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Diagnosing and Curing Sewer Gas Smells and Septic Tank Odors InspectAPedia®  -     How to prevent or cure sewer gas odors from septic systems, building plumbing, & other causes Sewer gas smell diagnosis - plumbing checklist & gas sniffer equipment advice Septic gas smell diagnosis - septic system checklist Other causes of sewage odors, septic odors, sewer gas, rotten egg, or other indoor gas odors

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. Also see ODORS & SMELLS DIAGNOSIS & CURE for procedures to diagnose and cure all kinds of odors in and around buildings.

Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links 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.

First Steps to Check for the Source and Cure of Sewer Gas Odors

What steps are useful to avoid smelling (or blowing up from) the gases that form and naturally want to escape from residential septic systems? How can we diagnose sewer odors at a building site to determine if it's a venting problem, a failure of the septic system, or the failure of a neighbor's septic system, or perhaps even a leak from a fuel gas line or appliance?

First Check for Very Dangerous Conditions: fuel gas leaks, sewer gas leaks, gas accumulation

• Observe septic system safety procedures When opening a septic tank for service or repair, observe the safety precautions I describe earlier on this web page.
• We have had reports of a septic pumper having his face burned by exploding methane when he lit a cigarette during pumping, a homeowner who generated an explosion of the septic tank that rocked the entire neighborhood when he built a brush fire atop the septic tank.
• We have also read of deaths occurring when untrained service people entered a pumped-out septic tank to inspect it.

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MVOCs & MOLDY MUSTY ODORS
ODORS & SMELLS DIAGNOSIS & CURE
PLUMBING TOPICS
GAS MEASUREMENT TOOLS
SEPTIC SYSTEMS HOME
SEPTIC & CESSPOOL SAFETY
SEWER GAS ODORS
  First Steps for Sewer Gas Odors
  Dangerous Conditions
  Building Drain & Sewer Line Odors
  Cure Odors in Septic Systems
  Other Causes of Odors from a Septic System
  Plumbing Fixtures or Traps
  Plumbing Vent Definitions, Types
  Plumbing Vent Defects
  Septic System or Sewer Piping
  Site, Weather, or Failing Neighbors
  Tests for Indoor Sewer Gas
  Tracking Odors to Source
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

• 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.

Guide to Tracking Sewer Odors to Their Source

See Tracking Odors to Source for the full detailed article on this topic.

• Plumbing vents which have been accidentally closed off or blocked: some plumbing vents include a screen which can slide down, closing off the vent and causing sewer gas backups in the building. We've also found plumbing bents blocked by wasps nests, birds, and once even a frog. Our photos above show how we simply raised the screen back up to open an old sheet-metal vent.
• Leaking plumbing vent system piping: check that all of the pipe connections in the building's plumbing vent system have been properly made and sealed. Particularly with plastic piping it's possible for an installer to push vent pipe sections together while forgetting the glue.
  
  Because the vent is passing only gas, no one notices that it's leaking until there is an odor complaint, the pipe falls apart (which may never happen), or there is a sewer gas explosion. An experienced plumber may spot a loose vent piping correction quickly, especially in an attic or basement. It helps if you've already traced the sewer gas smell to its strongest location.
  
  Pressure test the vent piping or drain piping systems: It is also possible to perform a pressure test of the vent piping system or the drain piping system - a procedure which will diagnose for sure whether or not the plumbing vent system is leaking sewer gas. A plumber installs seals at appropriate locations and pressurizes the piping to check for leaks. [Thanks to J.V. for this detail.]
• Missing plumbing vents: I've seen systems with no vents at all - which often leads to sewer gases reentering the building through sink and shower traps.
• 
  Plumbing vents which are too short in their above-roof projection can be blocked by snow and ice in freezing climates.
• Plumbing vents which are blocked by ice freezing inside the vent line: in freezing weather if a lot of water vapor is passing up the vent line, for example from long steamy showers or perhaps from a hot water fixture left running or leaking into the drain system, frost can form in the vent piping where it passes through a cold attic and outside. When the frost becomes heavy enough it can block the plumbing vent system completely.
  
