How to Separate Plumbing Fixtures from Plumbing Drain System as the Source of Sewage or Septic Odors InspectAPedia® -
Separate Plumbing Fixtures from Plumbing Drain System as the Source of Sewage or Septic Odors
Causes and cures for sewer gas odors related to wet or cold weather
How to find and cure bad smells in buildings
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This article discusses the diagnosis and correction of sewer gas or septic odors (and other building smells and odors with focus on diagnosing odor sources and causes in cold weather.
Some of the diagnostic steps pertain to all seasons. The photograph provides a lot of septic odor diagnostic information. Also see ODORS & SMELLS DIAGNOSIS & CURE for procedures to diagnose and cure all kinds of odors in and around buildings.
Also see our broad-scope article on diagnosis and cure of sewer gas and septic odors: Sewer Gas Odors diagnosing, finding, and curing septic tank and sewer line smells.
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to the author. Technical reviewers are welcome and are listed at "References."
This is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.
Diagnosing fixture versus plumbing system blockage problems for correcting septic or sewer gas odors
How to tell a local problem at one fixture from a system wide plumbing problem:
If one fixture or one group of fixtures is not draining well, suspect a local problem with blockage or venting at an individual plumbing fixture or at one or two plumbing fixtures in the same room, sharing a common (blocked) drain, while plumbing fixtures in other areas of the building are performing normally.
Odors that occur only at one bathroom when no drains are slow might be due to a leaky toilet seal, or to trap or drain problems serving just that bath.
The photos at left show a toilet wax ring being replaced. Even if the toilet is not rocking on the floor - that is, it appears securely bolted down, if the wax ring is damaged sewer gases may leak out in that bathroom around that individual fixture.
If all building drains are slow or blocked throughout a building then we suspect a system waste line blockage or blocked septic system.
If lower floor plumbing fixtures back up while upper floor fixtures appear to drain normally, the problem may still be a system blockage, such as a blocked main drain. The upper floor fixtures are simply draining into the sewer line with apparently no problem, but when the volume of wastewater in the sewer line is great enough the sewage backs up into the lowest fixtures in the building, often in a basement.
Odors caused by slow drains or septic drainfields: Included here but not common is the odor causing effects of a slow drain or drainfield: sometimes there will be a partial clog, causing septic effluent to back up and then recede as the drain empties.
In this case there may be no visible actual sewage backup into the building. The same thing could occur if the backup was caused by a failing drain field - the system may back up when there is a large volume of wastewater but then slowly recede. Or it might just create enough back pressure to push gas through the traps but no actual effluent. See Septic Inspection Testing for details.
How to diagnose partial drain blockages: If a partial drain blockage is the problem (watch out it could be due to a damaged pipe or tree roots in which case simply snaking the line is just a band-aid short-term fix), you might notice the problem more when the volume of wastewater surges, such as when doing laundry or emptying a bathtub. See CLOGGED DRAINS.
An increase in the number of occupants in the building (and thus using the bathrooms) will also cause a surge in wastewater volume and may disclose these conditions.
Septic Blockages and Sewer Odors in Wet Weather
A very common source of drainfield failure as well as an occasional source of septic tank back-flooding and occasional push of sewer gases (or sewage) back into a building is the failure to protect the septic tank and drainfield from surface runoff and ground water. An intercept drain may be needed. If this is the case septic problems (odors or backups) will be associated with heavy rainfall or wet weather.
Septic odors in the house could indicate that effluent is backing up into the house past the traps which normally intercept the odors, or entering at a building window or door which has had a plumbing vent terminated too close by. Such odors may or may not mean the septic system is failing. A backed up septic system does not always equal a failing septic system
Is there a municipal sewer?: Make certain that your building and your neighbors are served by private septic systems as opposed to a municipal sewer. Gases from a municipal sewer are also explosive and can be present in large quantity.
Is it a sewer gas or a fuel gas leak?: Make certain that what you're smelling is sewer gas and not a fuel gas leak such as LP gas or natural gas from a gas appliance, fixture, or gas line. There are serious explosion hazards with gas fuels. If you suspect a fuel gas leak leave the building and call your fire department from a safe location.
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Herb Reed County Extension Director, Agricultural and Natural Resources Educator, Calvert County Maryland - private email to DF 9/5/2006 adding comments about odors and partial blockages.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and
Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by
Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)
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