All wastewater from the household enters the septic tank. The solids settle to the bottom of the tank and form a sludge layer. The fats and greases collect at the surface, forming a scum layer. Micro-organisms slowly break down the matter in both the scum and sludge layers. Any material that cannot be broken down will gradually build up. As a result the tank needs to be pumped out periodically by an accredited waste service provider (usually every 3-5 years).
The wastewater then flows, or is pumped, to a sub-soil bed or sub-soil trench for disposal. The wastewater is distributed along the length of the bed or trench through a system of pipes.
Eventually, the wastewater is absorbed by the soil and/or plant roots around the disposal area.