Water Treatment > Quality Water Solutions > Operational Issues > Feed Point Optimization

Some water treatment chemicals react with coagulants. Most operators know which chemicals cannot be mixed safely, but they may not realize that these same chemicals would be more effective if addition points in a plant do not cause interactions.

Coagulant and lime offer a good example. Plants using lime for softening or pH control often add it along with alum or iron salts. Unfortunately, the two interact so more of each must be added. The result is a huge volume of chemical floc and sludge but inefficient particle removal. A simple solution is to add them at different times, say 30 seconds to a few minutes apart. Addition sequence also is important and depends on plant configuration and raw water type. Lime is often added first for softening and coagulant is added first for clarification.

Another example is adding fluoridation chemicals to the rapid mix area if aluminum (alum or PACl) and calcium (lime or calcium chloride) are present. Aluminum and calcium remove fluoride, usually forming tiny particles that are not settlable or filterable. If excess coagulant is present, fluoride demand may not show up. This is especially true of enhanced coagulants, which, as highly cationic products, greedily complex fluoride. The best practice is to separate fluoridation chemicals and coagulants as much as possible or as allowed by local regulations.

Other treatment chemicals, such as taste and odor control chemicals like potassium permanganate and powdered activated carbon, can increase coagulant demand by forming particulates. Potassium permanganate oxidizes materials in the water, forming manganese dioxide precipitate. Coagulants act on this precipitate and powdered activated carbon as they do on any other particulate. Coagulant addition should compensate for the solids generated by taste and odor chemicals.

Mixing energy is critical because the more particulates and coagulant molecules come in contact, the faster and more efficient charge neutralization and flocculation proceeds. Improved mixing can lower coagulant dosage, improves clarification and enhances the action of other treatment chemicals like carbon and permanganate.