Antimicrobials accumulate in the municipal sludge used to fertilize crops
CHRISTOPHER R BRODIE / American Scientist 1jan2007
More than a million pounds of antimicrobial chemicals from soap and other products flow into the nation's sewers every year. Do these compounds pose a risk? Product manufacturers say no, pointing to data that show only traces of the two most common antibacterials, triclosan and triclocarban, in treated wastewater. What happens to the remainder is less certain. The stock explanation has been that the majority is broken down during the treatment process. The fraction released into surface water was thought to meet the same fate sooner or later. Thus, much of the claim that these products are safe rested on the fact that they were rendered harmless in treatment plants or just beyond.
New data puncture that conclusion: 50 percent of triclosan and 76 percent of triclocarban remain unchanged by aerobic and anaerobic digestion in a typical wastewater facility, according to a pair of recent reports. This large intact fraction isn't going out with the treated water—the old estimates are correct in that respect. Rather, it is trapped in the sludge at the bottom of the treatment tanks. Most of that sludge gets spread on the ground to fertilize pasture, forests and human food crops.
Rolf U. Halden, a scientist and engineer at Johns Hopkins University, and his coworkers are the chemical detectives behind this work, which appeared in the June 1, 2006 issue of Environmental Science & Technology and in Chemosphere, published online June 9, 2006. By comparing the amounts that entered a wastewater plant with the amounts that exited or were broken down, the Hopkins team pieced together a much more complete picture of the life cycle of these compounds in the environment.
Triclosan and triclocarban are small organic molecules that give antimicrobial properties to personal-care products such as soap, deodorant and toothpaste as well as durable goods such as cutting boards, baby carriers and socks. Overall, Halden's team estimates that more than 100,000 pounds of triclosan and over 300,000 pounds of triclocarban are spread on the ground as sludge each year in the United States, based on data from a dozen sites around the country. Of the total mass that enters a typical sewage-treatment facility, two percent of triclosan and three percent of triclocarban remain in the clean-water output. Thus, only 48 percent of triclosan and 21 percent of triclocarban are transformed or lost in the treatment process—much less than industry estimates. At 50 and 76 percent, respectively, sludge is the biggest repository.
According to a 2002 report by the National Research Council, 63 percent of the 5.6 million tons of dried sludge made in the United States each year is applied to the land. (When used as fertilizer, municipal sludge goes by the more polite name of biosolids.) This recycling is viewed as being good for the environment, because the alternatives are incineration, burial or (before 1992) offshore dumping. But combined with the numbers from the National Research Council, Halden's analysis indicates that hundreds of thousands of pounds of triclosan and triclocarban are spread on the ground every year. Remarkably, this massive contamination is unregulated and unmonitored. The ecological effects are similarly unexplored.
In fairness, the reason that no one noticed the organic compounds (such as triclocarban) in sludge is that the technique used to measure such things wasn't up to the job. Sludge is a complex matrix that adheres to and masks molecules that are strongly hydrophobic, as are triclosan and triclocarban—so much so that this municipal gunk acted as a "chemical black hole," according to Halden. "It used to be you could dump [manmade] chemicals in the sludge and they'd disappear," he explains, referring to how they became invisible to detection. But in 2004, Halden's group described a method that allowed chemists to peer inside the sludge, something he describes as "one of the last frontiers in analytical chemistry."
What they saw was an accumulation of triclosan in the sludge up to concentrations of 30,000 micrograms per kilogram—more than 6,000 times more concentrated than the incoming sewage. The numbers were even higher for triclocarban: 51,000 micrograms per kilogram in the sludge, which worked out to 8,400 times the concentration of sewage and 300,000 times that of the sewage treatment plant's outflow.
The biosolids industry is regulated by the U.S. Environmental Protection Agency, which dictates the conditions under which the substances can be used. But in terms of sludge composition, the EPA only set limits for metals and certain pathogenic bacteria. There is no oversight of organic chemicals and no categorical prohibition of the use of biosolids on food crops. Current rules do govern the types of food that can be grown with biosolids fertilizer, the amount of time between application and harvest, and other practical details. But the EPA's official stance is that the practice of growing food in dewatered municipal sludge is acceptable.
And perhaps it is. Manufacturers consider triclosan and triclocarban to be safe—even healthy, to judge by the tone of their advertisements. Some bar soaps, for example, are five percent triclocarban by weight. With the exceptions of triclocarban causing outbreaks of "blue-baby syndrome" in the 1960s and ‘70s (pediatricians still advise against exposing newborns to triclocarban) and the trace amounts of dioxin, a known carcinogen, that tag along with triclosan, the compounds have a clean safety record in people. The problem, as any toxicologist will tell you, is that the dose makes the poison.
The estimated annual production of triclocarban exceeds one million pounds. From this massive starting amount, consider that triclosan and triclocarban resist degradation (Halden estimates their half-life in sediments to be 540 days) and that their chemical structure suggests that they build up in fat, and it's easy to see the potential for accumulation in the food chain.
Chemical stability by itself is hardly damning, and neither compound has killed anyone. But direct effects on hand-washers and tooth-brushers are not the only relevant outcomes. For example, triclosan disrupts the functions of the endocrine system in cultured cells. Furthermore, the risk of fueling the evolution of antibiotic-resistant bacteria—as yet unproven—remains plausible. Halden's concern is that in sludge, the combination of concentrated microorganisms, some of them capable of causing disease, with extremely high concentrations of antimicrobials is a recipe for drug resistance.
In the end the decision of what to do about sludge will be up to the risk managers. The only thing environmental scientists like Halden can do is to show their data as best they can with the resources they have. Fortunately, the EPA seems to be listening. Their 2001 survey of biosolids examined only metals and dioxins, but the 2006 survey (as yet unreleased) will be more comprehensive, checking for more metals, a short list of organics and, at least in some samples, triclocarban specifically. Getting rid of the things that get rid of microbes may turn out, paradoxically, to be the healthy thing to do.