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Suffolk, VA Develops Effective Process for Measuring and Analyzing Pump Station Flows Discharging Into Manifold Pressure Mains

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The City of Suffolk, Virginia (City) entered into a Special Order by Consent (SOBC) with the Virginia Department of Environmental Quality (DEQ), the Hampton Roads Sanitation District (HRSD), and other area localities to reduce sanitary sewer overflows (SSOs) from wastewater collection systems. As part of the SOBC, the City was required to develop and conduct a flow monitoring program to determine which service areas exhibit excessive infiltration and inflow (I/I). Service areas found to have excessive I/I would be required to undergo a sanitary sewer evaluation survey (SSES) and subsequent rehabilitation. The City would also need to develop a hydraulic collection system model and integrate it with HRSD's regional model. The flow monitoring, SSES, and hydraulic modeling work was to be conducted according to Regional Technical Standards (RTS) prepared by the HRSD and localities.

The City operates a conveyance system that includes approximately 264 miles of gravity, 60 miles of force mains, and 130 pump stations whereby flow is discharged to HRSD force mains and conveyed to HRSD regional wastewater treatment facilities. HRSD has major manifolded force main interceptors located throughout the Hampton Roads region and communities like Suffolk connect and discharge into the interceptors via pumping stations. Ninety eight percent of the City's flow is pumped into the regional force main interceptors. Of the City's 130 pump stations, 94 pump directly into the regional force main interceptors.

The City was initially challenged with how to set up their flow monitoring program since it was not feasible for them to retrofit flow monitors on their existing stations primarily because of the space and access restrictions on the discharge piping. Additionally, the City wanted to monitor the flows at each of the 94 terminal pump station sites that connect directly into the regional force main interceptor. The City needed to know how each of the stations performed under wet weather conditions and how the stations would respond to less frequent and more intense projected wet weather conditions.

The City performed a demonstration project at pump station number PS 136 to verify and document their unique flow measurement process. Upon successful completion of the demonstration project, the flow measurement process was applied to the City's other 93 stations. In general terms, the pump station's flow was measured by combining data from pressure sensors located at hydraulically important locations in the station with key data such as the station's wet well and sewer system dimensions and elevations, and station operating performance. PS 136 was selected because it pumped directly into an HRSD force main and was considered a terminal pump station; it served a typical, small service area of about 78 acres; and represented the City's typical duplex station configuration.

A performance objective of the flow measurement process was that the data flow among the different pieces of monitoring equipment and supporting analytical tools needed to be efficiently managed so that data quality control and evaluation could be optimally automated. This objective influenced the selection of equipment. A Telog Data Acquisition Unit was selected to store and transmit data collected from the pressures gauges installed on the discharge piping; in the wet well to measure elevation; in the pipe bedding to measure trench water elevation; and in a groundwater well to measure groundwater elevation. The Telog unit also recorded pump on and off times. The wet well elevation sensor enabled the wet well wastewater volume to be calculated and also the wastewater storage volume in the incoming pipes was calculated when the wet well elevation increased above the pipe inverts. A stage-storage curve was developed using a computational tool programmed using Microsoft SQL Server.

This paper describes in detail how the data from the flow monitoring devices were combined with the system's physical attributes and processed through a flow algorithm to compute the pump station influent flows. Station operating curves were developed to define the relationship between the pump station total dynamic head and the discharge flow. This process differed from traditional drawdown testing that creates a pump curve for each pump and for the combined pumps. Instead, the City's process required that a station operating curve be developed because of the station's variable downstream discharge pressure in the interceptor force main. The station operating curve differs from a traditional pump curve in that it includes minor head losses experienced in the piping between the pump and the location where the pressure is measured by the transducer sensor on the discharge piping.

One of the lessons learned from the demonstration project was the need to effectively coordinate and manage the large volume of monitoring data prompted by the need to measure low flow periods. For instance, data from each device must be on the same time reference and synchronized in the same time step sequence. Data anomalies must also be addressed in the flow algorithm or at other appropriate process steps.

Once the station's flows were calculated, the City analyzed the relationship between the station influent hydrograph and the rainfall events and quantified the rainfall derived infiltration and inflow (RDII) component. The analysis was performed using a computer program call Sliicer™ which is a product of ADS Environmental Services. Sliicer is a collection of tools that help to perform efficient analysis and visualization of dry weather and wet weather flow components such as RDII. The RDII component was particularly important because the RTS uses the RDII metric as an indictor of excessive I/I under certain wet weather conditions.

The RTS states that a service area would be deemed excessive and require subsequent SSES inspections if the peak hourly flow projected for a 10-year 24-hour rainfall recurrence interval exceeded a peak flow threshold criteria of 775 gallons per equivalent residential unit plus three times commercial flow plus actual major industrial flows. RDII had to be measured and characterized for the PS 136 sewer basin using other engineering methods and tools before it could be projected to the 10-year 24-hour storm event. PS 136 flows were calculated and analyzed using monitoring data from nine significant rain events and projected to the 10-year 24-hour storm event using the RTK method with a groundwater routine. The 10-year projected peak flow for PS 136 exceeded the peak flow threshold criteria and was characterized as an SSES Basin.

The demonstration process showed that a very accurate representation of the station's flow was developed and provided credible data for subsequent analysis and project planning. The City applied the same flow calculation and analysis process to the remaining 93 pump stations. The City also found the monitoring and data collection equipment and website data access will serve a long term system monitoring role as well as a short term flow computational role.

Explanation of this flow monitoring process will benefit other utilities looking at feasible alternatives for measuring pump station flows especially when they discharge into a manifolded force main.
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Keywords: Flow monitoring; RDII; SSO; demonstration project; excessive I/I; feasible alternatives; flow analysis; peak flow; pump station; stage storage; wet weather event

Document Type: Research Article

Publication date: 2009-01-01

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