Clever compost clears pollution

By BBC News Online's Damian Carrington

A hi-tech composting technique in which natural soil bacteria and fungi are "turbo-charged" by chicken droppings is now clearing polluted US sites of DDT and other toxic pesticides.

Dr Neil Gray, of AstraZeneca in Ontario, Canada, said that digging up the polluted soil and creating carefully-controlled compost heaps allows soil bacteria to munch their way through a whole range of dangerous chemicals.

The technique received patents this week, and because it involves adding about 20% organic material, it can transform wastelands into well fertilised fields in just six weeks.

A large-scale trial in Tampa, Florida, has already been successfully cleaned up , with 90% of the DDT being broken down to water, carbon dioxide and salts. The test processed 16,000 cubic metres of soil contaminated with 31 different chemicals.

Dr Gray said that, at $200 per tonne, the method cost just a third of low temperature heat treatments, which evaporates the pesticides into activated carbon, and a fifth of incineration.

Crafty composting

Speaking at the British Association¿s Festival of Science, he added that some other methods of treating DDT still left toxic breakdown products.

The key to the method is optimising conditions so that all the different sorts of naturally-occurring bacteria can take a turn at attacking DDT, toxaphene, chlordane and others.

First the soil is examined to find out its temperature, nutrient level, acidity and moisture and oxygen content. These factors control which bacteria grow.

Then, for example, manure or grass is mixed into the excavated soil using giant machines. The organic material feeds the aerobic bacteria. These can drive up the temperature to 50 C and will use up all the air. Then the anaerobic bacteria start their work.

These cycles can be repeated and varied to optimise the destruction of the particular suite of pollutants found in each site.

The US banned the production of DDT in 1973 but it is still in widespread use around the world, killing the mosquitoes that carry malaria.