Rapid mixing with HEV static mixers: The effects on aluminum hydroxide precipitation and coagulation with alum

The application of an HEV static mixer in the "Alum Rapid Mix" stage for the water treatment industry was evaluated by measuring and comparing the effects of its mixing intensity and dispersion characteristics to a continuous stirred tank and a pipe in turbulent flow.; Experimental results indicate that the rate of change of pH, which occurs as North Saskatchewan River water is mixed with liquid alum, determines the morphology of the aluminum hydroxide precipitate: a result of the degree of supersaturation created by a combination of the dilution effect on pH and the pH sensitive thermodynamic solubility interaction of the alum/water mixture. The literature review revealed that a low degree of supersaturation forms precipitates 100 times larger than a high degree. Mixing conditions designed to produce the larger precipitates, will have flocculation rates over 10 times faster. Production of the larger precipitate is obtainable with an HEV static mixer.