By Rick Andrew

Arsenic is a well-known and widely encountered naturally occurring contaminant in groundwater. For over 20 years, the EPA has promulgated a value of 10 μg/L as a maximum con­taminant level (MCL) for arsenic. This MCL has been enforced since January 23, 2006.

Over the last 15 years since the MCL for arsenic was lowered to 10 μg/L, many communities have been working to improve their water treatment capabilities to meet the requirement. Private well owners have also been dealing with this lower MCL for arsenic. This issue of arsenic in private wells affects home­owners both because they are dealing with a health concern regarding the arsenic in their well water and because they have had to address the issue when they consider selling their homes. These private well owners have certainly been seeking reliable point-of-use (POU) treatment systems to address arsenic contamination.

Arsenic reduction performance in POU treatment systems is addressed by multiple NSF/ANSI Drinking Water Treatment Unit (DWTU) Standards that include arsenic reduction requirements, complete with very robust and detailed test methods and required helpful information to educate end users. One such DWTU Standard is NSF/ANSI 53, which covers active media POU systems.

Manufacturer Claims of Arsenic Reduction
There are two forms of arsenic in drinking water—arsenic III, or trivalent arsenic; and arsenic V, or pentavalent arsenic. Although arsenic V is generally easier to treat in drinking water by various media, it is not universally true. Another consideration is that arsenic III and arsenic V in water can respond to treatment differently at low vs. high pH. So, the evaluation of arsenic reduc­tion must consider multiple variables including the form of the arsenic, the pH, the media characteristics, and other factors.

Taking these factors into account, NSF/ANSI 53 includes two variations on the claim of arsenic reduction. They are:

  • Pentavalent arsenic reduction.
  • Arsenic reduction.

Claims of pentavalent arsenic reduction require successful testing for reduction of arsenic V at pH 6.5 and pH 8.5. The more generally applicable claim of arsenic reduction requires successful testing for reduction of arsenic V at pH 6.5 and pH 8.5, and also successful testing for reduction of arsenic III at pH 6.5 and pH 8.5.

Interestingly, there is no claim of trivalent arsenic reduction only. The reason is the equilibrium between arsenic III and arsenic V is such that some water supplies with arsenic issues are contam­inated by arsenic V entirely. But very few water supplies with arsenic issues are contaminated solely by arsenic III; and even in these few cases, the presence of any oxidizing agent, such as oxygen (air), quickly oxidizes the arsenic III to arsenic V. Arsenic III is very easily oxidized to arsenic V. Because of this prevalence of arsenic V and the tendency of arsenic III to be oxidized to arsenic V, is important that any claim of arsenic reduction must address arsenic V.

Arsenic Reduction Test Methods
Test methods and water characteristics for arsenic reduction are described in Figures 1 through 4. The figures also indicate that there are two options for influent challenge:

  • 0.30 mg/L.
  • 0.050 mg/L.

The level of influent challenge is at the discretion of the man­ufacturer. The default influent challenge is the 0.30 mg/L level. The option of the 0.050 mg/L influent challenge is provided so that consumers of water from supplies that were previously in compliance with the 50 μg/L (0.050 mg/L) arsenic MCL in place prior to 2006 are able to select POU systems that have been demonstrated to effectively treat water contaminated at the level of the old MCL.

The water characteristics outlined in Figures 2 and 4 specify the concentration of various ions present in the test water in addition to the arsenic. The concentrations of these ions are specified because they are commonly present in drinking water, and they can influence the effectiveness of active media when treating arsenic in water. Thus, they are included in the test water at specific concentrations to enhance reproducibility of the test and also to represent real-world conditions.

The arsenic V reduction test water specifies that free available chlorine must be present, while conversely the arsenic III reduc­tion test water requires that dissolved oxygen cannot be present. The reason for these specifications is to have an oxidizer present in the arsenic V reduction test to ensure the arsenic remains in the arsenic V form; whereas in the arsenic III reduction test water, oxidizing chemicals, such as dissolved oxygen, cannot be present to make sure that reduction of arsenic III is being tested instead of reduction of arsenic V.

Two POU systems are tested in parallel at the highest achievable flow rate with 60 psi inlet pressure, using on and off cycling such that water is flowing 50% of the time. Testing in this manner occurs 16 hours per day, with an 8-hour rest period overnight. Samples of the influent and treated water are taken at specific points throughout the life of the product and beyond, to 200% of the manufacturer’s rated treatment capacity. If the system includes a performance indication device (PID, or end-of-life indicator), the test terminates at 120% of the manufacturer’s rated treatment capacity. Each sample of treated water must contain no more than 0.010 mg/L of arsenic. This test method assures that even under heavy usage beyond the product’s life, the system will perform effectively.

Trusted Options for Private Well Owners As a widespread contaminant in the U.S., arsenic can be a health concern and can cause impacts to property values where it is detected in private wells. The POU and POE industry understands this situation well, and has responded by researching, developing, and manufacturing POU treatment systems to meet their needs. The industry has collaborated with other stakeholders, including end users and regulators, to ensure the NSF/ANSI DWTU standards adequately address the regulatory requirements associated with arsenic in drinking water. Once again, the POU and POE industry listened to and understood their customers, helping them to add value to their property, conform to regulatory require­ments, and perhaps most importantly, assure that they can provide healthy and safe drinking water for their families

Andrew_Rick_mugAbout 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:


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