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: