The water we drinkKeeping water clean by using sound to filter bacterial spores

Published 7 July 2010

Acoustic trapping can remove bacterial spores from water, according to a new set of experiments funded by the U.S. Army; the idea is to allow the water to flow through a cavity in which a transducer sets up an acoustic standing wave

Measuring the quality of water on an ongoing basis is an important task but not one that is always straightforward. Taking samples for chemical analysis is simple enough and can be relatively easily automated. Determining what kind of bacteria are present, however, is a little more tricky because these need to be filtered and removed from the water before testing. Because filters quickly become clogged and useless, this requires human intervention on a regular basis.

Technology Review reports that the U.S. Army is funding a project to determine whether sound can help. The idea is to allow the water to flow through a cavity in which a transducer sets up an acoustic standing wave. Any bacterial spores in the water are then subjected to three forces: buoyancy/gravity, the drag of the fluid as it flows along, and the acoustic pressure from the standing wave.

Having previously worked out how to balance these forces to trap micro-sized polystyrene beads, a group from Western New England College and Andover, Massachusetts-based Physical Sciences Inc have now perfected the trick for water-born spores of bacillus cereus bacteria.

The technique captures some 15 percent of the spores passing through the acoustic trap in water traveling at between 40 and 250 ml per minute (that is, very slowly). The trap then needs to be sealed to stop the fluid flow so that the spores fall under gravity into a collection chamber below. “The acoustocollector is ideally suited for large-volume sampling of water supplies for concentration of spores,” says the team.

The spores can then be analyzed using spectroscopy of some sort.

This seems potentially useful. The team, however, will need to test the device using water from a realistic source which is bound to contain all kinds of gunk in addition to the spores of interest. Whether the spores can be separated in these circumstances remains to be seen.

The U.S. Army clearly has an interest in being able to monitor its own and other people’s water supplies at low cost and remotely if necessary, TR notes. It looks as if acoustic trapping may well have the potential to help them do it.

—Read more in B. Lipkens et al., “Separation of bacterial spores from flowing water in macro-scale cavities by ultrasonic standing waves,” arXiv 1006.5467v1 [physics.flu-dyn] (28 June 2010)