Due to a number of factors, like lower water temperatures, less turbulence in the pipes and chemical reactions, fats oil and grease (FOG) hardens when it reaches the sewer system and builds up on the insides of the pipes. The compound (or “soap compound”) that builds up inside the pipes leads to an ever decreasing diameter for the water to flow by; until it finally completely blocks the passage.
The blockages, which are time consuming and expensive to clear, can lead to flooding and corrosion of the pipes. Accumulated FOG also causes bad smells which attracts rats.
Here you can see a video from Yorkshire Water showing the flooding problem affecting households and Real Estate companies:
And here one of the rats that benefit from this problem (click the picture to see a video)
In the summer of 2013, a fatberg the size of a bus was found in the London sewer system. The blockage took weeks to clear. http://www.thameswater.co.uk/media/press-releases/17205.htm