The Environmental Protection Agency doesn't know if the treated sewage sludge that farmers use as fertilizer is safe, according to a report from its internal watchdog.

The treated sewage known as biosolids is chock full of nutrients, which is what makes it so good at enriching soil. But it also can be chock full of pollutants, from heavy metals such as mercury and arsenic to pharmaceutical compounds, flame retardants and disease-carrying organisms.

And the EPA doesn't know enough about hundreds of pollutants found in the material, the agency's inspector general said in a report Thursday.

The EPA's controls over using biosolids as fertilizer are "incomplete" or have "weaknesses" and "may not have fully protected human health and the environment," said Jill Trynosky, a project manager with the inspector general's office. "The EPA is unable to state whether, and at what level, the pollutants found in biosolids pose a risk to human health or the environment," Trynosky said in an agency podcast describing the investigation.

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