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Old rules of thumb and mixer design methods based on mixing power intensity today still circulate in the waste water industry. A critical review reveals that alleged required levels of mixing power intensity, expressed in for instance W/m3, hp/103 or 106 US gal, or hp/1000 ft3, are not well-defined. Furthermore, they are based on a variety of mixing technologies and out-of-date mixing requirements. Designing mixers with a momentum approach and using Flygt submersible mixers or top-entry agitators is shown to require only a fraction of these intensities in for instance biological treatment and sludge handling.
