5.0 Lift Station and Forcemain
Design of the lift station generally consists of a single wet-well configuration, approximately 20 feet deep, which will house two submersible pumps. It is anticipated that the two pumps will each have the following specifications:
- 5-7.5 hp
- 120 gpm @ 50’ Total Dynamic Head (TDH)
- 1,750 rpm
- Able to pass 3” solids (non-clog pumps chopper style pumps)
A magnetic flow meter will be installed in this lift station. Weather-proof enclosures will be used to house electrical and I/C equipment. Wet well level will be measured by ultrasonic non-contact equipment; float switches will be provided in low and high positions for redundancy. Odor control measures for the grease interceptor and wet well will include the installation of either a liquid chemical feed system and chemical storage tank and/or a passive air filter with replaceable activated media.
A force main with an approximately 4” ID will be constructed from the lift station site to an interceptor manhole (see map in Appendix B) a distance of approximately 1,300 linear feet. Design calculations used to size the sewer force main, sewage pumps and emergency storage tanks are included in Appendix F. Further calculations dependent on wet well size, including cycle time, # motor starts per hour, will be included in the Basis of Design Report. Security Sanitation District limits the flushing velocity to 6 ft/s. The velocity at 120 gpm for 4” DR11 HDPE is 3.26 ft/s, which is adequate to effectively flush the force main while remaining below the maximum of 6 ft/s.
RMG Engineers performed a geotechnical investigation of the River Bend Creek development site, which includes the proposed lift station site and prepared a report based on the results of the investigation. The report, dated April 2, 2018, is included as Appendix H. The geotechnical investigation included eleven (11) bore holes which were drilled to a depth of twenty (20) feet. Bore hole R-11 was located on the proposed lift station site. The R-11 bore hole and soil sampling indicated that soil at the lift station site is comprised of sand, silty to clayey to a depth of approximately ten (10) feet, and sand, silty with gravel to an approximate depth of fifteen (15) feet. Claystone and shale bedrock was encountered to a depth of fifteen (15) feet and continued through the remaining depth of the bore hole. The installation of the wet well may require minimum excavation of this bedrock. Based on geotechnical recommendations, over-excavation and replacement with compacted structural fill would be provided between unsuitable soil and foundation elements. The bore hole also revealed a ground water depth of seventeen (17) feet below the ground surface. This water table will require the wet well manhole and auxiliary storage tank to be ballasted to resist the buoyant force of the groundwater. Further evaluation of ballasting requirements will be completed and summarized in the design submittal.
A preliminary cost estimate for the lift station and force main are included in Appendix I.