It's a water tight tank, when waste enters it, organic matter floats to the top where bacteria converts it to liquid. Inorganic matter of bacterial digestion sink to the bottom of the tank, making a SLUDGE LAYER. Water entering the tank takes up the space between the floating layer and the sludge layer, this is needed to have a good working septic tank. If your septic is not maintained on a regular basis, usually every two to three years, solids from the sludge layer can go up and out of the outlet pipe into your leach field. This will continue to plug up your leach field and you may have to replace it in the future which is a costly item.