What's the downside to clean water? Dirty sludge. A nationwide survey of sewage treatment plants shows that the sludge they produce--the residue from cleaning up wastewater--contains a wide variety of toxic metals, pharmaceuticals, flame retardants, and other compounds, including some antibiotics in surprisingly high concentrations. That's significant because every year more than half of the roughly 7 million metric tons of these so-called biosolids produced in the United States are applied as fertilizer to farm fields.
Whether the concentrations of these chemicals pose any health threat isn't known, but the new data, released last week, will allow the U.S. Environmental Protection Agency (EPA) to begin assessing the risks. "This is a very important study," says Rolf Halden, an environmental scientist at Arizona State University, Tempe.
Wastewater treatment plants remove excess nutrients and pollutants that would otherwise harm aquatic life. A side benefit is that the nitrogen and phosphorus in sewage is recycled and used as fertilizer on some U.S. agricultural land. But biosolids also contain chemicals, such as carcinogenic dioxins, that don't break down during treatment. In 2001, EPA conducted a survey of wastewater treatment plants and concluded the concentrations of dioxins were too low to pose a health threat. Five years later, prompted in part by public concerns over drugs in the environment, the agency started testing for a much broader range of biosolid contaminants, including 97 pharmaceuticals and related compounds.
Many of the 145 chemicals tested for were present nationwide. Biosolids from all of the 74 large treatment plants surveyed contained the same 27 metals, but only zinc, molybdenum, and nickel exceeded standards for application to fields. Almost all of the 11 flame retardants on the list were present in every sample. Twelve of the 72 pharmaceuticals were similarly ubiquitous.
Two of the most common drugs were the antibiotics triclocarban and ciprofloxacin. Although the average concentrations were similar to those in previous small-scale studies, several samples harbored up to 440 parts per million of triclocarban, which is added to antimicrobial soap and other personal care products. That's almost 10 times higher than ever reported in biosolids and "astonishingly high," Halden says. One question is whether the antibiotics harm soil microbes, or aquatic life if enough leaches into streams, Halden says. "We really don't have the answer."
An EPA official, who has not received permission to speak about the results, agrees that the concentrations of triclocarban "intuitively look high" but says the agency can't yet say whether they or any other of the detected chemicals pose a risk to vulnerable humans or wildlife. "It's not appropriate at this point to speculate on significance of the results," he says. In the next year, EPA expects to complete risk assessments on 10 chemicals in the survey.