Popular antibacterial soap ingredient’s presence in environment could be cause for concern
New research suggests that large quantities of a popular ingredient in antibacterial soap could be entering the environment through recycled sewage sludge.
Researchers looking for emerging contaminants in wastewater treatment plants (WWTPs) face a formidable challenge—to “find the needle in the haystack,” says Rolf Halden, an assistant professor of environmental health sciences at the Johns Hopkins Bloomberg School of Public Health. In a study posted today to ES&T’s Research ASAP website (DOI: 10.1021/es052245n), Halden and his coauthors report that approximately 75% of the mass of a popular antiseptic called triclocarban that enters a typical WWTP persists in the treated sludge.
Their work is the first peer-reviewed field study of triclocarban in sewage sludge and one of the few reports of the compound’s fate during wastewater treatment. The findings suggest that—given the high rate of sewage sludge reuse as fertilizer—a single WWTP can return more than 1 metric ton of triclocarban to the environment. The authors agree that their findings also raise the question of whether triclocarban could be promoting antibiotic resistance in bacteria in the sludge or elsewhere in the environment.
Triclocarban, commercially known as TCC, is a pesticide used extensively as an antimicrobial additive in soaps and body washes; one survey of commercial products found it in 30% of bar soaps. With annual production estimated at 1 million pounds or more, triclocarban is classified as a high-production-volume chemical by the U.S. EPA.
Despite triclocarban’s widespread usage—and its chemical similarity to triclosan, another common antiseptic additive that has come under scrutiny—relatively little is known about the compound’s fate during wastewater treatment. “I think it’s surprising that no one has looked at it before,” says Shane Snyder, R&D project manager at the Southern Nevada Water Authority. “People have done a tremendous amount of work on triclosan.”
Halden and his coauthors found that triclocarban was almost entirely removed from the WWTP’s effluent after activated sludge treatment—with an average efficiency of 97%. The compound tends to attach itself to particles in the sludge because of its hydrophobic nature. Therefore, it remains in the sludge; even after 19 days of anaerobic digestion, levels were as high as 51 milligrams per kilogram (mg/kg). According to Snyder, the concentration of triclocarban in treated sludge is higher than he would have expected.
The authors propose that triclocarban’s chemical structure may explain its resistance to degradation. “Some chemicals don’t withstand the anaerobic digestion component of wastewater treatment,” explains Halden, “but this chemical does because it looks foreign to microorganisms.”
In previous studies, also published in ES&T, Halden and his colleagues documented the presence of triclocarban in streams, groundwater, wastewater, and drinking water around Baltimore, Md. They found concentrations ranging from 3 nanograms per liter in treated drinking water to more than 5 micrograms per liter in streams with significant raw sewage contamination. While doing so, they developed a state-of-the-art analytical technique to selectively identify and quantify triclocarban against the complex background of sewage sludge.
The presence of triclocarban in sewage sludge raises questions about its fate when the sludge is reused as biosolids. Federal regulations classify treated sludge as biosolids on the basis of levels of pathogens and other contaminants. According to EPA statistics, about half of the biosolids generated from wastewater treatment are recycled to land for agriculture and other uses.
Hans Sanderson, director of environmental safety at the Soap and Detergent Association, comments that the research is technically sound and provides helpful information about triclocarban’s fate. He emphasizes, however, that the results do not necessarily apply to the biosolids that meet the strictest pathogen requirements—known as Class A biosolids—which are subject to fewer usage restrictions. The heat treatments designed to kill pathogens in the sludge may also change the concentration of triclocarban in the finished biosolids. “That concentration, we don’t know,” says Sanderson, “but in all likelihood it’s not going to be higher than the concentration you have in the sludge—it’s probably going to be lower.”
The authors say that the sludge sampled in this study would meet the Class B biosolids standards, but they are in the process of measuring triclocarban levels in Class A biosolids. According to Halden, “Unpublished data from the National Biosolids Repository study show that triclocarban can withstand additional treatment steps and remains detectable at mg/kg levels in Class A biosolids.”
The researchers are also investigating whether triclocarban is bioavailable in soil, or whether it stays attached to particles, which would limit its potential impact. Although relatively few data exist about the toxicity of triclocarban, the authors report that it has been found to impair reproduction in laboratory rats and that some of its degradation products are carcinogenic.
To provide researchers with more information about the composition of sludge, Halden has established the Johns Hopkins University National Biosolids Repository. This nationwide sampling project seeks to identify synthetic compounds—such as PCBs and PBDEs (polybrominated diphenyl ethers)—that persist in municipal sludge and to estimate the chemical input to agricultural soils by sludge reuse.
Halden suggests that triclocarban’s antimicrobial properties give additional cause for concern, noting that the related compound triclosan has faced scrutiny since the late 1990s, when researchers first found that it could promote growth of resistant strains of bacteria. The conditions under which microbes encounter topical antiseptics like triclosan and triclocarban—at low concentrations, for extended periods of time—are considered the most likely to encourage resistance. But Stuart Levy, a professor at Tufts University and the president of the Alliance for the Prudent Use of Antibiotics, cautions: “With triclocarban, we don’t have any data of its relation to antibiotic resistance. So that makes this paper more speculative as to what [the residue might] do in the microbial world.”
The authors plan to continue their investigations into triclocarban, and they emphasize that many unanswered questions remain. “It’s kind of a detective story,” Halden says. “We noticed [triclocarban] in the streams, and we followed it to the wastewater treatment plant and see it’s not being degraded. We’re not at the end of the story.”