  In freezing climates, check that a plumbing vent is not being blocked by frost or by snow-cover. Waste line sounds heard in piping or at plumbing fixtures, such as "drain gurgling" or "glub glub" sounds, or similar noises may indicate defective or clogged plumbing, We discuss how to diagnose and cure drain sounds at Plumbing Drain Noises. . Also see Wet Weather or Cold Weather Septic Odors or Sewage Odor Diagnosis & Repair Guide

• Plumbing vent distances to fixtures: Plumbing codes require that plumbing vent lines be installed sufficiently close to plumbing fixtures to vent those drains (typically 5' or less) and that the vent system conducts any sewer gases outside and above the building (not in the attic).
• House Traps and Sewer Gases: Some residential plumbers like to install a trap in the main house sewer line at the point where the line exits the house wall to head for the septic tank. Such traps may protect copper waste lines from corrosive sewer gases. They also may prevent gases from a septic tank from passing back up the waste line and up the house plumbing vent stack.

Problems With Plumbing Fixtures or Fixture Traps Can Release Sewer Gas Odors Indoors

See Plumbing Fixtures or Traps for the full detailed article on this topic.

• Look below the sinks for antiquated or un-vented drains - if you see an "S" trap rather than a modern "P" shaped plumbing drain trap, the fixture is almost certainly not properly vented. Don't forget to check for smells at your garbage disposer drain too. If that is the odor source you may be able to remove the odor by cleaning liquids or even simple vinegar.

Building Drain & Sewer Line Defects Can Release Sewer Gases & Odors Indoors

See Building Drain & Sewer Line Odors for the full discussion of odors from building drains & drain piping. Also see Septic System or Sewer Piping for odors from septic systems and sewer lines.

Septic System or Sewer Piping Blockage or Failure Can Produce Sewer or Septic Gases Outdoors OR Indoors

See Septic System or Sewer Piping for odors from septic systems and sewer lines.

• Check for a sewer line vent through the building foundation wall: I often find that a vent has been placed at a house foundation wall just above where the sewer line leaves the building. I suspect the plumber thought that this would aid drainage into the septic tank. Millions of homes have been built without this vent so I doubt it's needed. See if that's an odor source, and review the installation with your plumber.
• Install a gas diverter in the septic tank: the sludge in the bottom of a septic tank is being digested by anaerobic bacteria and perhaps also other microbes which form methane gas as a product of the decomposition of sewage. In some septic tanks the sludge layer may produce an occasional "burp" of gas that forms a large methane/hydrogen sulfide bubble. In a single compartment septic tank such bubbles can force unwanted solids up the tank outlet tee and into the absorption system. If this is suspected, special gas deflection devices (much like an upside down chimney cap) can be installed on the tank outlet tee.
• Sudden and unusually large septic system system usage that causes a large volume of water to enter the system, such as many loads of laundry or many visitors to a property which served by a small septic system might temporarily dilute the bacteria in the septic tank. It would be odd for this problem to persist. Normally bacteria would recover, probably in a day or so. If a septic system drain, tank, or drainfield are partly blocked, usage surges can produce this condition more often and at an increasing rate as the system deteriorates.
• Prolonged cessation of usage of the septic system, such as a home being left vacant for years. It would be odd to have to do anything special about this condition as when the system is returned to use it should recover quickly. However if the home has been vacant for a long time and/or if you simply don't know the condition of the septic system, it would be smart to have a septic contractor locate the tank, open it, pump it, and report on its condition both for operating confidence and more, for safety. (Is the septic tank cover safe?
• Acidic septic tanks: An Australian website suggested that (for the Australian climate) the septic tank contents may become too acidic. They recommend the following remedy, which I do not recommend in other areas without first consulting with local septic experts. And certainly do not attempt this process nor any process using any other septic additive or product before you know in fact where the odor is coming from. If, for example the odor is from improper plumbing vent installation, flushing lime down a toilet is not a fix it's simply silly.

