Ultrasound cleans ceramic filters

Engineers at Ohio State University have discovered how to clean high-tech ceramic water filters at low cost with ultrasound.

Though early in its development, this technology may one day enable water treatment plants to purify water with ceramic membrane filters instead of harsh chemicals.

‘If water treatment plants could clean water with membrane filters, they could minimise the cost, safety and disposal issues associated with the use of harsh chemicals,’ said Harold Walker, an assistant professor of civil and environmental engineering and geodetic science at Ohio State.

As an alternative to chemicals, researchers are studying ceramic membrane filters, which are honeycomb-like 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.

Over time, however, the membranes become clogged with contaminants and must be cleaned.

Linda Weavers, herself an assistant professor of civil and environmental engineering and geodetic science, and Walker took note of recent research involving ultrasound and bubbles. The idea, most recently 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 that is easily obtainable with typical ultrasound equipment. The foetal ultrasound tests that 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 explained.

‘Frequency has nothing to do with power. Think of it as bass sounds versus 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 for 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 suspects that the jets sprang from vibrational nodes, which are locations along the surface of the filter where ultrasonic waves merge together and magnify each other.

With ultrasound as a cleaning method, water treatment plants wouldn’t have to remove filters from use to clean them, Walker said. Loosened contaminants would wash away in an exhaust flow separate from the clean water.

‘If you left the ultrasound running, you could clean a filter while it was still in use, and keep it from ever getting clogged in the first place,’ Weavers added.