1 June 2002
Ultrasound could clean water, eliminate chemicals
Columbus, Ohio—Using ultrasound to clean high-tech ceramic water filters will not only save money but also one day enable water treatment plants to purify water with ceramic membrane filters instead of harsh chemicals, said engineers at Ohio State University.
"If water treatment plants could clean water with membrane filters, they could minimize the cost, safety, and disposal issues associated with the use of harsh chemicals," said Harold Walker, who along with Linda Weavers, both associate professors of civil and environmental engineering and geodetic science, and doctoral students Dong Chen and Mikko Lamminen worked on the project.
As an alternative to chemicals, researchers are studying ceramic membrane filters—honeycomblike networks of tiny channels separated by thin ceramic films, or membranes. When water flows through the channels, the membranes act as sieves to catch contaminants such as clay, iron oxide, bacteria, and viruses.
The problem occurs when, over a period of time, membranes clog up with contaminants, and users have to clean them out.
Weavers and Walker took note of recent research involving ultrasound and bubbles. The idea, reported by scientists at Oak Ridge National Laboratory, is that sound waves can form and collapse bubbles inside a liquid, releasing heat and energy.
To test whether collapsing bubbles could clean a ceramic filter, the engineers submerged a filter in water that contained latex and silica particles.
They used particles in a range of sizes to mimic the contaminants found in water treatment.
They used an ultrasonic probe to vibrate the water at 20 kilohertz, or 20,000 vibrations per second—a low frequency easily obtained with typical ultrasound equipment. For example, the fetal ultrasound tests women undergo during pregnancy employ much higher frequencies—on the order of 10 megahertz, or 10 million vibrations per second.
But that doesn't mean a 20-kilohertz probe is less powerful, Weavers said. "Frequency has nothing to do with power. Think of it as bass sounds vs. soprano sounds. Both can be louder or softer. Whether a sound is bass or soprano depends on frequency, whereas loudness and softness depend on power," she said.
The 20-kilohertz vibrations caused bubbles to form and collapse and kept the ceramic filter clean.
"The bubbles seemed to scour the surface of the filter," Weavers said. "Where the bubbles collapsed, tiny water jets formed and flushed away the contaminants."
Though the engineers are still not certain exactly how the process works, Weavers said she suspects the jets sprang from vibrational nodes-locations along the surface of the filter where ultrasonic waves merge and magnify one another.
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