The minimum size and the location of a septic tank and its absorption field must comply with state regulations. Sewage flow coming from the house is separated into three fractions after it enters the tank. Grease, oils and other light materials accumulate at the top of the tank in a layer of scum. The heavier solids settle into a sludge layer at the bottom of the tank. The intermediate layer is wastewater.
Wastewater passes on to the absorption field.
The scum and sludge are held in the tank by a set of baffles. Since the settling process requires time, tanks are designed so that liquids are retained for about 24 hours, under normal flow conditions, before discharging to the absorption field. Even though it has no moving parts, the septic tank requires regular maintenance. Annual inspections of the baffles are recommended to ensure that scum is not leaving the tank and entering the absorption field. Similarly, accumulated sludge must be removed on a regular basis.