<human>: What's the most common type of septic tank in the U.S.?
<bot>: The most common type of septic tank in the United States is a gravity-based septic tank, which uses the force of gravity to move waste and water through the system. This type of septic tank typically consists of a large underground concrete tank with a lid or cover that sits on top of it. Wastewater from the home flows into the tank via pipes or a drain field, and then the wastewater moves downwards towards the drain field, where the solids are removed by bacteria. The clean water then moves upwards back into the home.