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News

Using biosensor bugs to spot heavy metals in sewage sludge fertiliser

Society For General Microbiology : 06 April, 2005  (Company News)
Sewage sludge is a cheap fertiliser, and spreading it on forest floors should be a good way to dispose of it. But sewage can contain metals like copper or lead that can be dangerous at high concentrations, so it needs a safety check first. Researchers think they have finally found a cheap way to test its toxicity, scientists will hear at the Society for General Microbiology's 156th Meeting at Heriot-Watt University, Edinburgh.
Sewage sludge is a cheap fertiliser, and spreading it on forest floors should be a good way to dispose of it. But sewage can contain metals like copper or lead that can be dangerous at high concentrations, so it needs a safety check first. Researchers think they have finally found a cheap way to test its toxicity, scientists will hear at the Society for General Microbiology's 156th Meeting at Heriot-Watt University, Edinburgh.

'You need somewhere to dispose of sewage and people aren't too happy about it being used on crops that we might eat, but it's a pity to waste it as it can be a good fertiliser, so forests should be ideal. The problem is that you cannot dig sewage sludge into the soil in a forest without causing extensive damage to the tree roots,' explains Dr Jacqui Horswell from the Institute of Environmental Science and Research in Porirua, New Zealand, who is leading the research.

'But if you just spread sewage waste on the forest floor, you might get a build up of heavy metals and other toxic elements in the surface leaf litter layer,' says Dr Horswell. 'That's because sewage contains just about every element and compound found in domestic and industrial waste.'

Leaf litter on forest floors contain masses of fungi and bacteria, all living on and breaking down the organic matter and releasing nutrients back into the soil. The scientists used bacterial and fungal 'biosensors' developed by Aberdeen University to test for toxic metals in sewage sludge when it was spread under pine trees.

'The Aberdeen biosensors act like microscopic canaries for coal mines. The special bacteria and fungi contain a light-producing gene. The light gets switched off and they don't glow as much when they are affected by poisonous chemicals. Putting it simply they only glow when they are happy!' says Dr Horswell. 'We added high levels of copper, zinc and nickel to test what might happen if sewage sludge was spread in forests for many years, and showed that the biosensors could accurately pick up toxic levels of heavy metals.'

The researchers found that after as little as 20 years application of sewage, heavy metals can build up to poisonous levels in forest soils. The information will be used to guide policy decisions on disposing of sewage sludge on land, and safeguard the life-supporting capacity of soils.

'Our next job is to look at the way heavy metals from sewage sludge affect the rest of the ecosystem - insects and small animals as well as plants and bacteria,' says Dr Horswell. 'We also need to look at what happens when the forest is cut down, and the land-use changed - perhaps to grow food crops. Will the metals get into our food? Will this be a risk to human health and limit future land use?'
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