If the homeowner notices that the toilet isn't flushing as fast as it used to, or the floor drain is backing up when the clothes washer discharges, the problem may be a partially plugged house sewer pipe. There are also many other causes but this is one place to start looking. The house sewer becomes the inlet pipe to the septic tank. As the sewage reaches the tank it drops into the liquid in the tank with a downward flow direction. This drop tends to move the sewage into the depth of the tank.
Most states specify an inlet device, either a baffle or a sanitary tee.
The purpose of the inlet device is to prevent the floating solids, called the scum layer, from building up and plugging the end of the sewer pipe. Some states don't require inlet devices and many devices get knocked off if the house sewer is improperly rodded with a plumbing snake to open up a plugged pipe. The bottom of the inlet baffle or sanitary tee should extend below the surface of the liquid at least six inches and not more that 20 percent of the tank liquid depth see Figure 1. So for a tank that has 60 inches of liquid, the baffle or tee should extend at least 6 inches but not more than 20 percent of 60, or no more than 12 inches below the surface of the liquid. If the baffle or tee is not deep enough, the floating scum layer may tend to plug it.