Flow Metering - A Powerful Tool to Build Smart Sewer Systems

Flow Metering -- A Powerful Tool to Build Smart Sewer Systems Benefits and Objectives Measuring real-time flow rates in a sewer collection system is like a doctor taking a pulse for a patient. Rapid advancements in flow metering and data gathering technologies have led to substantial improvements for real-time operational support and decision-making systems. This study undertook a review of current available flow metering technologies to optimize the operation and maintenance (O&M) of sewer collection systems, and also to identify potential methodologies and applications where flow metering provided significant insights and benefits to system evaluation. This paper provides a summary of the results from this study, and covers the following specific objectives: - Summarize the uses of flow metering techniques to reduce infiltration and inflow (I&I) and sanitary sewer overflows (SSOs), improve level of service, and reduce O&M costs - Provide general guidance and lessons learned for municipalities and utilities to develop and conduct flow metering programs for sewer collection systems - Equip operators, regulators, engineers, and scientists with tools to accurately characterize sewer flows and assess a sewer collection system's capacity and performance - Present several unique case studies of flow metering and flow data analysis. Methodology Flow metering principles and technologies -- Area/velocity flow meters typically utilize pressure or ultrasonic sensors to measure flow depth and doppler-type velocity sensors to measure velocity. The flow rate is determined through depth (from which area is calculated) and velocity measurements. Metering location selection -- The selection of flow metering locations is fundamental to assuring accurate representation of flows throughout the system. Flow meters are typically located along trunk sewers near major confluences. For inflow and infiltration studies, flow meters can be focused in the areas with known high I&I. Data analysis -- Dry weather and wet weather average and peak flows are calculated using flow metering data. Sanitary sewage flow, base infiltration, rainfall-derived infiltration and inflow (RDII) are calculated as well. The rainfall data from a rain gauge is also analyzed. General Applications Hydraulic model calibration -- Flow monitoring is often conducted to support hydraulic model calibration for master planning. Hydraulic models can be calibrated using both dry weather and wet weather flow metering data. Capacity assessment -- Simulations of dry weather and wet weather scenarios based on flow metering data provide information for capacity assessment, which ultimately can be used to identify alternatives for addressing capacity limitations. I&I study -- I&I analysis using flow metering data is able to identify areas of concern that need to be addressed. Design wet weather flow projections -- Flow metering and rainfall data could be used to develop a model to predict the wet weather flow for a 2-year or a 5-year storm event at a specific basin. Unique Case Studies Sewer Clogging -- A section of sewer pipe upstream of a lift station in a coastal city in New Jersey often experienced surcharges during high flow events. A level sensor (Telog) and a HACH Flodar meter were installed to evaluate the issue (Figure 1). A hydraulic model for this section of sewer was developed and calibrated using level sensor data and HACH Flodar data. The modeling results demonstrated that a clogged bar screen at the lift station appeared to be the cause of the sewer backup. The hydraulic modeling with flow metering data is a powerful tool to illustrate the impact of hydraulic restriction and head loss on upstream water levels. Plant influent flow meter validation -- A temporary flow metering study was conducted to verify the accuracy of an influent flow meter at a wastewater treatment plant in California. The comparisons of temporary flow metering data, plant influent, and effluent flow data are shown in Figure 2. It was found that the influent flow meter readings were higher than the temporary flow meter results. Recommendations were made to improve the accuracy of the existing plant influent flow meter. Hurricane event -- A major hurricane occurred during a temporary flow metering study in a coastal city in New Jersey in 2011. The flow metering data and the hydraulic modeling simulation provided a unique scenario for the hurricane event (Figure 3). Because of the magnitude of the storm and the regional power outages that transpired, all sewer lift stations were disabled during the hurricane. However, the monitoring and modelling revealed that the collection system handled the high flow very well, with only a small area of flooding due to the lift stations off-line. Pump station design flow determination -- A linear regression model (Figure 4) was development by plotting RDII volume against rainfall precipitation and used to predict wet weather flows under a 5-year storm event for a pump station. Lessons learned Field verification -- Before installation of flow meters, field verification should be conducted to validate the actual locations of the manholes for installing flow meters, as well as size, slope, invert elevation, and condition of the pipes. Meter maintenance -- Routine maintenance and regular inspection of flow meters (e.g., routinely check for clogging, meter cleaning, replacement of desiccants and batteries, etc.) is required to keep meters functioning properly. QA/QC -- The Manning equation can be employed to conduct QA/QC of the flow metering data. A scatter graph (velocity vs. depth) for a flow metering study in Indiana was created to compare the differences between metered flow and theoretical flow (Figure 5). Further field inspection found that a nearby lift station had experienced occasional back flow issues causing erroneous meter readings. In addition, the scatter graph indicated that the sewer experienced surcharges or potential overflows at this location. Rainfall data -- It was recommended to install a continuous-recording rain gauge for future studies. Conclusion The paper shows how adoption of a flow metering program enables wastewater utilities to improve productivity for inspection, condition assessment, optimizing sewer O&M efforts, and more effectively implementing asset management programs. With anticipated future advancements in sewer flow monitoring, utilities will be even better able to identify solutions to address capacity limitations or performance challenges.