Water Treatment > Quality Water Solutions > FAQ > What should I consider when evaluating alum and PACl for total organic carbon and enhanced coagulation?

Coagulation programs designed for TOC removal may have trouble meeting solids disposal, corrosion prevention and other operating goals. To gain data on how to balance TOC removal and other treatment priorities, we looked at how 35 municipalities removed TOC through coagulation and how they could improve performance with their current coagulant or an alternative one. Most of the plants applied alum, while the rest used ferric chloride, ferric sulfate, polymers or PACl. The study found that:

• Lower pH values improved TOC removal with alum and ferric salts, except in cold, low-alkalinity, low-TOC waters where no clear relationship emerged.
• Acid alum reduced TOC at 0.1 to 0.5 higher pH values than alum alone and at 1.4 to 2.5 higher pH values than iron compounds.
• PACl and formulated coagulants may offer significant cost and operating benefits because they removed TOC better at ambient pH than alum or iron salts.
• Blended coagulants performed as well or better at lower metals dosage than alum or iron salts. They also generated less sludge and appeared to improve pH adjustment.

Plants should conduct trials on TOC treatment alternatives to understand how a change can affect treatment overall. Factors to evaluate include: ability to handle a lower pH if enhanced coagulation with alum, ferric sulfate or ferric chloride is indicated; availability of safety systems for acid handling and feed systems, if needed; how to cope with excess residues, especially if heavy metal content rises; and whether or not the new coagulant causes coagulant-induced color problems or manganese residuals in the effluent due to contamination.

If TOC problems emerge during spring or fall runoff events, separate seasonal coagulation programs may be better for TOC removal than irrevocably changing a coagulant. This provides the flexibility to treat diverse water quality conditions, rather than forcing one approach to work all year.