Imagine a product so ubiquitous that it is made in every city in the nation, yet so under the radar that the Environmental Protection Agency admits it does not understand the risks of the 135 potentially hazardous substances that the product may contain. Imagine that the product’s manufacturers accept EPA’s admittedly limited safety guidelines and devote little, if any, resources to finding out more about its hazards. Now imagine that the farmers who benefit from using the product the most are also said to be the very people at greatest risk to be harmed by it.

That’s a snapshot of America’s current system for dealing with sewage sludge, the bi-product left over after the more than 3000 public wastewater treatment plants in the country clean everything households and industries flush down the sewer. The EPA and municipalities like New York City have embraced using sludge as a fertilizer product since ocean dumping was banned in 1988. Today the national program for “beneficial re-use” provides an outlet for more than half of the seven million tons of sludge produced in the U.S. each year.

There is a powerful environmental argument for using sludge this way: the material contains useful plant nutrients and if it is not recycled on land, it has to be buried in landfills or burned in incinerators. Yet this booming recycling process is fraught with uncertainty about its long-term environmental impact. EPA’s rules for regulating sludge have not been comprehensively updated in nearly two decades, despite the fact that a laundry list of potential contaminants have been shown to be present in sludge. Instead of focusing on revamping the rules in recent years, the agency has devoted resources elsewhere. Meanwhile New York City, like other governments across the country, tacitly accepts EPA’s flawed rules as it sends thousands of tons of sludge to American farms.

“We don’t do our own research into it,” said Vincent Sapienza, deputy commissioner for wastewater treatment in the New York City Department of Environmental Protection: “Whatever direction we are given by EPA and some of the other organizations, we will follow that lead.”

In a January 2009 survey of sludge from treatment plants across the country, EPA found the presence of 135 potentially dangerous metals, pharmaceuticals, steroids, hormones and other substances that it does not regulate. While some scientists say the rules for using sludge protect the public from immediate risks, the presence of these chemicals raises questions about whether spreading it on farmland might contaminate soil over the long term.

If the right precautions are taken, “certainly the land application program does not present significant risk to the general public, at least in the short run,” said Murray McBride, a soil scientist who heads Cornell University’s Waste Management Institute. “The risk is really being accepted by farmers, and they might not know that.”

At the core of public distrust about applying sludge to farmland are the EPA’s rules for regulating the practice. Known as “Part 503”, these rules were finalized in 1993 and underwent minor changes in 1994 and 1995. They require sludge to be treated and tested for nine heavy metals and a short list of disease-causing bacteria before it is applied to farmland.

Despite being told eight years ago by a national panel of scientists to update the scientific basis for Part 503, EPA has not yet done so. As an EPA source with knowledge of the sludge program admitted recently when asked about the potential contaminants in sludge, “There’s a lot we don’t know.” Meanwhile, the staff and budget for the agency’s sludge program are now shrinking.

Still the official, who spoke on condition of anonymity because he was not supposed to speak with the press, maintained EPA’s position that putting sludge on farms with the existing standards is safe. “We believe that the standards for use and disposal of sewage sludge are protective of public health,” he said.