Why does my septic tank smell?

Septic tanks usually smell a little bit. However, the odour should not be noticeable in a well-installed septic tank. If you’re noticing a bad smell from your installation, there are a few common causes that could be contributing to the build up of pungent gases.

In this blog post, we’ll look at some of the common causes of a bad septic tank odour. At the end of the article, we’ve also suggested some next steps to help you deal with the issue.

  1. Food waste may be entering the septic tank. We break food down in our bodies, removing a lot of nutrients and leaving only waste. This waste enters the septic tank and settles to the bottom, bacteria in the septic tank will break down a portion of it and the rest has to be removed once a year. If you add food (by using a food disposal unit) you are adding waste that hasn’t already undergone a process of being broken down (in our bodies). There isn’t the right type of bacteria in a septic tank to fully break down food and therefore it rots. When this happens, it will smell of rotting eggs. If you have a pungent smell from your septic tank the first thing to do is avoid any food entering the tank (this is the same for milk and liquids).
  1. You may be using too much detergent or chemicals. Detergents and chemicals can kill bacteria and stop the breakdown of waste, avoiding or minimising these will help with any smells.
  1. There could be incorrect ventilation. Most houses are fitted with a soil stack, which allows smelly air from a sewer to go up through the vent and out above the roof. When you have a septic tank you have to ensure that there is air flow up to the soil stack. A good analogy here is imagine leaving the front door open in your house, if no other windows or doors are open then nothing happens. If you however open the back door, you let airflow through the house and a door might bang shut. This is similar to what needs to happen with your septic tank ventilation, you need to have a way for fresh air to come into the septic tank to enable the poor quality air to flow up and out of the soil stack. If you don’t have a low level vent on your septic tank to let air in, try introducing one (speak to your builder or a specialist if you are unsure). Also consider adding an active wind ventilator to pull air up through the soil stack
  1. The system may need de-sludging. Each year a septic tank should be de-sludged, this process removes the build up of solids in the tank. You may have heard people say “it’s been in 50 years and only emptied once”, this persons septic tank may not smell because their ventilation is good, but their drainage field may be really blocked up! So please keep on top of emptying the septic tank.
  1. The water in a toilet U-bend may have dried up. If you have been away for a long time, or haven’t used a certain toilet for a long time, the water in it can evaporate. This can cause smells to come up and out of the toilet, this can easily be cured by flushing the toilet.
  1. There could be a crack in the soil stack/pipe. If you are experiencing smells in the house and you have ruled out point 5, then it is possible that air could be getting in via a crack in the soil stack running up through the walls. If it is a new build then contact the builder to discuss this.

Take home message

If your system is smelling, then firstly look to what you are putting in it and how frequently it is being emptied. If you are very satisfied that you are using it correctly then you should look to the ventilation and ensure there is good airflow up through the soil stack and out of the roof. Installing an active wind ventilator on the soil stack is a good option but only if there is a way for fresh air to come into the septic tank.

If you don’t fully understand how they work, you can jump to our article that explains how a septic tank works.

What’s next?

When faced with a smelly septic tank, there are a number of different options available to you, from replacement parts, to desludging, to replacing the tank altogether if it’s no longer fit for purpose.


Last Update: October 2, 2019  

January 4, 2018   11993    Septic Tanks  
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