Emasesa is combining the oranges with other waste products, including sewage and cast-offs from olive factories and abattoirs, to produce biogas, which is then used to power Seville’s water purification plants.

The gas can be burnt to generate electricity and southern Spain’s largest purification plant is now almost entirely self-sufficient in energy.

In an added bonus, the project will help reduce the millions of euros Seville spends every year disposing of oranges after they fall to the pavement.

“The city has a problem with oranges,’’ Benigno López, the head of Emasesa’s environmental department, told The Times. “They’re bitter oranges that can’t be eaten. We put together the problem the city had with the solution we had devised to become self-sufficient.’’

Mr López is drawing up plans to use the entire 450,000-tonne harvest to generate electricity for 73,000 homes. The biogas can also be used as fuel for the city’s fleet of buses.