By Greg Gilles
Summary: Early in 2002, the USEPA is expected to hand down the final ruling of the arsenic MCL standard. At that time, a multitude of small community water systems will be presented with a minor dilemma—how to treat their water. Many of these efforts will concentrate at the point-of-use. Two more viable alternatives will be adsorptive media and RO, or a combination of both.
The debate over the public health effects of arsenic contaminated drinking water may seem somewhat recent to Americans. New research, however, suggests concern over health effects due to arsenic contaminated drinking water began as early as the foundation of our country. According to pathologist Frank Hancock, arsenic contaminated drinking water may be responsible for killing more than 90 percent of the original settlers at Jamestown.1 Since the 1600s, little attention was given to arsenic contaminated water in America until 1942, when the U.S. Public Health Service set the drinking water standard at 50 parts per billion (ppb). While this standard is still in place today, it has been the subject of debate and controversy in the federal government for more than 20 years. With increased public awareness and press coverage, arsenic has become a workplace “water cooler” conversation topic. The final maximum contaminant level (MCL) is currently slated for February 2002. This is almost a year after the Bush Administration, citing the need for further review, halted a 10 ppb MCL approved by the previous administration that would have matched World Health Organization guidelines and European standards.
Increasing consumer demand
Given the elevated consumer awareness of arsenic, the public’s response to regulatory delays has been to take action by purchasing in-home treatment through point-of-use/point-of-entry (POU/POE) devices. Interestingly, 65 percent of the respondents in Water Quality Association’s 2001 National Consumer Water Quality Survey said they would pay more for a home water treatment device to remove arsenic if it were present in their water. This number was slightly greater than the number willing to pay more on their monthly utility bill, indicating a growing sentiment of mistrust of centralized treatment systems to provide safe drinking water.
While POU options are commonly used by homeowners to reduce hardness, odor, off-taste and heavy metals, the option of using a POU/POE solution to meet an enforceable drinking water standard is a relatively new approach. The current arsenic treatment rule expanded by the Safe Drinking Water Amendments of 1996 allows utilities to consider the use of POU/POE technology for compliance purposes. In fact, U.S. Environmental Protection Agency (USEPA) administrators are required to consider POU/POE devices as potentially affordable means of achieving compliance, particularly for small drinking water systems. Of course, for utilities choosing this option, there are certain statutory conditions placed upon the management, operation and design of such treatment units since utilities are ultimately responsible for protection of public health. Nonetheless, whether affected water is from a regulated utility or private well, there seems to be an overwhelming consensus that the current 50-ppb MCL is unacceptably high. As such, concerned consumers are seeking home water treatment solutions and aren’t content waiting for respective utilities to act.
Technology choices
Adsorption-based products and reverse osmosis (RO) are two key options cited as available for POU applications to remove arsenic from drinking water. Although other options exist, these are considered by most, including the USEPA, to be Best Available Technologies (BAT)—effective and affordable ways for individual homeowners to lower the concentration of arsenic in their drinking water. Over the past 15 years, the USEPA has sponsored and/or tracked a myriad of case studies using these POU technologies for removing a range of contaminants including arsenic, fluoride, copper, radium and others. For arsenic removal, case studies were performed in Oregon, Alaska and New Mexico. In addition to these, over 20 other case studies were performed for other contaminants, although very few in the past five years. The findings from these studies clearly indicate that these two POU choices offer affordable options to meet the anticipated new drinking water standards. Two new studies sponsored by the American Water Works Association Research Foundation (AWWARF) were planned to begin this month to look at POU options and implementation for small community water systems.
Adsorptive media
Adsorptive media is an effective technology to remove 90-99 percent of arsenic from drinking water. Adsorbents such as activated alumina (AA) have been cited in several previous USEPA case studies in the 1980s for POU reduction of arsenic as arsenate—As(V)—and fluoride adsorption. Both chemical sorption and physical mechanisms (exchange and sorption) are at work with products having high porosity and high surface area to volume ratios. The most common POU device consists of a granular adsorbent packaged within a conventional filtration cartridge for undercounter use. A pressure rated housing may or may not be needed. The disposable adsorbent cartridge can be used as a stand-alone device connected to a dedicated faucet or in conjunction with other undercounter water treatment equipment. Newer, more advanced adsorptive media technology has been developed with significant performance advantages over conventional adsorbents. Several benefits of one new adsorptive media include reduction of arsenic as arsenite—As(III)—and As(V) species, higher efficiencies for arsenic removal and enhanced capacities allowing annual cartridge replacement. Depending on the specific media, adsorption-based products can offer other secondary benefits including lead reduction, fluoride reduction, taste/odor benefits and other features. Figure 1 shows some typical alternative size configurations for adsorptive media cartridges.
Reverse osmosis
Common undercounter RO units, which are by far the most popular type in the United States, can effectively remove 85-95 percent of arsenate in drinking water. The semi-permeable membrane is the heart of a POU RO system and rejects most of the dissolved and non-dissolved species in the feed water. Anionic species such as As(V) in normal pH ranges are rejected along with other total dissolved solids (TDS). Thin-film composite membranes typically have a higher rejection rate over the TDS range (90-98 percent) than cellulose triacetate (CTA) membranes, which are more typically in the 85-95 percent range. Household water pressures of 50-60 pounds per square inch (psi) can yield one gallon of treated drinking water (at a rate of 0.1 gallon per minute, gpm) from every 3-5 gallons of feed water. The units commonly include a sediment pre-filter and carbon filter to remove larger particulate matter and free chlorine, which can foul and deteriorate the membranes over time. Maintenance of the system involves replacement of the RO membrane every two to three years along with pre- and post-carbon filter changes every six months. Table 1 shows a comparison of some of the features of these two POU methods for arsenic removal.
