Septic tanks need to be watertight. The riser should be sealed to the top of the tank and the riser cover should be sealed to the riser with butyl rubber or some other flexible sealant. No liquid should enter or leave the tank.
Where do you vent a septic tank?
The Role of the Vent As the septic tank fills with waste and water, the air has to go somewhere or the pressure will stop the flow and back up into the structure. To solve this problem a vent is connected to the top of the tank to release the waste gases and air outside.
How do you vent a septic system?
https://www.youtube.com/watch?v=SHB6sq4li_M
How should a septic tank be vented?
There is not a great deal of gas generated in a septic tank, but the gas must be released so pressure does not build up in the tank. If the septic tank has inlet and outlet baffles, they must be vented. If the inlet and outlet are tees, they must have upward extensions and be vented.