Why Sewer Gas Odor Increases After Replacing a Septic System

How to locate and mitigate sewer gas odors inside or outside a home — and what to discuss with the homeowner

Why Sewer Gas Odor Increases After Replacing a Septic System

You replace a failed or failing system and then you get the call: “I have bad sewer gas odors and did not have them before you replaced my system, so you must have done something wrong.”

It’s always better to warn customers before you replace their system. “Replacing a failed/failing system can result in increased sewer gas odors inside or outside your house if pre-existing venting issues exist."

The reason this happens is fairly simple. When a system is failing pipes are typically full of water. The more water in a pipe, the less air movement through the pipe. In a properly functioning system, the pipes are mostly empty, with the water merely flowing along the bottom. This gives a lot of potential for air movement — and with air movement comes potential for sewer gas odors if their vent system is compromised in any way. New and newly replaced systems have a lot of air movement, and thus will expose shortcomings in vent systems by allowing gas to escape, resulting in
sewer gas odors.

Sewer gas odors inside of a house 

A properly functioning plumbing vent system will prevent sewer gases in a home by directing them out of the roof vent. 

Sewer gas odors inside of a home could be caused by any of the following:

1. Is there a bathroom that is rarely used or a plumbing fixture that does not get used very often?

    a) Traps in drains are a method of preventing sewer gas inside of a home by standing full of water. If a drain such as a floor drain that does not typically get water in it, a guest bathroom sink that isn’t used, or a bathroom tub that isn’t used, the water in those drains’ traps will evaporate over time allowing sewer gas to come through the drain. This is stopped simply by running water into the unused drain for a few minutes, refilling the traps and preventing sewer gas
from escaping. 

2. The No. 1 cause of sewer gas odor inside a home is a sanitary sump crock that is not sealed airtight.

   a) A sanitary sump pump is a pump in the basement floor that pumps basement-level wastewater (floor drain, laundry tub, basement bathroom, etc.) up to a sanitary sewer exiting through the basement wall as opposed to gravity beneath
the floor.

   b) The cover of the sanitary crock must be sealed airtight. If smoke could get out of the sanitary crock, sewer gases would easily escape from the sanitary crock.

   c) The crock should have a bolted-down top with a rubber gasket. Check grommets around wires and pipes that penetrate the top of the crock. Some plumbers caulk the grommets for extra security.

Other sources of sewer gas odors inside a house can include:

 1. Compromised vent pipes within walls or attic spaces.

   a) Faulty joints or fittings in the vent pipes that are not properly sealed.

   b) Penetration by drywall screws or other accidental construction-related penetrations into vent pipes.

 2. A toilet wax ring or seal is compromised and requires replacement.

 3. Pipes sticking out of a basement floor for "future plumbing."

Properly functioning vent systems will prevent sewer gas and odors. 

How to find the vent leak

If all other attempts failed, everything has been tried to no avail, there are methods to locate the vent leak. Plumbing companies and other leak detection companies can perform a smoke test where they cap off the vent system and incorporate a safe smoke into the vent system. Any leak in the system will allow smoke out, indicating
the problem area(s).

Sewer gas odors outside of a house 

Sewer gases outside of a home could emanate from the following sources:

Roof vent(s): Because some of the constituents of sewer gas are heavier than air, sewer gas tends to flow off the edge of roofs in a downward pattern from roof vents. Many people walk outside, smell sewer gas and their eyes are immediately drawn to septic tank manhole covers, not realizing they are standing directly below a roof vent. 

  a) Seasonally a roof vent could be frozen or covered in snow.

  b) Animal or animal nests could be in the vent.

Solution: There are carbon filters that work well in most cases on roof vents and easily filter out the odor.

Septic tank manhole covers: It is possible that some manhole covers will allow sewer gases out if not seated properly. This can be tested by pushing down on the manhole cover along the edges to see if you can get the cover to "rock," indicating that it is not seated flat.

Solutions: Replace the cover, or seal the cover with a type of gasket that while impervious to gas will not stick to the cover and allow removal of the cover for maintenance/pumping. There are several methods of sealing, one is to use a butyl mastic, but to cover the top of the mastic with wax paper so it does not stick to the cover. 

A high water level in the septic tank, which also indicates there is a further problem as the tank level should not be above the bottom of the outlet pipe. 

Discharge of wastewater to the ground surface, which is also considered a failed system.

A vent on a pump tank (or other) manhole cover. The solution to this would be a carbon filter.

Please note: The natural way for sewer gases to be removed from the area is merely by wind. If the area in question is low, or surrounded by large trees, or encumbered in any other way such as outbuildings, the ability for wind to naturally blow away the sewer gas is diminished.

Remember to let your customer know about the potential for sewer gas odors before you replace their system. 

About the author
Todd Stair is vice president of Herr Construction, Inc., with 34 years’ experience designing, installing, repairing, replacing and evaluating septic and mound systems in southeast Wisconsin. He is the author of The Book on Septics and Mounds and a former president of the Wisconsin Onsite Water Recycling Association.


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