FLOW INTO AND OUT OF THE TANK

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).

Inlets

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.

Outlets

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.)

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