How does my conventional septic system work?

This is how a conventional septic system works:

1. All water runs out of your house from one main drainage pipe into a septic tank.
2. The septic tank is a buried, water-tight container that holds wastewater long enough to allow solids to settle down to the bottom, while the oil and grease floats to the top.
3. The liquid wastewater then exits the tank into the drainfield.
4. The drain field is a shallow, covered, excavation made in unsaturated soil. Pretreated wastewater is discharged through piping onto porous surfaces that allow wastewater to filter though the soil. If the drainfield is overloaded with too much liquid, it will flood or create backups in toilets and sinks.
5. Finally, the wastewater percolates into the soil, naturally removing harmful substances.