Reed beds which turn a sludge by-product into drinking water have been created at an Essex reservoir.
Essex & Suffolk Water has built 16 reed beds covering 10 acres (four hectares) at Hanningfield Reservoir near Chelmsford.
The sludge is created during the drinking water treatment process and the reeds can break it down - providing water which goes back in the reservoir.
The water company said it replaced a mechanical process with a natural one.
Heidi Mottram, chief executive of Northumbrian Water, which owns Essex & Suffolk, said: "These are the first reed beds of their kind in the world and the first we've ever put into our drinking water treatment process.
Essex & Suffolk said any remaining sludge, which can be sold as agricultural fertiliser, would not need to be be cleared from the reed beds for 20 years.