The sewage works is next to a badger sett and the animals regularly forage for insects in the compound overnight.
Mrs Hockley-Byam said: “It’s not uncommon for badgers to fall into slurry pits, but this is a first for us.
“Most badger groups around the country have dealt with it one time or another – it’s a known hazard.”
The group’s volunteers have previously visited the sewage works to reunite a lost cub with its parents, but “that call-out was a lot less smelly”, she added.