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How does an absorption trench work?

What is an absorption trench?

An absorption trench is a depression or excavation in the ground wherein the wastewater is distributed to reduce odour and filtration of pathogens, toxins, and other pollutants. Once the wastewater is treated by the primary method (septic tank) or secondary method (wastewater treatment plant), the effluent is gravity fed/pumped into the absorption trench so it can be further treated by the bacteria present in the soil.

The depth, length and size of the trench depend on the volume of wastewater to be treated. The absorption trenches are narrow and deep and the rate of absorption of the effluent depends on the type of the soil.

Absorption trench diagram

How does an absorption trench work?

In a septic system, the wastewater flows to the septic tank where the solids settle to the bottom and form a sludge layer. The fats, oil, and grease settle at the top to form a scum layer. The middle layer is called the effluent layer and it is gravity fed or pumped to the absorption trench. The effluent is distributed throughout the length of the trench via pipes. The holes in the absorption trench allow effluent to seep into the surrounding gravel. Gravel allows oxygen to reach the bacteria and water to be absorbed by the soil.

The absorption trench works in the following way:

  1. Effluent is broken down by in the soil by a combination of sunlight, bacteria and plants.
  2. Bacteria uses two methods to reduce the wastewater pollutant levels - Aerobic and Anaerobic digestion. 
  3. In aerobic digestion, the wastewater is purified and broken down into more environmentally friendly chemicals in the presence of oxygen.
  4. Anaerobic digestion breaks down the organic matter in the absence of oxygen. 
  5. The treated wastewater can evaporate from the soil, seep into the groundwater or is used by plants or grasses in the drain field. 
Absorption Trench Operation ( Source: Hornsby Shire Council)

Size of absorption trench

The size of the absorption trench depends on:

  1. Size of your household
  2. Type of soil
  3. Draining capacity of the soil
  4. Size of the drain field
  5. Vegetation

It is important to design the trench according to household demands - high load on the septic system would require a large absorption trench.

Why do absorption trenches fail?

Common reasons for the failure of absorption trench are as follows:

  1. Incorrect installation
  2. Large plants or tree roots 
  3. Poorly maintained septic system
  4. Uneven trench base 
  5. Too much water - both wastewater and storm water can overwhelm the drain field 

How long do absorption trenches last?

Absorption trenches can last for 25-30 years if they were designed, installed and maintained properly. However, a poorly constructed trench or poorly maintained septic system can fail after a couple of years. Proper maintenance, right location and a well-designed trench will ensure your septic system lasts for years without a problem. 

Maintenance of septic systems

  1. Regular cleaning/pump out (every 3-5 years) of the septic tank ensures the solid waste doesn’t flow into the absorption trench. If solid waste flows to the trench, its ability to distribute effluent will be severely affected.
  2. Using water efficiently
  3. Proper disposal of waste in the household
  4. Regular maintenance of the drain field. 
  5. Regular inspection of the drain field for leaks and odours
  6. Keeping the vehicles or heavy machinery away from the drain field.

Sources

Septic Tank and Absorption Trench Manual

Water NSW Absorption Trench

Operation and Maintenance Manual - Septic Tank and Absorption Trench System

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