A septic tank does not technically “fill up” with wastewater because liquid effluent continuously flows out of the tank into the drain field. However, solid waste and sludge slowly accumulate inside the tank over time. The rate at which a septic tank fills with solids depends on household water usage, the number of people living in the home, and the size of the septic tank. Understanding how septic tanks work helps homeowners know when pumping is necessary and how proper maintenance protects the entire septic system.