Coffs Harbour & Coast. Septic Tanks Full & Absorption Trenches Collapsed Overflowing Backing Up – Solved

Septic Tank Problems. What could be the cause?

Common Reasons For Septic Tank Overflowing

I believe that in most cases the water can not drain to the absorption trench due to either the pipe being blocked right near to the septic tank outlet (not very likely), the distribution box or the 3-way diverter is blocked or damaged(more likely) or my main belief based on experience is that the adsorption trenches have collapsed.


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Overflowing adsorption trenches

What is a Septic Tank Adsorption Trench?

Absorption Trench: This is an open cavity trench that are about 600mm under the surface and can be 10 or so metres in length. These are called adsorption trenches. Usually, 2 or 3 trenches all feeding from the diverter box or other names are splitter box. These trenches allow the water to disperse over a large area all under the surface, but if they have collapsed, they block the water from draining away.

This leaves the wastewater flowing from the septic tank only a very limited area to drain too and therefore the ground becomes saturated and cannot be evaporated fast enough.
The way a properly working adsorption trench system works is that the heat on the ground draws the moisture up from the large wet area underneath. The larger the trench area the greater the evaporation can occur. The reverse is also true.

Adsorption Trench System Today. Remember older systems were not the same quality or the same strength. This is a Everhard System

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Adsorption Trench Collapsed. Try to see the dipping in the soil

Please note the dips in the ground levels if possible. This seems to indicate the adsorption trenching position an also the collapsed nature of the trench. As a side point, it is always not recommended to have cattle walking over the trenching as the soil coverage above the trenching is quite shallow and damp in nature. The castles weight and skinny legs have a tendency to push down and through the trenching or forcing it to fold in on itself over time. Refer to the image of the plastic half moon used on septic trenching above.

Collapsed Adsorption Septic Tank Drain
Collapsed Septic Runoff Drain or Adsorption Pit

Water Coming out of the ground. It may be important not to have the cows walk on top of the trenches due to the wet soft soil conditions.

Sunken Drainage Line for a septic tank Adsorption Septic Tank Drain

Important Note:

The septic system and the trenches should never be near bore water source. The Bore water tables can be within 3 metres below the surface. The NSW Regulations on this is that a Bore and Septic tanks need to be 100 metres apart.

https://www.waternsw.com.au/__data/assets/pdf_file/0003/58251/Designing-and-Installing-On-Site-Wastewater-Systems-WaterNSW-CRP-2019.pdf
Pages 22-24