What Makes a SSO Storage Tank a Cost Effective Alternative for Sanitary Relief Projects

Crawford, Murphy & Tilly, Inc. (CMT) was selected by the Metropolitan St. Louis Sewer District (MSD) to conduct a pre-design study of the Coldwater Creek trunk sewer which serves the nearly 50-square mile Coldwater Creek Service Area. The purpose of the predesign study was to evaluate all facets of a sanitary trunk sewer relief project that would provide the best alternative to eliminate sanitary sewer overflows (SSOs) and basement back-ups. The sections of trunk sewer evaluated are approximately 9.4 miles long and range in size from 24-inch to 72-inch diameter.MSD's conventional solution to most cases of surcharging sanitary sewers and basement backups that occur under wet weather conditions has been the construction of relief sewers to provide more sewer capacity. With a relief sewer, the excess wet weather flows could be transported to the wastewater treatment plant site where they would be stored until the wet weather event ended and the treatment plant had available capacity to treat the stored wastewater. MSD's conceptual design for this project was a deep tunnel relief sewer with an estimated cost of $120 - $160 million.However, the results of the pre-design study showed that the preferred alternative included a combination of two off-line flow equalization storage tanks out in the collection system, some relief sewers to be installed by microtunneling methods, and rehabilitation of the existing trunk sewer. By being able to store the excess wet weather flows out in the collection system instead of at the treatment plant, over 9 miles of relief sewers ranging in size from 30-inch to 96-inch diameter would not be needed. This preferred alternative, estimated to cost $87 million, had a significant cost savings compared to the second best alternative consisting of construction of a relief sewer by deep tunneling, estimated to cost $168 million.Our paper will discuss the four phases of the pre-design study: 1) Investigative Phase, 2) Development of the Selected Alternatives, 3) Preferred Alternative Phase, and 4) Preliminary Design Phase. We will also discuss some details of the alternatives evaluated. We will discuss several factors to consider when determining whether off-line storage out in the collection system will be feasible and cost effective for a given watershed. We will describe the hydraulic modeling used to size the flow equalization storage tanks, including a continuous simulation using 54 years of historical rainfall data to evaluate the effects of sequential storms on the required storage tank volumes.MSD subsequently selected CMT for final design of the first of the two recommended storage tank facilities, called Wet Weather Storage Facility Tank A. Our paper will discuss details of the final design of this storage facility and why certain features were selected.The Tank A facilities will include a 6 million gallon storage tank that is 180 feet in diameter with side water depth of 32 feet, a diversion structure, a pump station with a 30,000 gpm capacity to pump into the tank, an odor control system, and a real time control system that will automatically operate the facilities. The water levels in the trunk sewer will be monitored at manholes approximately 2, 4 and 6 miles downstream from Tank A. This data will be transmitted by radio back to the Tank A site for control of gates and valves that will divert wastewater into the tank and allow it to drain back out of the tank at the appropriate times to eliminate sanitary sewer overflows.Several “green” features have been included in the design. Instead of using potable water, rainwater that falls on the domed roof of the tank is being collected in a cistern for use in cleaning the tank with spray wash nozzles after each wastewater storage event. A rain garden detention basin is being constructed to allow infiltration of runoff from the impervious paved areas to be constructed on the site.The site for the Coldwater Tank A is very visible to the public, located immediately adjacent to the Cypress Road ramp onto I-70 across from Lambert St. Louis International Airport. MSD and CMT worked with a citizen's committee from the City of St. Ann to develop aesthetic enhancements for the storage tank to make it more visually appealing.The SSO storage tank facilities for the Coldwater Tank A project will be the first of its type in MSD's collection system. MSD staff has recognized this as a cost effective solution for eliminating sanitary sewer overflows and have plans for a number of additional SSO storage tanks in their upcoming multi-billion dollar capital improvement program.