Septic odors can occur near the vent pipes atop house roofs when the smell drifts down to ground level, as well as inside bathrooms, and near the septic tank or leach field. But bad odors do not necessarily mean the septic system has failed. Most septic odors are part of the natural biological decomposition that takes place inside the tank before the effluent makes its way to the septic's leach field. For odor problems inside your house, look to plumbing issues.

Waste water treatment tank
credit: Chatmali/iStock/Getty Images
Large blue septic tank

Plumbing Traps

The P-trap and S-trap are part of the plumbing for every drain inside the house. Toilets have them automatically built in, while showers, tubs and sink drains must have one of these traps installed in their plumping waste pipes. With similar features, these traps hold enough water to create a temporary seal in the pipe. The sewer gases -- found in all sewer and septic pipes -- cannot pass this water seal unless the trap becomes dry through lack of use or leaks and allows the sewer gases to escape into the room. Try filling the sink or tub the odor is emanating from and draining it to fill the P-trap. If the odor goes away, that was your problem.

Clogged Vent Pipes

The standing vent pipes that go through the roof as part of the drain, waste and vent plumbing system allow the system to breathe and the sewage to move through the pipes. These pipes also release sewer gases that usually dissipate quickly, unless the vent pipe is clogged. Small animals or even leaves can get trapped inside these pipes, or older iron pipes can become encrusted over time. Working cautiously on the roof, insert a garden hose into the vent pipe -- if water comes out the top, the pipe is clogged and requires fixing.

Malfunctioning System

If you overwhelm your septic system by adding too many household chemicals or too much organic matter -- such as from a garage disposal -- to the system, you can upset the anaerobic decomposition cycle of the organic matter inside the tank. This slows the septic system down, and may cause it to quit functioning altogether. In an ideal situation inside the tank, the enzymatic action of the bacteria digests the solids, turning them into liquids that disperse in the leach field. If you add women's feminine products or male birth control products to the system by flushing them down the toilet, they will not break down and could also clog the tank. Make certain you use septic-safe toilet tissue for this same reason.

Full Tank

Tanks that have quit functioning or are the wrong size for the number of bathrooms in the house can become full and start to smell. In these cases, you need to have a septic pumping company pump out the tank to get it functioning again. If the tank is too small for the house, you will need to have it pumped out on a regular basis. This usually only occurs when homeowners add unapproved bathrooms to the house to avoid building code requirements.

Tips and Warnings

Septic odors in small doses are not harmful, but can lead to headaches and nausea. Inside the closed tank, however, methane may be concentrated and deadly. If you suspect a problem with your septic tank, after eliminating all other possibilities, seek the services of a professional to examine the insides of the tank. This is not a project for a do-it-yourselfer at all. Special personal protection equipment is required.