There's definitely still a sewage treatment plant near the site of the accident:

https://www.google.com/maps/place/Sheerness,+UK/@51.5112905,...

https://en.wikipedia.org/wiki/Beckton#Sewage_works

But you can't discount the difference between the dictionary definition of "sewage" versus the reality of incidental pollution as discovered in-situ.

Case in point:

https://en.wikipedia.org/wiki/Gowanus_Canal

http://nymag.com/daily/intelligencer/2010/03/post_25.html

https://www.timeout.com/newyork/blog/ten-feet-of-black-mayon...

https://ny.curbed.com/2018/7/19/17590126/brooklyn-gowanus-dr...

http://nautil.us/blog/in-the-black-mayonnaise-of-brooklyns-g...

https://www.brownstoner.com/development/gowanus-canal-black-...

https://www.dnainfo.com/new-york/20161123/gowanus/gowanus-ca...

Also:

https://en.wikipedia.org/wiki/Newtown_Creek