• Reduce the septic tank acidity by preparing a mix 0.5kg of lime with 10 liters of water.
• Flush the lime mixture down the toilet 2 or 3 times a day for 3 to 4 days, until a total of about 5kg is used.
• Alternatively the 5kg of lime to 10 liters of water can be inserted into the septic tank in one dose, however this can be more difficult to flush through the system, especially with new dual flush cisterns.
• If the odours persist, repeat this process after 7 days.

This Australian septic system advice was paraphrased/near-quoted from advice given by Adelaide Hills Council, Australia. In some areas such as South Australia, aerobic wastewater treatment is used to treat effluent to a sufficient level of sanitation that it can be used for irrigation within the property - in other words it is dispersed on the soil for the purpose of both disposal and for irrigation.

How to Diagnose, Prevent, or Cure Odors and Hazards from Gases in Septic Systems

See   Cure Odors in Septic Systems for the full detailed article on this topic. Also be sure to see Other Causes of Odors from a Septic System.

A variety of mistakes or just plain bad luck about site terrain shape and prevailing wind, or something more serious like a failing septic system can, however, produce sewer odors at a property. Here are some steps to diagnose and correct gas odors at properties served by septic systems. Some of these steps also apply to homes connected to a municipal sewer as well.

At SEPTIC & CESSPOOL SAFETY we've already explained that gases produced in a septic tank are dangerous, as a potential source of explosion and as a cause of death by asphyxiation if someone falls into or deliberately enters a septic tank.

The gases that form in septic tanks are primarily two, methane, CH₄, and hydrogen sulfide H₂S. It's the H₂S (a "rotten egg" smell) that people mostly notice if gases from a sewage system are not properly vented at a building. Other gases produced by the decaying organic matter in the tank are also mixed into this brew as well.

Experts [Burks/Minnis, Kahn et als, Jantrania] will tell readers that septic tanks and their covers and access covers and piping fittings should all be sealed air-tight with proper rubber gaskets. In nearly 50 years of looking at septic tanks and systems, I've yet to see a conventional concrete tank which is sealed with gaskets. Some steel and certainly some of the newer fiberglass tanks may be in fact more precisely designed and built, but concrete septic tanks and covers are a bit rough and will be leaky in most installations.

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

Please see Site, Weather, or Failing Neighbors for the full detailed version of this article.

• 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 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.
• 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.

Other Causes of Odors from a Septic System

Please see Other Causes of Odors from a Septic System for the full detailed version of this article.

Septic system odors may occur from a few other common conditions which you may not have considered:

• Inspect the building water heater: if the "sewer gas smell" is noticed particularly when hot water is being run at a sink, tub, or shower, have your plumber check for a bad water heater sacrificial anode. The sacrificial anode rod in a hot water tank is intended to slow corrosion in the water heater tank but when the anode deteriorates it can cause a "rotten egg" smell in the hot water supply which may be mistaken for sewer gas. We discuss the hot water tank sacrificial anode and dip tube in more detail at Check the Sacrificial Anode & Dip Tube of Your Water Heater Tank.
• Check the building water supply: if the "sewer gas smell" is a "rotten egg smell" and if it is noticed particularly when hot or cold water are being run at a sink, tub, or shower, your water supply may contain excessive hydrogen sulphide or H2S. This is not likely to be the case if you are on a municipal or community water supply but could occur if your water is supplied by a private well. Have the water tested for sulphur and H2S. If this condition is confirmed, the H2S is not a health concern but remains an aesthetic one. It can be corrected by installation of a proper water treatment system such as a chlorine injection system, a potassium permanganate filtration system, or by other methods. More information about causes of and cures for hydrogen sulfide odors in water is at Identifying Odors in Drinking Water.
• Use of improper or excessive cleaning chemicals in the building, or perhaps excessive use of antibiotics (such as by a nursing home served by a septic tank and whose residents consume a lot of antibiotics) can kill off bacteria in the septic tank. This is an unusual condition unlikely to occur in a normal homeowner environment.

Tests for Indoor Sewer Gas & Septic Odors - Indoor Gas Leaks using the TIF 8800

Please see Tests for Indoor Sewer Gas for the full detailed version of this article.

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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