Common antimicrobial could thwart microbes that clean up sewage sludge
Alex Marshall/Clarke Energy/Wikimedia Commons (CC BY 3.0)
Triclosan, after being flushed down the drain, may muck up sewage treatment. In wastewater treatment plants, the omnipresent antimicrobial can sabotage some sludge-processing microbes and promote drug resistance in others.
The antimicrobial, a common ingredient in personal care products such as hand soaps and toothpaste, accumulates in municipal treatment plants across the country. In lab experiments, researchers have now found that concentrations of triclosan present in wastewater can destabilize the microbial communities that help treat sewage solids, which can then be used as fertilizer.
Triclosan may also encourage the growth of microbes that are immune to drugs, increasing the chances that drug-resistant microbes will spread in the environment via fertilizers, the researchers found. The results appear June 10 in Environmental Science & Technology.
In municipal wastewater treatment plants, the solid waste from sewage is collected in stocky oxygen-free silos. There bacteria and other microbes called archaea digest the solids into a more compact, better smelling and less infectious sludge. Civil engineers monitor the microscopic workers and their chemical transformations, including how much methane gas the microbes release, which is a measure of sludge break down.
Much of the 100 metric tons of triclosan estimated to wash into U.S. wastewater treatment plants each year ends up in this sludge. A 2009 Environmental Protection Agency survey found that treated sewage solids held a median of about 4 milligrams of triclosan per kilogram of sludge; the maximum level was about 130 mg/kg. But researchers didn’t know whether triclosan affected sewage plant microbes.
In the lab, civil engineer Patrick McNamara of Marquette University in Milwaukee and colleagues simulated sewage sludge treatment, starting with microbes scooped from a local treatment plant that had already been exposed to triclosan. The researchers then fed triclosan to the microbes at three concentrations, spanning today’s median triclosan level to four times as high as the current maximum found in sewage solids.
At 5 mg/kg, the researchers saw no inhibition of microbial methane production compared with no-triclosan conditions. But at 50 mg/kg, the microbes released wildly variable amounts of gas, sometimes low, sometimes high. At 500 mg/kg, the microbes’ methane production crashed to just above half of control levels.
McNamara speculates that the medium level of triclosan could represent a tipping point for the microbes before the community’s sludge-processing power goes down the drain.
The finding alarmed but did not surprise McNamara. Triclosan is intended to disrupt microbes in our homes, he says, and it can have the same impacts in treatment plants.
In a separate sludge-treatment test using microbes that had never been exposed to triclosan, McNamara and colleagues found that high levels of the antimicrobial caused a spike in the amount of a particular gene present in the microbial community, called mexB. The gene makes microbes resistant to triclosan as well as to antibiotics such as ciprofloxacin, McNamara says. Though the researchers don’t yet understand what caused the rise in the gene’s prevalence in the microbial population, it could mean that the nonresistant microbes died out, leaving behind resistant bugs, or that some microbes transferred mexB to others, or both.
Currently, the only way to reduce wastewater levels of triclosan is if consumers use less of the antimicrobial. The U.S. Food and Drug Administration is reviewing the use of triclosan in consumer products.
Civil engineer Rolf Halden of Arizona State University in Tempe says the finding warrants attention. “Regulating antimicrobials in wastewater should be a priority for the nation,” he says.
Editor's Note: This story was updated on June 25, 2014, to correct the name of the journal in which the results were published.
P.J. McNarmara, T.M. LaPara and P.J. Novak. The impacts of triclosan on anaerobic community structures, function, and antimicrobial resistance. Environmental Science & Technology. Published online June 10, 2014. doi: 10.1021/es501388v.
US EPA. Biosolids: targeted national sewage sludge survey statistical analysis report fact sheet. 2009,EPA 822-F-08-006.
J. Heidler and R.U. Halden. Mass balance assessment of triclosan removal during conventional sewage treatment. Chemosphere. Vol. 66, January 2007, p. 362. doi: 10.1016/j.chemosphere.2006.04.066.
B. Mole. Triclosan aids nasal invasions by staph. Science News. Vol. 185, May 17, 2014, p. 12.