While many us keep an eye on how much fat we eat, we give little thought to what happens to waste cooking oil and fat - especially when it's poured down the drain. Deep below London's streets, the results are all too clear.

Warm leftover fat slips easily down the plughole, but it then cools and solidifies, causing so-called fatbergs. These clog up the city sewers.

Now waste oil and fat is to be used as renewable energy to power a new electricity plant, making life easier for flushers like Rob Smith. He explained that all households dispose of some fat down the drain when washing up, but much more comes from restaurants and goes into the sewers. The team of flushers travel through the sewers to check whether the fatbergs have been flushed naturally through the sewer system, or if they are causing blockages. If they are causing blockages, they need to be flushed out, as Smith explained:

"Flushing out means that we have to pump the down stream down to get the level down below the pipe levels so that we've got a clear avenue for the fat to go through because fat will just float on the top of the sewage."

Thames Water supplies water and wastewater services to London and the Thames Valley region, serving around 9 million customers. The company has to deal with 80,000 blockages a year in the sewer system, and it says half of those are contributed to by fat. It costs them £10 million a year to clear all blockages.

In future, the company wants to collect so-called FOGs - fat, oils and grease, directly from fat traps, installed under the sinks of 10,000 restaurants in central London.

That fat will power the world's biggest fat-fuelled power station. It will be built at Beckton, in east London, next to the Beckton sewage works, which is the biggest sewage treatment works in Europe.

Simon Evans, Media Relations Manager at Thames Water said collecting the fat directly from restaurants should prevent so many fatbergs clogging up the system:

"We'll take that fat off to what will be the biggest fat-fuelled power station in the world. That will be right next to our Beckton sewage works and that will, all the electricity that produces will run the plant and everything that we don't use will be pushed out to the national energy supply grid and it will go to run homes and business in the area."

Developer 2OC has to get the power plant up and running by 1st April 2015, to qualify for full UK government subsidies.

Thames Water has agreed to provide half the fuel for the power station in the form of fat. They will also purchase 60% of the power that the plant puts out.

The project is expected to be closely watched by other major cities keen to slim down their fat deposits.

Presented by Adam Justice