When the small team at the Chinook (Montana) Wastewater Treatment Plant selected new mixers for a plant upgrade in 2004, they could not have known that the same mixers would still be dutifully working 16 years later — and contributing to nutrient removal.
The mixers were purchased so that a 50 hp surface rotor aerator could be switched off during night hours of low BOD loading. The two 5 hp submersible mixers have saved $18,000 per year on electricity, achieving payback on the entire installation in slightly less than 4.5 years.
In addition, by trying the mixers on different on-off cycles, plant
