Water Treatment > Quality Water Solutions > Treatment Issues > Soluble Residuals

The appearance of soluble aluminum or iron in finished water may indicate that the coagulant is either overfed or underfed. Overfeeds of alum and iron coagulants place too much metal in the system, while underfeeds generate metal-coated colloids that pass through filters and emerge in the finished water. This can cause a number of problems:

• Soluble aluminum can postflocculate, increasing distributed water turbidity.
• Residual iron can cause 'red' water, staining, and corrosion.
• Residual aluminum and iron can complex with other ions in the finished water to form deposits that reduce the carrying capacity of the distribution system.
• Both metals can precipitate the phosphates in phosphate-based corrosion chemicals, reducing their effectiveness.

General Chemical studies show that soluble aluminum residuals are reduced by using polyaluminum hydroxychloride (PACl) because it is used in lower dosages than traditional coagulants (so less metal is present to begin with). PACl is also more efficient, so more of the aluminum undergoes reaction and less exits the system.

Soluble manganese is also of concern in water systems because it can cause black water and staining. It can occur naturally or enter a plant due to the addition of potassium permanganate or iron-based coagulants, in which it is a common contaminant. Manganese also builds in sludge, which releases manganese when it becomes anoxic. Plants may not notice the release of this metal from sludge because raw water generally contains low amounts of manganese.