Ageing of bleached kraft mill effluent studied by degradation of chlorinated phenolic compounds and Selenastrum algal assays

Bleached kraft mill effluent (BKME) from a Finnish kraft pulp mill with a full bleaching system was aged under laboratory conditions. The ageing solutions were prepared in nutrient solution and three types of lake water. The changes in acute toxicity of the BKME were studied by Selenastrum algal assays and degradation of chlorinated phenolics. Chlorinated phenolic compounds were analysed with a gas chromatograph from the subsamples taken from the ageing solutions on the following days: 0, 4 and 7 or 8. The toxicity of the ageing solutions was bioassayed on the same days using Selenastrum capricornutum as a test organism. The response of the test algae to ageing solutions was measured by cell counting after 72 and 96 h.

During the ageing period the amounts of chlorinated phenolics decreased in the following order: (1) chlorocatechols, (2) chlorophenols and (3) chloroguaiacols. After the ageing period the EC₅₀ values of BKME ageing solutions to Selenastrum algae ranged from 6% to 10% (v/v) depending on the test medium and the effluent sample used. The correlation between the EC₅₀ values and total chlorinated phenolics was not statistically significant, so it seemed that, in these studies, chlorinated phenolics were not the primary reason for the acute toxicity of the BKME to Selenastrum algae.