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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.

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  • Clarification: let's imagine we're talking about a single non-toilet fixture that has neither a vent or a trap and does not meet up with any other drain plumbing. So there's no vacuum seal to impede the flow or cause pressure issues. Just a super simple pipe.
    – iLikeDirt
    Commented Feb 25, 2014 at 16:38

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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.

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  • Hehe... payload...
    – longneck
    Commented Feb 25, 2014 at 14:17
  • Right, and the vacuum lock is caused by the trap. But what if there's no trap either? In that case, it would be like pouring water down the straw without putting your finger over the top, right? Same with the wikipedia quote answer below: since vents are to protect traps, what if you have neither?
    – iLikeDirt
    Commented Feb 25, 2014 at 16:35
  • But no trap, and you'll have an open line to a cesspool. Whether it's vented or not, it's still going to stink.
    – Chris Cudmore
    Commented Feb 25, 2014 at 17:13
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    Everything above is correct, though I don't think the problem with not having traps is being stated clearly enough. In addition to noxious fumes, it's flammable gas! Methane and other flammable gases come up from sewer lines, and those gases are easily ignited. If you don't have traps, you need to solve that problem immediately; it is very dangerous.
    – pbristow
    Commented Feb 25, 2014 at 17:31
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    In addition, having a puddle of water in the bottom of a toilet is helpful for when the payload is deposited and not yet flushed because being submerged in water prevents out-gasing. Otherwise, a trip to the bathroom will get extra smelly. Without a vent, the puddle may not form because the effluent traveling down the drain can suck the bowl dry.
    – longneck
    Commented Mar 7, 2014 at 16:57
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According to Wikipedia there's slightly more to it than venting excess gas that has been produced downstream. You have to consider possible hydraulic/pneumatic effects when you empty your bath.

A sewer pipe is normally at neutral air pressure compared to the surrounding atmosphere. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or it will push back on the waste stream and downstream traps' water seals. As the column of water passes, air must flow in behind the waste stream or negative pressure results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.

Excessive negative air pressure, behind a 'slug' of water that is draining, can siphon water from traps at plumbing fixtures. Generally, a toilet outlet has the shortest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gasses to enter a building.

On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal, with dire hygiene and health consequences if too forceful.

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