When wastewater enters your septic tank, it naturally divides into three parts:
Solids sink to the bottom and form a sludge layer.
Liquids stay in the middle and form a layer of mostly water.
Oils and fat rise to the top and form a scum layer.
The liquid layer accounts for 90 percent of your tank's capacity, meaning excess water use can affect the tank's performance over time.
Ideally, water in your tank flows through in the course of several days while materials on the bottom are broken down by bacteria. Water is then carried through drain pipes to the drainage (or leach) field, where it is distributed into the soil. The size of your drainage field depends on the type of soil. Clay, for instance, holds a limited amount of water.