The inside of the tank is sectioned into two compartments. The inlet side, where our flush went, is twice as big as the outlet side. Bacterial and enzyme action within the tank breaks down the solids and grease to form a sludge which settles to the bottom. Regular pumping keeps the sludge from building up in the tank and eventually getting out to the leaching area. The only opening in the wall forming the two compartments is a slot across the middle of the wall. This slot, about 1 1/2" wide, allows only the liquid from the middle of the first compartment to enter, as the grease floats at the top and the solids and sludge settle to the bottom.
Since our flush increased the volume of liquid in the first compartment and water "seeks its own level", a proportional amount of liquid has passed through the slot into the second compartment, the outlet side. The pipe that will bring our flush to the leaching area penetrates the wall of this compartment at an elevation three inches below the inlet pipe. Attached to the pipe is a vertical pipe forming a sideways "T" called a T-Pipe or outlet baffle.
In the case of recently installed tanks there is instead a similar "T" containing a re-usable plastic filter insert, called an outlet filter, which should be cleaned each time the tank is pumped. The filter element is simply pulled out from the top of the filter housing which forms the outlet "T" and hosed off. It is then re-installed.