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Diagnosing and Curing Sewer Gas Smells and Septic Tank Odors Septic Systems - Design, Inspection, Testing, & Maintenance SepticAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

How to prevent or cure sewer gas odors Sewer gas smell diagnosis - plumbing checklist Sewer gas smell diagnosis - septic system checklist Other causes of sewage or 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. This is a supplementary chapter to our online book: "Septic Systems - Design, Inspection, Testing, & Maintenance" whose chapters are shown at the left of this page. 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." This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems. 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. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

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

In a properly constructed drain-waste-vent system and septic tank installation, sewer gases produced in the septic tank find their easiest escape path back up through the scum layer and into the septic tank inlet tee and from there up the DWV system to the building plumbing vent stack system where gases are vented above the building roof. More gases may escape and make their way through the tank's outlet tee top into the soil absorption system where the gases are distributed over a larger (leachfield) area and further filtered and deodorized by the soil. Where a building is connected to a municipal sewer, the building drain-waste-vent system (DWV) includes traps and vents to be sure that any sewer gases passing back up waste lines are vented safely above the building roof.

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.

Helpful 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?

• 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. I've 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, and I've read of deaths occurring when untrained service people entered a pumped-out septic tank to inspect it.

• 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.

• 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.

• 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. This rod 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. More information about causes of and cures for hydrogen sulfide odors in water is at Identifying Odors in Drinking Water.
• 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 filteration system, or by other methods.

• 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.
• 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 me 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

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

• 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.
• 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 litres 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..

The Septic Systems Online Book - Where Are We?

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

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author/editor of the InspectAPedia ^© Website
• Thanks to Slade Franklin for the reminder that a leaky wax ring at a toilet can lead to septic odors in bathrooms. 11/2007
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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.

More expert information on this topic

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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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