Aqueous solution containing ahumic acid was treated by ozonation, which was followed by chlorination with HCIO. The TOC level in preozonated samples was varied by changing ozone dosage. When 50% of the TOC was removed, the THMFP and TOX were destructed to ca. 1/30 and 1/20 of the respective initial values. The effect of preozonation on the mutagenic activity of nonvolatile organic chlorides generated by the chlorination was evaluated by the Ames test after the solute was separated on the basis of affinity for hydrophobic XAD-2 adsorbent and fractionation with various solvents. The overall mutagenic activity, defined as the number of revertants per unit volume of the solution considered, was expressed by the product of the following factors: concentration of nonvolatile matter in solution, mass fraction of hydrophobic component in the nonvolatiles, and number of revertants per mg of dosed hydrophobic component. The contribution of each factor was evaluated experimentally. Preozonation was effective for the selective destruction of precursors of the hydrophobic component.

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