Screening biosolids for environmental contaminants

Duke University researchers have developed a method to determine the environmental impact of various chemicals in biosolids. Their research has been published in the journal Environmental Science & Technology.

Every year, waste treatment facilities in the United States process more than eight million tonnes of biosolids, about half of which is recycled into fertiliser and spread on crop land. But as industry invents new materials and chemicals for modern products, many find their way into our skin, our bloodstream and our biosolids.

More than 500 different organic chemicals have been identified in the biosolids used as fertiliser across the US; thus, the researchers say, “there is a need for a rapid, high-throughput method to evaluate their ecotoxicity”. Federal law regulates remnant levels of heavy metals and pathogens in the biosolid fertiliser, but according to Professor Claudia Gunsch, chemicals are not currently accounted for because it has been prohibitively expensive to even begin sorting out which ones might be ecologically unfriendly.

Professor Gunsch and colleagues from Duke’s Pratt School of Engineering developed a cost-effective method for screening chemicals for potential environmental impact. The first author on the study, Ryan Holzem, said the team “wanted to develop a method where you could check [the chemicals] quickly and efficiently, and flag the most potentially dangerous ones for more complex measurement”.

An important benefit of fertilising soil is replenishing nitrate levels, which are crucial to growing plants. One indicator of the soil’s health is the rate at which native bacteria are breaking down those nitrates through a process called denitrification. If antimicrobials or other chemical agents are affecting the bacteria’s ability to complete this process, the soil’s quality is degraded.

The screening technique involves growing a bacterium commonly found in soil that is important to the nitrogen cycle - Paracoccus denitrificans - in pure laboratory cultures. The researchers then add various amounts of the chemicals in question to determine the minimum amount that affects the denitrification process.

“Typically you have to use a complex, $50,000 piece of equipment to measure the gases that are produced by the active bacteria,” said Holzem. “But our method isolates denitrification on its own so that we can use simple processes to measure it.”

Holzem and Dr Gunsch worked with Associate Professor Heather Stapleton to examine six commercial antimicrobial chemicals: triclosan, which is found in most antibacterial soaps and toothpastes; triclocarban, which is also abundant; and four emerging antimicrobial compounds used in applications such as pesticides, paper mills, deodorants and antimicrobial household goods.

The team found that triclosan, which is already under fire from environmentalists, has troubling concentrations in the environment, given its effects on the Paracoccus bacteria. Three of the other five compounds tested were also found in concentrations high enough to warrant concern.

Dr Gunsch said the researchers “hope that companies developing new chemicals might use this method to start looking at potential environmental threats before incorporating them into consumer products”. She and Holzem further indicated that the technique could be used to test a variety of compounds through many different ecological indicators as well as the nitrogen cycle.

“The increased sensitivity, low cost and high-throughput adaptability make this method an attractive alternative for meeting the initial testing regulatory framework for the Federal Insecticide, Fungicide, and Rodenticide Act, and recommended for the Toxic Substances Control Act, in determining the ecotoxicity of biosolids-derived emerging contaminants,” the researchers said.