By Rick Andrew
NSF/ANSI 58 Reverse osmosis drinking water treatment systems sounds from the title as if it is quite comprehensive in terms of its scope. And in many respects, it is. It covers material safety for contact with drinking water, through formulation review and extraction testing. It addresses structural integrity through cyclic and hydrostatic pressure testing. There are requirements for reduction of total dissolved solids and optional requirements for testing additional contaminant reduction claims, including pentavalent arsenic, trivalent and hexavalent chromium, copper, cysts, lead and perchlorate, to name a few. NSF/ANSI also requires that specific information be included in product literature to enable users to understand the installation, operation and maintenance requirements of the system.
NSF/ANSI 58 does, however, have some limitations in scope, in terms of the types of reverse osmosis systems that are included. Looking to the standard itself, the scope is defined as: “The point-of-use reverse osmosis drinking water treatment systems addressed by this Standard are designed to be used for the reduction of specific substances that may be present in drinking water supplies (public or private) considered to be microbiologically safe and of known quality (except that claims for the reduction of filterable cysts may be permitted). Systems covered by this Standard are intended for reduction of total dissolved solids (TDS) and other contaminants specified herein. Systems with components or functions covered under other NSF or NSF/ANSI Standards or Criteria shall conform to the applicable requirements therein.” This obviously excludes certain types of reverse osmosis systems, as follows.
RO systems for community water supplies
Because the scope of NSF/ANSI 58 is limited to POU RO systems, it does not address large-scale RO systems designed to treat water for community water supplies. These larger RO systems fall under the scope of NSF/ANSI 61 Drinking Water System Components – Health Effects. Conformance to this standard includes formulation review and extraction testing, similar to the material safety procedure under NSF/ANSI 58, although with a different protocol for exposing the product to simulated drinking water. The contaminant reduction performance of these systems is typically evaluated as part of system start-up and then through ongoing water quality monitoring, as opposed to being covered under a specific product standard.
POE RO systems
Again, with the scope of NSF/ANSI 58 limited to POU RO systems, POE RO systems are excluded. POE RO systems, like RO systems designed to treat water for community water supplies, fall under the scope of NSF/ANSI 61 Drinking Water System Components – Health Effects for evaluation of material safety. If there were a desire to also include POE RO systems within the scope of NSF/ANSI 58 for contaminant reduction performance, structural integrity and user information, it could be done. One major issue to address would be the details of the test protocols not being appropriate for POE systems. For example, the TDS reduction test under NSF/ANSI 58 requires sampling on days two through four of the seven-day test to include water draws of five percent of the daily production rate. If a POE RO system had a daily production rate in excess of 1,000 gallons (3,785.41 liters), this would mean sample draws of over 50 gallons (189.27 liters) to assess TDS reduction performance, which is unwieldy in the laboratory and not representative of typical use. So, this would need to be modified to address POE RO systems.
Another issue would be the possibility of RO water causing corrosion issues with certain types of household plumbing. Corrosion is a complex subject beyond the scope of this discussion, but suffice it to say that currently NSF/ANSI 58 does not address corrosion at all because of the scope being limited to POU. There may be many cases in which special considerations to guard against corrosion of the plumbing system would not be required if using a POE RO system, but nonetheless, consideration would need to be given to this issue if adapting NSF/ANSI 58 to address POE RO systems.
Desalination
From time to time, NSF receives inquiries regarding whether RO systems intended to be used for seawater desalination fall under the scope of NSF/ANSI 58. They do not. The issue is that all of the test protocols in NSF/ANSI 58 utilize conditions typical of fresh-water sources. For example, the TDS reduction test under NSF/ANSI 58 specifies a TDS concentration of 750 mg/L using sodium chloride as the source of TDS. Typical seawater TDS concentrations, however, are in the range of 35,000 mg/L—about 45 times higher that the concentration used for testing under NSF/ANSI 58. There are other issues that would need to be considered if desalination applications were added to the scope of NSF/ANSI 58. For example, higher pressures are typically required to achieve reasonable production rates given the high TDS concentrations of seawater. So, structural integrity requirements would need to be reconsidered. There are other differences between treatment of seawater and treatment of fresh water that would also need to be addressed.
Microbiological purification
NSF/ANSI 58 includes RO systems intended to provide treatment of chemical or physical contaminants present in otherwise potable drinking water. The idea is that persons utilizing a private well that has contamination resulting from naturally occurring arsenic could use this type of system, or consumers concerned about possible lead contamination resulting from a lead service line or other lead-containing plumbing components. Note that cyst reduction is included in NSF/ANSI 58 because chlorinated potable water derived from a surface-water supply can become contaminated with viable protozoan cysts if adequate physical barriers or ultraviolet treatment are not in place.
However, RO systems intended to be used for treatment of water that is microbiologically unsafe or of unknown quality are outside the scope of NSF/ANSI 58. This includes RO systems intended to be used on unimproved surface water sources, intended to protect consumers from microbiological contamination resulting from incursions into the drinking water supply and intended to be used for similar purposes. Effective treatment of water that may be microbiologically contaminated with bacteria and viruses is addressed by the scope of NSF Protocol P231, based largely on the 1987 US EPA Guide Standard and Protocol for Testing Microbiological Water Purifiers.
RO—a technology with broad applications
In considering all of these applications of RO which are not covered under the scope of NSF/ANSI 58, it becomes obvious that RO is a technology that is useful for many applications in drinking water treatment (and many applications beyond drinking water treatment, as well). Each of these has unique considerations that preclude them from being included in NSF/ANSI 58, at least as it is currently written. However, when it comes to POU RO systems intended to be used to treat chemical or physical contaminants that may be present in potable drinking water, NSF/ANSI 58 is a comprehensive standard addressing safety of materials in contact with drinking water, structural integrity, contaminant reduction performance and user information requirements.
About the author
Rick Andrew is the General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols), and Biosafety Cabinetry Programs. He has previously served as the Operations Manager, and prior to that, Technical Manager for the pro- gram. 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: [email protected]