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Using Sound to Filter Bacterial Spores from Water

Acoustic trapping can remove bacterial spores from water, according to a new set of experiments funded by the U.S. Army.

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. But determining what kind of bacteria are present is a little more tricky because these need to be filtered and removed from the water before testing. And because filters quickly become clogged and useless, this requires human intervention on a regular basis.

So 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 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 a company called Physical Sciences, both in Massachusetts, have now perfected the trick for water-borne spores of bacillus cereus bacteria.

The technique captures some 15 percent of the spores passing through the acoustic trap in water travelling at between 40 and 250 ml per minute (ie 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 analysed using spectroscopy of some sort.

That seems potentially useful. However, the team 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. And it looks as if acoustic trapping may well have the potential to help them do it.

Ref: arxiv.org/abs/1006.5467: Separation Of Bacterial Spores From Flowing Water In Macro-Scale Cavities By Ultrasonic Standing Waves

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