Sound cleans up water purification

High-energy bubbles scour municipal water filters.

Sound waves could provide a greener way to make tap water taste better.

Ultrasound can make bubbles in water that could clean ceramic filters quickly and cheaply, say Linda Weavers and colleagues of Ohio State University, Columbus¹. When the bubbles burst, they release energy that makes tiny, but fiercely powerful, jets of water that scour the filter’s surface and flush away debris.

Most municipal water treatment relies on slow, environmentally risky, chemical purification. A greener, more efficient alternative is to use membrane filters. These have pores so minuscule that they sieve out particles and microorganisms as small as viruses; but they get clogged easily. Fouled filters must be removed and either scrubbed or replaced, slowing the process and raising its cost.

Ultrasound cleaning could make membrane filtration the rule rather than the exception, hopes the Ohio team. “If you left the ultrasound running you could clean the filter while it is still in use, and keep it from ever getting clogged in the first place,” says Weavers, who believes that, scaled up, the process could replace chemical water purification.

Philip Brandhuber of McGuire Environmental Consultants in Denver, Colorado, who specializes in membrane filtration of drinking water, agrees, saying he is pleased to see this “completely new approach to the fouling problem”.

One potential problem is that the bursting bubbles could damage the filters. Ceramic filters are hard-wearing and heat resistant. But ultrasound cleaning could degrade the cheaper polymer membranes that are more widely used, points out Menachem Elimelech an environmental engineer at Yale University.

Others think that the benefits of a switch to ceramic filters would justify the costs. Tom Allsop, Water Department superintendent for Pinesdale, Montana – whose plants currently use sand and paper filters – says he’d jump at the chance to install an ultrasound-ceramic combination. “If it works as well as they claim, it would save us enormous amounts of money each year,” says Allsop.

Brandhuber doubts that Allsop’s enthusiasm is representative of the drinking-water industry as a whole. The industry is inherently conservative, Brandhuber says, being concerned with public health and publicly funded. Before municipal plants are likely to invest, the technology must undergo much more rigorous investigation, he adds.

Weavers’ group intends to do just that. They are planning more tests to determine how well bubbles remove different contaminants and how different types of filter withstand the sound-induced scouring.

References
Chen, D., Weavers, L. K. & Walker, H. W. Using ultrasound to reduce ceramic membrane fouling by silica particles. Presented at the 2002 National Meeting of the American Chemical Society, Orlando, USA. (2002).