Heavy snowmelt and rain events send runoff into a city’s sewer system and can overwhelm the affected sewers and wastewater treatment plants, potentially causing overflows into nearby streams and rivers. One solution is to build storage tunnels and pump stations deep underground to capture and store the overflows, which allows the utility to control the rate at which wet weather flow is conveyed to a treatment facility.
GS&P recently designed such a pump station for the Metropolitan St. Louis Sewer District’s (MSD) 3.9-mile Deer Creek Sanitary Tunnel, which will soon be constructed in a layer of limestone rock 180 feet below the surface. Design, construction, operation and maintenance of a pump station of this depth pose many unique and interesting challenges, which are discussed below.
Configuration versus Costs
Developing an effective and economical pump station layout is a significant challenge given the dynamic tension between excavation costs and interior space requirements. This was particularly evident during the design of the Deer Creek Pump Station (DCPS) when considering the requirement to provide adequate room at pump level for operation and maintenance while minimizing expensive deep-rock excavation. The configuration of the DCPS went through five conceptual design iterations before a final solution was chosen. The selected pump station configuration separates the wet well and dry well into three distinct 36-foot diameter shafts, which utilize the limestone bedrock to provide hydraulic separation. This layout not only accommodates concurrent construction of the 19-foot diameter tunnel and the pump station shafts, but reduces excavation costs by an estimated $5 million to $10 million when compared to the more commonly used single-shaft configuration. Another advantage of the three-well configuration is hydraulic redundancy: if one dry well floods, it does not impact the functionality of the second dry well.
Selecting the Pumps
It is difficult to find a wastewater pump with solids-pumping capability that can accommodate static head (pressure) ranging from 15 feet to 125 feet. However, this was one of the design challenges posed by the operational criteria for the DCPS. Water levels in the Deer Creek Sanitary Tunnel and associated collection system will vary dramatically from near-empty (low level) to flood conditions (high level). In addition, the DCPS must be capable of providing a wide range of flow rates in order to respond to available treatment plant capacities. In an average year, St. Louis receives 107 days of measurable precipitation, which equates to one or two precipitation events a week. Given this potential frequency of storms, it is critical that tunnel dewatering begins as soon as possible following a storm event. Consequently, the DCPS must have the flexibility to pump at low flows when available downstream capacity is limited, and to pump at higher rates as sewer capacity becomes available. The DCPS has the flexibility to pump from 2 million gallons per day (MGD) to 25 MGD depending on how much downstream capacity is available.
To address these and other pump performance criteria, GS&P elected to use seven variable speed pumps to maximize the flow range, accommodate static head conditions, standardize pumping equipment, and provide superior operational flexibility and redundancy. After considering a variety of pump types, GS&P recommended the use of vertically oriented, dry-pit, submersible pumps with cooling jackets. The submersible type pump provides a level of protection in the unlikely event of dry-well flooding. Additional benefits of the vertical pump orientation are reduced space requirements and ease of removal.
Managing debris is another considerable design challenge of a deep pump station. For the DCPS, two static, stainless-steel bar screens will be installed in the wet-well shaft at the downstream end of the tunnel. The static screens and raking system will capture and remove large solids from the flow stream prior to the flow entering the pump station influent sump, which contains the pump suction bells. Debris that accumulates on the screens will be removed by an automatic screen rake, which is specifically designed for a deep-well-type application. The clam-shell raking mechanism will be lowered into the 180-foot shaft to remove debris from the static screens. The system is capable of operating with varying water depths and flow rates. When the pump station is in operation, the screening system will automatically clean the screens at least once each hour. The wet well also contains a grit pit at the upstream base of each static screen. The automatic screen rake will also remove grit that accumulates in the grit pit with each pass.
The Right Access
A fourth critical piece of the puzzle was to provide equipment and personnel access to the tunnel and wet well for inspection and maintenance. One option considered was to provide a permanent crane, personnel basket, blowers and environmental monitoring equipment to facilitate tunnel and wet-well access. Considering the extremely corrosive conditions, three- to five-year periods of inactivity between recommended maintenance/inspections, and specialized training required to perform tunnel maintenance, it was determined that permanently installed equipment was not the best solution for the DCPS. Instead, strategically located roof hatches provide mobile crane access to the tunnel and wet well. This solution allows equipment and personnel access without the liability and maintenance associated with utility-owned access equipment.
These are just a few of the major design challenges associated with deep pump stations for sanitary-sewer and combined-sewer storage tunnels. Overcoming these challenges for the DCPS required some out-of-the-box thinking, but the end result is a cost-effective and flexible design solution capable of emptying the 44-million-gallon Deer Creek Sanitary Tunnel in less than 48 hours.
Senior Water Resources Engineer Ken Roseman, P.E., also contributed to this post.