All wastewater generated from the dwelling enters the septic tank where primary treatment occurs. Anaerobic bacteria (bacteria found in environments without oxygen) thrive in the septic tank where they organically break down solids, creating sludge. Lighter materials such as grease and soap float to the top of the tank and form a layer called scum. This simple setting process allows virtually all of the wastewater (effluent) to flow out of the septic tank free of grease, soap and solids. It is important to remember that septic tank inlet and outlet pipes are near the top of the tank. Thus, the septic tank always appears full.
The relatively clear wastewater exits the septic tank outlet and flows into the distribution box (D-box). The purpose of this distribution box is to evenly distribute the wastewater flow through the drainage field. Once in the drainage field, the wastewater passes through perforated lateral pipes, a layer of crushed stone and finally through several feet of unsaturated soil. As the treated wastewater travels through the soil, bacterial die off and the water is purified.