We proposed a method for describing and predicting the changes in the total toxicity based on the concentrations of toxic-controlling chemicals during the ozonation of landfill leachates. In a model experiment, DOC of the sample landfill leachate decreased but its cytotoxicity increased with the increase of ozonation time. Molecular weight fractionation of the most toxic water samle (ozonated for 300 min) showed almost all the toxicity was attributable to the low-molecular-weight (less than 100) fraction. Further chemical analysis revealed that this strong toxicity was mostly explained by the presence of formaldehyde in that fraction. The toxicity reduction by removing this fraction from the whole water sample was predicted using multi-chemical toxicity analysis, and the result was well correlated with the observed toxicity. We attempted to describe these phenomena using a simple numerical model assuming three groups of toxic chemicals; that is, a group consisting of originally-contained chiemicals, a group consisting of moderately-toxic chemicals emerging at a relatively early stage of the ozonation, and a group whose toxicity is explained by formaldehyde. This simple assumption was largely successful in describing the changes in the total toxicity during the ozonation. On the basis of this simple model, toxicity reduction was predicted for pre-aggregation treatment before ozonation and for ozone concentration during the ozonation. The method proposed in this study would be useful in optimizing water treatment processes and their running conditions in terms of the toxicity reduction efficacy.
