Macerating/dumping into septic system vent stack.

korbe wrote:
As far as I know,a normal septic tank isn't vented. ...

Yes, a septic tank is vented, it vents up through the house stack and out the roof just like municipal connected systems. At least that's how they are supposed to be plumbed.

I would find it hard to believe the OP has true septic vent stack at ground level as this would be venting methane gas low to the ground. The chances of fire or explosion would be high as well as the constant smell of septic fumes.