Following a major plant upgrade, a historic city begins testing more efficient mixing
Albion is located in south-central Michigan. Tenney Peabody, Albion’s first permanent settler, arrived in 1833. In 1835, other early settlers followed from Albion Township, N.Y., from whence the city derives its current name.
Albion completed the update of several components of its 4-million-gal-per-day (mgd) plant in the fall 2017. Afterward, plant superintendent Kent Phillips became interested in testing the impact that optimizing his mixing might have on other aspects of the plant’s conventional activated sludge process with enhanced biological phosphorus removal.
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 dissolved oxygen (DO) in this part of the process. Reduced DO could improve biological phosphorus removal and allow the plant to reduce chemical and energy use.
An opportunity to refine the treatment process arose when Phillips contacted Xylem distributor Kennedy Industries to arrange a field demonstration of the company’s new Flygt 4220 mixer.
The Flygt 4220 Series mixers represent a significant departure from present-day compact mixers, even though they look almost exactly the same, and they do the same job in approximately the same way—a rotating propeller creating thrust and, ultimately, bulk flow in the tank. The radical part of the new mixer design is that it employs an IE4 equivalent permanent magnet motor, enabling previously unobtainable motor efficiency.
Also, integrated advanced controls and power electronics in the motor allow the adjustment of thrust and energy consumption to adapt to process changes, as well as provide continuous communication with the operator. The mixer basically cares for itself, and operators or other systems such as SCADA or system controllers via ModBus can easily control it remotely.
Removing an old mast-mounted mixer and replacing it with a Flygt 4200 Series is easy:
- The 4220 mixers do not create an inrush current (soft start by design) so the electrical provisions for a higher power-rated mixer can easily accommodate a 4200 of similar thrust; and
- The 4220 mixers do not need a motor starter, so installation is simplified, especially when starting from scratch.
Tank-side controls provide easy access to a host of mixer functions.
The Flygt 4220 was placed in a 16-ft-long-by-13-ft wide-by-15-ft-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 4220 was drawing only 0.53 kW 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 4220 down from 180 to 100 rpm, where it drew a mere 0.10 kW while still providing sufficient mixing in the experimental tank. The test showed that dramatic opportunities could be used for right-sizing mixer thrust when the equipment is available to take advantage of it.
The Flygt 4220 comes in 1.5-, 2-, 3-, and 4-hp models. A 4-hp mixer can operate at any lower power level and perform with the same high efficiency as a lower-power model.
The old mixer was drawing 1.3 kW. In the end, Phillips found he could turn the 4220 all the way down to 0.1 kW—8% of what the old mixer used—and still have satisfactory mixing.