Composting industrial waste
The first U.S. survey of wastewater treatment solids destined to become fertilizer turns up numerous emerging contaminants.
Rich in nitrogen, phosphorus, and other nutrients, the sludge left over from wastewater treatment often ends up as fertilizer that is spread on farm fields or packaged into bags and sold at garden shops. This material—known as biosolids—also carries an unknown load of potentially harmful chemicals. In the first comprehensive analysis of biosolids from a variety of wastewater treatment plants in the U.S., published today on ES&T’s Research ASAP website (DOI: 10.1021/es0603406), researchers document emerging contaminants in biosolids collected at treatment plants.
Biosolids are often available for public purchase, says Chad Kinney of Eastern Washington University, who conducted the study along with a team of U.S. Geological Survey (USGS) researchers. The scientists examined 9 biosolid products from 8 wastewater treatment plants scattered across the U.S. and tested them for 87 compounds. They found that the concentrations of some compounds remained relatively constant over time. Of the 87 compounds, 55 cropped up in one or more of the biosolids studied, with concentrations ranging between 10 and 20,000 µg/kg dry weight.
“One of the things that surprised us was the diversity of compounds that we found,” says Ed Furlong of USGS, a coauthor of the research. For example, because of the volatility of fragrance chemicals, he expected their concentrations to be low. Instead, the fragrance molecules seemed to stick around, despite extensive processing of the biosolids. The team hypothesized that the organic matrix of the biosolids acts to retain these chemicals.
As researchers with Environment Canada documented in 2002, biosolids can retain pharmaceuticals and other pollutants at concentrations high enough to raise red flags. For example, their report found nonylphenol, an endocrine disrupter, at a median concentration of 232 µg/g dry weight.
Kinney says that analyzing biosolids for contaminants is difficult for numerous reasons. Wastewater treatment plants use different methods and take in waste that changes over time. The processing and variable makeup of biosolids—from liquid to pellet to compost—also alter which chemicals are destroyed and which survive. In the end, how the biosolids are applied can also affect chemical survival. Folding the waste into soil may increase biodegradation, Kinney says, and photodegradation may come into play if the biosolids are simply dumped on soil surfaces.
Furlong emphasizes that all biosolids must follow U.S. EPA regulations. He points out that the team found triclosan at levels of only a few thousand micrograms per kilogram. “That’s a low concentration,” he says, but “if they’re persistent, or not degraded, which we don’t yet know, it just shows that there’s not enough information to say what that [concentration] means.”
Biosolids may contain residues from many commercial products, not just pharmaceuticals and consumer products, says Rolf Halden of Johns Hopkins University. Halden recently published results in ES&T showing that biosolids contain triclosan and triclocarban, antibacterial compounds commonly used in soaps. He says the new work provides a challenge and a first step in identifying the chemicals that might be present in biosolids. He also notes that sludge is difficult to analyze and that the reported levels are probably quite conservative.
“It’s very hard to think of anything more difficult to analyze than sludge. It’s clear that this is not an easy matrix to work with,” Halden says. “We’re just beginning to understand what the consistency of the sludge is, ... what it contains, and what it means for human health.”
