HomeMy WebLinkAboutSP201900006 Correspondence 2019-12-16 •SHIMP :ENGINEERING, RC..
Design Focused Engineering
.December 16, 2019
Frank Pohl
County Of Albemarle
Department of Community Development
401 McIntire Road,North Wing.
Charlottesville,Virginia 22902
RE: ZMA2018-01$ River's Edge
Pump Station Preliminary-Design
Dear Mr. Pohl,
As part of the plan requirements,we have completed a basic preliminary design for the sewer pump
stations.
Flow Calculations:
A design population of 100 2-person units has an average:design flow of 100 gpd/person=200 gpd/unit.
(SCAT regs -:9YAC25-790-460-F).
The design peak capacity of the system.shall be 2.5 times this avg. design flow.
(SCAT regs:: 9VAC25-790-310-D)
Avg Design Flow= 100.units x 200 gpd/units:=20,000 gpd
Peak Design Flow=2.5 x Avg Design Flow= 50,000-gpd (34.7 gpm)
Minimum design pump rate must exceed 35 gpm to allow for the Peak Design,Flow,
Basic Pump Station Design:
The pump station shall bean inline underground wet well with submersible pumps that will pump
effluent through a force main at select times when the sewage level reaches set.elevations in the wet
well. The pump station will consist of a remote monitoring system that will be.managed by a contracted
professional third party, a backup generator in case of loss of power, and an.onsite alarm in case of
failure.
Preliminary Wet Well Sizing:'
The wet well will be a 60"precast lined watertight manhole. For a wet well with 10'sdepth well and a
.design lead pump switch"on" elevation at 4' above the invert,the Well storage would teach 540 gal. of
effluent before the lead pump engaged. In this scenario,the pump would operate roughly 37 times per
day (using the avg. design flow estimate of 20;000 gpd for the 100 units from above). With this basic
design,the lead pump would engage on average every 39 minutes.This cycle period minimizes potential
nuisance from the pump station by ensuring the sewage:does not sit long enough to go anaerobic and
also ensures potential adverse noise impacts on future residents are mitigated:through cycle frequency
management.
912 E..1 ighl:St.Charlottesville,VA 229021 434'.227.5140ii shutip-engirieerioq:Coin
Additional Design Considerations:
The following considerations are based on previous design experience with pump stations, and the final
design shall account for these considerations.
1. Since the pump station will be handling raw sewage,pumps and force main must be capable of
passing 2" solids. Thus, the minimum force main size should be 3". A minimum scour velocity of 2
fps must be achieved within the force main.
2. To remove cumbersome.maintenance requirements, submersible grinder pumps shall be used.
3. Two robust pumps shall be chosen to provide redundancy. Multiple switches shall be implemented
which will trigger first one,then both pumps when sewer levels reach critical elevations within the
wet.well.
4. CCS Tracer wire shall be installed along the force main to allow for maintenance to immediately
identify the location from above ground.
5. To prevent unpleasant odors, which are mainly caused by sewage going anaerobic, a small air pump
and diffuser shall be installed for the wet well.
6. Fencing and vegetative screening shall be provided to make the pump less conspicuous for residents.
If you have any questions please do not hesitate to contact me at your convenience. I can be reached at:
justin@shimp-engineering.com or by phone at 434-227-5140.
Regards,
Keane Rucker -
keane@shimp-engineering: (434)299-9843
Copy:justin.@shimp-engineering.com