Antimicrobial resistance is one of the major concerns in human and veterinary medicine today. A variety of agricultural activities may contribute to this issue, especially when wastewater is used for agricultural irrigation.
Dr. Daniel Ashworth, research soil scientist with USDA-ARS, has been researching ways to lower the levels of antibiotics and other potentially harmful substances in irrigation water to mitigate antimicrobial resistance. He is also looking at whether such substances move from soils to plants and enter the food chain.
“Agriculture uses a lot of water,” said Ashworth. “Globally, about 70% of fresh water is used for agriculture. In the U.S. it’s about 45%.” He added that ag water recycling isn’t widely practiced, so any research in this area is unique.
Biochar may have a role in the mitigation and remediation of ag water. Biochar is the product of heating carbon-rich organic matter in the absence of oxygen in a process called pyrolysis. Ashworth’s research focuses on how to optimize biochar for absorbing antibiotics.
“We can control feedstock choice, pyrolysis temperature and post-production modifications that influence the characteristics of the biochar,” said Ashworth. “These characteristics influence the adsorptive properties.”
When biochar is used in a filtration medium, it may have the capacity to remove antibiotics and per- and polyfluoroalkyl substances (PFAS). This would produce cleaner water for ag irrigation and prevent contaminants from getting into agriculture. PFAS are widely-used, manufactured chemicals that break down very slowly. The EPA is currently researching ways to better detect and measure PFAS in the environment.
“Treated wastewater may be contaminated,” said Ashworth. “Contaminants from various industrial processes include pharmaceutical residue, microplastics and disinfectants. But antibiotics are the big issue.”
It’s important to note that treatment plants don’t remove contaminants – water treatment is designed to remove organic matter, pathogenic bacteria and nutrients.
The antibiotic aspect is important because these compounds are frequently used to treat bacterial infections in humans and animals. “Human use accounts for 20% of antibiotic use; the other 80% is agricultural use,” said Ashworth. “Up to 90% of an antibiotic dose is not retained by the body (of humans and animals) and ends up in a wastewater system.”
Ashworth explained a greenhouse study that was conducted to determine if there was uptake of antibiotics in both the above- and below-ground portions of plants. Radishes and spinach were used for the experiment, and soil was irrigated with treated wastewater that contained antibiotics.
When the plants had grown to harvestable size, plant material was used to feed earthworms, then the earthworms were evaluated to determine whether they had accumulated antibiotics. Ashworth found there was uptake by earthworms, so the question became “How can we leverage biochar to interrupt antibiotic transfer to agricultural systems?”
Biochar is a low-cost solution compared to other strategies, and it’s agriculturally relevant – many biochars are made from ag waste products. Ashworth said farmers could potentially produce waste that can be turned to biochar, which could then be used in a biochar filtration system.
In another study, Ashworth made small quantities of biochar for research trials. Nut shells, rice husks, dairy manure and grass clippings are among the materials effectively used for biochar – and which successfully reduced antibiotics in treated water.
One system that worked well was a sandwich-style arrangement in which a layer of biochar was placed between the influent and effluent and replaced as necessary. The sandwich method allowed the removal of the biochar layer in a filtration system, with the option of reheating the biochar for reuse.
While there were differences in removal efficiency among various feedstocks, Ashworth said increasing pyrolysis temperature improved removal efficiency. A highly effective biochar, which Ashworth refers to as “rogue biochar,” was comprised of 80% softwood, 15% hardwood and 5% nut shells treated with a high pyrolysis temperature.
In small-scale trials, Ashworth found that biochar effectively removed six commonly used antibiotic compounds from water. Compounds in the trial included sulfamethoxazole, trimethoprim, erythromycin, amoxicillin, cephalexin and tetracycline at 500 ppb.
“We got greater than 95% removal for all six antibiotics tested,” said Ashworth. “We aren’t sure why it did such a good job but we used it in several bench-scale studies. Treated wastewater influent was 500 ppb for the six antibiotics, and what came out was around 5% to 10% ppb.”
In another trial, Ashworth set up a greenhouse-scale system to filter PFAS, with treated wastewater held in a barrel connected to another barrel containing biochar layers. Rogue biochar was used because it was known to be effective. Ashworth said it would be worth trying other biochars in such a system.
“We’ve seen that antibiotics can disseminate through agricultural systems,” said Ashworth. “The main concern is the impact on the development of antibiotic resistance in food chains.”
There isn’t a clear correlation between biochar characteristics and antibiotic removal efficiency, but it appears that a higher pyrolysis temperature would improve efficiency. Ashworth believes there are yet unidentified biochars that may be highly effective; however, he isn’t sure why they are effective or how they would be identified.
“Certain biochars may be available that offer high removal efficiency but identifying them without conducting batch studies may be tricky,” said Ashworth. “We think that overall, using biochar for water filtration offers a potentially viable mitigation strategy for reducing the input of antibiotics into irrigated agriculture. We’re protecting the agricultural environment and human health.”
by Sally Colby
