Coagulant mixing revisited: theory and practice

Are rapid-mixing tanks with intense mixing necessary? Intense mixing applies to the coagulation mechanism of charge-neutralization of negatively-charged particles, but this is not a common coagulation method. More common is precipitation of aluminum or iron for coagulation of particles and natural organic matter, which is often called sweep-floc coagulation. Coagulation by precipitation is indifferent to mixing intensity. Energy and cost savings are realized by designing mixing systems for coagulation that do not use tanks with mechanical mixers. Systems examined are hydraulic methods (weirs, Venturi meters, others) including the non-mechanical systems of pipe and open-channel static mixers. These systems provide sufficient mixing intensity for sweep-floc coagulation and for certain conditions provide sufficient mixing intensity for charge-neutralization coagulation with much less energy input. Mixing systems designed specifically for coagulant addition, such as rapid-mixing tanks and pipe and open-channel static mixers, should be sized based on the average daily flow not on the plant capacity flow. This produces sustainable designs with lower capital costs for all systems and lower operating costs for rapid-mixing tanks.