In 2017, a wastewater plant in the city of Albion began testing Xylem’s new Flygt 4220 mixer to see how it could improve mixing efficiency. The plant found that the Flygt 4220 mixer could produce the same results as the plant’s old mixer using 92 percent less energy.

The city of Albion – located in Michigan, US – completed an upgrade of its four-million-gallon-per-day wastewater plant in the fall of 2017. Afterwards, plant superintendent Kent Phillips became interested in testing the impact that optimizing his mixing might have on other aspects of the treatment process.

The plant is well configured for this type of experiment. It is a conventional activated sludge process with two parallel trains. The parallel trains make it easy to tweak one and observe how it impacts the effluent from that side, while holding the other train constant.

Phillips’ initial theory was that modulating the speed to maintain mixing while reducing surface “churning” would reduce the dissolved oxygen (DO) in this part of the process. Reduced DO could improve the biological phosphorus removal and allow the plant to reduce the chemical and energy use.

Testing the Flygt 4220 adaptive submersible compact mixer

An opportunity arose to refine the treatment process when Phillips contacted trusted Xylem distributor Kennedy Industries to arrange a field demonstration of Xylem’s new Flygt 4220 mixer.

The Flygt 4220 mixer represents a significant departure from present-day compact mixers.

The mixer’s design includes a new, smooth cone shape that prevents ragging around the blades – so maintenance is minimized. The mixer design also employs an IE4 equivalent permanent magnet motor, enabling previously unobtainable motor efficiency.

In addition, the Flygt 4220 mixer is unique in having an integrated variable frequency drive. These advanced controls enable the mixer to adjust thrust and energy consumption based on process changes. It also provides continuous communication with the operator. The mixer can easily be controlled remotely by operators or other systems like SCADA or system controllers via ModBus.

Easy mixer installation with no motor starter needed

Removing an old mast-mounted mixer and replacing it with a Flygt 4220 mixer is easy. The Flygt 4220 mixer does not create an inrush current (soft start by design) so the electrical provisions for a higher-power rated mixer can easily accommodate a Flygt 4220 of similar thrust. The Flygt 4220 mixers do not need a motor starter, so installation is simplified, especially when starting from scratch.

Result: mixer reduces energy consumption from 1.3kW to 0.1 kW

The Flygt 4220 mixer at Albion was placed in a 16-foot long by 13-foot wide by 15-foot deep anoxic tank, replacing an existing compact mixer that drew a continuous 1.3 kW. Installation was completed August 2, 2017, after which the mixer was run at the recommended thrust for the first few days based on the manufacturer’s mixer sizing tool.

Right away the new mixer was drawing 0.53kW to produce the same thrust as the original did for 1.3 kW. After that, Phillips began experimenting. He found he was able to run the mixer at much lower speeds and still maintain adequate mixing.

In the end, he was able to turn the new mixer down from 180 to 100 RPM, where it drew a mere 0.10 kW while still providing sufficient mixing in the experimental tank. That is 8 percent of the energy use of the old mixer. The test showed that there can be dramatic opportunities for right-sizing mixer thrust, when the equipment is available to take advantage of it.

Xylem’s Flygt 4220 mixer comes in 1.5-, 2-, 3-, and 4-horsepower models. A four-horsepower mixer can operate at any lower power level and perform with the same high efficiency as a lower power model.