Oct. 4, 1997 Cincinnati -- Researchers in the University of Cincinnati College of Engineering have shown that a modified filtration system along interstate highways can prevent heavy metals from polluting nearby water supplies.
The system, known as a partial exfiltration trench (PET), was designed and built by research assistant professor John Sansalone as part of his doctoral research in the department of civil and environmental engineering at UC. The PET replaces the normal sand used in highway storm drainage systems with an iron oxide coated sand. That makes the sand signficantly more effective at trapping heavy metals such as cadmium, copper, lead and zinc.
"Polluted water flows in, and clean water flows out," explained Steven Buchberger, associate professor of environmental engineering and Sansalone's thesis adviser.
Sansalone presented data from a year-long field test during the recent World Congress of the International Association for Hydraulic Research (IAHR) in San Francisco. A prototype PET system was installed along a stretch of Interstate 75 near downtown Cincinnati. It is the second busiest stretch of interstate highway in the state of Ohio.
The effectiveness of the PET system varied for each specific metal, but the overall trapping efficiencies ranged from 82 to 97 percent. The PET even holds up well during heavy rainstorms. The system can handle up to one inch of rain per hour. That's when Sansalone discovered a side benefit to his novel system.
"The PET not only works as a water quality device, but it can act like a water quantity control device to reduce surface flooding," said Sansalone. That discovery was completely unexpected in the Cincinnati area where clay soils are common.
Sansalone will continue his research by looking at ways to make the system more economical and efficient. It took ten tons of coated sand to treat 20 meters of highway during the field test, so Sansalone must find a consistent and simple method for producing huge quantities of coated sand.
"When we made the prototype, we made more coated sand than has ever been artifically produced on Earth. It was a real undertaking," said Sansalone.
He will also work on modifying the coating itself to increase its trapping efficiency and lifetime. The goal is to have a system which can last as long as the typical highway pavement about 15 years. Lab tests indicated Sansalone's coating could last approximately 40 years. The field tests indicated a much shorter life expectancy, but one very close to the final project goal.
"Conditions in the field are always more severe than you can simulate in the lab," said Sansalone. "Based on the results we've seen so far, a 10 to 15 year life is reasonable."
Even more important than lifespan is the ability to clean or recharge the PET system easily. In the next phase of the project, Sansalone will try to demonstrate that the trapped metals can be removed by a simple back-washing. That's important, because if you can't remove the toxic pollutants for disposal, you wind up with tons and tons of toxic waste.
Lab-scale experiments indicate the back-washing process is feasible. However, field tests are required to test the procedures on a large-scale operation.
Sansalone's research is funded by the Ohio Department of Transportation and the National Science Foundation. His presentation in San Francisco was recognized by the IAHR's John F. Kennedy Award for Hydraulic Research. The award is named for an engineering researcher who specialized in hydraulics.
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