Saddle Creek RTB - From Concept to Reality

INTRODUCTION The Saddle Creek Retention Treatment Basin (RTB) is a significant project as part of the City of Omaha's water quality program 'Clean Solutions for Omaha' to address compliance with their Long Term Control Plan to address combined sewer overflows. Prior to the construction of the facility, over 65 times per year, untreated combined sewage overflowed into the Little Papillion Creek (LPC) from the City's CSO 205 outfall structure. With the new facility in place, combined sewage will be diverted to the RTB to receive grit and screenings removal, disinfection, solids settling, and dechlorination before being discharged back to LPC. The Saddle Creek RTB project began planning and design activities in April of 2011. Construction commenced in Spring 2019 and is anticipated to start operations in the summer of 2023. This facility will provide storage and equivalent to primary treatment of combined sewage flows up to the permitted design flow rate of 160 million gallons per day (MGD) with the ability to screen, remove grit, and disinfect flows up to 320 MGD. Lower volume flows will be captured within the basin and will be pumped into the Little Papillion Creek Interceptor (LPCI) and conveyed to the Papillion Creek Water Resource Reclamation Facility (PCWRRF) for full secondary treatment. Larger volume flows will receive grit and screenings removal, disinfection, solids settling, and dechlorination before being discharged to the LPC. The facility will result in a significant reduction in the volume of untreated CSO, total suspended solids (TSS), and E coli bacteria entering the LPC. This presentation will describe how the Saddle Creek RTB will effectively and reliably capture and treat untreated combined sewage in compliance with regulatory requirements. FACILITY OVERVIEW The Saddle Creek RTB facility consists of a 3.3 million gallon (MG) underground concrete storage basin, with grit removal, mechanical screening, chemical disinfection, gravity effluent discharge, a dewatering pump station, as well as Headworks, Operations, and Chemical Buildings. The facility also includes a diversion structure, sewers, odor control, stormwater BMPs, driveways, and other appurtenances. Refer to Figure 1 for a Process Flow Diagram illustrating the various features of the RTB facility below ground. The facility operates during wet weather events. Prior to the completion of the Saddle Creek RTB project, an existing dry weather diversion weir directed dry weather and a portion of wet weather flow, up to 40 million gallons per day (MGD), was sent to the existing Dupont Grit Facility and ultimately to the LPCI. Prior to this project, any excess flow was discharged untreated to the CSO 205 Channel and eventually the LPC. The existing dry weather diversion pipes will be abandoned as part of this project and replaced with a new 60-inch sewer which will pass through a grit pit within the new facility's Headworks Building. The diversion weir will send flows using real time control and smart sewer technology to provide gate control of up to approximately 60 MGD to the LPCI before overflowing into the RTB influent sewer. This new diversion chamber will divert wet weather flow up to the peak treatment design flow of 160 MGD to the RTB. This diversion chamber configuration ensures that flow rates up to the design flow can pass through the RTB and does not increase the risk of upstream flooding during larger events. Captured volume in the tank will be pumped into the LPCI and conveyed to the Papillion Creek Water Resource Recovery Facility (PCWRRF) for full secondary treatment. Flows more than the facility capacity will be routed around the RTB and discharged into the LPC. Dry weather flow and some wet weather flows up to approximately 60 MGD will pass into a new grit pit, the diversion flow grit pit, constructed in the proposed Headworks Building. For flows passing through the RTB, large grit and other heavy materials will drop out via gravity into a second, larger grit pit, the RTB grit pit, prior to entering the screen channels. Downstream of the RTB grit pit, mechanically cleaned screens will provide for the removal of objectionable solids with the intent of protecting downstream equipment and controlling floatables. The facility includes three (3) continuous duty multi-rake screens with a clear opening size of ¾-inch. Disinfection must result in an effluent that meets E. coli and total residual chlorine (TRC) permit limits. Liquid sodium hypochlorite and sodium bisulfite will be used for disinfection and dechlorination. Above ground improvements include a building to house controls, grit and screening equipment, and chemicals. Refer to Figure 2 for a rendering of the finished facility. PROJECT COMPLETION STATUS In December 2018, the City received bids within the budget for the project and in February 2019, a contract valued at approximately $90 million was approved to build the Saddle Creek Retention Treatment Basin facility. Construction began in April 2019. Wade Trim performed design engineering on the project and is currently providing construction management services. The project is scheduled for Substantial Completion in Spring 2023. Refer to Figures 3 and 4 for construction progress photos. CONCLUSIONS The project's innovative use of a 'gravity in, gravity out' design for the RTB facility allowed for a reduction in pumping of significant flows. The use of real time controls and smart sewer technology will allow this facility to operate efficiently and effectively over a wide range of conditions, which have been evaluated through extensive hydraulic modeling and development of process control strategies. The use of instrumentation for flow and level measurement used in process control must incorporate redundancy and be flexible to allow for changes in available technology and must be compatible with other technologies being used at the facility and within the collection and treatment system.