Researchers at a number of U.S. universities are developing relatively simple technologies that may someday protect hundreds of millions of people worldwide whose drinking water is tainted with arsenic. These technologies may also assist mining and industrial firms in removing arsenic from discharges into groundwater and surface waters.
Arsenic, commonly found in nature as the mineral compound arsenopyrite, is released into water from soil and rock erosion, and is prevalent in the southwest United States, eastern Michigan, and parts of New England, among other places. Southern Asian locations such as Bangladesh, India, and Taiwan also have high levels of naturally occurring arsenic. Arsenic is also a by-product of industrial processes, including semiconductor manufacturing, petroleum refining, and mining and smelting operations, and is used as a wood preservative, in herbicides, and in animal feed additives.
According to the EPA's Office of Ground Water and Drinking Water (OGWDW), most people in the United States are exposed to arsenic through food or drinking water. Based on national surveys from the late 1970s to the mid-1980s, the OGWDW estimates that approximately 150 community water systems (serving at least 25 people or 15 service connections) do not meet the current drinking water standard for arsenic of 50 micrograms per liter (µg/l). The EPA has determined that exposure to high levels of arsenic increases the risk for skin cancer, but it has not quantified an increased risk of mortality from arsenic-induced liver, kidney, lung, or bladder cancers. Data also suggest that arsenic may affect the vascular system in humans and may be associated with the development of diabetes.
Although existing technologies can effectively treat arsenic in surface water, one new technology, dubbed AsRT by its developers, appears to be more cost-effective than traditional methods at removing arsenic to below a detection limit of 1 µg/l. The new method is also inexpensive enough that it might someday be used on drinking water wells in less developed countries such as Bangladesh, where arsenic appearing in groundwater is producing skin disorders among those exposed to high levels of arsenic (around 500 µg/l). University of Connecticut (Storrs) associate professor of civil engineering Nik Nikolaidis, who developed the AsRT technology with Jeff Lackovic, a doctoral candidate at UConn, and Greg Dobbs, a senior consulting scientist with the United Technologies Research Center, estimates that 77 million people in Bangladesh alone may live in areas where at least some of the well water is contaminated with arsenic. Nikolaidis is working to scale down the size of the filter so that it may be used on individual wells. The problems of arsenic in developed countries such as the United States also provide a potent incentive for developing new technologies. Large populations in the United States, especially in California and Nevada, have water supplies containing more than 10 µg/l, and some even have water sources containing above the current standard of 50 µg/l. U.S. researchers are working to untangle questions about the health effects of arsenic exposure to help shed light on whether the U.S. drinking water standard is too high. The EPA is expected to issue a new proposed standard for arsenic in drinking water by January 2000; the World Health Organization recommends a standard of 10 µg/l.
Comparing Approaches
A reduction in the U.S. drinking water standard will underscore the need for technologies such as the UConn project that remediate arsenic effectively and at a low cost. Although arsenic may combine with inorganic or organic compounds, it is inorganic arsenic compounds that are most commonly found in water. In water, two species of inorganic arsenic, arsenite (As[III]) and arsenate (As[V]), are most common. Arsenite, the more toxic of the two, contains less oxygen than arsenate, and is more mobile. Arsenate is less soluble and is more readily remediated by existing technologies. Although AsRT is more effective on arsenate, field tests involved arsenite-contaminated water and the removal results were still impressive. The new technology can be used to clean groundwater, surface water, and wetland sediments contaminated with arsenic, according to the AsRT developers. Its only drawback may be a net export of iron from the iron filings used in the treatment, which may require further remediation, although this drawback is not an issue in groundwater or surface water systems where the iron can be oxidized and precipitated out of the water.
The AsRT technology involves pumping arsenic-contaminated water through a bed of sand and iron filings. As the water passes through the iron filter, arsenic is removed from the solution through an as-yet undefined mechanism. The arsenic may be removed as part of the iron precipitation or coprecipitation, or may attach to the iron filings that have corroded, or finally, may attach to the iron oxides that coat the sand. Further studies are underway to elucidate the exact mechanism. In field tests, groundwater containing 300-400 µg/l is pumped, bottom to top, through a tube filled with the sand-iron mix at a rate of about 1 gal per minute. As of late July 1998, 380,000 l were processed resulting in less than 1 µg/l of arsenic effluent, according to Nikolaidis. The team's findings are available on the Internet at http://www.eng2.uconn.edu/~nikos/asrt-brochure.html.
