Ashes to ashes, mould to mould – New Scientist

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By Andy Coghlan
16 November 1996
A FUNGUS which usually makes citric acid for the food industry could find a new job cleaning up the metal-choked ash left over from incinerating municipal waste. Ash decontaminated by Aspergillus niger could be turned into building materials, and the metals could be recycled.
Fly ash from incinerators is usually buried in landfill sites because of contamination with metals such as lead, nickel and cadmium. The ash is first mixed with materials such as concrete to stop the metals seeping into rivers and streams. But the cost of landfill is rising and the search for alternatives is intensifying.
In experiments at the University of Zurich, Helmut Brandl and his colleagues in the Institute of Plant Biology have shown that Aspergillus niger can tease out most of the toxic metals in incinerator ash. “The fungus produces citric acid, which leaches metals out of the waste,” says Brandl.
It took the fungus a week to extract between 80 and 100 per cent of the lead, cadmium, manganese, zinc and copper from the ash. Typically, half the metal was removed in a day. Fluid containing the dissolved contaminants was drawn off and treated to extract the metals. The inert silicates and aluminium ores left behind in the ash could be turned into building materials such as paving slabs.
One disadvantage is the cost of adding glucose, a sugar that the fungus needs for survival. “It’s relatively expensive, so we’re seeing whether the fungus can live on cheaper materials such as wastes from the food industry,” says Brandl.
He was also disappointed by the low amounts of nickel leached out by the fungus, typically less than one fifth of the amount in the ash. But he is confident that it will be possible to get almost all the metal out by treating the ash with blends of different fungi, or with bacteria. Already, he has had some success treating the ash with Thiobacillus ferroxidans bacteria.
The next step is to reconfigure the laboratory rig so that the fly ash and fungus are easier to separate from one another. Within three years Brandl hopes to have a small demonstration plant running.
His work with the fungus is one of 14 projects in Switzerland to develop alternative ways of treating fly ash. A ban on the export of fly ash from Switzerland comes into force in 2000.
Tony Dean, a director at the waste treatment firm Cleanaway in Ellesmere Port, Cheshire, says that the approach is interesting. “It’s almost unbelievable they could get half the metal out in a day,” he says. But he warns that it will remain impractical in countries such as Britain, where disposal to landfill is still reasonably cheap.
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