organization since 2006, and has worked in the water and drinking water industries since 1999. Bolda’s professional background is in analytical chemistry and product testing, and she holds a BS Degree from the University of Iowa. She can be reached by phone (630) 929-2534 or email [email protected] to provide additional information concerning WQA’s Laboratory or Gold Seal Certification Program.


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