Design of a sewer sump station w/ additional flows from upstream pump stations

[PBW2]

How do you handle design of a pump station that has gravity flow plus pumped discharge from upstream pump stations?

We have (2) pump stations that pump into the gravity sewer system (an 8" pvc sewer @ 0.33% slope). The (2) existing upstream pump stations are 100 gpm each. A conservative approach would be to calculate the average daily flow into the pump station multiply by peaking factor and then add 200 gpm from the pump discharges. I believe this would oversize the pumps but would be appropriate for discharge directly into the wetwell.

Since the upstream pumps discharge into the gravity system then flow through 3000 lf of 8" sewer, I'm thinking that a more realistic approach is to calculate the gravity flow (gpm) from the full sewer pipe into the wetwell and then add that to the average daily flow into the pump station multiplied by peaking factor.

We'd have 2.5xADF (peaking factor x average daily flow)+ maximum flow from an 8" pipe.

 

[bimr]

Is the 3000 lf sewer an interceptor? It may be helpful to provide a sketch.

 

[PBW2]

No the 3000 lf of gravity sewer is a sewer system within a neighborhood. There are manholes every 300 to 400 ft.

 

[77JQX]

I think you're splitting hairs. While I agree the travel time might attenuate flows, if the travel time is large compared to the upstream pump run times. But I doubt a complete analysis will result in a significant downsizing of the pumps compared with the first method. I'd go with the first method.

 

[PBW2]

Wouldn't the max flow from the gravity pipe flowing full will attenuate the flow? With a 100 gpm pump discharging into a manhole with an 8" gravity sewer pipe @ 0.33% slope, max flow out would be approximately 40 gpm. There would be some surcharging at the first manhole.

 

[77JQX]

What happens to the other 160 gpm going into the manhole?

 

[bimr]

You should be sizing the new wetwell at a maximum volume of approximately 30 minutes. Repumping of wastewater has the potential for developing anaerobic conditions and odors. You may need odor control equipment if you are close to a residence.

Try to accommodate the peak demand with two or three duty pumps. It may make sense to use three pumps for this application. Try to accommodate the normal demand with one duty pump.

The realistic approach would result in a better design. Otherwise you will be oversizing the pumps in the new wetwell with long periods at night with minimal flow. Investigate the pump run times from the old lift station. The flow from the old lift station will be the pump run time, not the maximum pump capacity.

 

[PBW2]

bimr/77JQX - thanks for your replies.

A little more info and a sketch...the pump station (PS1) is an existing pump station that we are looking at replacing/upgrading the pumps within the wetwell. The existing ADF into the wetwell for PS1 from the gravity sewer has been determined to be 24 gpm (60 gpm w/ peaking factor). Another neighborhood pump station (PS2) pumps 100 gpm into a manhole in the gravity sewer system for PS1. The client is proposing a new pump station (PS3)to pump an additional 100 gpm into the sewer system of PS1. Conservatively (or maybe not) we could size the new pumps @ PS1 for 260 gpm (100 gpm PS2 + 100 gpm PS3 + 60 gpm (2.5xADF)). See the attached sketch. I hope this makes sense.

 

[bimr]

You have not mentioned possible I?I which may or may not be an issue. Generally, the design peak flow for sanitary sewers shall be selected based on one of the following methods:

A) The ratio of peak to average daily flow as determined from Appendix D, Figure No. 1.

B) Values established from an infiltration/inflow study acceptable to the Agency.

ftp://

http://www.ilga.gov/JCAR/AdminCode/035/03500370ZZ9996dR.html

Peak flow for small systems is generally higher than 2.5:1.

It would be overkill to just add all the peak flows together. Your realistic method makes more sense.

Consider installing 3 pumps. A 3 pump system will provide more flexibility over the range of expected flows.