Coagulation/filtration, the standard treatment for remediating arsenic and other contaminants from surface water, uses iron, which reacts with arsenic to create a solid that precipitates from the water. Jeff Kempic, team leader for treatment technology in the OGWDW, says coagulation/filtration can remediate arsenic to levels of 2-5 µg/l, but doing so requires more coagulate or using a different coagulate. "It's not dramatically expensive to remove more arsenic in a coagulation/filtration system," he says, "if the treatment plant is already in place." However, this treatment system produces an arsenic-contaminated sludge that might need to be disposed of in a hazardous waste landfill if the EPA lowers its arsenic standard. One advantage of the AsRT treatment appears to be a smaller waste stream than traditional approaches, according to Nikolaidis, because most of the arsenic removal occurs on the surface of the iron filings and sand in the filter, and this mixture of iron and sand should be usable for several years, depending on conditions at a particular treatment site.
Other common water treatment approaches, including reverse osmosis, also perform well in removing arsenic, but also produce waste streams. Reverse osmosis involves pushing water through a membrane that captures contaminants, and is commonly used in Florida as a way of removing minerals from drinking water. Although effective in removing contaminants to below 2 µg/l, reverse osmosis is a more expensive technology than coagulation/filtration and produces a brine that must be treated for arsenic contamination. Moreover, reverse osmosis produces a larger waste stream than other treatment methods, which may make the method impractical where water is scarce.
Anion exchange technology is also effective for removing arsenic, but offers its own disadvantages. This method involves passing water with anions of arsenate through a column of resin beads containing exchangeable, innocuous ions such as chloride, resulting in a swap that leaves the arsenate in the water column and the chloride in the water. Although it is a relatively inexpensive technology, it will not work on arsenite because that compound is uncharged, and removing contaminants at lower levels will affect how soon the exchange column must be regenerated before breakthrough (the point at which removal levels begin to deteriorate). This process also produces an arsenic-contaminated brine.
For some municipalities, a lower arsenic standard could require the installation of a treatment system for groundwater that presently is simply chlorinated, according to Dennis Clifford, a professor of environmental engineering at the University of Houston in Texas. Applying conventional treatment approaches to groundwater is problematic on a number of fronts. For one thing, coagulation/filtration requires large settling tanks. A community of 10,000 people uses an estimated 1.4 million gallons of water per day. To meet this need would require pumping about 60,000 gal of water an hour from groundwater sources. At the settlement stage of the treatment process, in which the arsenic precipitates from the water, the water would have to be pumped into a 180,000-gallon tank and remain there for 2-3 hours. For some cities, where wells are widely disbursed or located in neighborhoods, it may be impractical or downright impossible to build a treatment facility on such a scale.
Clifford and his associates from the University of Houston have developed a modified version of the coagulation/filtration process that has been tested in Albuquerque, New Mexico. Their iron coagulation/microfiltration approach uses traditional iron coagulation, but employs a vigorous 10- to 20-second mixing process, after which treated water is passed immediately through a membrane filter with a pore size of 0.2 microns or less. Passage through the membrane filter immediately removes arsenic, which eliminates the need for large settling tanks and enables water systems to produce water as demand requires it. "We've developed the process and shown that it works," Clifford says, but the cost of installing such a system is not yet clear.
Clifford has also experimented with modifications to the anion exchange process in an effort to reduce the quantity of brine produced, and has discovered that brine water from recharging can be reused up to 25 times without hindering arsenic removal. Although brine reuse will reduce the overall quantity of brine to be discarded, the reused brine will be heavily contaminated with arsenic, necessitating further treatment or disposal.
The least expensive treatment option, activated alumina adsorption, involves passing acidified water through columns of activated alumina that adsorbs the arsenic. Regenerating the column, however, requires running hazardous chemicals--sodium hydroxide and sulfuric acid--through the system. The hazards of handling sulfuric acid, a drop of which can dissolve a paper napkin in 10-20 seconds, or sodium hydroxide, which quickly burns, may make this approach impractical for water systems serving a few thousand people.
