Wastewater and the solids it carries travel into the septic tank. Upon entering the tank the wastewater hits a baffle. The baffle stops the wastewater from rushing in and stirring up the sludge. Another baffle at the outlet keeps the scum layer, greases etc. In the tank, most solids settle to the bottom, where bacteria partially break down the materials, forming sludge.
Some material floats on the water, forming a layer of scum.
The wastewater flows into the disposal field, carrying viruses, bacteria, nitrate and any chemicals added to the water. The most common disposal field design uses perforated pipe laid on a bed of screened or crushed stone. The size of the disposal field depends on the number of people using the system and on the permeability of the soil. Each perforated pipe, or distribution line, carries the wastewater into the field where the water moves through the stone and into the soil. The soil acts as a filter by removing larger particles from the wastewater, and slows the rate of travel of the water.