Known as the “Land of 10,000 Lakes,” Minnesota takes its environment and its water resources seriously. That is why the city of Mayer, a town of about 1,800 in Carver County, Minn., was proud to have its wastewater treatment plant operator recognized by the Minnesota Rural Water Association (MRWA) for implementing innovative energy savings using variable frequency drives (VFDs).
The recipient of the award, Greg Kluver, had worked for Mayer in several capacities since 1990 and is currently the wastewater contract operator.
“During the last few years, municipal budgets have been under a lot of pressure,” he said. “In 2010, I proposed that Mayer invest in variable frequency drives to cut the cost of running the wastewater treatment plant’s biggest energy users, blower motors.”
The Plant’s Operation
The treatment plant uses three blower motors to perform aeration, one of the most important functions of the wastewater treatment process. Blowers agitate the sewage and inject oxygen into the water so that microbes can remove contaminants. However, the amount of oxygen must be precise. Too little oxygen will prevent the microorganisms from breaking down the organic waste. Too much will cause the microbes to cannibalize themselves and die.
Kluver noted that Mayer’s water treatment plant was designed as an extended aeration activated sludge facility, which depends on maintaining the proper balance of microorganisms. In this plant design, the influent, incoming wastewater flows into a pretreatment building to remove physical material, such as grit and sand. Then primary treatment begins. Sediment is settled out, and phosphorus is detained by using an anaerobic and anoxic tank in which the bacteria, in the absence of oxygen and nitrates, accumulate phosphorus.
In the secondary treatment phase, the wastewater flows into two aeration basins, in which it is continuously agitated and injected with air. The added oxygen triggers the microorganisms’ aerobic digestion process, feeding on the organic matter and removing nitrogen and ammonia. As the microbes flourish in the oxygen and nutrient-rich liquid, they clump (flocculate) together to form a mass of organic solids, known as a biomass. This biomass is also called a mixed liquor.
The mixed liquor flows to a secondary clarifier (settling tank), in which the activated sludge settles out. A portion of the sludge is returned to the head of the aeration basins to maintain a high population of bacteria to break down the organic material and maintain a constant flow rate. The remaining sludge is pumped to a digester, where it is stored. It is then transferred by truck during the spring and fall to another facility for final treatment and disposal. The remaining clear liquor passes through bridge sand filters and ultraviolet disinfection before being discharged to the south fork of the Crow River.
The VFD brings the blower up to speed by ramping the voltage. The newly installed VFDs can also be programmed to receive a signal from the dissolved oxygen sensor and adjust the blower speed accordingly.
The Addition of the VFDs
Together, the two concrete aeration basins can handle a combined volume of about 320,000 gallons per day. To supply enough oxygen into the wastewater, the blowers must deliver up to 650 cubic feet per minute of air.
“We have three 40-horsepower rotary blower motors configured in parallel,” said Kluver. “They are 60-hertz, positive-displacement root blowers. The blowers run one at a time for 24 hours per day, for a predetermined time period. Then we cycle on the next blower to equalize the run time between all three.
“This application requires constant torque (CT). In CT applications, torque is directly related to current. That means a VFD maintains CT by increasing the voltage in a linear manner as speed increases.”
The VFD slowly ramps the voltage to bring the blower up to speed. The newly added VFDs can be programmed to receive a signal from the dissolved oxygen (DO) sensor and adjust the blower speed accordingly.
“The great thing about a positive displacement blower, as soon as the rotor turns, it is pushing air into the process,” said Kluver. “This creates an almost perfect linear performance curve. The higher the voltage, the faster the rotor speed, and the greater the airflow. On the other hand, if we only need half the air, we can reduce the blower speed and the horsepower by 50 percent and cut the kilowatts in half.”
Kluver contacted an equipment supplier near St. Paul with a VFD specification, and the supplier recommended a VFD that was dedicated to water and wastewater applications. This allowed for simplified implementation. The VFD had many built-in features—including a cascade controller to radio frequency interface filter to input the choke to real-time clock.
On this plant’s basic positive-displacement blower project, the equipment supplier and Kluver took advantage of the built-in water/wastewater intelligence. They were able to input the DO value into the on-board, closed-loop control of the drive. The VFD display then showed the actual DO value, DO set point, motor speed and kilowatt consumption. As a result, an integrator was not needed, which helped with cost savings.