Over 100 years ago, the first wastewater treatment plant was built in Madison, Wisconsin — about 50 years before most other communities had such a facility. The Madison Metropolitan Sewerage District was created in 1930 by state statute to protect the environment in the region of lakes and streams that comprise the upper Yahara River watershed. And since that time, they’ve seen a lot of equipment come and go.

In the fall of 1999, the district decided to replace its aging electromagnetic flowmeters (magmeters) at 32 different points in its Nine Springs Wastewater Treatment Plant, which serves about 300,000 people in the Madison area. The magmeters are used to measure flows for waste activated sludge, clarifier underflow, return activated sludge, and primary effluent.

District staffs are responsible for the administration, and for design and maintenance of the interceptor collection sewers and pumping stations that convey wastewater from over 40 municipal customers to the Nine Springs plant. The MMSD handles an average of 40 million gallons of wastewater a day.

After carefully weighing its options, the district decided to purchase UniMag™ magmeters from Isco, Inc. in Lincoln, Nebraska.

Electromagnetic flowmeters are based on a principle of physics known as Faraday’s Law of Electromagnetic Induction. Simply put, Faraday’s Law says electrical energy (voltage) will be created across a conductor (water or other conductive fluid in this example) as it passes through a magnetic field produced by coils energized by an electrical current. The induced voltage is directly proportional to the magnetic field and velocity of the fluid, allowing the flow rate to be accurately measured by measuring the voltage across electrodes in contact with the fluid.

One of the weaknesses in conventional magmeter design has been the requirement for a liner of non-conductive material such as rubber inside the flow tube, to prevent an electrical short-circuit between the electrodes used to measure the voltage.

Over time, this liner material gradually degrades due to its microporous nature, as well as abrasion caused by the sediment naturally present in wastewater — particularly activated sludge.

This was the case at the Nine Springs plant. Paul Nehm, director of operations and maintenance at the treatment plant, commented, "We chose the Isco Unimag magnetic flow meters because there was no requirement for a liner. The magmeters that we replaced showed serious deterioration over the years to the point where we could not rely on the accuracy of the measurement. The Isco Magmeters also could be sized to fit the various locations in our plant without the need for pipe lay-length modifications."

H&H Industries of Madison, with Greg Schnelle as project manager, installed the flow elements with little difficulty. The wiring between the flow elements and the converters was performed by the district’s electrical maintenance section. The Isco service department, along with Mulcahy Tech Sales (the local Wisconsin representative), checked the connections and calibration of the first four meters. The training they provided allowed the district personnel to complete the electrical installation of the remainder of the units.

Isco UniMag magmeters feature an exclusive, patented flow tube design with self-contained sensors that can be interchanged on-site quickly and easily, without recalibration. The patented, high energy UniPulse excitation maintains measurement accuracy even under conditions of electrode fouling, and low or variable media conductivity. UniMag flow tubes are also available in PVC (polyvinyl chloride).

The newly installed Isco Magmeters confirmed that the previous meters had not been accurately measuring the flows, Nehm noted. This had resulted in an excessive amount of mixed liquor being wasted to the digesters, and a higher than necessary sludge production rate. "The new meters also allowed us to ensure that we are splitting our flows equally to our aeration tanks and withdrawing underflow equally from our clarifiers," he stated. "Although we have had the meters installed for a relatively short period of time, the Isco support as well a as support from Mulcahy Tech sales has been excellent. We do not anticipate having to replace these meters for a very long time."

For additional information, contact Isco, Inc. in Lincoln, Nebraska — phone 800-228-4373, FAX 402-465-3064, or e-mail info@isco.com.