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