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.