Working at Cross-purposes
The appropriate treatment approach will depend on a particular water system's needs, resources, location, and other factors, Clifford notes. But any arsenic treatment system is bound to exceed the typical cost of $0.30-1.00 per 1,000 gal for conventional treatment approaches, he says, though costs will also be affected by what standard the EPA sets. Jim Taft, chief of the targeting and analysis branch of the OGWDW, says a reduction in the standard, for example to 45 µg/l, would not affect many water systems, but an arsenic standard as low as 5 µg/l would affect most communities with arsenic in their source water. The EPA, however, has not settled on a particular number or range, he says.
Steve Borell, executive director of the Alaska Miners Association, fears a lower drinking water standard will drive a reduction in arsenic discharges under the Clean Water Act. His association challenged the EPA on a 0.18 µg/l discharge limit for Alaska mine discharges and other industrial discharges. In February 1998, the EPA changed the rule to allow industrial discharges to meet the drinking water standard. Under an industrial discharge standard of just a few micrograms per liter, there would be little mining in Alaska, Borell says, not simply because some small mines would be forced to close, but because the costs of meeting the standard would discourage new mining ventures.
Because of uncertainty about health effects, the EPA developed a research plan, the full results of which will not be complete by the January 2000 deadline for issuance of a proposed arsenic standard. However, the National Academy of Sciences' review of arsenic health effects will be completed, which should provide a key piece of information for the EPA in establishing its standard, according to Taft. "The agency is committed to meeting the deadline. We'll take advantage of the best information available at the time of the [arsenic rule] proposal," he says.
The incompleteness of the health effects data worries the American Water Works Association (AWWA), which represents 4,000 water utilities nationwide and 51,000 individual members. "The biggest hang-up we have is that the science is not there yet to justify lowering the standard," says Dan Pedersen, a regulatory engineer for the AWWA. "We don't think there's controversy over whether arsenic causes health effects. The controversy is over what level [of exposure] produces those effects."
Allan Smith, a professor of epidemiology at the University of California at Berkeley, believes the debate over the dose-response curve for arsenic effects diverts attention from prudent public health decision making. He notes that studies he and his group have conducted in Chile and Taiwan demonstrate that at concentrations of 500 µg/l, arsenic poses the highest known environmentally related cancer risks. At exposures to such concentrations of arsenic, 1 in 10 people will die of lung, bladder, and skin cancers. Linear extrapolations from these concentrations to lifetime consumption of water containing 50 µg/l (the current drinking water standard) results in risk estimates of 1 in 100. Even if the dose-response curve is not linear and the actual risk is 10 times lower than this, he says, the cancer mortality estimate would still be 1 in 1,000 for the current U.S. drinking water standard, far higher than the 1 in 100,000 or 1 in 1 million goals often invoked for standard setting. "That is an extremely high risk [for drinking water]," he comments. In fact, he says, "Setting standards for drinking water contaminants has usually involved extrapolating potential risks way below levels that are scientifically demonstrable." Smith says no data exist to prove or disprove that arsenic produces no health effects at or below a threshold level of exposure. He therefore believes that an arsenic standard of 10 µg/l should be adopted at least as an interim standard, without waiting on scientific proof of the exact levels of risk, which he believes will be extremely difficult to obtain.
Ruth Hund, senior project manager at the AWWA Research Foundation, says a greater understanding of how arsenic works to produce disease will help settle questions about the dose-response curve. Until then, policy makers must determine whether the health risks posed by the current standard justify the costs of meeting a reduced arsenic standard. "The drinking water industry's number one concern is to protect public health," Hund says. "The question is, where should we put our resources to most effectively protect public health? Arsenic really brings this question to the forefront. How expensive would it be to treat to remove arsenic at lower levels, and are those resources better used to do something else?"
Suggested Reading
Lewis DR, Southwick JW, Rench JD, Calderon RL. Assembly of a cohort to examine drinking water arsenic in Utah. Epidemiology 7(4): S66 (1996).
Smith AH, Goycolea M, Haque R, Biggs ML. Marked increase in bladder and lung cancer mortality in a region of northern Chile due to arsenic in drinking water. Am J Epidemiol 147(7):660-669 (1998).
Smith AH, Hopenhayn-Rich C, Bates MN, Goeden HM, Hertz-Picciotto I, Duggan HM, Wood R, Smith MT, Kosnett MJ. Cancer risks from arsenic in drinking water. Environ Health Perspect 97:259-267 (1992). |
Karen Breslin
Last Updated: October 29, 1998