The incident occurred at the WASA water treatment plant in Hathazari Upazila’s Mudnaghat area.
The dead have been identified as Nazrul, 30, and Yusuf, 22. Additional details were not available on the victims.
Mudnaghat Police Investigation Centre Inspector Md Abdul Karim told bdnews24.com that welding work had been done at the septic tank 15-20 days ago.
“Two workers took off the lid of the septic tank and entered it this morning. Ammonia gas had built up inside. They fell unconscious due to the effects of the gas. They were rescued and taken to Chattogram Medical College Hospital (CMCH).
CMCH Police Outpost Naik Amir Uddin told bdnews24.com that doctors declared them dead when they arrived at the hospital.
“The doctors said they were suffocated by the ammonia gas and died,” said Naik Amir Uddin.