The household wastewater flows into the tank where the average detention time is 30 hours. The tank should have at least 1,000 gallon capacity. Three layers are formed in the septic tank. There are baffles in the tank which keep the wastewater flowing though the tank at a level near the middle of the tank.
Solids settle to the bottom forming a layer of sludge.
The grease and foam float to the top forming a scum layer. Both the sludge and scum layers remain in the tank where bacteria work to break them down. In time, the layers build in size. When the layers become too large and take up too much space in the tank, the tank needs to be pumped. Every 3 years the tank should be pumped out by a licensed septic system pumper.