What is a septic trench?

Once the primary treated (traditional septic tank) or secondary treated (wastewater treatment plant) effluent moves through the tank treatment process the resulting wastewater is gravity fed or pumped into the septic trench or bed for absorption into the surrounding ground.

The septic or absorption trench is located below the ground and used to effectively distribute wastewater while reducing odours and contamination of gardens or the surrounding property, gently filtering out pathogens, toxins, and other pollutants.

The septic trench is usually narrow and deep, while septic beds tend to be wider and shallower. The absorption method used will depend on the soil and layout of your property.

Absorption trench design

Septic trenches are often around 500 to 700 mm deep and around 600 mm wide, however, check with your local council for the specific specifications in your area. Dimensions, including trench length and number, can also vary depending on the soil type of your property, the expected wastewater volume, the local climate, and site features.

The absorption trench can be media filled or fitted with a durable self-supporting arch placed on gravel or coarse sand. Care needs to be taken during the design and installation of your trenches to ensure:

• Effluent is evenly distributed along and between trenches
• Effluent effectively reaches the end of the trench
• All pipes and arches are laid to the manufacturer's recommendations.
• Inspection ports are correctly placed

Septic trench process

1. Treated effluent flows into the trench from the septic or wastewater system
2. The wastewater moves along the absorption trench seeping through the slotted distribution pipes
3. The effluent is then filtered through the surrounding layer of gravel and sand to the underlying soil.
4. The nutrient-rich wastewater is taken up by the nearby grass and small plants.

What is absorption trench failure?

Absorption or septic trench failure is when the trench becomes compacted, blocked or collapsed. Trench failure can be caused by:

• vehicles driving over the trench area
• large animals
• incorrect installation
• areas of soft soil
• frequent foot traffic
• Blocked pipes
• Root intrusion - large plants or trees located too close to the trench system
• Too much other water in the area - eg stormwater runoff
• Uneven trench base - causing one area to receive too much water
• Infrequent septic tank cleaning - causing untreated waste to enter the trench system

It is recommended that property owners regularly inspect septic system trench areas for signs of trench failure and address any issues promptly.

A well-designed professionally installed and regularly maintained septic trench system can last more than 20 years without significant issues.