The method uses biogas, which is produced when waste water sludge is broken down by microbes in a process known as anaerobic digestion.

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Prosecutors said the company also illegally stored and disposed of hazardous waste without a permit, instructing workers to mix coal tar sludge, a listed hazardous waste that contains benzene, on the ground.

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This is because the waste consists of barrels of sludge containing compounds such as nitrides, oxides and water that are not, themselves, radioactive.

ECONOMIST: Nuclear waste

The company has said the incident was a "natural catastrophe" and insists on its website that the red sludge is not considered toxic waste according to European Union safety standards.

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