By Eleanor Williams
BBC News, Beckton, London
The "poo cakes" have a high calorific content and burn easily
The UK is firmly in the grip of one of the coldest winters on record and many households are seeing their fuel bills soar.
Although the government has dismissed fears over dwindling gas supplies during the big freeze, the weather highlights the desirability of finding more "green" fuel.
One water firm thinks it may have the answer - and it is not wind or hydro power. Thames Water has successfully been using human toilet waste to make electricity for the past decade.
Long before you get to Beckton sewage treatment plant in east London the smell tells you it is near.
Not strange considering it is the biggest sewage plant in Europe and treats a large majority of the waste coming from London's toilets.
Human waste has long been seen as a by-product, but Thames Water claims it saved £15m last year, and generated 14% of its power, from either burning sludge or methane derived from its 13 million customers' toilets.
In total the firm treats 2.8 billion litres of sewage every day from people in London, Berkshire, Oxfordshire, Hertfordshire and Surrey.
Harjeet Singh is the plant manager for the sludge-powered generator at Beckton.
Before it, and its sister plant Crossness on the other side of the River Thames, was built in 1998 at the cost of £165m, the sludge was simply deposited in the sea.
"It was taken on big barges out into the North Sea and dumped," Mr Singh explained.
"A tanker was leaving every two minutes. Now we only have about four lorries of ash [the only by-product] a day."
The solid sludge is emptied out of the compressor and fed into a hot furnace
Mr Singh describes the process which every day turns tons of human waste into electricity as "totally green".
The raw sewage comes into the plant and is filtered, with anything that is not excrement or toilet paper removed, leaving a mix of 95% water and 5% solids which is then pumped into industrial 50m (165ft) compressors where the water is squeezed out, leaving solid "poo cakes".
These cakes, which still contain a large amount of water but burn easily, are fed into a gigantic hot furnace which produces steam that drives a large turbine, creating electricity.
"In the same way our domestic boilers work," Mr Singh explained
The electricity generated has the capacity to power 10,000 homes but here it is fed back into Beckton's own system to keep the sewage plant running.
Fat for lipsticks
Mr Singh said: "A computer monitors emissions so we don't emit any nasties.
"That's not happened in the past 12 years, we've not exceeded [limits] since we opened.
"At the end of the process we're left with about 30% ash."
Ash, the only by-product, could be used to make concrete
He said the firm was looking into ways of using the ash in the future.
"We don't want it to go to landfill and one thing we are looking at making with it is concrete", he said, showing a small test block he "made earlier".
He said they were also toying with the idea of using the fat in the toilet sludge to make lipsticks and other cosmetics.
However a more likely use may be as fertiliser for farmers and gardeners.
"[The sludge] contains lots of nutrients and has a high calorific content.
"But you can see a difference during the school holidays and Christmas; there's less waste coming into the plant because people are away from the city.
"There is also a difference in the summer holidays in the calorific value of the sludge; it's lower because people eat more salads."