Relationship between raw water TOC and the optimum coagulant dose (iron III chloride)

The general scope of this study is the removal of organics from highly colored impoundment and river waters by coagulation with ferric iron. The aim of this research was to define the relation between the optimum coagulant dosage, the organic content, the coagulation pH and the origin of the raw water. The results showed that the optimum pH for coagulation with Fe (III) was between 4 and 5 for organics removal measured by TOC and UV-absorbance at 254 nm and between 5 and 6 to reach the lowest residual iron. Depending on the origin of the water and the date of the sample, TOC removal by coagulation at pH of 5.5 ranged between 60 and 80 %. Whatever the origin of the raw water was, the optimum dose of coagulant was 2.1 +/- 0.2 mg Fe per mg of TOC. In a previous study we obtained the same result with solutions of fulvic acids. Residual iron, after coagulation, clarification and filtration increased when the dose of coagulant was below the optimum dose. On line analysis of residual iron could be a way to control the coagulation stage and to reach a maximum TOC removal. Another efficient way is to measure continuously the TOC in the raw water and adjust iron dosage according to the optimum ratio Fe/TOC = 2.1 g/g.