By Rick Andrew
NSF’s Consumer Affairs Office fields thousands of inquiries each year from everyday people concerned about contaminants in their drinking water. Although lead has consistently been the most popular inquiry over the last several years, arsenic is always in the top-10 list of contaminants about which consumers ask.
A big part of the reason for the popularity of these inquiries is that arsenic is naturally occurring. According to Wikipedia, arsenic is the 53rd most abundant element in the earth’s crust and comprises about 1.5 ppm of it. This leads to arsenic being present in groundwater in many local regions across the United States. 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 USEPA maximum contaminant level (MCL).
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. Active media systems and POU RO systems have special considerations when it comes to treatment of water contaminated with arsenic. This column will focus on those considerations related to POU RO systems.
Pentavalent and trivalent arsenic
The first consideration related to treatment of water contaminated with arsenic is to understand that arsenic occurs in water in two different forms, or oxidation states: pentavalent arsenic (also called As[V], As[+5] and arsenate) and trivalent arsenic (also called As[III], As[+3] and arsenite). These forms have different chemical properties and as such, respond differently to treatment technologies, resulting in these technologies having varying effectiveness depending on the form or oxidation state of the arsenic. Generally, trivalent arsenic is more difficult to treat in drinking water than pentavalent arsenic. Fortunately though, trivalent arsenic can be oxidized to pentavalent arsenic through the use of typical disinfectant chemicals, the most common being free chlorine. Other oxidizing chemicals are also capable of converting trivalent arsenic to pentavalent arsenic.
RO has limited effectiveness for treatment of trivalent arsenic, so, NSF/ANSI 58 includes requirements for claims of pentavalent arsenic reduction only. It does not include 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.
The general approach to treatment of groundwater contaminated with trivalent arsenic by POU RO, therefore, is to include a chlorination device just upstream of the POU 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.
These complexities of treating arsenic in drinking water by RO can lead to confusion by end users. In order to help educate them, NSF/ANSI 58 requires specific information to be included in product literature for POU RO systems with pentavalent arsenic reduction claims. In fact, the Performance Data Sheet for these systems is required to include an Arsenic Facts Sheet that:
- 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 and
- highlights the importance of proper system maintenance, including replacement elements.
It provides a convenient guide for end users to understand how their treatment system is working, and what must be done to keep it working.
Test method for reduction of pentavalent arsenic by POU RO
The test water for pentavalent arsenic reduction for RO systems starts with RO/DI water, with sodium chloride 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. In either case, the system must reduce the arsenic to ≤ 10 µg/L. The test itself is conducted over the course of a week, using sampling and operational cycles that are designed to cover a variety of usage patterns and assess the impact of those usage patterns on the 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 involving:
- Completely emptying the storage tank and taking a sample, followed by allowing the tank to refill
- Emptying the storage tank to the point where the automatic shut-off valve is activated and taking a sample, then allowing the tank to refill from this point
- 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 storage tank and taking a sample, then allowing the tank to refill.
Whenever samples of treated water are collected, samples of the challenge water are also collected. These treated water 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.
Effective solutions for common problems
Finding out that one’s well is contaminated with arsenic is not good news and it happens relatively frequently in certain states and regions due to the natural occurrence of arsenic. Looking into the issue and finding out that arsenic occurs in different forms and can be difficult to treat in the trivalent form can be unnerving, especially to those who are not familiar with water treatment technologies and techniques. A number of these folks reach out to resources such as NSF or their local water treatment professional looking for help.
Fortunately, when people inquire about treating arsenic in their drinking water, there are solid answers and recommendations that can be made. For those private well owners who decide to go with RO treatment, there is fortunately a tried-and-true approach to arsenic reduction, as well as a rigorous standard to evaluate and certify POU RO systems to be effective. Using the advice of water treatment professionals or consulting online resources or help-lines, such as the ones offered by NSF (as well as certification listings), can offer reassurance and peace of mind for private well owners faced with the reality of arsenic contamination. This is one problem that can be solved very effectively.
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