The effectiveness of the two biological treatment systems operating at a New Zealand bleached kraft softwood integrated pulp and paper mill was assessed. The two systems operate in different configurations. Treatment system A, which receives general mill wastewaters and chlorination stage bleaching discharges utilises deep, aerated lagoons and has a 4.5 day retention time. Treatment system B, which receives alkali extraction bleaching wastewaters and foul condensates, uses a lagoon system with a retention time of 45 days.

Detailed chemical analyses of the untreated and treated wastewaters were made. Mass balances were calculated for a range of physical parameters and for specific chlorinated and non-chlorinated organic constituents. Significant differences in the treatability of various constituents were found. In particular, while system A was able to reduce levels of AOX by 65%, no significant reduction in AOX occurred in system B. In contrast, system B reduced levels of chloroacetic acids by 84% while system A did not achieve any statistically significant removal of these compounds. The treatability of chlorophenolic compounds also differed. System A was unable to remove chlorophenols and chloroguaiacols while system B did not reduce levels of chlorocatechols.

These results confirm that the treatability of bleached kraft pulp and paper mill wastewater constituents is dependent upon the characteristics of the treatment systems and the compositions of the wastewaters.

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