Most modern septic tanks, whether made of concrete or plastic, have two compartments separated by an internal baffle and an inlet and outlet port. When the tank is first installed, each port usually has a preinstalled 4-inch sanitary tee fitting, and you connect the waste line from the building to the inlet fitting and the drain line to the outlet fitting either by gluing it or using a mechanical flexible coupling (often referred to as a Fernco coupling). If these tees aren't preinstalled, you have to install them before you can connect pipes to the tank.
The Reason for the Tees
In days gone by, septic tanks had only one compartment. Tees were placed at the inlet and outlet to force incoming waste to the bottom of the tank and to prevent the scum layer at the top, which contains greases, oils and other lighter-than-water contaminants from flowing out to the drain field and clogging the soil. Modern tanks have baffles that perform this function, but the practice of installing tees persists, even though some professionals wonder if they're actually necessary, as evidenced by a recent discussion on Plbg.com.
Video of the Day
Whatever your stance on the necessity of the tees, they are insurance against the septic tank baffles failing, and it's prudent to install them. When properly installed, they direct waste downward and allow sewer gases to escape through the top of the tee, which terminates just below the lid. Some plumbers install a grate on the upper portion of the tee to keep out debris, but it's not a necessity, and under no circumstances should a grate be installed on the lower portion or the pipes will clog.
How to Install Septic Tees
If the tees don't come preinstalled on the septic tank, you have to do it yourself from inside the tank. The inlet and outlet holes are predrilled or, in the case of concrete tanks, preformed, and they are usually sized to hold a 4-inch tee tightly. You don't always need adhesive, but it isn't a bad idea to use some anyway, and a bead of butyl or silicone caulk around the perimeter of the tee on both sides of the tank will do the job.
The top of the tee should have a short length of pipe attached to extend the opening above the scum layer in the tank, while the bottom of the tee must extend below the scum layer or about 2 feet below the tee. You can glue a length of 4-inch pipe to the tee to extend the bottom or you can install a septic tee pipe, which often includes an internal filter.
Connecting Inlet and Outlet Pipes
The trenches for the waste and drain pumps slope toward and away from the tank, respectively, with a slope between 2 and 10 percent. If the slope is gentle enough, you can glue the pipes directly to the tee, but if the slope is steep, it's a good idea to glue a 22 1/2-degree bend onto the tee to make the glue connection watertight. If needed, the bend faces upward on the inlet side and downward on the outlet side.
Even though the pipes fit tightly in the fittings, it's important to glue them. If you don't, the tee could become detached and fall into the tank, and that's a fix that requires professional services. Falling into a septic tank or even looking into one too closely can be fatal, so never try to make this repair yourself.