Why Does My Septic Tank Smell?

Knowing precisely what a septic tank is and how it operates, you’d think they would be quite a smelly thing to install in such close proximity to your home. But the fact is that your tank should not smell.

If you’re noting a bad smell akin to rotten eggs or sewage, in general, coming from your septic system or the drains within your home, this could be a sign that there’s a problem and that certain action needs to be taken to get things back to how they should be – odourless.

Why Does My Septic Tank Smell?

Common causes of septic tank smells are down to ventilation, contaminants, food waste and more. However, the most common cause of septic system smells is that the tank is full and needs to be emptied. However, we will discuss more potential causes throughout this post.

Should a Septic System Smell?

We’ve already alluded to this answer, but septic tanks are designed to keep the odours from our organic waste within the confines of the tank.

With that said, there are certain things that might cause foul smells to seep from the tank, and on occasion, even make their way into your home.

The good news is that these can be addressed and stenches from your septic tank can be eliminated once you know the root cause.

Causes of Septic Tank Smells

Bad smells emit from septic tanks when our household waste isn’t processed properly, or it’s time to have the tank pumped to remove long-standing oils, grease and solids that have culminated over time.

Other common causes of septic smells include:

A Crack in the Soil Stack/Pipe

If the septic odours have seeped into your home, one cause could be a crack in the soil stack.

Once the household waste has made its way into the septic tank and separated into three layers (scum, effluent and sludge), the wastewater is carried out through a horizontal and vertical pipe into the drain fields.

The soil stack is the term given to the vertical pipe and runs the length of the building. A crack in this pipe could be what’s causing your drains to smell.

Anaerobic Bacteria

The unsung heroes of a septic tank are the aerobic (beneficial) bacteria that work their magic naturally. If your septic system doesn’t have a good supply of oxygen, these bacteria can transform into Anaerobic bacteria.

The problem with this is that Anaerobic bacteria is incredibly ineffective at digesting organic waste, something that Aerobic bacteria is spectacular at.

Poorly digested organic matter will cause a build-up of sludge (solid waste), and that build-up can be the cause of your septic tank smell.

Blocked Drains and Soakaways

If your drains or soakaways are partially or fully blocked, there’s a good chance that this is the cause of the bad smell within your home. An additional check you can make to confirm whether this is the case is to take a look at your drains. Are they taking longer than normal to remove water? Is your toilet flushing more slowly? If so, this is likely the problem.


Certain chemicals (in cleaning products for example) or even detergents and other household products can result in our septic systems starting to smell.

Other household items like wet wipes and sanitary products that a septic system isn’t designed to handle can also result in a smelly septic tank.

Even your loo roll can have an impact. While your tank is designed to deal with toilet tissue, some products are easier for the system to handle than others. Opt for recycled or biodegradable tissue to minimise potential problems and bad fragrances.

Dried up U-Bend

Some properties now have multiple toilet facilities, such as the main bathroom, a downstairs toilet or an en suite. If you have a particular toilet that doesn’t get much use, the water in the U-bend can evaporate, resulting in an unsavoury scent.

Failing Septic Tank

Sadly, as with most things in life, septic tanks don’t last forever. A well-maintained system can have a lifecycle of 30 years, so if you’ve inherited a tank when you moved in, it may need to be replaced.

Food Waste

Food scraps and coffee grounds can easily be washed down the drain and find themselves in your tank. The problem is that food waste should not be put into a septic tank, the result of which will be odours and potential blockages.

Although sludge (the solid matter that sinks to the bottom of the septic tank system) can and should be emptied out regularly, food scraps can increase how often this has to be done, create blockages and emit a foul aroma.

Incorrect Ventilation

Septic tank gases such as ammonia, carbon dioxide, hydrogen sulphide, methane, nitrogen dioxide, sulphur dioxide and traces of carbon monoxide need to be released from the system, but this isn’t exactly what’s causing the odour.

If the soil stack isn’t above the roofline, the gases won’t be able to escape in the designed manner, which is one of the possible reasons septic tanks smell.

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 to maintain the Aerobic bacteria and ensure that they continue functioning properly.

The Tank Needs Desludging

Tanks need to go through a specific servicing called septic tank desludging once a year. Because the tank only allows effluent to travel through the pipes and into the soakaway, there will be a build-up of sludge and scum over time. The only way to remove this is through desludging, where a vacuum tank comes onto your property to remove the top and bottom layers that have accumulated within your system.

Not organising this can lead to septic tank problems such as nasty odours both outside and within your home.

Your Vent Needs to be Replaced

Finally, external vents that have been blocked by leaves or even birds nests and internal vents that have seen better days may need to be replaced. This will stop the smells in their tracks.

Your plumbing vent pipe could also be causing a problem with odours if it isn’t long enough. As an additional measure, you could incorporate a carbon filter into your vent pipe to help control the smells. These filters will have to be changed every one to five years but can be a worthwhile investment.

How to Get Rid of Septic Tank Smells in Your House

Septic tank odours entering your home are generally caused by the following:

  • Backups in the pipes caused by grease, oils and fats.
  • Disproportionate water use that’s washing away the good bacteria in your tank, resulting in too much sludge and scum.
  • Dried toilet wax seal at the base of your loo. This is fairly easy to resolve and just needs a new wax ring to be installed.
  • A full tank.

To stop septic tank smells from entering your home, you can try some treatments that will flush out any Anaerobic bacteria but leave the Aerobic bacteria behind.

If you think it could be a plumbing problem, try checking the U-bend of any rarely-used toilets and the wax seal. Flushing the toilet to add more water to the U-bend or changing the seal may stop the septic tank smell in your house.

If the cause of a backup in the pipes is your tank being full, we can attend your property and resolve the issue on your behalf.

How to Get Rid of Septic Tank Odours Outside

Unpleasant, sewer-like smells outside of your property can be caused by:

  • A full tank
  • Sewer gas escaping.

As we mentioned earlier, septic tanks create certain gases which need to be released.

Sometimes these sewer gases are released and the direction of the wind blows it near your home, causing an unsavoury odour. Carbon filters, which we mentioned earlier on in this post can help resolve this issue.

If the problem is caused by any of the other factors mentioned in this post, you may need to enlist the help of professionals such as ourselves to fix any ventilation problems or system cracks, desludging or upgrading your system.

Sewage Treatment Emptying Service

While any of the issues mentioned in this post could be the cause of the septic smell both within and outside of your property, the most common will be that your tank needs to be emptied.

At Owls Hall, we offer septic tank servicing, repairs and emptying and desludging services. Please don’t hesitate to get in touch to book any of these services, or to ask additional questions if you can’t figure out what’s causing your odour problem.