"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 (US$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.

And experts say sewers could be exploited elsewhere.

"The technology is there, so if the infrastructure is also there, this is a feasible solution in many cities worldwide," said Monica Axell, head of the International Energy Agency's heat pump centre. The agency advises 26 industrialised nations.

She said a bigger heat pump in Sweden, with a 160 MW capacity, exploited heat from treated sewage. And in Finland, a 90 MW plant ran on waste water.

In Oslo, untreated sewer flows -- from toilets, bathtubs, sinks and rainwater from the streets -- runs into the system past a filter that keeps out big objects such as dead rats.

Sewage was flowing into the system at 9.6 Celsius (49.28 Fahrenheit) on Friday and coming out at 5.7 Celsius after heat is extracted with a refrigerant.

The energy in turn goes to warming the water in the 400 km (250 mile) pipe system, fed to offices and homes, to about 90 C from a temperature of 52 C when it reaches the sewerage plant. Other plants, burning industrial waste, also heat the water.

FUEL PRICE

Similar heat pumps can be run on any stable source of water -- in Paris the Seine River is tapped to run air-conditioning systems. Sea water can also be exploited.

Sewer power is less polluting than burning fossil fuels but more than renewable energy like wind power. About a third of the heat energy comes from electricity to drive the system and the other two-thirds is the heat from the sewer.

"Oil prices have an impact on investment willingness, but more important is the ratio of fuel price and electricity price," Axell said. "A high fuel price and a low electricity price is a strong driver to invest in heat pump technology."

Among other sewage energy projects worldwide, US scientists are looking to exploit sewage-eating bacteria to generate electricity.

"The microbial fuel cell work is going well, but we still are not out of the lab on this technology," said Bruce Logan of Pennsylvania State University.

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.