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Flowrate required to prevent sludge buildup in raw sewage pipeline

silverfox8028 (Civil/Environmental) (OP)
19 Jan 11 11:52
I have an 8" HDPE raw sewage pressure main 13,000' long running uphill a total of 110'at almost a constant slope with a few 5' dips in elevation (under culverts, etc.). What flowrate must be maintained to prevent settlement of sludge from plugging the pipeline over time(scouring)? Currently the pumps only run a few hours per day until future housing developments tie in (years from now) and I'm insisting they run long enough each time to totally flush the line plus 10-15% more.
Do I have a problem or not?
coloeng (Civil/Environmental)
19 Jan 11 12:37
According to ASCE Manual of Practice No. 37, "Design and Construction of Sanitary and Storm Sewers", velocities normally fall within the range of 3-5 fps and velocities of 2.5-3 fps are required to resuspend solids that have accumulated.  So your looking at a minimum flowrate of 0.83 cfs (375 gpm) to resuspend the solids.
bimr (Civil/Environmental)
19 Jan 11 14:26
Most references state that you need approximately 3.5 ft/sec to resuspend solids that drop out when the force main flow stops. If you operate continuously, you need approximately 2 ft/sec velocity to keep solids in suspension.

When starting the force main, your pumps should operate at the higher flow to resuspend the solids. You can then throttle back to the lower flow rate.

You may also have a problem with air pockets at the lower velocity. You need at least 3.5-4 ft/sec to force the air pockets through the force main.

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