For decades sewage sludge from waste treatment works has been used as a fertiliser on agricultural land. But File on 4 hears serious concerns over whether it could pose a risk to human health and whether tougher regulation is needed. The practice is perfectly legal. Treated sewage known as 'sludge' or 'biosolids' provides a rich and cost-effective source of nutrients for soil which is then used to grow crops. The process saves more than three and a half million tonnes of human waste going into landfill or being incinerated. But reporter Claire Bolderson hears from scientists worried about the chemicals, plastics and medicines that could be damaging soil and making their way into the food chain. And she investigates the process of regulating the treatment, storage and use of sludge, amid claims from experts that rules are outdated and oversight lacking. Recycling sewage as fertiliser fits today’s environmental agenda for waste. But do we know enough about what the potential impact of the practice might be in the future? Reporter: Claire Bolderson Producer: Ben Robinson Editor: Carl Johnston