87Coagulation
The optimal pH range for coagulation is 6 to 7 when using alum and 5.5 to 6.5 when using iron. For
high alkalinity water, excessive amounts of coagulant may be needed to lower the pH to the optimal
pH range. In these cases, it may be beneficial to use acid in addition to the coagulant to reduce the
amount of coagulant needed and effectively lower chemical costs. Enhanced coagulation is now
widely practiced for removing disinfection byproduct (DBP) precursors, and it also removes
inorganics, particulates, and color causing compounds.
Removing these contaminants using coagulation depends on the amount of coagulant added. It is
important to determine the optimal dose for coagulation; insufficient doses will not effectively
destabilize the particles and adding excessive doses can cause detrimental effects such as re-
stabilization, excessive sludge production, or corrosion.