LONDON, Dec 7 (AFP) - Britain's biggest water company lifted the lid Monday on how it slashed costs by burning customers' faeces to make electricity.

Thames Water said "poo power" saved them 15 million pounds (24.6 million dollars, 16.6 million euros) last year by generating renewable energy from the foulest of sources.

The company, which handles the waste of 13.6 million people in London and the River Thames valley, said it had met 14 percent of its energy needs by either burning sewage or methane derived from it in 2008-2009.

Energy is generated through either "thermal destruction", where dried blocks of "poo cake" are burned, or "anaerobic digestion", where the methane from sewage sludge is burned.

"There's no polite way of saying this but what we produce -- our poo -- isn't simply waste, it's a great source of energy," said Keith Colquhoun, Thames Water's climate change strategy manager.

"That's good news because we treat 2.8 billion litres of sewage every day at our 349 sewage works. The solids in sewage have a high calorific content that we use to generate electricity."

Colquhoun said everyone had to muck in and urged delegates at the UN climate talks in Copenhagen to produce more than just hot air.

"It's about all of us taking action -- and in our case, that includes poo power," he said.