University of Maryland Extension

Septic System Odors

Odors Inside the House

Many people have asked me about a problem with a sewage odor inside their house. Usually this odor eminates from a dry drain.  Each drain in your house has a trap associated with it. The trap is the U-shaped pipe you see under your kitchen sink. See diagram below.

The purpose of this pipe is to keep the gases from your septic system from being able to come back inside the house. The trap works because water sits in that U-shaped section of pipe, and therefore the gases can't get back up through the drain.

However, if the water in that trap should ever dry up, then the gases are able to escape. The water evaporates out of these traps if they aren't used for a period of time. Usually it's an unused guest bathroom, or basement shower stall, or some fixture that just isn't used very regularly.

The solution of course is very simple: Add water to the drain!

Drain Trap

Odors Outside the House

Many people have asked me about a problem with an occasional sewage odor outside their house. Usually this odor eminates from the vent stack on the roof.  Each drain in your house has a vent stack associated with it. The vent stack is the small pipe(s) you see sticking out of your roof. These pipes are designed to vent the gases from the septic system out through the roof. They also help with the drainage of wastewater through the plumbing system, keeping air-locks from forming.

Depending on where the vent pipe comes out of the roof, the height of the pipe, the height and shape of the roof, and the direction and speed of wind, the gases could wind up back down on your patio due to a downdraft. See diagram below.

Vent Stack Diagram

This problem can be alleviated by extending the height of the vent stack, or placing a charcoal filter over the top of the stack.

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