'Oslo's citizens are helping to warm their homes and offices simply by flushing the toilet'

Large blue machines at the end of a 300-metre long tunnel in a hillside in central Oslo use fridge technology to suck heat from the sewer and transfer it to a network of hot water pipes feeding thousands of radiators and taps around the city.

"We believe this is the biggest heating system in the world using raw sewage," Lars-Anders Loervik, managing director of Oslo energy company Viken Fjernvarme which runs the plant, told Reuters. The plant opened this week.

The heat pump, a system of compressors and condensers, cost 90 million Norwegian crowns ($13.95 million) and has an effect of 18 megawatts (MW), enough to heat 9,000 flats or save burning 6,000 tonnes of oil a year.

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In Oslo, a problem is that the flow in the sewers is irregular -- Monday mornings between 4-6 a.m. are especially dry because people go to bed early on Sunday. But at weekends, the flow is good.

"When people have been out to parties there's a lot of beer going into the sewer," said Oyvind Nilsen, the project manager for the Oslo plant.

At the opening ceremony for the plant, Oslo mayor Per Ditlev-Simonsen was given a new toilet seat for his office. "It will be an inspiration," he said.

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