This was one of those happenstance visits when I had a walk to take some photos in the area, and found the gate open! So, taking the opportunity to have a look around, I met the operative who was working inside the plant. He very kindly gave me a quick tour and explained the workings. The visit was in June 2009.
In the photo below, I was shown the outlet for the treated water.
Edit: Whilst copying my pages during the changeover to a new website builder I discovered this page unfinished. So I've made a note of it and will be adding more information and photos as soon as I can (along with a huge backlog of other pages to bring up to date).
There are a few more photos in the Photo Gallery album. Unfortunately it was an extremely hot and bright middle of the day, and most of them came out badly, so I haven't included them all. There are a few atmospheric ones from the perimeter which I took during different visits, so l might include them later when I finish the page.