Solving Combined Sewer, Flow Metering Problems with Ultrasonic Meters

March 20, 2015
The Easterly Wastewater Treatment Plant (WWTP), located on the northeast side of Cleveland between Lakeshore Boulevard and Lake Erie, is part of the Northeast Ohio Regional Sewer District (NEORSD). The piping in the plant featured venturi (Dall) flow meters, but the differential pressure lines of these devices were directly exposed to the wastewater, causing various problems. As a solution, NEORSD ultimately selected the clamp-on ultrasonic meter from Flexim Americas, which had a strong signal and a good signal-to-noise ratio.
The Easterly plant provides wastewater treatment services for more than 300,000 residents and businesses in northeastern Cleveland and the surrounding suburbs, treating an average of 94 million gallons of wastewater per day.


By Jack Sine

The Easterly Wastewater Treatment Plant (WWTP), located on the northeast side of Cleveland between Lakeshore Boulevard and Lake Erie, is part of the Northeast Ohio Regional Sewer District (NEORSD). The utility provides wastewater treatment services for more than 300,000 residents and businesses in northeastern Cleveland and the surrounding suburbs, treating an average of 94 million gallons of wastewater per day.

The Easterly WWTP also receives and treats stormwater from combined sewers and recently found an innovative way to solve two challenging problems simultaneously. Given the number of combined sewers in its service area, the plant needed to add capacity and overflow storage but also needed to address flow metering needs -- an area of growing importance for the near future.

Combined Sewer Overflows

The city of Cleveland and some of its older suburbs have combined sewers that handle both sewage and rainwater. However, the problem occurs when there are heavy rains.

"We simply did not have the capacity to handle all of the water produced by large storms," said Paul McGuire, senior project engineer for NEORSD. "We could expand our capacity some but not enough to handle the runoff from major weather events. The resulting overflow finds its way to Lake Erie and results in pollution. Since we couldn't add enough capacity, we decided to add storage."

NEORSD developed a detailed plan and eventually entered into a consent decree with the Environmental Protection Agency and other related organizations to implement its combined sewer overflow (CSO) long-term control program known as Project Clean Lake. A portion of the program includes two storage tunnels 24 feet in diameter that run two miles long and 200 feet underground.

"We're also in the process of enlarging our Easterly plant's secondary treatment capacity to 400 million gallons a day," said McGuire. "Since our average treatment rate is 94 million gallons per day, we will be able to handle most rain events." But when large storms come, portions of water and sewage will be diverted to the storage tunnels and held until after the rain stops. Then, the stored combined sewage will be pumped to the plant for full treatment. "That means Easterly is doing its part to keep Lake Erie pollution free," said McGuire.

A Metering Solution

The existing piping in the plant featured venturi (Dall) flow meters, but the differential pressure lines of these devices are directly exposed to the wastewater, causing various problems, explained McGuire. "Since solids are in the wastewater being measured, the meters would frequently become clogged. Even clean water purges didn't keep the impulse lines clean, so we would have to clean them manually."

McGuire noted that plant staff eventually stopped cleaning these lines, as the process became a nuisance. "Back then, metering wasn't critical to plant function," he said. "But now, as we ramp up plant efficiency, we will be using metering to control flow, and metering will be critical to our efficiency." After several demonstrations from various flow meter manufacturers, NEORSD ultimately selected the clamp-on ultrasonic meter from Flexim Americas, which had a strong signal and a good signal-to-noise ratio.

"This was a good application for our Flexim meter," said Brian Papa of Chaltron, the manufacturer's representative for Flexim. "One of the problems for the Easterly facility is that there are no long straight pipe runs, which most flow meters need for accuracy." Flexim engineers installed two dual-channel meters with two pairs of sensors on the same line. The meter then averages the channels. "The multiple-beam approach maintains a high accuracy with limited straight pipe," said Papa. "It resulted in a much more consistent, repeatable measurement than the old venturi meters."

The non-intrusive clamp-on meters measure flow with ultrasonic pulses that resonate through the pipe's exterior and into the wastewater. Unlike the venturi meters, they are not exposed to the flow, which means plant operators didn't have to shut down the line during installation. "And there has been no maintenance required," said McGuire.

NEORSD selected the clamp-on ultrasonic meter from Flexim Americas, which had a strong signal and an efficient signal-to-noise ratio.

There were some initial concerns about the permanence of the clamp-on mount, but Flexim engineers used a non-grease solid couplant to protect the transducer. "This eliminates the need for maintenance usually associated with grease-based coupling agents," explained McGuire.

The twelve ultrasonic meters have been functioning trouble-free for three years.

How Ultrasonic Flow Metering Works

The technique most ultrasonic flow meters use is called transit-time difference, which exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium, similar to a swimmer paddling against a current. The signal moves slower against the flow than with it.

"When taking a measurement, the meter sends ultrasonic pulses through the medium, one in the flow direction and one against it," said Izzy Rivera, Flexim's product engineer. The transducers alternate as emitters and receivers. The transit time of the signal going with the flow is shorter than the one going against it. The meter measures transit-time difference and determines the average flow velocity of the medium. Since ultrasonic signals propagate in solids, the meter can be mounted directly on the pipe and measure flow non-invasively, eliminating any need to cut the pipe.

But the task of installing the meters was not without its challenges. "The amount of straight run that's usually required to obtain best accuracy is 10-5 upstream/downstream diameters," said Rivera. A 10-inch pipe, for example, would need 150 inches, or 12.5 feet, of straight pipe. "We find that we can squeeze this down to 5 diameters upstream and still maintain the meter accuracy with multi-beam meters." At Easterly, they used a two-channel meter with two pairs of transducers, averaging the flow profile. "We also have a four-channel meter that can deliver even higher accuracy under non-ideal piping conditions," said Rivera.

McGuire added that the pipe containing the venturi meters was replaced with a 48-inch-diameter spool piece of cement-lined ductile iron pipe. "[We] installed the Flexim meters on the spool piece," he said. "Now we had no restriction in the wastewater line and no pressure loss like we did with the venturis."

Even with the challenges of the shorter pipe runs, McGuire said the installation of the meters went smoothly. "They have provided consistent, reliable data," he said. "In the future, the data will be used by the process control and automation system to not only monitor but possibly control the flow and optimize the process."

Papa added that there are plans to install an additional 50 ultrasonic meters in Easterly and her sister plants. "That installation will give the entire NEORSD precision control over all their processes."

About the Author: Jack Sine is a freelance writer specializing in green projects, manufacturing, chemical production, and HVAC. He can be reached at [email protected] or 845-831-6578.

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