Part of a reverse osmosis system on a dairy farm. This treatment technology passes manure slurry through a series of membranes to purify and recycle water. Credit: Diana Aga
Part of a reverse osmosis system on a dairy farm. This treatment technology passes manure slurry through a series of membranes to purify and recycle water.

N&H TOPLINE: Remnants of antibiotics persist in treated farm waste

Many waste treatment systems — including those for handling municipal wastewater, hospital waste and manure — do not factor in treatment of antibiotics.

Each year, farmers in the U.S. purchase antibiotics that are approved for use in cows, pigs, poultry and other livestock, but when they repurpose the animals' manure as fertilizer or bedding, traces of the antibiotics may leach into the environment. New research from the University of Buffalo in New York may provide insight into the scope of this issue.

According to two new studies led by Dr. Diana Aga, the Henry M. Woodburn professor of chemistry in the University at Buffalo College of Arts & Sciences, two of the most elite waste treatment systems available on farms today do not fully remove antibiotics from manure.

Both technologies — advanced anaerobic digestion and reverse osmosis filtration — leave behind measurable levels of antibiotic residues, which can include the drugs themselves as well as the molecules into which the drugs break down.

In addition, the studies uncovered new findings about solid excrement, which is often filtered out from raw wet manure before the treatment technologies are implemented.

Researchers found that this solid matter may contain higher concentrations of antibiotics than unprocessed manure, which is a concern because this material may be released into the environment when it's used as animal bedding or sold as fertilizer, the announcement said.

"We were hoping that these advanced treatment technologies could remove antibiotics. As it turns out, they were not as effective as we thought they could be," Aga said.

She added, though, "On the positive side, I think that a multi-step process that also includes composting at the end of the system could significantly reduce the levels of antibiotics. Our earlier studies on poultry litter demonstrated that up to 70% reduction in antibiotics called ionophores can be achieved after 150 days of composting. Testing this hypothesis on dairy farm manure is the next phase of our project, and we are seeing some positive results."

The research on reverse-osmosis filtration was published online in January in the journal Chemosphere. The study on advanced anaerobic digestion, which was a collaboration between the University of Buffalo and Virginia Tech, appeared online in March in the journal Environmental Pollution.

"Neither of the treatment systems we studied was designed to remove antibiotics from waste as the primary goal," Aga said. "Advanced anaerobic digestion is used to reduce odors and produce biogas, and reverse osmosis is used to recycle water. They were not meant to address removal of antibiotic compounds.

"This problem is not limited to agriculture: Waste treatment systems today, including those designed to handle municipal wastewater, hospital wastes and even waste from antibiotic manufacturing industries, do not have treatment of antibiotics in mind," she emphasized. "This is an extremely important global issue, because the rise of antibiotic resistance in the environment is unprecedented. We need to start thinking about this if we want to prevent the continued spread of resistance in the environment."

Aga is a proponent of the "One Health" approach to fighting antimicrobial resistance, which encourages experts working in hospitals, agriculture and other sectors related to both human and animal health to work together, because humans and animals are often treated with the same or similar antibiotics.

Aga was an invited presenter last week at an international forum on the latest research about antimicrobial resistance. The event, in Vancouver, B.C., was co-chaired by representatives of the UK Science & Innovation Network, Wellcome Trust and the U.S. Centers for Disease Control & Prevention.

To conduct the research, the scientists visited two dairy farms in upstate New York.

Both facilities extract much of the solid matter from cow manure before subjecting the remaining sludge to high-tech waste management techniques. To process the remaining sludge, one farm uses advanced anaerobic digestion, which employs microorganisms and pasteurization to break down and convert organic matter into products that include biogas, while the other farm uses reverse osmosis, which passes the slurry through a series of membranes to purify water.

Both technologies reduced antibiotic residues in liquid manure but did little to cut down levels in the remaining solid matter, the researchers said. The study also revealed that antibiotic compounds tend to migrate from the liquid parts of the manure into the solids during treatment, making it arguably more important to treat the liquid than the latter, according to the announcement.

The following are key findings from each study:

* The research on advanced anaerobic digestion examined a popular class of antibiotics called tetracyclines and found that these drugs and their breakdown products migrated from the fluid part of the sludge into the solid part during treatment. At the end of the process, the solids contained higher levels of tetracycline antibiotics than the original raw manure. The study also found that both the liquid and solid parts of the sludge contained genes that confer resistance to these antibiotics.

* The study on reverse osmosis looked at how well this water purification technique removed synthetic antimicrobials called ionophores, which are used to promote production in dairy cows and to treat coccidiosis. The research found that reverse osmosis effectively filtered ionophores from the liquid portion of manure. However, low levels of the drugs persisted in "purified" water after treatment due to the deterioration of membranes used in the filtration process. Also, solid matter extracted from the water during reverse osmosis still harbored high levels of ionophores. Finally, the study found that prior to treatment, many of the ionophores appeared to have already migrated into the solid part of the raw manure that was removed before the reverse osmosis even began.

"Both of the systems we studied are a good first step in reducing the spread of antibiotics and potentially reduce resistance in the environment, but our study shows that more must be done," Aga said. "We need to look at different waste management practices that, maybe in combination, could reduce the spread of antibiotic compounds and resistance in the environment."

Aga pointed to composting as one area to explore. Her team is studying how advanced anaerobic digestion can be used in conjunction with composting of solid materials to remove antibiotics and their breakdown products from manure. The preliminary results of the research, which have not yet been published, are promising, Aga said.

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