Settling of solids in wastewater

[DC20]

We are designing a wastewater lift station having a force main which is going to be 28,000 ft long. The force main will discharge into a standpipe structure. This structure has the existing gravity sewer on the other side.

During low flows, when the pumps are stopped at the lift station it would take approx. 14 minutes to fill back the wetwell. During this period of no pumping we are concerned about settling of solids in the standpipe and blocking the entrance of stand pipe for the force main entrance. For now we don't have any data, but we may get it as the design progresses. So we have to rely on theoretical calculations (if possible) for now.

Is there is way to estimate the amount of solids that will settle during 14 minutes? I have estimated solids load (assuming typical solids concentrations), but have no idea how to make a reasonable estimate of the settling of solids.

Can anyone point correct direction?

Any help is appreciated.

 

[civilperson]

The primary pumps should be sized to run continuously at low flows and then the secondary pump should kick on at high level switch.

 

[coloeng]

In a typical sanitary system,there shouldn't be enough solids in the water to block the exit from the force main. Also, any solids that do settle shouldn't have time to consolidate in just 14 minutes and the flow from the force main should resuspend them.

If you design your standpipe structure so that the exit to the force main is above the outlet to the gravity sewer and slope it, you shouldn't have any problems.

 

[dicksewerrat]

If you use a high rpm pump you will have much smaller particles. Just remember to geta pump that will pass 3" solids. However 'solids' may be a misnomer. Most sewage solids are pretty soft. Execpt for the stuff that shouldn't be there. Diapers, towels, plastic toys and such.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

You should fillet the bottom corners of the standpipe with concrete so that there are no flat spots at the standpipe bottom where soilds will collect.

The recommmended angle of repose of the fillets is 55 degrees. Solids will not collect on a 55 degree angle.

If you fillet the bottom of the standpipe and if your sewer is exiting the bottom of the standpipe at a minimum flow of 2 ft/sec, then you should have no problems with solids.