Industrial wastewaters containing chlorophenolic compounds exhibit various degrees of resistance to biodegradation. Preoxidation of these wastewaters by using ozone as the chemical oxidant has been found previously to be quite effective in promoting their biodegradabilities.

This study was made to further investigate whether or not preozonation taking place under different pH conditions would bring about different effects over the following activated-sludge treatment systems. The results indicate that the pH conditions set for the preozonation step did play significant roles in the subsequent biological treatment processes. To the activated sludge system which had not been exposed to chlorophenolic compounds before, preozonation carried out at low or high pH both greatly improved the biodegradabilities of the influents, with the latter condition showing better improvements. Nevertheless, to the activated sludge system previously acclimated to the individual chlorophenolic compounds, the process of preozonation in some cases would then bring about retarding effects over the biodegradabilities of the influent. The phenomena of retardation were generally more apparently observed in the samples which had been preozonated under alkaline conditions.