Septic Tanks and Onsite System Components
The single most important idea
The septic tank is not where the wastewater is treated. The tank is a separator and a
store. The treatment happens in the unsaturated soil beneath and around the absorption
field, where aerobic organisms in the biomat and in the soil profile oxidise organic matter, and
where filtration, adsorption and time retain and destroy pathogens.
Everything else follows from that. It is why soil character and the depth of unsaturated soil
below the trench matter more to performance than anything about the tank, and it is why solids
carried out of the tank are so damaging - they blind the surface where the treatment occurs.
What the septic tank does
Two functions:
- Separation - heavy solids settle to form sludge; fats, oils, grease and light material
float to form scum. A clarified middle laye