Monitoring floc formation and breakage

Monitoring of flocculation processes is of great practical importance and can give very useful information on the action of different coagulants under various conditions. It is shown that a continuous optical monitoring technique provides a convenient means of following the formation, break-up and regrowth of flocs under standard mixing and stirring conditions. Examples of results using this approach are given, with alum and PACl coagulants as well as a cationic polyelectrolyte. PACl products give larger flocs than with alum, but they show about the same relative degree of breakage when the shear rate is increased. With all of these coagulants, floc breakage is not fully reversible; i.e. flocs only re-grow to a limited extent when the shear rate is reduced. This is in marked contrast to the action of a cationic polyelectrolyte, where floc breakage is almost completely reversible. Some possible reasons for these observations are discussed, but, so far, there is no adequate model available.