The toxicity of eighteen different untreated or secondary treated TCF (total chlorine free), ECF (elementary chlorine free) and conventional (Cl₂) bleaching effluents were assessed by a battery of biological tests. The toxicity tests used were: Pseudomonas putida growth inhibition test, Vibrio fisheri luminescence bacteria test, Selenastrum capricornutum algal growth inhibition test, Daphnia magna mobility inhibition (24 h) test and Branchydanio rerio zebra fish hatching and survival test.

In the P. putida growth inhibition test only conventional bleaching effluents and ECF birch pulp effluent gave a slightly toxic response. The V. fisheri test was more sensitive. The EC50 values of most untreated ECF and TCF effluents were under 10% effluent concentrations (conventional effluent 15%) and secondary treated effluents were not toxic. All untreated bleaching effluents gave a toxic response in S. capricornutum algal test. EC50 values varied between 12% and 46% effluent concentration. Treated effluents were not toxic and had a stimulative impact on algal growth. The results obtained by the Daphnia magna test showed that effluents of TCF, ECF (birch pulp) and conventional (pine pulp) bleaching were equally toxic (LC50-values about 40%). Untreated TCF and ECF (pine and mixed pine and birch pulp) like secondary and pilot treated effluents were all non-toxic. The results obtained by the egg/larvae test of zebra fish showed that the LOEC values differed between hatching while others to mortality. Secondary treated effluents did not have any significant effects. Of the untreated effluents TCF and conventional (both pine pulp) were the most toxic (LOEC values of hatching between 1% and 3.2% and of mortality between 12% and 6.8%). The effects of all the other samples were nearly the same (LOEC values varying between 5% and 25%). There was no signigicant difference in toxicity between untreated conventional, ECF and TFC bleaching effluents. The natural constituents of wood are probably responsible for the toxicity observed in ECF and TCF effluents.