Water Conditioning & Purification Magazine

POU RO Systems and Arsenic Reduction

By Rick Andrew

One of the technologies commonly employed to help reduce arsenic levels in drinking water, particularly for those people who have private wells that are impacted by naturally occurring arsenic, is reverse osmosis. Because arsenic does occur naturally in groundwater, it is a relatively common issue that homeowners with wells can face. In fact, Wikipedia states that arsenic is the 53rd most abundant element in the Earth’s crust and comprises about 1.5 ppm of it.1 Because of this relative abundance, it is not uncommon for arsenic to be present at detectable concentrations in groundwater. And there are health concerns related to arsenic in drinking water.

According to Michigan Department of Environment, Great Lakes and Energy, long-term exposure to low levels of inorganic arsenic in drinking water is known to cause human health problems including cancer, thickening and discoloration of the skin, problems with blood vessels, high blood pressure, heart disease, nerve effects including numbness and/or pain, and interference with some important cell functions.2 Because of these health effects, any public or private drinking water supply drawn from wells in aquifers with arsenic contamination at a level higher than 10 µg/L will need treatment in order to comply with the US EPA maximum contaminant level (MCL). Due to the abundance of arsenic and its prevalence in groundwater, coupled with the health effects related to drinking water contaminated with arsenic, NSF/ANSI 58 includes requirements for testing and establishing claims related to arsenic reduction capabilities of POU RO systems.

Forms of arsenic
It is important to understand that arsenic occurs in water in two different forms, also known as oxidation states. These are pentavalent arsenic, also called As(V), As(+5) and arsenate, and trivalent arsenic, also known as As(III), As(+3) and arsenite. These forms have different chemical properties and because of these different properties they respond differently to treatment technologies. Treatment technologies have varying effectiveness in treating water contaminated with arsenic, based on the form or oxidation state of the arsenic.

The general rule of thumb is that for most technologies, trivalent arsenic is more difficult to treat in drinking water than pentavalent arsenic. The good news is that trivalent arsenic can be quite easily and quickly oxidized to pentavalent arsenic through the use of typical disinfectant chemicals, the most common being free chlorine. Other oxidizing chemicals can also be used to convert trivalent arsenic to pentavalent arsenic. As per the rule of thumb, RO has limited effectiveness for treatment of trivalent arsenic; it’s much more effective in treating pentavalent arsenic. Accordingly, NSF/ANSI 58 includes requirements for claims of pentavalent arsenic reduction but not requirements for evaluation of trivalent arsenic reduction.

Claims of pentavalent arsenic reduction under NSF/ANSI 58 are limited to water supplies with a free chlorine residual present or water supplies demonstrated to contain only pentavalent arsenic. This approach in the standard is consistent with the typical approach to treatment of groundwater contaminated with trivalent arsenic by POU RO. A chlorination device just upstream of the POU RO is usually employed to assure that all of the arsenic present in the water is oxidized to the pentavalent form prior to being treated by the RO. This combination of technologies allows for effective oxidation of the trivalent arsenic to pentavalent arsenic, which in turn is effectively treated by the POU RO system.

Because of the issue of trivalent and pentavalent arsenic and the use of chemical oxidation as pretreatment, the topic of arsenic treatment in groundwater can be confusing to end users, especially homeowners. Recognizing the potential for confusion, NSF/ANSI 58 requires specific information to be included in product literature for POU RO systems that have pentavalent arsenic reduction claims. This information is required in the form of an Arsenic Fact Sheet included with the performance data sheet, which:

  • describes the forms of arsenic present in groundwater
  • explains that the system treats only pentavalent arsenic
  • refers to the use of free chlorine to oxidize trivalent arsenic to pentavalent arsenic
  • emphasizes the importance of testing the water periodically to verify system performance
  • highlights the importance of proper system maintenance, including replacement elements

With all of this information included, the Arsenic Facts Sheet provides a useful and readily available guide for end users to understand how their treatment systems function and what steps must be taken to maintain them.

Testing POU RO for reduction of pentavalent arsenic
NSF/ANSI 58 describes the test method for reduction of pentavalent arsenic by POU RO in great detail, beginning with the composition of the test water. This test water is created beginning with RO/DI water, to which sodium chloride is added to achieve a concentration of 750 mg/L. Pentavalent arsenic is added to this water at a concentration of either 50 ug/L or 300 ug/L. In either case, the system must reduce the arsenic to ≤ 10 µg/L.

Beyond the test water, the method also specifies how the POU RO system is to be operated. The test is conducted with the system operated over the course of a week. This operation incorporates sampling and operational cycles that are designed to cover a variety of usage patterns. By varying the usage patterns, the test can assess the impact of those usage patterns on the overall performance of the system. For example, for a typical POU RO system with a storage tank and automatic shut-off valve, there are operational cycles that test arsenic reduction when:

  • starting with the storage tank full, completely emptying the storage tank and taking a sample, followed by allowing the tank to refill
  • starting with the storage tank full, emptying the storage tank to the point where the automatic shut-off valve is activated and taking a sample, and then allowing the tank to refill from this point
  • starting with the storage tank full, drawing five percent of the daily production rate of the unit and sampling, then allowing the tank to refill
  • a 48-hour stagnation with no water drawn from the storage tank, followed by completely emptying the tank and taking a sample, and then allowing the tank to refill

Whenever samples of treated water are collected, samples of the challenge water are also collected. These treated and challenge water samples are analyzed to determine the effectiveness of the treatment. The standard requires that the average value for all of the treated water samples, as well as 90 percent of the individual treated water samples, must be ≤ 10 µg/L arsenic.

Solutions for private well owners impacted by arsenic
Dealing with problem well water is a relatively common challenge for rural homeowners. When the problem is arsenic contamination, things can become a bit more complex and also can have health implications. Fortunately, the POU/POE industry provides proven solutions for private well owners. One of those proven solutions for contamination with arsenic is POU RO treatment. The proof comes through testing and certification to NSF/ANSI 58 for arsenic reduction. The testing provides confidence in arsenic reduction performance and the certification adds ongoing assurance of this performance, plus requirements for a clear explanation of the nature of arsenic contamination, the function of the POU RO system and the responsibilities of the homeowner to properly maintain the system to assure it continues to provide effectively treated drinking water.


  1. Arsenic. Wikipedia. https://en.wikipedia.org/wiki/Arsenic#:~:text=Arsenic%20is%20a%20chemical%20element,Arsenic%20is%20a%20metalloid
  2. Arsenic in Well Water. Michigan Department of Environment, Great Lakes and Energy. https://www.michigan.gov/documents/deq/deq-wd-gws-wcu-arsenicwellwater_270592_7.pdf

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: Andrew@nsf.org

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