Water Treatment > Quality Water Solutions > Coagulation Control > pH Measurement

The primary coagulants used in potable water treatment -- alum, polyaluminum hydroxychloride (PACl), and ferric chloride and sulfate -- hydrolyze in water and evolve aluminum or ferric hydroxide floc. Hydrolysis speed and efficiency depend on pH, alkalinity and water temperature. Speed varies from less than 0.001 second to more than 1 second. Efficiency increases if more insoluble hydroxides form for entrapment and if the hydrolyzed products have a higher charge that enhances neutralization.

Alum removes turbidity best between pH 6.0 and 7.5. It is inefficient between 8.5 and 10 because hydrolysis continues until soluble aluminates predominate and little floc forms. Soluble aluminates react with calcium to form precipitates above pH 10, the level used in water softening. Iron compounds have a similar optimum pH range as alum in turbidity removal, but do not form soluble species at intermediate pH ranges.

PACls are prehydrolyzed and so have less sensitivity to pH, i.e., they can have a high charge developed in manufacturing. They can be used over the entire pH range generally found in water clarification (roughly pH 4.5 to 9.5).

Coagulation of DBP precursors and other organics is best done at a lower pH than turbidity removal, i.e., 5.5 to 6.5 for iron and 4.5 to 5.5 for alum. Alum is somewhat easier to optimize because the pH ranges for turbidity and organic removal overlap. The low pH can be reached by dosing a huge excess of coagulant, but this boosts sludge volume. Plants often add an acid or use acidulated alum instead. Acidulated alum may be preferred because it is a one-step approach and avoids use of a hazardous material. If pH is lowered in clarification, it may have to be raised after filtration.

Controlling pH uses off-the-shelf equipment -- a pH probe and meter and a signal transmitter linked to a chemical metering pump. Backup systems should be built in to prevent alkali/acid excursions if the primary system fails. A good laboratory pH meter will also be needed for calibration.

Use jar tests to approximate the optimum acid dosage by sequentially adding acid. Select the pH that gives the best reduction of the target parameter -- color, TOC, etc. Add in as many routine treatment chemicals as possible to account for additive and synergistic effects.