By Rick Andrew
Reduction of arsenic in well water is one of the most frequent types of inquiries to NSF’s Consumer Affairs office. It is always one of the top 5 most frequently inquired about contaminants, right up there with lead, hardness, Cryptosporidium and disinfectants (chlorine and chloramines). Fortunately, private well owners have many options when it comes to treatment of well water contaminated with arsenic. The NSF/ANSI Drinking Water Treatment Unit (DWTU) standards include requirements for claims of arsenic reduction for active media systems under NSF/ANSI 53, POU RO systems under NSF/ANSI 58 and distillation systems under NSF/ANSI 62.
Pentavalent and trivalent arsenic
When it comes to treatment of arsenic in drinking water, it is important to understand that there are two forms of arsenic: pentavalent arsenic [also called As (V), As (+5) and arsenate] and trivalent arsenic [also called As (III), As (+3) and arsenite]. Treatment technologies may have varying effectiveness based on the form of the arsenic. Trivalent arsenic is generally more difficult to treat in drinking water than pentavalent arsenic. Trivalent arsenic, however, can be converted to pentavalent arsenic through the use of an oxidizing chemical such as free chlorine. Other oxidizing chemicals are also capable of converting trivalent arsenic to pentavalent arsenic. In order to help convey a level of understanding of this issue to end users, active media systems and RO systems making claims of pentavalent arsenic reduction and/or arsenic reduction are required to include fairly extensive and specific information as detailed in NSF/ANSI 53 and NSF/ANSI 58 in their product literature.
NSF/ANSI 53 includes requirements for evaluating active media systems for pentavalent arsenic reduction. The standard requires that pentavalent arsenic reduction be tested at both pH 6.5 and pH 8.5. The test method includes a very specific recipe and procedure for preparation of the test water, starting with RO/DI water and adding minerals and other chemicals to achieve the required levels of pH, magnesium, fluoride, silica, phosphate, calcium, free chlorine and pentavalent arsenic. There are two levels of arsenic challenge included in the standard: 50 ug/L and 300 ug/L. The system must reduce the arsenic to ≤ 10 ug/L. Successful testing at both pH 6.5 and pH 8.5 is required to establish a claim of pentavalent arsenic reduction. The tests must be conducted to 200 percent of the manufacturer’s recommended treatment capacity, or 120 percent if the system has a performance indication device (PID) that lets the end user know when replacement of the treatment media is needed. Samples of the challenge water and treated water are collected throughout the test to establish treatment performance. Additionally, claims of pentavalent arsenic reduction are limited to water supplies with a free chlorine residual present or water supplies demonstrated to contain only pentavalent arsenic. The standard also includes requirements for systems to make arsenic reduction claims. This is a broader claim compared to the claim of pentavalent arsenic reduction. In order to make the broader claim, reduction of pentavalent arsenic must be tested as described. Additionally, reduction of trivalent arsenic must be tested. The standard includes a specific recipe for preparing the test water for trivalent arsenic reduction. There are the two levels of trivalent arsenic challenge at 50 ug/L and 300 ug/L. There is required testing at both pH 6.5 and pH 8.5 and the tests are conducted to 200 percent of capacity or 120 percent if the system includes a PID.
RO treatment of arsenic
Requirements for evaluating POU RO systems for pentavalent arsenic reduction are included in NSF/ANSI 58. The test water for RO systems starts with RO/DI water, but overall is much simpler than the test water for active media systems. For RO systems, sodium chloride is added to the RO/DI water to achieve a concentration of 750 mg/L. The pentavalent arsenic is added to this water at a concentration of either 50 ug/L or 300 ug/L. The system must reduce the arsenic to ≤ 10 ug/L. The test itself is conducted over the course of a week, with operational cycles varying to take into account the various operating conditions of a typical POU RO system. For example, for a typical POU RO system with a storage tank and automatic shut-off valve, there are operational cycles involving:
- completely emptying and filling the storage tank
- emptying the storage tank to the point where the automatic shut-off valve is activated and allowing the tank to refill
- drawing five percent of the daily production rate of the unit and allowing the tank to refill
48-hour stagnation with no water drawn from the storage tank
Samples of the challenge water and treated water are collected throughout the test period at points where water is drawn from the storage tank. These samples are analyzed to determine the effectiveness of the treatment. Because RO has limited effectiveness for treatment of trivalent arsenic, NSF/ANSI 58 does not include requirements for evaluation of trivalent arsenic reduction. Instead, claims of pentavalent arsenic reduction are limited to water supplies with a free chlorine residual present or water supplies demonstrated to contain only pentavalent arsenic.
Distillation can also be an effective treatment for arsenic in drinking water. NSF/ANSI 62 includes requirements for evaluation of distillation systems for arsenic reduction. These requirements indicate that based on the 1991 study, Evaluation of Total Dissolved Solids as a Surrogate Parameter for the Reduction of Inorganic Contaminants by Distillation Systems, conducted for the Water Quality Association by NSF International, TDS may be used as a surrogate for verifying the reduction of arsenic, barium, cadmium, chromium, copper, lead and selenium when the system is tested for TDS reduction, according to NSF/ANSI 62.
This test for TDS reduction involves test water prepared in a way similar to the test water under NSF/ANSI 58, except that sodium chloride is added to RO/DI water to a concentration of 1,000 mg/L instead of 750 mg/L. The distillation system must reduce the TDS by 97 percent over the course of the test protocol. The test protocol for distillation systems covers a one-week period, involving either batch or continuous operation per the operating characteristics of the distillation system and a stagnation period. Samples of the challenge water and distilled water are collected throughout the test to establish the TDS reduction performance. Unlike active media technologies and RO, distillation effectiveness is not sensitive to the form of the arsenic. So, a claim of arsenic reduction may be made for systems that meet the requirements for TDS reduction under NSF/ANSI 62. There is also no need for end user information about pentavalent and trivalent arsenic to be included in product literature.
It’s nice to have solutions!
As we engage in business and life in general, we sometimes find ourselves in a position in which people approach us for help. Unfortunately, we are not always as well equipped as we would like to be to able to help them. As a father of twin, fifth-grade girls, I can assure you that there are many times I am in this position! Fortunately, when consumers inquire to NSF about treating arsenic in their drinking water, we are in a very good position. We can let them know that there are multiple technologies that can work and there are well established American national standards for proving the effectiveness of these technologies. From there, these consumers can consult certification listings to choose from a wide array of independently tested and certified options to best fit their needs. This is indeed a good place to be.
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