Water Treatment > Quality Water Solutions > Coagulation Control > Residual Metals in Finished Water

Iron or aluminum residuals may show up in finished water if a coagulant program operates below peak efficiency. Iron residuals can cause staining and corrosion, while aluminum residuals can postflocculate and increase system turbidity. Iron and aluminum also reduce the effectiveness of phosphate-based corrosion control by precipitating the phosphates.

On-line colorimetric analysis is the primary way plants measure residual iron and aluminum. A colorimetric analyzer samples the water stream at regular intervals and adds reagents that react with the metals present. The type and intensity of the color formed by the reaction indicates the metal present and its concentration.

Excess residuals occur if coagulants are overfed or underfed. Iron compounds are somewhat more difficult to control for they typically have a narrower optimum dosage range than aluminum compounds. Alum and sodium aluminate are more forgiving on dosage overfeeds. Underfeeds of alum and iron coagulants may alter contaminant charge but not cause effective flocculation. This results in small metal-coated colloids that pass through filters and appear in finished water.

Colorimetric analyzers are easy to calibrate against laboratory values of the same chemical parameter (aluminum or iron) and are relatively inexpensive compared to other monitoring hardware. They are often placed after filtration. This location can be too late in the process because they provide information on the first step (coagulation) as the water is about to leave the plant.

Colorimetric analysis may signal a coagulant problem, but does not define its cause. Although it is a useful method, it is less effective in optimizing coagulation than more discriminating methods that define how well coagulation works, like streaming current analysis.