Water Treatment > Quality Water Solutions > Treatment Issues > TOC Removal

Total organic carbon (TOC) is present in all raw waters as either naturally occurring organic matter or synthetic organic chemicals, or both. Impending federal limits will compel water systems to reduce TOC. The challenge: how to balance TOC removal against other treatment priorities. Enhanced coagulation, which can more than double dosages, is often cited as the answer for TOC control. However it profoundly affects plant operation by increasing filtration, dewatering and disposal expenses, while increasing use of corrosion control and pH-control chemicals through higher pH.

Studies conducted by General Chemical at numerous treatment plants found that coagulation efficiency for TOC removal was generally highest with relatively low dosages of acid alum and low basicity polyaluminum hydroxychloride (PACl), depending upon water characteristics. Moderate dosages of alum and high dosages of iron salts also removed TOC effectively.

PACl coagulants combine adequate-to-excellent TOC removal with potential savings by lowering residuals and alkalinity supplementation and optimizing color, turbidity and residue. Acid alums can provide good TOC reduction at low metal requirements. They also offer the familiarity of alum and an easy switchover of storage and handling equipment.

The large coagulant doses needed to thoroughly remove TOC can cause a fluffy floc, especially in highly colored waters with low turbidity. Fairly low doses (below 5 to 10 ppm) of clay-based weighting agents can aid settling by enhancing floc weights, providing sites for floc growth and adding surfaces for adsorption.

Calcium also can improve TOC removal. Facilities using lime for pH adjustment might want to experiment with the best feed point for lime well upstream of coagulant addition.

Plants may benefit from a seasonal coagulation program to meet varying TOC loads. They should trial TOC treatment alternatives to evaluate how a coagulant change would affect the overall process.