What type of system relies on gravity to move wastewater through the treatment process?

A gravity system is designed to use the natural force of gravity to facilitate the movement of wastewater through the treatment process. In such systems, wastewater flows downhill through pipes, starting from the source—often a home or facility—toward the treatment area, typically a septic tank or drain field. This method is advantageous because it requires minimal mechanical energy, resulting in lower operational costs and reduced complexity in design.

In contrast, pressure distribution systems rely on pumps to distribute wastewater evenly across a treatment area. These systems can be more effective in certain terrains or where gravity systems are not feasible, as they can help manage the distribution of effluent to prevent flooding of any specific area. Advanced Treatment Units (ATUs) and step-down systems incorporate various additional technologies or configurations to manage and treat wastewater, but they also do not primarily depend on gravity as the primary means of moving wastewater.

Thus, the fundamental principle underlying gravity systems makes them the correct choice when identifying systems that rely solely on gravity for wastewater movement.