The production of drinking water requires the combined interaction of rapid mixing, flocculation, and settling. Treating cold raw water (that is, 40 °F water) is a challenge because:
- The effectiveness of rapid mixing is impaired by the increased viscosity of the cold water.
- The rate of floc formation is decreased because: a) the rate of the hydrolysis reaction is decreased and b) the mixing effectiveness of the flocculators is decreased by the water’s increased viscosity.
- The settling rate of floc particles is significantly impaired by the water’s increased viscosity.
- Note that these impairments are typically partially offset by the reduced water productions rates that normally occur during cold weather.
Methods for improving the effectiveness of cold water treatment include:
- Increasing the hydrolysis rate by adding lime ahead of the coagulant addition.
- Increasing the effectiveness of the rapid mix and the flocculators by increasing their rotational speeds to maintain their design mixing intensities.
- Using a coagulant that is relatively more effective for treating cold water - this generally involves switching to a blended alum (Clar+Ion®) or ACH/PACl based product during the cold water season. (Note: the floc formation rate of all coagulants decreases, to some extent, at temperatures < 40 °F. However, alum’s floc formation seems to be more adversely affected than ferric salts, while ACH and PACls are least affected because, by their very nature, they are partially “pre-hydrolyzed”).
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