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Study Shows Low Concentrations of Silver Nanoparticles Can Foil Wastewater Treatment

A study conducted at Oregon State University has given new insight on how a progressively common consumer product component - silver nanoparticles - can potentially hamper the treatment of wastewater.

According to the study’s findings, traditional toxicity testing techniques for silver concentrations at treatment plants may deliver results that produce a false sense of security.

The research is crucial because if silver, which possesses broad-spectrum antibacterial properties, impedes the work of the plants’ useful bacteria, then a lot of nutrients wind up in waterways.

That, in turn, can result in eutrophication: An overabundance of nutrients in a water body that leads to an explosion of vegetation, such as an algae bloom, and a squeezing out of animal life because of inadequate oxygen.

Silver nanoparticles are being incorporated into a range of products including wound dressings, clothing, water filters, toothpaste and even children’s toys,” said corresponding author Tyler Radniecki, an environmental engineering assistant professor at OSU. “The nanoparticles can end up in wastewater streams through washing or just regular use of the product.”

The research by Radniecki and collaborators in the College of Engineering examined silver nanoparticles, the ionic silver they discharge and an ammonia-oxidizing bacterium, Nitrosomonas europaea.

Ammonia-oxidizing bacteria (AOB) are essential because they change ammonia to nitrite to start the process of obtaining one of those nutrients, nitrogen, out of the wastewater. The research examined both free-floating or planktonic, N. europaea and also the biofilms they form.

The OSU study verified former observations that biofilms are better able than planktonic bacteria to protect against silver’s effects.

Biofilms showed higher resistance for multiple factors. One was simply more mass of cells, and the top layer of cells acted like a sacrificial shield that allowed the bacteria below not to be inhibited. Slow growth rates were also a protection from silver toxicity because the enzymes that silver prevents from turning over aren’t turning over as frequently.

Tyler Radniecki​, Corresponding Author

More notably, the study exposed a new wrinkle: That the obstruction of AOB’s ammonia-conversion capacity is more a function of silver exposure time than the level of silver concentration.

Most of the studies investigating the inhibition of wastewater biofilms by nanoparticles have been conducted in short-term exposure scenarios, less than 12 hours,” Radniecki said. “Also, they’ve used an equal amount of time for hydraulic residence and sludge retention.”

The difficulty with that, he explains, is that in a treatment plant that utilizes biofilms, the sludge retention time—the time the bacteria are in the plant—will be a lot greater than the hydraulic residence time, i.e. the time the wastewater is in the plant.

That allows, over time, for the accumulation and concentration of metal contaminants, including ionic silver and silver nanoparticles. The immobilized biofilm cells are exposed to a much greater volume of water and mass of contaminants than the planktonic cell systems. What that means is, the results of short-term exposure studies may fail to incorporate the expected accumulation of silver within the biofilm; wastewater plant monitors might be underestimating the potential toxicity of long-term, low-concentration exposure situations.

Tyler Radniecki​, Corresponding Author

The National Science Foundation supported this study, and findings were reported in Chemosphere.

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