Magnetic bugs cut sewage sludge
Adding iron dust to wastewater makes bacteria reusable.
Magnetic bacteria could cut sewage sludge. They could shave up to five tonnes of waste off the several hundred tonnes produced every day by a plant serving 100,000 people, say Japanese researchers.
Bacteria used to break down some harmful pollutants in waste water add to leftover sludge. Much of this ends up buried in landfills.
But microbes will cling to powdered iron sprinkled into the brew, find Yasuzo Sakai, of Utsunomiya University, and his colleagues. Magnets can then drag them out, reducing the sludge volume and enabling a plant to re-use the bugs.
A year-long test, in which 80 litres of raw sewage were passed through a rotating magnetic drum every day, produced no bacterial sludge, Sakai told this week's meeting of the American Chemical Society in New Orleans. "Magnetic separation is fast and reliable," he says.
Bacteria can be strained out or allowed to settle. Both approaches have disadvantages, says microbiologist Tom Curtis of the University of Newcastle upon Tyne, UK. "Gravity is cheap but not very effective; membranes are effective but expensive."
Magnetic separation "is ingenious, and it could find favour", he adds, but it's too early to say whether the running costs will make the technique competitive.
And there's no guarantee that Sakai's results will be repeated on a larger scale, warns chemical engineer Tom Stephenson of Cranfield University, UK. "I can imagine that a scale-up would be difficult and costly," he says.
The researchers have now teamed up with water-treatment companies and local government to build a pilot plant that will treat 30 tonnes of sewage per day.
3/03 Nature Science Update
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