CASE STUDY

Chaltron Systems, Inc. Solves Combined Sewer and Flow Metering Problems for Wastewater Plant

The Easterly Wastewater Treatment Plant recently found ways to solve two knotty problems at essentially the same time. The treatment plant is part of the Northeast Ohio Regional Sewer District (NEORSD) and is located on the northeast side of Cleveland between
Lakeshore Boulevard and Lake Erie. It provides wastewater treatment services for more than 300,000 residents and businesses in northeastern Cleveland and the surrounding suburbs. In addition to treating wastewater from homes and businesses, the Easterly
plant also receives and treats storm water from combined sewers. An average 94 million gallons of wastewater per day are treated at Easterly.  Because of the number of combined sewers in Easterly’s service area, the plant needed to add capacity and overflow storage, and management decided to also address a need that was going to grow in importance in the near future -- flow metering.

Combined Sewer Overflow (CSO):


Cleveland and some of its older suburbs have combined sewers that handle both sewage and rain water. The problem comes 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 run off 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.


“We drew up a detailed plan and submitted it to the EPA. In time we entered into a consent decree with the EPA and related agencies. Then we began
implementation of our CSO long-term control program known as Project Clean Lake.” A portion of the program includes two storage tunnels 24 feet in diameter, 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. When the rain stops, 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.”

 

A metering solution:


The existing 48” piping in the plant featured venturi (Dall) flow meters, but these had led to ongoing problems. “The problem with the venturi meters is that they are intrusive and the differential pressure lines are directly exposed to the wastewater,” said 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. “Eventually, it became too much of a nuisance and we didn’t bother. Back then metering wasn’t critical to plant function. But now, as we ramp up plant efficiency, we will be using metering to control flow and metering will be critical to our efficiency.
“We wanted meters that would be easy to install and maintain,” said McGuire. “We asked several flow meter manufacturers to demonstrate their technology onsite. Most did not perform very well. One couldn’t even get a signal. Fortunately, there was one meter that had a strong signal and a good signal-to-noise ratio. It was a clamp-on ultrasonic from Flexim Americas.

“This was a good application for our Flexim meter,” said Brian Papa of Chaltron, the manufacturer’s representative for Flexim Americas. “One of the problems for the Easterly facility is tha there are no long straight pipe runs which most flowmeters need for accuracy. But the Flexim engineers had a solution for that. They put a dual channel meter with two pairs of sensors on the same line and themeter averages the channels. The multiple beam approach maintains a high accuracy with limited straight pipe. It resulted in a much more consistent, repeatable measurement than the old venturi meters.”“The fact that the Flexim meters were non-intrusive clamp-on made our decision easy,” said McGuire.“They measure flow with ultrasonic pulses that go through the exterior of the pipe and into the wastewater flow. Unlike the venturi meters, they are not exposed to the flow. This has the added benefit that we did not have to shut down the line for themeter installation and there has been no maintenance required.  
 
“At first we were concerned about the permanence of the clamp-on mount, but the Flexim engineers pointed out that they used a non-grease solid couplant that protected the transducer. Its use eliminates the need for maintenance usually associated with grease-based coupling agents.” 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. It exploits the fact that the transmission speed of an ultrasonic signal depends on the flow velocity of the carrier medium,
kind of like a swimmer swimming against the 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. 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 initially the task of installing the meters was not without its difficulties. “The amount of straight run that is usually required to obtain best accuracy is 10-5 upstream / downstream diameters,” said Rivera. “So a 10-inch pipe needs 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 we 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” “We replaced the pipe with the venturi meters with a 48-inch diameter spool piece of ement-lined ductile iron pipe and installed the Flexim meters on the spool piece,” said McGuire. “Now we had no restriction in the wastewater line and no pressure loss like we did with the venturi.”

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


“There are plans in the works to install an
additional 50 ultrasonic meters in Easterly and her
sister plants,” said Papa. “That installation will give
the entire NEORSD precision control over all their
processes.

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