A BMA official, who asked not to be named, said the five workers of a private firm were checking the facility at the Chatuchak waste water treatment plant on Soi Inthamara 35 at 11.30pm on Friday.
The four survivors said they were checking the water tunnel when they saw water gushing in so they ran for their lives but one colleague got washed away and drowned before he could reach the entrance.
Officials had to drain the water to retrieve the body.