Some 30 years ago, as the United States began to tighten its environmental regulations on residential and industrial wastewater, operators of sewage-treatment plants embraced what seemed an eminently sensible idea. They decided to take the rich organic sludge left over after clean water is extracted and sell it to farmers as fertilizer. The programme might well be as sensible as it seems. It is possible that the millions of tonnes of sludge being spread across the rural landscape contain no significant levels of toxic chemicals, heavy metals or disease-causing organisms.