The function of a drainage system goes beyond removing wastewater from hydraulic installations. Drainage systems must also be designed to prevent backflow while keeping sewer gases out of interior spaces. The term “sewer gas” is used to describe the mixture of gases released from the sewer, and the individual gases can be toxic or non-toxic.
Many sewer gases are odorless and the characteristic smell when they leak can be attributed to hydrogen sulfide. This gas is also toxic and flammable, making it dangerous in higher concentrations.
There are many ways sewer gas can escape from a drain system, and the specific cause can only be determined with an inspection. This article describes the most common problems and how to fix them.
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1) Dry P-Traps in Drainage System
You've probably noticed that plumbing fixtures connect to the drainage system with pipe segments that are shaped like the letter P on the side. This is called a P-trap and it holds water to create a liquid barrier between the drainage system and interior spaces.
As water evaporates over time, the P-trap of a plumbing fixture that has not been used for a long time may become dry. When this happens, the water barrier that holds sewer gas is no longer present and unpleasant odors can escape.
This problem can be resolved easily – it's just a matter of allowing water to enter the P trap. No plumbing services are required and the sewer gas problem can be resolved in seconds. If dry P traps have been discarded and sewer gas is still escaping, it is recommended to call a plumber. However, it helps to know about the possible problems that may be encountered.
Note that siphons are not only used in plumbing fixtures but also in floor drains. Even if all fixtures have water in their traps, there may still be a dry floor drain allowing sewer gas to escape.
2) Damaged seals or piping
Plumbing connections use special seals to prevent leaks, and in the case of drainage systems, these seals also keep sewer gas inside the pipe. In particular, toilets have a wax ring that can become damaged over time, allowing sewer gases to escape.
Some damaged seals can be detected by observing water leaks, but this is not always the case. Professional plumbers use a smoke test to find leaks – pressurized smoke is injected into the drainage system and leaks through damaged seals.
The drain pipe itself can also be damaged, and this has a similar effect to a damaged seal: sewer gas can escape and wastewater can also leak, depending on the position of the cracked pipe. The smoke test is also effective in this case.
3) Damaged or blocked ventilation lines
A functional plumbing system not only has drain pipes but also ventilation lines. Just as sewer pipes remove wastewater, vent lines reach the roof to vent sewer gas outside.
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Without vent lines, there is no way to stabilize pressure within a drainage system.
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If a drainage system is installed without vent lines, pressure differences may prevent water from entering or may even cause wastewater to back up from the sewer pipes.
A blocked vent line can build up pressure and sewer gas can seep into areas of the building, even when P-traps contain water. Ventilation lines can become blocked if leaves and debris fall or if a small animal climbs onto the roof and dies inside. In either case, a professional plumber can clean the vent lines to resolve the issue.
Consider that ventilation lines can also be damaged, as can sewer pipes, and sewer gas can escape through cracks. Plumbers can use smoke testing to detect these problem spots.
Additional Recommendations
When an indoor space is affected by an unpleasant smell, do not immediately assume there is a sewer gas leak. Mold is another common problem that produces odors and can grow on many surfaces when indoor humidity is not properly controlled.
Mold and sewer gas produce very different odors: while sewer gas is often described as “rotten eggs,” mold odors are described as “musty.” Mold can also become a serious problem if left unchecked, as it damages many materials and causes allergic reactions in sensitive individuals. Large mold stains require the services of a professional mold remediation company for removal.
When a building has a sewer gas leak, make sure dry P traps are discarded before calling a plumber for repairs – the problem can be resolved by simply filling the traps with water. However, if the drainage system or its ventilation openings are damaged or clogged, they must be repaired by professional plumbers.
