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The velocity needs to be 20 centimeters per second to keep feces moving. A 4 inch pipe has a cross-sectional area of 80 square cm. At a velocity of 20 cm/sec, that's 1600 cubic centimeters per second = 1600 ml/sec
= 1.6 liters/sec
= 0.42 gallons/sec
= 1500 gallons per hour minimum flow to move feces.
3 inch pipe has cross-sectional area of about 45 cm2 x 20 cm/sec = 900 cm3/sec = 0.9 l/sec = 0.24 gal/sec = 864 gal/hr minimum flow to move feces.
A related and more frequent problem is bio-fouling inside the pipes. Snails, insect larvae mud tubes, and other things attach themselves to the inside of the pipe. This traps additional debris and the process snowballs. The process is somewhat self-correcting since the water velocity increases as the pipe becomes constricted. The higher velocity makes it harder for more material to accumulate. However, on a gravity fed drain, the pipe and flow can become constricted enough to lower the standing water level in the sump.
In designing the bottom drains, its nice to have some provision for reaming out the pipe if it becomes fouled. If there are no clean-out plugs, you can attach a string to a plastic golf ball and let the water flow carry the ball through the drian line and into the sump. Then attach a large abrasive scrubby thing (technically called a 'pig') to the string and pull it back and forth through the pipe to clean it.
-hopkins
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