Modeling tools yield cost-effective solutions for wastewater collection and treatment, particularly for peak flow during wet weather.
By Jeff Keller, PE
The Little Blue Valley Sewer District provides wastewater collection and treatment for approximately 320,000 customers in Missouri's Jackson and Cass counties. The district provides this essential service for residences and businesses through an extensive interceptor sewer network serving over 270 square miles, pumping stations and a main wastewater treatment facility located east of Independence, Mo. Since the district's inception in the early 1970s, Burns & McDonnell has provided design and consulting services on a variety of treatment and conveyance projects.
Wet-Weather Storage Planned to Hold Peak Flows
As a part of a regional plan to assume operation of an existing sewer utility in Cass County, Mo., the district is providing wet-weather storage to temporarily store peak flows during storm events. Sewer systems typically experience high flows during storm events due to infiltration and inflow (I&I), where leaks in the system allow rainwater and groundwater to enter the pipe network. The planned excess-flow basin site was an existing pumping station with earthen basins available to hold wet weather flows temporarily until the storm events pass and the treatment plant has adequate capacity to treat the additional water. Prior to the plan for excess flow storage, this existing pump station pumped all flows into the upper extremities of the district's gravity collection system. These flows ultimately were received at the existing treatment plant.
Wet Well Sizing Presents Major Challenge
To transfer wet weather flows into the existing earthen basins, the existing pump station would be required to deliver 24.9 million gallons per day (MGD). However, the original design condition of the pump station was only 5.7 MGD. The significant increase in pumping rate posed a challenge, as the sizing of the existing concrete wet well structure was based upon the original duty point of 5.7 MGD, not the new delivery of 24.9 MGD. Traditional design approaches would have found the existing pump station wet well to be too small to accommodate the flow.
Wet well sizing is primarily driven by flow velocities in the wet well and pump operating time requirements. The higher pumping rate and velocities in this undersized structure would lead to eddies, vortices and other turbulence that entrap air in the water.
Such entrained air can be drawn into the pump suction end where the compressible air bubbles can cause a condition known as cavitation, which damages pump impellers and other "wet end" internal pump components. Depending upon the severity of the cavitation, the effects can range from vibration, loss of efficiency, pump damage or catastrophic pump failure. (See Figure 1.)
The traditional solution to this sizing problem is construction of a larger wet well. However, this approach was hindered by site constraints, including the presence of other structures surrounding the deep well and the electrical conduit and piping on all sides of the structure. This congestion would have made the traditional approach difficult and costly.
Computational Flow Dynamics Models Used to Find Solution
A more cost-effective solution was needed. Engineers devised the solution using a computational fluid dynamics (CFD) model, which provided a tool to develop design modifications to control turbulence and uniformly distribute the higher flows to multiple pumps. CFD modeling allows different fluid flow scenarios to be simulated. This digital modeling is performed in many industries to analyze various scenarios from combustion performance in automobile engines to spacecraft heating during atmospheric re-entry. CFD is occasionally applied in the wastewater industry to confirm scenarios such as optimum mixing in basins and fluid flow conditions in clarifiers.
How CFD Works
CFD models use complicated fluid mechanics equations applied to models where the water flow is broken into millions of tiny elements that sequentially relate to one another to replicate actual flow conditions. The sequence is repeated over all the elements in the model for thousands of discrete time intervals to create a dynamic representation of the fluid interactions that would be expected to occur in the real-life scenario. For this application, the CFD model could be used to test various flow control devices such as baffles, walls and ramps that could help reduce the potential for turbulence and uneven flows produced due to the high velocities in the wet well. Multiple CFD modeling firms were contacted to discuss the utility of this technique. Ultimately, the pump manufacturer was able to provide the modeling through a third-party firm that had previously performed CFD modeling for other pumping applications.
Existing Conditions Simulated
Burns & McDonnell, the pump manufacturer and the modeler developed the wet well geometry for the model that would represent the existing condition without any flow control structures in place. Alternative scenarios were then evaluated using differing water depths and pumping combinations. The final model showed that the wet-weather flow was conveyed into the wet well in an unbalanced manner, resulting in the bulk of the flow landing near one side of the pump station, creating highly turbulent conditions on one side and potentially starving the pump on the other side of the wet well. Without improvements, the cavitation of one or more pumps was likely, with resulting dire consequences. (See Figure 2.)
Control Structures Designed and Evaluated
Additional modeling runs evaluated multiple flow-control structures to determine what type of structure would provide uniform, laminar flow to all pumps. The final solution included a combination of a flow distributing baffle wall in the wet well and flow guide vane located in the entrance to the well. The wall includes slotted openings at the bottom of the wall to promote uniform flow towards the pump intakes. The guide vane redirects and centers the flow.
Concerns with debris building up on the guide vane precipitated the design of a guide vane shaped more like a sharks' fin, where debris can slide over the vane and continue into the wet well.
Modeling results indicated that these modifications would mitigate most potential problems and allow the small wet well to convey the very high flows needed in this application.
The result is a design for pump station modifications that capitalizes on the infrastructure already in place, saving an estimated $700,000 compared with building a new wet-well structure. The project is scheduled to be built in 2012-2013 with startup in late 2013. The renovated facility will allow the new conveyance system to manage wet weather conditions without swamping the new wastewater treatment plant, which ultimately receives all the collected flow. With new electronics, pumps and SCADA equipment, the renovated facility should continue to serve the district for many years as it expands service into other areas of western Missouri.