By Rob Herman
Certification of a device for conformance to a national standard requires laboratory testing. The device’s intended engineering and design specifications are also a necessity, but it must be proven through testing that the device actually performs as designed. Ask any engineer about the surprises they’ve had when a design was tested and the results were much different than predicted.
This doesn’t mean, however, that every system is tested for every aspect of the standard. Since the inception of the Drinking Water Treatment Unit (DWTU) program here at NSF International, systems submitted for certification under one of the ANSI/NSF Standards are first evaluated for design similarities between the new candidate systems and existing certified systems from that manufacturer. This allows the creation of families of systems, where one of the products in a family is tested and the test data qualifies the rest of the family.
The design similarity evaluation has criteria that must be met for the transfer of data to be acceptable:
- The candidate system must be equivalent or more conservative than the tested system in the technical aspects effecting performance,
- The relationship between the candidate system and tested system must be evaluated using basic scientific and engineering principles, and
- The test data must be in conformance with the current requirements of the standard.
Test data transfers
Until recently, this process has only been applied to complete systems. Today, there’s a process where a manufacturer can have performance claims tested on its system component and transfer the test data to its customers, the system manufacturers and assemblers. It’s common practice for component manufacturers to have components certified for material safety and when appropriate, structural integrity. It’s not permitted, however, for a component manufacture to be listed for performance claims under the standard. This is because the application of the component in the final system directly affects the performance of the component. However, a manufacturer can have performance testing conducted on its component, and when that component is used in a system, the data can be transferred to that system if it meets the same criteria mentioned previously for families. This allows the component manufacturer to prove the capability of their component to the system manufacturer and provides guidelines to the system manufacturer regarding the application of that component. This is a great advantage to the system manufacturer, since less testing is required on their system to achieve certification.
This process was first developed for reverse osmosis (RO) membranes used in RO systems with shutoff valves and storage tanks. The first step in developing the process was to determine what design factors affect the performance of this type of RO system in the ANSI/NSF Standard 58 performance test. Design-dependent factors affecting the performance of RO systems include membrane characteristics, net driving pressure and reject water percentage.
Membrane performance characteristics are very complex and this is often a difficult factor to resolve. The membrane is the central element in an RO system and its characteristics have the greatest effect on the system performance as a whole. Since this is such a critical factor, the membrane must be of the same manufacture between the tested and candidate systems. This allows the negation of membrane characteristics, since they will be identical between systems.
Net driving pressure
The second design-dependent factor is net driving pressure, or the result of several design components working together. Since the inlet pressure and chemical makeup of the water to the system is set by the standard, the concern is how the system affects net driving pressure. The two components in this type of RO systems affecting net driving pressure are the shutoff valve and the storage tank.
Shutoff valves directly affect net driving pressure by the pressure where the valve shuts off influent flow to the membrane. This is usually set at a minimum of 20 pounds per square inch gauge (psig) differential pressure across the membrane. The pressure that the valve turns back on also affects the performance. In general, the higher the net driving pressure during the time the membrane is producing water, the higher the performance in both production rate and rejection of contaminants. Shutoff valves that turn off at higher pressures (less net pressure across the membrane) will cause the system to perform poorly.
Storage tanks have a direct effect on net driving pressure by influencing the pressure versus volume curve between the shut off valve on-and-off pressures. This curve is affected by the geometry of the tank, the flexibility of the bladder (if one is used) and pre-charge pressure. All of these factors influence the net driving pressure during the refilling of the tank and the volume of water used before the shutoff valve opens.
Since net driving pressure is very difficult to measure during the operation of the system, production rate can be used as an indication of the overall net driving pressure of a system with the same area of membrane. This is possible since the production rate of a membrane is dependent on the net driving pressure and the pressure vs. volume curve of the storage tank. So the production rate per square inch of a membrane in the candidate system must be equal or greater than the production rate per square inch of the membrane in the tested system.
Reject water percentage can have a significant effect if the percentage is above 40-to-50 percent. Most residential RO systems have reject water percentages in the 20 percent range and small variances in this percentage has little effect on performance. So the reject water percentage on the candidate system must be in the same range or lower than the tested system.
If this process is followed, the component RO membrane element manufacturer can test its membrane in a typical system for all contaminants under the standard. This data can then be transferred to a system manufacturer when the above criteria are met when the candidate system completes the total dissolved solids (TDS) test under ANSI/NSF Standard 58.
This process has been implemented for RO systems and has been accepted by California, Iowa and Wisconsin for state product certification. A similar procedure is available for carbon or mixed adsorptive media systems under ANSI/NSF Standards 42 and 53. For more information, contact NSF’s Drinking Water Treatment Unit (DWTU) program.
About the author
Rob Herman is technical manager of NSF International’s Drinking Water Treatment Unit program and has been with NSF since 1985. He holds a bachelor’s degree in chemistry from Lawrence Technological University in Southfield, Mich., and is currently in the environmental sciences graduate program at the University of Michigan.
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