Dynamic Pump Station Capacity Evaluation Using Available Gravity Storage Achieves Improved Operations and Capacity Planning

Background: The City of Virginia Beach Department of Public Utilities (DPU) operates a 3,200 kilometer (2,000-mile) sanitary sewer system, including 414 pump stations and 300 kilometers (190 miles) of force mains that pump flow to the Hampton Roads Sanitation District (HRSD) collection systems. DPU uses hydraulic models of each pump station's upstream gravity pipes (referred to here as 'service areas') to support operations and capacity planning. The current methodology for analyzing pump station capacity was developed to streamline master planning and allow DPU to rapidly assess potential impacts from proposed development. However, the current methodology does not reflect the dynamic and time-dependent characteristics of rainfall-derived infiltration and inflow (RDII) that occur in response to a storm. Consequently, a pilot study was developed to evaluate a revised methodology that considers dynamic flow conditions, pump head conditions, and uses the upstream gravity system for temporary storage of peak flows. In future master planning efforts, this revised methodology will be utilized in a hybrid approach to conduct deeper analysis when warranted to size pump station improvements and confirm how the interaction of manifolded pump stations affects capacity. This paper and presentation discuss the findings of the pilot study, which focused on the 2.83 square-kilometer (700-acre) Resort Area subsystem containing four manifolded pump stations and their associated service areas along the oceanfront in northeastern Virginia Beach (Figure 1). This subsystem was modeled in InfoWorks ICM. A boundary condition was applied at the connection point where the manifolded force mains from the four pump stations discharge to the HRSD regional force main (Figure 2). Current vs Revised Methodologies: The current DPU methodology for evaluating pump station capacity has three key characteristics, as well as a defined deficiency criterion for insufficient capacity. First, the current methodology assigns constant wastewater flows to each pump station, calculated by summing average flows from all developed parcels based on their land use class codes and applying a peaking factor based on land use type for peak flow. As this method does not account for any time-varying characteristics from RDII in response to rain events, the revised methodology used unit hydrographs developed for each service area from the regional hydraulic model and scaled them to reach the same service area-specific peak flow for a design storm with a 2-year average recurrence interval (Figure 3). The current methodology also assumes a constant boundary condition at the connection to the HRSD system. The level for this boundary condition is determined by HRSD's typical simulated pressures at each connection point during wet weather conditions. However, using a constant boundary condition potentially overestimates head during the rising and falling limbs of a storm while underestimating head at the storm's peak. Instead, the revised methodology utilized a variable water level time series, derived from a regional hydraulic model using the same 2-year storm (Figure 4). This variable boundary condition dynamically changes throughout a rainfall event, meaning that pump stations will operate against higher head during peak conditions and will pump more flow once the head decreases after the storm. The dynamic water level boundary condition was applied at the Resort Area subsystem's connection point to the HRSD system. In the current methodology, only the specific pump station under review when evaluating manifolded systems is simulated; all other pump stations in the subsystem are excluded. This does not allow for interaction between pump stations within a subsystem and head conditions created by simultaneous operation of manifolded pump stations. The revised methodology addresses this limitation by always simulating all pump stations in the manifolded subsystem. Finally, according to the current methodology, pump station capacity is considered inadequate if the firm pumping capacity, defined as the intersection of the pump curve and the system head curve, is less than the constant peak inflow when the largest pump is out of service. However, if the constant modeled peak inflow (Figure 3) exceeds firm pumping capacity, then the pump station operates higher on the pump curve, leading to elevated wet well levels and potential backup into the upstream gravity system (Figure 5). The pilot study defines 'gravity storage' as this flow that is backed up from the pump station into the upstream gravity system and is temporarily held until the wet well level drops and flow can enter the pump station without constraint. If a pump station in a manifolded subsystem is deemed deficient using the current methodology, the resulting slated upgrade will cause a higher pressure head against which the manifolded pump stations will operate. This could result in more inefficient operation at these pump stations, as they will operate higher on their head curves, and subsequently a chain reaction may occur where these pump stations in turn could be deemed deficient. Ultimately, prematurely upgrading one pump station may lead to early capital costs for upgrades and higher operational costs at all nearby manifolded stations. The revised methodology alters the deficiency criteria to include upstream gravity storage, which is maximized due to the area's flat topography and resulting minimal pipe slopes. Under this definition, a pump station is flagged as deficient only if the upstream gravity system cannot store peak flow without dropping below a minimum 0.46 meters (1.5 feet) of freeboard (Figure 6). Findings: To illustrate the effects of this pilot study, the capacity of pump station 129 within the Resort Area subsystem was analyzed within three scenarios: Scenario 1: The current methodology was applied with constant peak inflow to the pump station during a wet weather event (Figure 3), a constant boundary condition at the connection to the HRSD system, exclusions of pump stations 116, 117, and 120 during the analysis, and following the deficiency criteria of peak inflow exceeding firm pumping capacity. Under these conditions, pump station 129's constant peak inflow greatly exceeded the firm pumping capacity (Figure 7). Therefore, the pump station was considered deficient under the current methodology and would be slated for replacement. Scenario 2: As an intermediate case study, this scenario uses three of the four conditions present in the revised methodology. A unit hydrograph was used in lieu of a constant peak inflow (Figure 3), a variable water level boundary condition was applied at the connection point to the HRSD system (Figure 4), and the revised deficiency criteria was applied to include upstream gravity storage. However, only pump station 129 was simulated in this intermediate scenario to illustrate the importance of allowing interactions between manifolded pump stations. Due to influence from the unit hydrograph and variable boundary condition, the pump station 129 wet well began to back up into the upstream gravity system, but the minimum freeboard upstream of the pump station remained greater than 0.46 meters (1.5 feet), so the pump station was not considered deficient (Figure 8). Scenario 3: The full revised methodology was applied. This scenario included all four Resort Area subsystem manifolded pump stations operating at full capacity, creating head conditions against which all pump stations operated. This caused more inefficient operation, higher wet well levels, and therefore more backup into the upstream gravity system. The minimum freeboard upstream of pump station 129 was less than 0.46 meters (1.5 feet), so this pump station would be considered deficient under the revised methodology (Figure 9). In a hybrid approach to master planning, since pump station 129 was considered deficient under both the current and revised methodologies, it would be slated for replacement with confidence that an increase in capacity is necessary. Significance: This pilot study is primarily applicable to utilities with low-slope gravity systems, as well as utilities with high densities of manifolded pump stations. The dynamic, time-dependent evaluation of pump station capacity and incorporation of upstream gravity storage into the revised deficiency criteria provides a more realistic analysis of pump station performance and helps inform cost-effective operational and capital decisions. Specifically, fewer pump stations will be prematurely replaced due to capacity limitations and improved pump operation will reduce long term maintenance costs. DPU will use this revised methodology for pump station master planning. The methodology could be used by other utilities with similar topography and operations to support informed decisions regarding pump station capacity evaluation and replacement.