We have the 2nd largest (soon to be the largest) sewage works in Europe. The only bigger one is in Paris I think, or somewhere in France anyway.
I know it sounds disgusting, but it's actually quite fascinating , and having been round it I found it amazing. The purification process is incredible, the end result is pure clean water, and I was totally amazed to be confronted by this block of concrete that looked and felt like concrete but was quite literally made from....you know what
Google Beckton sewage treatment works - it's in London and is the size of a small town!!