Testing POU RO Systems for Pentavalent Arsenic Reduction
By Rick Andrew
Arsenic is a naturally occurring mineral that globally on average makes up about 1.5 parts per million of the earth’s crust. As such, it is the 53rd most abundant element. Because it is present naturally in the earth’s crust, it can enter into groundwater. Many North American consumers have no worries about arsenic in drinking water, as surveys of US drinking water indicate that about 80 percent of drinking water supplies have less than two ug/L of arsenic. There are some North American consumers, however, who do have health issues related to drinking water and arsenic because two percent of US drinking water supplies exceed 20 ug/L of arsenic. These issues arise from documented, potential adverse effects of arsenic on human health.
Prolonged exposure to inorganic arsenic in drinking water at concentrations above 300 ug/L can cause stomach and intestinal irritation, including stomach ache, nausea, vomiting and diarrhea. Other effects of oral consumption of inorganic arsenic may include decreased production of red and white blood cells. This condition can lead to abnormal heart rhythm, blood-vessel damage, fatigue and impaired nerve function. In addition to these effects of arsenic consumption, probably the most characteristic effect of long-term oral exposure to inorganic arsenic is skin damage. The skin can develop dark spots and discoloration, and corn or wart-like areas can appear on hands, feet and other areas. These corn or wart-like areas have the potential to develop into skin cancer. Oral ingestion of arsenic may also increase the risk of lung, liver, bladder, prostate or kidney cancer. The Department of Health and Human Services (DHHS) has determined that inorganic arsenic is a known carcinogen. The International Agency for Research on Cancer (IARC) has determined that inorganic arsenic is a human carcinogen. And, US EPA and the National Toxicology Program (NTP) have classified inorganic arsenic as a known human carcinogen.
On January 22, 2001, US EPA adopted a new standard of 10 ug/L as a maximum contaminant level (MCL) for arsenic, replacing the previous MCL of 50 ug/L. Public water systems were required to comply with the 10 ug/L MCL by January 23, 2006. This new regulation forced many communities to upgrade the treatment of their public drinking water supplies. Additionally, many private well owners were suddenly faced with the fact that their well water did not meet the new MCL for arsenic. Not only were these consumers dealing with health concerns regarding the levels of arsenic in their well water, but they also have encountered difficulties in selling their homes because of the arsenic.
Small communities and individual well owners alike need proven POU technologies and products to help them treat drinking water contaminated with arsenic. This need for proven arsenic reduction performance in POU water treatment devices is addressed by several of the NSF/ANSI Drinking Water Treatment Unit (DWTU) Standards including NSF/ANSI 58 for POU RO systems.
Arsenic exists in water primarily in two forms: trivalent arsenic and pentavalent arsenic. POU RO systems are much more effective for treating pentavalent arsenic than trivalent arsenic. This does not mean that RO cannot be used to treat trivalent arsenic. In fact, a free chlorine residual will oxidize trivalent arsenic to pentavalent arsenic with minimal contact time. Oxidation of trivalent arsenic to pentavalent arsenic can also be accomplished using ozone or potassium permanganate. The oxidized pentavalent arsenic can then be effectively treated by the RO system.
NSF/ANSI 58 does not include a test method or a claim for trivalent arsenic reduction. Keeping in mind that water supplies contaminated with arsenic can include either trivalent arsenic,
pentavalent arsenic or both, the standard requires that education be provided to consumers. This education is provided in the form of an arsenic fact sheet that describes differences between trivalent and pentavalent arsenic, and the need for effective oxidation of any trivalent arsenic that may be present in their water source prior to RO treatment (see Figure 1 for an example of the consumer arsenic fact sheet required by NSF/ANSI 58). The test protocol includes chlorine-free deionized water with sodium chloride added to achieve a concentration of 750 mg/L.
Pentavalent arsenic, in the form of sodium arsenate, heptahydrate (Na2HAsO4•7H2O), is added to make up a challenge water containing 300 ug/L pentavalent arsenic. Two test POU RO
systems are operated for seven days and multiple samples of influent and effluent are collected throughout that period to evaluate system performance. The test systems must reduce the arsenic so that the arithmetic mean of all product water sample results and 90 percent of the individual product water samples are less than or equal to 10 ug/L, which is the MCL for arsenic. Alternatively, a 50 ug/L challenge of pentavalent arsenic may be used instead of the 300 ug/L. The rationale for the 50 ug/L challenge level is that consumers of water from supplies that were previously in compliance with the 50 ug/L arsenic MCL would be able to choose systems that have been demonstrated to treat water at that level or below to the new requirement of 10 ug/L.
Solutions for consumers through POU
The US federal government promulgated a new maximum contaminant level for arsenic in 2006. With this new regulation, many consumers were faced with issues of non-compliance, as well as concerns about possible health impacts. In addition to being a health concern, arsenic contamination of private wells can be a financial concern for those seeking to sell their homes. (Mortgage lenders in affected areas of Michigan are requiring arsenic analyses on private wells prior to approving loans.) The POU/POE industry saw this need and developed products to meet it. They worked with other stakeholders to make sure the NSF/ANSI standards for POU and POE products adequately addressed the situation. Once again, the POU/POE industry was proactive and successful in serving consumers, helping them to improve their health, conform to regulatory requirements and add value to their property.
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