FLOW INTO AND OUT OF THE TANK
The inlet and outlet ports of the tank are generally equipped with devices such as baffles, concrete tees, or in more recent years, sanitary tees (T-shaped pipes with one short and one long leg).
The inlet device dissipates the energy of the incoming flow and deflects it downwards. The vertical leg of the tee extends below the liquid surface well into the clear space below the scum layer. This prevents disturbance of the floating scum layer and reduces disruptive turbulence caused by incoming flows. The inlet device also is supposed to prevent short-circuiting of flows across the water surface directly to the outlet.
The upper leg of the inlet should extend well above the liquid surface in order to prevent floating scum from backing up into, and possibly plugging, the main inlet pipe. The open top of the inlet tee allows venting of gases out of the tank through the inlet pipe and fresh air vents of the household plumbing.
The outlet device is designed to retain the scum layer within the tank. A sanitary tee can be used with the lower leg extending below the scum layer. The elevation of the outlet port should be 2 to 3 inches below the elevation of the inlet port. This prevents backwater and stranding of solids in the main inlet pipe during momentary rises in the tank liquid level caused by surges of incoming wastewater.
Typical inlet/outlet tees Gas Deflection Baffle
Gases are produced by the natural digestion of sludge at the bottom of the tank, and particles of sludge can be carried upward by these rising gases. Some tanks have a gas deflection baffle, which prevents gas bubbles (to which solid particles often adhere) from leaving the tank by deflecting them away from the outlet and preventing them from entering the drainfield. (See drawing, p. 6.)