Getting the Most Out of Your Storage Basin: Optimization Strategies for Planning, Design, and Construction

摘要

Introduction The United States Environmental Protection Agency (EPA) estimates there are at least 23,000 to 75,000 sanitary sewer overflows (SSOs) per year nationwide (not including sewage backups into buildings), discharging a total volume of three to 10 billion gallons to waters of the United States. Additionally, combined sewer overflow (CSO) volume discharged to waters of the United States is estimated to be 850 billion gallons annually. Providing for storage within the collection system is not a new concept but it continues to be a cost-effective solution to mitigate both SSOs and CSOs. However, there are new approaches to get the most of a storage facility and this presentation will explore one practical approach through a case study. This presentation will describe the concepts, methods, and tools used to optimize the design and performance of a storage facility located in a challenging environment. A 2.4 Million Gallon (MG) sub-surface storage facility, located in a park adjacent to an existing Lift Station (LS02) in Racine, WI, was designed to store wet weather flow that exceeds pump station capacity. The basin is surrounded by the Root River on three sides, producing elevated groundwater conditions that create uplift pressures on the tank. These pressures needed to be counteracted during the design to prevent float, particularly when the basin is empty and river levels / groundwater are high. The basin is also located in the 100-year floodplain, which created additional engineering and regulatory challenges. The storage facility was designed to meet the desired level of service (LOS), but also to serve as a flow-through system when the tank is full. This approach provided primary treatment of overflows more than the desired LOS, and significantly improved the level of protection for nearby home owners against basement backups. Optimization principles were applied throughout the planning, design, and construction phase of the storage facility to reduce cost and improve performance. The presentation will show how optimal sizing and reduced costs for the project were achieved by: 1. Sophisticated hydraulic modeling used to optimize the performance through all phases of the project (planning, preliminary engineering, and final design) 2. Thorough evaluation and selection of the appropriate LOS based on risk factors 3. Commitment to improving performance while reducing costs throughout all phases of the project 4. Evaluation of O&M strategies to reduce long-term O&M costs 5. Risk management strategies implemented during construction. Planning The concept of storage was first proposed during a system-wide study conducted by the Racine Wastewater Utility (RWU) in 2009. The system-wide plan evaluated a combination of storage and conveyance measures that best met the evaluation criteria: 1) the lowest unit cost for total bypass elimination ($/gallon removed); 2) the lowest unit cost for system-wide surcharge reduction (expressed as a system-wide average surcharge); and 3) the lowest total cost to meet a flow threshold at the WWTP. The LS02 storage option was identified as the most cost-effective solution to reduce nearby basement backups and overflows to the Root River. Preliminary Engineering In May of 2014, a short-duration, high-intensity storm produced peak flows into LS02 well above the combined pumping capacity of LS02 (approximately 6 million gallons per day (mgd)) and the adjacent bypass pumping station (permitted at 3 mgd). This condition caused flow to back up into the upstream interceptor sewer, producing approximately 45 basement backups in the nearby neighborhood. Immediately after this event, the RWU commissioned a preliminary engineering study to further evaluate and refine the storage solution provided in the 2009 system-wide plan, to ultimately reduce the potential for future basement flooding and SSOs to the Root River. Through this refined evaluation, the conceptual design was further optimized such that it reduced the storage basin size required to achieve the desired LOS (approximately half the volume of that proposed in the planning study). Design Continued optimization during the design was key to improving the overall LOS, reducing cost, and minimizing risk. To minimize risk, considerable site investigation work was conducted to understand the site's history and environmental conditions that could otherwise delay or prevent the project from moving forward. This included environmental assessment and testing, archaeological surveys, geotechnical investigation (including test pits), and wetland delineation. To maximize the level of protection, extensive hydraulic modeling was conducted to improve hydraulic efficiency and increase the level of protection against basement backups, even when the storage tank is full. Key to this was eliminating the current mechanical bypassing at LS02 and replacing it with an overflow weir at the end of Basin 2. By doing so, the combined flow capacity into LS02 and the proposed storage basin increased from 9 mgd to over 20 mgd, even when the storage basin is full. A secondary benefit to moving the overflow to the end of the storage tank is the enhanced primary treatment (settling) of solids that would otherwise be discharged to the river though bypass pumping. Reducing long-term operation and maintenance (O&M) costs was also a key consideration in the design. This included configuring the storage basin into two compartments so that the first compartment, designated as Basin 1, captures the majority of wet weather flows and associated sediment. Basin 1 overflows to Basin 2 through a baffled weir when Basin 1 is at capacity. Because most of the sediment and floatables will be remain in Basin 1, various self-cleaning technologies were evaluated for Basin 1. The recommended technology consists of four automated tipping trough sediment flushers, each with a dedicated flushing lane. The tipping trough is filled with wastewater after each event (and after the basin drains by gravity) and tips to produce a flushing wave across each Basin 1 flushing lane, to effectively flush sediment from the floor of Basin 1 to a sump. This water is then pumped automatically back to LS02 using 200 gallons per minute (gpm) centrifugal solids-handling pumps. Optimization during the design allowed for the overall storage basin volume to be increased by 40% for the same cost as estimated during the preliminary engineering phase. This additional volume not only increased the overall LOS but gave the Utility flexibility to hold more water at times, allowing for reduced LS02 discharge rates to reduce the risk of basement flooding and overflows in the sewer system downstream of the pump station. Construction Although the opportunity for optimization is highest during the planning and design phase, strategies to reduce costs were also implemented during construction whenever possible, including adaptive and risk management approaches that ultimately reduced the potential for costly change orders. This paper also discusses the strategies used to control costs.