The General Mills facility in Wellston, Ohio, is the world’s largest producer of frozen pizzas. In the summer of 2004, Darin Davis, General Mills Wellston environmental manager, needed to figure out a way to turn 90 million gallons of murky pond water into clear water with no more than 8 mg/l of suspended solids, the level required by the city’s publicly owned treatment works (POTW).
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“We needed to drain the entire 33-acre lagoon to make it possible to remove the sludge and land-apply it,” explains Davis. “However, we wanted to avoid trucking the water because of the high cost and the potential environmental impact.”
Treating the lagoon water in the plant’s wastewater treatment system wasn’t an option. General Mills has a permit to discharge 700,000 gallons per day of treated wastewater into the creek, but nearly all of this capacity is required for the plant’s manufacturing operations. Two options remained: trucking the water away or discharging it into the city’s POTW.
The POTW required that total suspended solids (TSS) not exceed 8 mg/l, but the lagoon water typically measured in excess of 200 mg/l, and readings could double after spring runoff or during high-algae growth periods in the summer months.
Davis and his team approached GE Water & Process Technologies (www.gewater.com). “We had a history of working closely with GE at our wastewater plant, and turned to them for assistance in solving this unusual surface water treatment challenge,” says Charles Camp, supervisor of wastewater treatment plant, Wellston General Mills.
GE engineers proposed using entrapped air flotation (EAF), a relatively new technology. EAF improves upon conventional dissolved air flotation (DAF), a nearly-century-old wastewater treatment method widely used in a host of industrial applications primarily for liquids, solids and oils separation. Conventional DAF uses pressurized gas to separate solids and oil from a water stream. EAF doesn’t rely on pressurized air, but rather air introduced under atmospheric conditions with treatment chemistry that allows precipitated contaminants to entrap the air bubble for separation from the bulk water.
Engineers conducted laboratory tests on water samples from the lagoon using a wide variety of chemical formulations. As a result of this testing, GE Water & Process Technologies recommended application of their KlarAid IC1187, an inorganic coagulant, and PolyFloc AE1138, a high-molecular-weight flocculant.
One of the many challenges was a lack of potable water at the site, so unfiltered water from the lagoon was employed as “drive water” in the EAF process. Despite concerns that the unfiltered water would be harmful to pumps and other components, the EAF systems performed without failure.
“The EAF system proved to be an excellent solution,” said Camp. “Despite the highly variable and difficult-to-treat feedwater, we maintained an average flowrate of about 300,000 gallons per day, right through the cold winter, and we never exceeded the stringent TSS standards set by the POTW.”
Davis calculates that General Mills saved approximately $5 million by avoiding the expense of trucking the lagoon water. “We also saved another $150,000, because the water removal project was completed three months ahead of schedule,” Davis says. “It turned out that we needed this additional time for the sludge removal operation, because we ended up removing double the amount of sludge from the bottom of the lagoon compared to our original estimates.”