Selecting a flow meter for your wastewater collection system can be a dizzying task. There are several manufacturers in the market today, and at first glance the different monitors appear to perform virtually the same.
Major advancements in flow monitoring technologies occurred in the late 19th and 20th Centuries including developments such as the Manning Equation; primary devices such as flumes by Parshall and Palmer-Bowles; and the development of Doppler velocity measurement, which allows flow to be measured with Area-Velocity meters in nearly all conditions using the Continuity Equation. Today, there are six major manufacturers producing Area-Velocity (Open Channel) flow meters.
Four different technologies are used to calculate flow with Area-Velocity meters: Doppler, Transit Time, Electromagnetic and Primary Flume Weir. Doppler technologies are the most widely used, and some innovative configurations have been developed in the last few years.
Each of these technologies utilizes the Continuity Equation, which requires a depth of flow to determine cross sectional area and a velocity sensor to determine velocity of flow. Today, only two depth-of-flow technologies are in practical use—pressure transducers and ultrasonic sensors.
- Pressure transducers are simple strain gauges with the “strain” coming from water pressure pushing on a membrane. An air tube offsets the effect of changing air pressure. Pressure sensors are commonly found on the least costly flow meters and they are simple to operate;
- Ultrasonic depth sensors are timing devices that measure the time for an ultrasonic “pulse” to return back to the sensor after reflecting off the water surface. These devices are generally not subject to drift and measure with greater precision than pressure transducers. They are more costly and are found on the more expensive, and more accurate, flow meters; and
- Velocity sensors fall into four different categories: 1) electromagnetic; 2) continuous wave Doppler; 3) gated Doppler; and 4) time of transit. The Continuity Equation requires that the average velocity be known. This is very difficult to do because of the velocity profile of moving water. The velocity near the pipe wall is lower than at the center and the velocity profile varies widely from pipe to pipe. Choosing which technology is right for your agency can be difficult and there is little objective information available to assist in the selection.
The EPA recognized the difficulty faced by municipalities as they try to sort through the performance claims by manufacturers of environmental equipment, and several years ago developed the Environmental Technology Verification (ETV) Program to verify performance of several types of air- and water-related equipment including flow meters.
Until all manufacturers agree to submit their equipment to a common method of verification, such as the EPA’s ETV Program, most consumers will continue to depend on their own experience or the experience of their peers to distinguish between the different technologies.
Currently, ADS is the only wastewater flow monitoring company that utilizes Peak Velocity Doppler as a means of determining velocity. This technology is the only wastewater measuring technology that is verified by the ETV Program and the only one that is successful with measurements in a wide range of both low flow and large pipes.
Peak Velocity Doppler is currently being used to collect sufficient and accurate flow information for the San Francisco Combined Sewer System to calibrate the city’s hydraulic model. The city is in the process of developing a 30-year Capital Improvement Program for their wastewater system, and it is vitally important that the best flow monitoring data be collected during a wet weather window to successfully calibrate the model.
The project requires a +5% accuracy as substantiated by third party verification; a minimum 90% up-time for all meter locations; as well as service and equipment performance under quality control guidelines such as ISO 9001. The ranges of pipe heights being measured is from 15 to more than 120 in. and many are odd shaped including egg-shaped, basket-shaped and box configurations. A total of 78% of the pipes measured in this project are 36 in. and larger.
In Santa Barbara, seasonal winter rains added unwanted flow volumes as extraneous rainwater seeped into defects in pipes and manholes. The threat of sanitary sewer overflows motivated an I/I study utilizing 45 Peak Velocity Doppler flow monitors and eight rain gauges strategically selected at the outfalls of the 43 mini-basins.
In mid-March of 2003, one of the largest storms in four years blanketed the Santa Barbara area. Performance information from each basin was collected; flow analysis was conducted; and Santa Barbara was presented with a list of basins ranked by the severity of I/I entering each basin. With a service area of 1.1 million linear ft of sewer lines, the city estimated that 500,000 linear ft of sewer does not need immediate field reconnaissance work. They were able to target the 10 identified basins and eliminate the 500,000 linear ft of sewer that performed well during storms from expensive sewer survey fieldwork.