What Happens in the Trench?

  • The effluent is distributed through the pipes/gravel or chambers, then percolates down into the soil.
  • Oxygen is present and aerobic bacteria break down the waste.
  • Viruses are held by soil particles and die over time.

Wastewater Treatment/Groundwater or Bedrock Depth

  • This chart illustrates the concentrations of materials entering the septic tank, leaving the septic tank, 1 foot below the trench, and 3 feet below the trench in a properly working septic system.
  • Adequate unsaturated soil depth prevents bacteria and pathogens from entering the groundwater.
Parameters Raw waste Septic tank effluent 1' below trench 3' below trench
Viruses *(PFU/ml) unknown 100,000 to 10 million 0 to 1000 0
Fecal coliform 1 million to 100 million 1000 to 1 million 0 to 100 0
Nitrogen mg/l 100 to 500 50 to 60 50 to 60 50 to 60
BOD5 (mg/l) 270 to 400 140 to 175 0 0
Phosphorus mg/l 10 to 40 10 to 30 0 to 10 0 to 1
  • Note that bacteria and viruses are not found 3 feet below the bottom of the trench.
  • Therefore, to protect groundwater, there must be at least 4 feet of soil between the bottom of the drainfield and the highest expected level of groundwater or bedrock.
  • Note that the wastewater treatment system does not have much effect on nitrogen levels.

*(PFU/ml)- Plaque Forming Unit

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