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This page describes how to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well. What makes the smell in sewer gas? Sewer gases are more than an obnoxious odor.

Because sewer gas contains methane gas (CH₄) there is a risk of an explosion hazard or even fatal asphyxiation. Sewer gases also probably contain hydrogen sulfide gas (H₂S) In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).

Depending on the sewer gas source and other factors such as humidity and building and weather conditions, mold spores may also be present in sewer gases. Also see Wet Weather or Cold Weather Septic Odors or Sewage Odor Diagnosis & Repair Guide for additional odor tracing and cure advice for odors occurring during wet or cold weather.

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Site Conditions, Weather Conditions, or Failing Neighboring Septic Systems or Plumbing Can Produce Local Sewer Gas Odors

• Check for sources of ground water or surface water that are flooding your septic tank or drainfield. Runoff from roof drainage or surface or subsurface water can easily enter and flood a septic tank and drainfield, including at surprising locations such as an opening at the septic tank top, a crack in the septic tank, or an opening in a drain line entering or leaving the septic tank.
  • Excavate and the soil around the drainfield: a few test holes, combined with a septic loading and dye test can tell us if the water flooding a septic field is coming from the building's septic tank and wastewater or from invading surface runoff.
    
    Septic loading and dye test procedures are given in detail at this website. See Dye Tests: how to perform a Septic Loading and Dye Test - the complete procedure for septic loading & dye testing, a septic function test.
  • Inspect the septic tank for evidence of flooding from invading ground water. See TANK INSPECTION PROCEDURE
  • Use an intercept drain: Probably the most common problem, however, is ground water or surface water flooding the leach field, causing it to fail or back up. It may be possible to install intercept drainage around the septic field to salvage the system.
• Check neighboring properties and drains: before digging up your yard and septic tank, walk around the neighborhood. Is the smell getting stronger as you walk away from your property? Perhaps a neighbor has a failing septic system or is having work performed on their septic system.
• Check nearby storm drains for odor sources.
• Check for unusual site and wind conditions: I've encountered buildings where all of the plumbing vent installations appear to be to code and complete, but where unusual terrain shape (house at the bottom of a large hill) and prevailing winds conspired (in some weather conditions) to blow septic gases back down from above the roof to a bedroom window or even to ground level.
  
  Depending on the building roof shape, orientation, and prevailing or even uncommon wind direction, wind blowing at the building can cause downdrafts around a plumbing vent stack, sending normal sewer gases and odors back closer to the ground or even into the building.
  
  If your sewer gas odors seem to correlate to windy conditions I'd check this out further. Extending the plumbing vents higher or installing a wind block at the vent top might help.
• Weather: rain and flooding can cause sewage odors at a site:
  • Flooding the drainfield causes sewage effluent to reach the surface
  • Even a drainfield that is not flooded may be smelly if it is improperly constructed or has lost its ability to treat septic effluent
  • Flooding the septic tank, by invasive ground water, surface runoff, or from actual flood conditions is likely to also leave the system not functional and possibly releasing septic gases See SEPTIC FLOOD RESPONSE
• Other Sources of Smells that are less like "sewer gas" odors (in my opinion) include the odor of burning electrical components. If you trace odors to an appliance or fixture or switch, shut off electricity to that device (or un-plug it if it's an appliance) and have the system or appliance checked by a licensed electrician. Burning electrical components and insulation, and overheating florescent light ballasts can make quite an odor but that's the just a warning sign of an unsafe condition that needs prompt attention.
• Submissions are invited: Contact Us if you have other examples of tracking down septic or sewer gas smells to their source. Credit and link-exchanges given.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

MVOCs & MOLDY MUSTY ODORS
ODORS & SMELLS DIAGNOSIS & CURE
PLUMBING TOPICS
GAS MEASUREMENT TOOLS
SEPTIC SYSTEMS HOME
SEPTIC SYSTEMS ONLINE BOOK
SEPTIC SYSTEM DESIGN BASICS
SEPTIC DESIGN ALTERNATIVES
SEPTIC INSPECTIONS
SEPTIC & CESSPOOL SAFETY
SEWER GAS ODORS
  First Steps for Sewer Gas Odors
  Dangerous Conditions
  Tracking Odors to Source
  Plumbing Fixtures or Traps
  Building Drain & Sewer Line Odors
  Septic System or Sewer Piping
  Cure Odors in Septic Systems
  Site, Weather, or Failing Neighbors
  Other Causes of Odors from a Septic System
  Tests for Indoor Sewer Gas
SEWER GAS ODORS in COLD WEATHER
  Short Answer in Cold Weather
  Diagnosing Sewer Odors
  Cold Weather Plumbing Vent Blockage
  Trap Siphonage and Sewer Gases
  Building drain odor source
  Fixture versus system blockage
  Odors in Wet Weather
  Indoor Septic Odors Outdoor Causes
  Outdoor Odor Sources
  Sewer Odor Tracking
  Remedies for Sewer Odors
  Odors and Drain Lines
  Failed Drainfields and Odors
Toxic Gas Test Procedures

The Septic Systems Online Book - Where Are We?

• This Chapter: SEPTIC SYSTEM SAFETY WARNINGS
• Next Chapter: SEPTIC SYSTEM DESIGN BASICS
• First Chapter: SEPTIC SYSTEMS ONLINE BOOK

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
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• Additional technical contributors & reference sources for this article are listed below.
• Thanks to Slade Franklin for the reminder that a leaky wax ring at a toilet can lead to septic odors in bathrooms. 11/2007
• Thanks to J.V. (privacy protected) for the reminder to make a detailed inspection of the plumbing vent system when sewer gas odors are present. 07/2008
• Thanks to Roger Hankey & Cheryll Brown, www.hankeyandbrown.com, ASHI home inspectors in Minnesota, for the deteriorated transite pipe gas flue vent photograph and comments. Mr. Hankey is a past chairman of the ASHI Technical Committee, serves as co-chairman of ASHI legislative committee, and has served in other ASHI professional and leadership roles. 7/2007.

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.
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More Information on Building Diagnostic Inspections and Repairs

• Cold Weather Septic Odors or Sewage Odor Diagnosis for additional odor tracing and cure advice focused on sewer gas odor tracking in cold or wet weather.
• Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds


• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
• 
  What is a Septic System An Engineer's View - Types of treatment tanks, adsorption systems, pumps, and other special equipment are discussed in some further detail in this text
  Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
  A Toxic Gas Testing Sampling Plan for Residential Indoor Air Investigations

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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