September 2011, Vol. 23, No.9

Research Notes

Silver nanoparticles don’t stand a chance in wastewater treatment

A team of Swiss researchers broke out the microscopes — electron microscopes, that is — to measure how silver nanoparticles behave through wastewater treatment. Their research, published in April, is featured in the journal Environmental Science and Technology.

“The majority of [silver nanoparticles] in consumer products will be released into sewer systems and thus reach wastewater treatment plants,” according to the study.

The antibacterial properties of silver have led to its development on a nanoscale as a biocide in products such as paints and textiles. Its release into the environment, the study says, is unavoidable.

However, “there is only a slim chance that the [silver nanoparticles] survive the wastewater treatment,” said the study’s lead author, Ralf Kaegi.

The researchers used a pilot wastewater treatment plant and municipal wastewater to observe how the metallic silver nanoparticles would behave through the treatment process. They found that the particles were almost completely transformed into silver sulfide, a very insoluble material, and removed along with the wastewater biosolids. They found that this transformation to silver sulfide occurred during denitrification.

“The data we have collected so far indicate that the [silver nanoparticles] are very efficiently removed during wastewater treatments,” Kaegi said.

This is a positive finding, as silver and its nanoparticles can be toxic. The study also noted that due to the insolubility of silver sulfide, it is much less toxic than other forms of the metal.

Scientists in California and Thailand team up to improve gasification process

The concept of waste-to-fuel is centuries old, but a new collaboration between research groups on opposite ends of the globe looks to expand the reach of a promising technology.

In May, researchers from the Bourns College of Engineering at the University of California, Riverside (UC-Riverside) gathered with representatives from the Thailand Institute of Scientific and Technological Research (TISTR; Pathum Thani, Thailand) to sign a memorandum of understanding for a collaboration involving steam hydrogasification technology, a waste-to-fuel process developed at UC-Riverside (UCR).

The agreement will allow TISTR to install a gasifier in its new laboratory, said Joe Norbeck, who leads the UCR group that developed the technology.

The U.S. Department of Energy rated steam hydrogasification as the least capital-intensive and most energy-efficient among similar technologies. The presence of steam during hydrogasification enhances methane formation. The overall process efficiency is estimated to be 62%, according to Norbeck.

The steam process also is innovative in that while existing technologies require dried feedstocks, steam hydrogasification can handle wet feedstocks.

“We have a very large program funded by the California Energy Commission that will use wet biosolids from a wastewater facility and mixed with biomass to make either electricity or transportation fuels,” Norbeck said, adding that TISTR researchers will aid in this project.

According to Norbeck, the technology is nearly ready to move up to pilot-scale testing. The collaboration with TISTR will help advance the technology toward commercialization.

“This specific collaboration will provide a means for graduate and personnel training on this technology, [and] explore new potential feedstocks and new fuels for both Thailand and Asia,” Norbeck said. “It’s really an exciting project because Thailand is showcasing the UCR technology. It’s a showcase for all of Asia.”

Improving performance of permeable reactive biobarriers

Researchers from North Dakota State University evaluated the use of an enricher reactor (ER) to maintain performance of a permeable reactive biobarrier (PRBB) system. ERs are off-line reactors used to enrich contaminant degraders by supplying necessary growth materials, the study says.

The team studied the operation of the combined ER–PRBB system to treat groundwater contaminants that appear intermittently. For the study, benzene was intermittently dosed into the system. Benzene, one of the major aromatic compounds in petroleum hydrocarbons, is a known carcinogen, the study says. The U.S. Environmental Protection Agency established that a maximum level of 5 mg/L of benzene is allowed in drinking water. 

They conducted experiments measuring PRBB’s effectiveness with and without bacterial supply from the ER. The experiments included benzene absence periods of 10 and 25 days. Two different scenarios during benzene absence were considered, one scenario without a carbon source and the other with ethanol as a carbon source. Ethanol was used because it is a common additive to gasoline and commonly is found with benzene in contaminated groundwater.

Performance of the ER–PRBB system proved better than performance of the PRBB without bioaugmentation. Performance without bioaugmentation decreased from approximately 65% to 30% after benzene reappeared.

The study also found that longer absences of benzene resulted in greater performance losses upon reappearance, but that ER–PRBB recovered more quickly than PRBB alone.

The study, “Groundwater Remediation Using an Enricher Reactor — Permeable Reactive Biobarrier for Periodically Absent Contaminants,” was published in the July issue of the Water Environment Federation (Alexandria, Va.) research journal, Water Environment Research. The article can be downloaded free at


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