A large shell-shaped structure is sucking tonnes of rubbish out of a river for free.

The Inner Harbour Water Wheel uses solar power to run a water wheel, into which garbage from the surrounding harbour is funnelled.

A raking system shoves it through the structure’s conveyor belt, where floating garbage is separated from the water and deposited into a dumpster barge.

The remarkable structure is the brainchild of US engineer John Kellett, who has spent over a decade designing similar systems for low-energy water-powered refuse infiltrators.

The wheel has been in place in the city of Baltimore for several months.

From May 16 to June 16 this year, the wheel pulled out 46 tons of garbage from the harbour, which city officials said was a major contribution to efforts to prevent plastic and debris from reaching the Atlantic Ocean.

Officials are also pleased with the effectiveness of the garbage-munching nautilus.

They say it collects around 95 per cent of garbage, which would otherwise have to be picked up manually.

Baltimore city officials hope the device will make Inner Harbour a swimmable body of water in under six years’ time.

More details are available in the informational graphic below, or the video below that.