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Septic Tank Odour

We regularly get asked about how to solve the problem of a smelly septic tank purchased from a competitor, and how our systems manage to operate without producing odour. Here we try to briefly give some advice.

Possible Causes of Septic Tank Odour

There are a number of possible causes of odour. The first possible cause is where large volumes of material that should not enter a sewage treatment plant make their way into your system. A good example is grease. An overflowing grease trap or a poorly installed grease trap can allow large volumes of fats, oils and grease [FOG’s] to enter a tank. Grease and oil can clog pipes and build up in a tank resulting in performance issues. FOGs can also result in a very bad smell by allowing anoxic conditions to form, and by causing toxic shock and killing the good bacteria in your septic system. So by making sure that only permitted substances enter your wastewater treatment plant, you can minimise the risk of septic tank odour.

The main cause of odour relates to the actual treatment process. Most systems on the market use an aerator to mix air into the wastewater. This air then provides a source of oxygen for the bacteria to grown and break down contaminants in the wastewater. But this air must then exit the tank. Poorly designed tanks do not allow this air to travel back the sewers and to then exit via the soil vent stack. If the air cannot travel back to the house and up the soil vent pipe for dispersal into the atmosphere at roof level, the air containing odour leaks out of the tank and nearby manholes causing a smell. All of our systems like the Biocell QuickOne are designed to prevent odour and to make sure air is ventilated correctly. This is why our systems are recognised as ‘odour free’ and are so popular with installers.

Another cause of bad septic tank smells can be a situation where there is no soil vent stack present at the house. The soil vent pipe is a pipe that runs up the side of a building and terminates at the roof level. As air passes over this pipe opening at roof level, it creates a chimney effect, drawing air out of the sewers and up the soil vent pipe. If you do not have a soil vent pipe, then by installing one you can help deal with any odour issues you may have. The soil vent stack should be positioned at the final manhole around the building where the sewer goes in the direction of your septic tank or sewage treatment plant. Larger buildings may need multiple soil vent pipes. Soil vent stacks should be regularly checked for blockages. If you are in an area that is very sheltered and the air existing the soil vent pipe cannot get away, we can offer activated carbon filters to place on soil vent stacks to neutralise the odour.

Solutions For Odour Problems

Some companies will offer a quick fix solution for smelly septic tanks, such as enzymes and bacteria. Our advice is ‘buyer beware’. If you have a problem it’s better to identify the cause rather than trying a quick short-term fix with no guarantees.

If you get a smell of sewer gas in a building then you should exit the building immediately and call for professional advice. Sewer gas is extremely dangerous if it builds up in an unventilated area and can result in serious illness and even suffocation/death. Incorrectly installed traps and seals on toilets, sinks, showers, etc. can also allow sewer gas to enter a building. Similarly, a soil vent pipe terminating close to an upstairs window can also allow this ventilated air from the sewers to get back into a building when the window is open.

So if you are installing a new sewage treatment plant – choose a Biocell system for guaranteed odour free operation.

4 thoughts on “Septic Tank Odour”

  1. Hello
    We have a condor sewage system servicing 3 houses. We have a very bad smell which our current service engineer cannot get on top of (he doesn’t know what is causing it)
    Will you service in this area and this system ?

  2. Hi,

    We have a biocell system at our new build home, and we occasionally have a reflux of waste gas smells. Just wondering if there is anything we can do to eliminate the problem. Also the system has been operating for around two years now, do I need to have any sludge removed or is this unnecessary?

    • Hi Russell, we would recommend having any treatment plant desludged at least every 18 months. If the tank is working fine, a problem with odour is usually a result of inadequate ventilation. See our post on common septic tank problems for further information.


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