To tune means to adjust for optimal or improved performance. Musical tuning entails adjusting an instrument to achieve a perfect pitch. Depending on the instrument, this continual process requires one to consider many fluctuating variables—changes in temperature, for example, or the varying acoustics of physical spaces. But much relies on intuition. Accurately determining lift station volumetric flow rates requires tuned precision, but on a more sophisticated scale.
As wastewater collection systems age it becomes important to accurately monitor inflow and infiltration at lift stations; engineers and utility managers rely heavily on flow-data accuracy in making sound decisions to improve and maintain their systems’ efficiency. But, as the EPA acknowledges, today’s wastewater utilities find themselves faced with increasingly tight budgets, and utilities search for cost-effective ways to monitor and record lift station flow rates—data that has never been easy to calculate.
Before the advent of SCADA (supervisory control and data acquisition) or telemetry systems, the municipal industry had two ways to measure flow rates in collection systems. Utilities could install a magnetic flow meter in the effluent line or use several types of metering devices in the influent pipe of the lift station. These techniques included volumetric, open channel, and closed conduit flow measurements. Implementing these flow techniques have myriad cost variables associated with them: installation, manpower, site preparation, and accuracy verification.
However, in the Science and Ecosystem Support Division’s Operating Procedure entitled “Wastewater Flow Measurement” made effective on August 12, 2011, the EPA clearly states that “volumetric flow measurement techniques are among the simplest and most accurate methods for measuring flow.” But calculating volumetric lift station pumping capacities to produce influent flow rates may be misleading. Pump capacities constantly change due to the applied head pressure and pumping conditions. Only tuning/updating pump curves while reacting to these changing conditions can improve volumetric flow accuracies. Using this tuning method, Maid Labs, the creator of the MerMaid system, has achieved true volumetric accuracies.
A magnetic flow meter provides the most accurate measurement for flow data. Some manufacturers offer an overall accuracy of 0.15 percent with 0.25 percent as the standard for these meters. A typical meter would be installed on the effluent side of a lift station. From there, it accurately monitors flow output from the pump(s) and totalizes the volumes. The downside rests with the expense to install a magnetic flow meter, and the fact that few lift stations have the required 100 percent full-pipe flow. Today, a magnetic flow meter can cost as much as ten to twenty thousand dollars, and many utilities only have 10 to 20 percent of their lift stations that even qualify for these meters. While this is the ideal system to monitor flow rates, it’s not practical or cost effective to install at most lift stations.
The second type flow monitoring takes place on the influent line/pipe approaching, or at, the lift station. If the influent piping at the lift station is submerged, or could become submerged, a suitable location upstream becomes necessary to monitor the flows traveling to the station. This often requires several meters to monitor the flows to one lift station. This method also requires various types of primary devices such as V notch weirs or level/flow-measuring devices which use the Manning Equation. The problem, however, resides with the overall accuracy of these devices: as the conditions change, accuracy erodes quickly. The logistics to provide this type of technology alone requires large amounts manpower. Additional workers would be needed for the installation, removal, physical maintenance, and relocation of these meters, but also for data collection and troubleshooting problems. Considering the number of additional employees, most utilities generally cannot afford to continuously monitor lift station flow rates—even if the DEP or the EPA mandates this monitoring.
The first SCADA and telemetry systems aimed to solve the cost and practical application issues; these systems had the capability to monitor all the lift stations within the municipality on a continual basis. Telemetry systems communicated this data to the host through an assortment of methods primarily using radio or phone networks. This new method used the pump capacities supplied by the pump manufacturers or a drawdown test to set a capacity for each pump in the lift station. By recording the amount of time each pump was active and multiplying its capacity, these systems could produce a derived flow and totals for each lift station. The HMI could then report the flow data, display those totals, and store the information for future use in historical or modeling programs.
The majority of municipalities today still use this derived-flow method to record flow rates, but even this system presents a problem: limited accuracy. To derive means to deduce by reasoning or an assumed conclusion. When it comes to monitoring flow rates, deduction can never equal solid calculation, reasoning becomes little more than guesswork. A calculation based on assumption, this method derives flow rates with an overall accuracy of only 10 to 15 percent of the actual flow in a lift station. This method scores points for cost effectiveness; the system does not require additional manpower other than the maintenance on the telemetry system. The telemetry system also adds value to the utilities by providing critical data for their lift stations’ maintenance divisions. However, many engineering consultants are skeptical of the utilities’ flow-data accuracy and some municipalities are uncertain of their recorded data’s accuracy.
In the August 2011 “Wastewater Flow Measurement” Operating Procedure, the EPA further stated that “any continuous flow measurement system that cannot measure the wastewater flow within ±10 percent of the actual flow is considered unacceptable for use in measuring wastewater flow.” The problem here is certain: a system based on the derived-flow method is just not precise enough for the task at hand. To meet today’s EPA standards wastewater utilities require improved methods to provide lift station flow accuracies greater than the derived 10 to 15 percent. In part two of this article, we will explore the development, difficulties, and solutions presented by tuned flow lift station monitoring. ■
Salguero, Louis. (August, 12 2011). Wastewater Flow Measurement (SESD Publication No. SESSPROC-109-R3). Athens, GA: U.S. Environmental Protection Agency Science and Ecosystem Support Division. 1-16. Stoner, Nancy and Cynthia Giles. (October 27, 2011). Memorandum: Achieving Water Quality Through Integrated Municipal Stormwater and Wastewater Plans. United States Environmental Protection Agency. Washington, D.C: Government Printing Office.
Gary L. Kerr is the national product manager for Maid Labs and brings with him over twenty years of experience in product performance and monitoring of wastewater pump station. For more information, visit
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