Sienna Varela, eight, was violently ill after falling into the tank at a pub beer garden. It is believed vandals had removed the lid.
The accident happened while the family were in the River and Rail beer garden in Hampton Loade, near Bridgnorth.
The pub said that area of the garden containing the manhole was “clearly” not open to the public, and that vandals had been responsible for removing the cover.
Paule Varela, Sienna’s father was alerted after hearing his eldest daughter scream for help.
Mr Varela said: “It was a complete shock as it’s quite a drop and she was absolutely covered.
"I believe she went completely under the surface as she had sewage in her eyes and mouth.
"I quickly flung myself onto the ground and leant forward as far as I could, I had to crawl into the tank and was just able to reach Sienna with my finger tips and drag her out.
“There were two manhole covers in the garden. One was covered by a grate and the other was just left open. As you can imagine my family were really distressed and concerned.”
Sienna, who is a pupil at St John’s Catholic School, was taken to Bridgnorth Hospital twice and then to Telford A&E the following day after she became violently ill with diarrhoea and vomiting.
The family were left to wash down their daughter in the pub’s bathroom after the accident. Sienna was also left with scratches on her legs from where she fell.
Mr Varela said they had been out celebrating his wife’s birthday when the accident took place.
Wayne Penn, owner of the River and Rail, said vandals had been responsible for the manhole cover not being in place.
He said: “We have been visited by both the police and environmental health, and they have deemed us safe.
"This specific part of the garden is clearly not open to the public; it is wild and overgrown with tall grass and reed beds, while the beer garden has a cut lawn.
“The manhole is usually covered, however I found the lid had been removed and chucked in a nearby brook, which I believe was an act of vandalism over the busy bank holiday weekend.
"We have now put out additional barriers and signs.”
Shropshire Council said it had carried out an inspection after the accident, on May 7, was reported.
Matthew Aldridge, health protection team manager, said: “At the time of the visit remedial works had been put in place, and the team are now reviewing the situation, but the risk to public is now low.”