Plumbing – Does the vented septic tank even require traps and vents near the actual fixture connections


My home has a septic system with a vented lid to let the sewer gasses escape from the tank itself. As a result, it seems like far less gas (if any) would travel up the waste plumbing and exit through the vents downstream of the P-traps. So I've been wondering: given the vented septic tank, is there any actual technical reason why my home plumbing needs to have traps and vents?

CYA provision: I'm not asking this because I'm planning to remove all the traps and vents or anything wacky and illegal and pointless like that. There would be no benefit at all to anyone, and it might even hurt future owners if the house is ever put on the city sewer or something. I'm just curious and I want to understand the systems a little bit better. Because it sure seems like traps and vents are designed to solve a problem that my septic tank solves on its own.

Best Answer

Take a straw. Suck up some liquid, and put your finger over the top. Notice how the liquid doesn't fall out the bottom of the straw. That's a vacuum lock, and it is one of the reasons why you need to vent your drains. Poorly vented drains don't drain well, and you'll get clicking and bubbling. Your toilet may fail to clear the payload. So regardless of any venting in the septic tank, you need to have appropriate drain venting.