Water Treatment > Quality Water Solutions > Treatment Issues > Alkalinity Considerations

Metal-based coagulants have two primary actions in water treatment: they neutralize negative particles in raw water and they react with alkalinity present to form a metal hydroxide floc. The floc then precipitates and carries suspended solids with it.

Traditional metal-based coagulants are acidic and so decrease system alkalinity. Different coagulants affect the need for alkalinity adjustment to different degrees, e.g., alum consumes half the alkalinity of ferric chloride.

If the coagulant demand of a low alkalinity water exceeds the alkalinity available, the metal hydroxide cannot precipitate completely and clarification will be incomplete. This can shorten filter runs due to early filter breakthrough and increased headloss. It can also cause post precipitation of the coagulant.

Most plants compensate for low raw water alkalinity prior to coagulation by adding a supplemental alkali like lime, soda ash, caustic soda or sodium bicarbonate in the raw water line and/or the rapid mix. The location(s) should allow good mixing before the coagulant addition point. Some plants also add alkali just before filtration and in the clearwell. While this does not affect coagulation, it elevates pH and contributes to finished water stability.

The choice of alkali can depend, in part, on the feedpoint. Lime is typically added as early as possible, usually in the raw water line or rapid mix. It is generally not added to the clearwell because this can drastically elevate filtered water turbidity. Although lime can be fed prior to the filters after most of the settling is completed, some plants have seen lime carryover. Caustic soda and soda ash can be fed at any of these feed points without the difficulties encountered with lime. They dissolve readily and do not add significant turbidity. They can be more expensive than lime, but they are easily fed and preserve finished water quality.

The coagulant used can affect the alkalinity adjustment in low alkalinity water. The alkalinity consumed by different coagulants can vary greatly. For example, ferric chloride (dry basis) consumes 1 ppm alkalinity/ppm coagulant, alum (dry basis) consumes 0.5 ppm alkalinity/ppm coagulant, while a high basicity PACl requires only 0.088 ppm alkalinity/ppm coagulant.