An important issue for arsenic reduction is the understanding of arsenic speciation and its affects on selected technologies. Effective POU systems must consider the presence of soluble arsenite (III) in the water, which, as an uncharged species, often co-exists with the oxidized or precipitated form, arsenate (V). Disinfectants such as chlorine will rapidly oxidize soluble As(III) to precipitated As(V) as demonstrated in recent USEPA-sponsored studies. Other oxidants such as permanganate, ozone and certain catalytic media have been shown to be effective as well with some limitations such as the presence of other reducing agents, dissolved oxygen and total organic compounds (TOC).2
In discussions with many water treatment professionals, a common misconception is that their POU-RO systems are adequately removing arsenic at high efficiencies. While this is true for arsenate, it’s not so for arsenite. Using an RO system alone should provide adequate reductions as long as the form of the arsenic is in the oxidized form (e.g., derived from a chlorinated distribution or source). The trouble is, nearly all consumers (and dealers) have little idea what form of arsenic exists in their water. As(III) and As(V) co-exist to some degree in many public and private groundwater or well water supplies in the United States as documented by the USEPA and others. Since many small community water systems and private wells have no form of disinfection (or oxidation), indiscriminate use of RO systems leave the possibility of inadequate arsenic treatment. The limited publicly available data on arsenite reduction for POU membrane-based systems indicate this technology to be rather inefficient and unpredictable at removing total arsenic. NSF verification studies also document this fact. Where careful speciation methods were employed, the reduction of As(III) across RO membranes typically ranged from 0-40 percent.
Two choices in one
Unless the speciation of arsenic is known, water treatment professionals must be prepared to remove both forms of arsenic. Manufacturers are now offering new adsorptive media for reduction of both forms of arsenic, without the addition of secondary chemical oxidants to the drinking water. Another attractive option is to combine RO with an adsorption media capable of removing As(III). Hundreds of thousands of RO systems are deployed in the United States, many in areas with known sources of arsenic above the current or anticipated MCL. Since it can no longer be assumed that an RO system serving a private well, or from a non-chlorinated water supply, is effectively removing arsenic, the two technologies can be synergistic. For instance, using an adsorbent to treat an RO permeate could have several advantages: 1) the membrane can serve as the first defense, removing some or much of the total arsenic; 2) the pH of the permeate is lower due to reduction of natural alkalinity, thus providing greater total adsorption, and 3) lower flow rates provide longer contact time and enhanced adsorption efficiencies.
Figure 2 illustrates the results from a laboratory experiment performed on a relatively new (less than 500 gallon usage), conventional undercounter RO system (equipped with a thin-film membrane and a new sediment prefilter). A target 50 ppb arsenite challenge solution was fed to the unit in the absence of oxygen (0.0 dissolved oxygen). A standard 10-inch adsorbent media cartridge was installed immediately after the membrane to treat the product water before entering a hydropneumatic tank.
The arsenic-containing feed water was speciated using ion exhange-based methods. Approximately 10 gallons of product water were generated per day during the one-week evaluation. The results across the RO membrane indicate an average of 30 percent reduction of As(III) during the 50-gallon test. Subsequently, the water was passed through an adsorbent cartridge reducing the arsenite to below detection of 3 ppb, representing an additional 93 percent reduction.
POU—affordable solutions
With an understanding of the proper use and application of adsorptive media and RO, each POU option is very applicable and capable of meeting the needs of three main groups of consumers served by community (municipal, regulated) water systems with elevated levels of arsenic; non-community, non-transient systems; and private well owners in rural communities.
Table 2 shows a basic comparison of costs for POU adsorptive media and RO. Cost assumptions are provided as a basis for comparison and, of course, can vary depending on the type of unit and life. While recognizing some of the attractive features of RO technology, adsorptive media-based cartridge solutions for arsenic removal offer the lowest overall cost considering initial investment, installation and annual maintenance.
Conclusion
A newly revised MCL for arsenic certainly appears imminent in early 2002. Thousands of public utilities will be facing the reality of implementing centralized treatment or other options for compliance. For many small and very small systems, implementing a centralized treatment system will be complex, cost-prohibitive and quite challenging given the lack of technical expertise and resources.3 Undoubtedly, the business of removing arsenic is more complex than removing chlorine, odor and improving taste, which are well understood. POU solutions for arsenic today offer hope for the increasingly educated consumer who wants to reduce this known toxin in his or her drinking water. RO and adsorptive media solutions each have advantages and limitations, which need to be understood to make an educated choice. The consumer checklist will surely include selection criteria of proven performance, simplicity, reliability and affordability.
References
- “Secrets of the Dead, Death at Jamestown,” Public Broadcasting Service, http://www.pbs.org/wnet/secrets2/case3_clues.html, July 2001.
- Laboratory Study on the Oxidation of Arsenic III to Arsenic V, USEPA Office of Research and Development, EPA/600/R-01/021, March, 2001.
- POU/POE Implementation Feasibility Study for Arsenic Treatment, AWWARF Project 2730, Narasimhan Consulting Services Inc., 2001.
About the author
Greg Gilles is vice president of applied technology at Apyron Technologies Inc., of Atlanta. Apyron is a developer and manufacturer of innovative adsorbent, catalyst and anti-microbial technologies. He can be reached at (678) 405-2705, email: [email protected] or website: http://www.apyron.com.