This is Why Your Septic Tank Fails, Backs Up and Smells Bad…


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Why do Septic Tanks Fail?

Like most systems within a home, a septic system needs to be regularly maintained. A well-maintained septic system should provide the household with a hassle-free sewage treatment service for a long time.

However, if the system is not properly maintained it can result in septic tank failure, including bad odours and sewage backing up to the home, which can pose a risk to the environment and to human and animal health.

Knowing the signs that indicate the system has failed or is about to fail, can enable swift action to prevent the potential health risks mentioned above, keeping your family and the surrounding environment safe.

why do septic tanks fail

When a septic tank fails, untreated sewage flows out of the system and ends up somewhere it shouldn’t.

Septic tank failure could cause sewage to back up in the sewage pipes leading from the building, resulting in toilets not flushing properly, and/or water from basins, showers or bathtubs may not drain efficiently.

It can also cause sewage to rise to the surface of the soil surrounding the septic tank or the drain field.

Septic tank failure is not only a nuisance due to unsightly sewage pools and bad odours, it is also a health hazard that can spread disease and pollute drinking water sources, freshwater systems and recreational water bodies.

Let’s look at some of the most common reason’s why septic systems fail, smell or back up.

Why do Septic Tanks Back Up?

Wastewater from a septic tank can back up for several reasons, as outlined below.

septic tank back up

Excessive Load

If the household generates more wastewater than the soil in the drain field can absorb, the excess will rise to the surface of the ground or back up in the pipes leading from the house.

This is usually caused by one of two things:

1) a poorly designed system that is either incorrectly sized, poorly constructed, or installed on impermeable soils; or

2) an increase in the household water use, either due to an increase in the number of people living there, or the addition of a water-hungry appliance such as a dishwasher or washing machine.

why do septic systems fail?


Surface water flowing off roof-tops, driveways and road surfaces into the soil in the drain field will also increase the load of the system and can cause the soil to become saturated.

Once the soil becomes saturated (no matter whether from wastewater or clean stormwater) it will not be able to absorb any more wastewater.

Any untreated wastewater added to the drain field will either back up to the house or rise to the surface of the soil, usually giving off a bad smell.

Groundwater intrusion:  During heavy rainfall events, spring snowmelt and a rising water table can often cause the septic drain field to become over saturated.  This can often lead to temporary system malfunction until the soils have had a chance to rest.  

Physical Damage

Physical damage to the soil or pipes in the drain field can result in septic system failure.

Soil can become compacted if vehicles are driven or parked on top, while drainage pipes can move or get crushed.

Roots from trees and plants can also cause the soil in the drain field to become clogged, so its best to cover the area with shallow-rooted plants such as grass rather than deep-rooted shrubs and trees.

septic system back up

Poor Maintenance

If a septic tank is not routinely pumped out, scum and sludge build-up in the tank can cause soil in the drain field to become clogged.

It is recommended that the septic tank is pumped out every 1-3 years, more frequently if the home disposes of food waste into the septic tank via a garbage disposal system.

septic tank pumping

Why do Septic Tanks Smell?

There are several things that can cause a septic tank to smell, a few of the more common causes include:

The tank may need to be pumped out to remove the accumulated solids which may be causing the drain field to become clogged and not functioning as efficiently as it should.

 If food waste is disposed of down the sink it can cause odours as bacteria break it down.

The reason for this is that unlike food that enters the system via human waste, food waste that enters the septic system directly has not been broken down by bacteria in our bodies.

The bacterial communities living in the septic tank are not efficient at breaking down food, and as a result, it rots, giving off a bad odour that smells like rotting eggs. If your septic tank smells bad, refrain from discarding food waste (including liquids such as milk) into the tank.

Excessive use of detergents, chemical cleaning and pharmaceutical products can kill the microbial communities living in the tank that are responsible for breaking down the waste.  

When the population numbers of these bacteria are reduced, so too is their ability to break down the waste.

 If a toilet has not been used for an extended period of time, for example, a toilet in a holiday house, the water in the P-Trap may have evaporated, which can result in odours from the septic tank rising up into the house.

A simple flush of the toilet will eradicate this problem.

Poor ventilation of the sewage system can prevent odours from escaping, causing a bad smell to linger.

To prevent a septic tank from smelling or failing completely, the system must be adequately designed and routinely maintained.

It is best to keep an eye on what goes into the tank and how often it is emptied to ensure that the system remains viable and is not overburdened.

Feel free to contact us with any questions if you have any of the above septic symptoms:

Luis Goncalves, ROWP, IN, PL


Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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