When the Moundsville (W.Va.) Wastewater Treatment Plant experienced a blower failure, the staff naturally assumed they should repair the failed unit to keep the system design intact. At the time, two 75 hp centrifugal blowers fed air to the plant’s four aeration tanks, and a third centrifugal blower unit served as a backup.

However, plant personnel decided to place the repair on hold pending an evaluation of tank conditions. The 2.43 mgd (design) activated sludge plant had been experiencing an issue with pin floc in the aeration tanks, indicating a problem with dissolved oxygen (DO) levels.

In the end, rather than simply repair or replace the old blower, the team installed a new aeration system using a blower and variable-frequency drive in a feedback loop with a DO sensor. The arrangement quickly solved the pin floc problem and saved substantial energy.

 

Studies first

Moundsville, a city of 10,000 along the Ohio River, is home to sites like Prabhupada’s Palace of Gold and the retired West Virginia State Penitentiary. The treatment plant serves the city, areas outside the city limits, and Glen Dale, W.Va., a neighboring community of 2,500. The facility had a solid compliance record leading up to the blower failure.

After a study with an online DO meter, plant operators determined that DO levels varied from about 0.5 to 6.0 ppm in a typical day. The low levels indicated that microorganisms were dying in anaerobic zones at times, while at other times the DO levels were excessive, which meant wasted energy.

With that data in hand, the team back-calculated the air requirements for the individual aeration tanks. The results supported a new solution: installing an inline DO meter and variable-frequency drive controlling a 100 hp positive displacement (PD) blower connected to the primary aeration tanks. The solution also called for a valve in the line to the secondary aeration tanks and adding a 50 hp rotary positive displacement blower.

The air system was carefully designed to respond to real-time conditions in the aeration tanks. The DO sensor would constantly monitor oxygen levels and output a control signal varying from 4 to 20 mA. The signal then would be sent to the variable-frequency drive, which controls the blower speed by changing the frequency of the motor power supply.

 

Quick results

Stu Harper of S. R. Harper, a Danfoss representative, recommended installing a Danfoss variable-frequency drive and Danfoss DO sensors, as well as a Kaeser Com-paK Plus EB290C 50 hp blower and a Kaeser Omega-paK FB620 100 hp blower with a maximum combined flow of 2,585 cfm. Harper chose the blowers for their wide turndown range, surge-free characteristics, and power almost directly proportional to speed in constant pressure applications like tank aeration.

“We installed the equipment ourselves and saved quite a bit of money,” says plant superintendent Larry Bonar. The staff programmed the drive to maintain a DO level of 2.2 ppm and a minimum 40 percent of blower speed. After four days of automatically controlled DO levels, the pin floc issue disappeared. In addition, the blower/drive combination operated at high efficiency and did not produce excess air. In just the first year, the plant saved about $24,000 in electricity.

Plant personnel went on to perform DO studies in the two remaining primary aeration tanks and again discovered excessive air levels. This time, they installed a second Kaeser 100 hp PD blower with a Danfoss variable-frequency drive and DO sensor to control airflow to the tanks. The two existing centrifugal blowers were still in working condition and did not go to waste — they were incorporated into the system as backups to provide full redundancy.

“All the system requires is routine maintenance,” says Bonar. “The repair enabled us to save $50,000 to $60,000 per year in energy costs, and helped to reduce solids in our effluent by letting us control the amount of dissolved oxygen in the tanks.”

 

About the author

Calvin Wallace is national sales manager for Omega blowers with Kaeser Compressors in Fredericksburg, Va.