The objective of this study is to evaluate the stability of activated sludge (AS) in the treatment of kraft pulp mill effluent exposed to black liquor shock, as well as the effect of its exposure on the morphology of Daphnia magna and DNA damage through mutagenicity and genotoxicity response in Salmonella typhimurium. To this end, we applied doses of 2-, 4-, 10-, and 30-mL black liquor/L influent-shock 1 (S1), shock 2 (S2), shock 3 (S3), and shock 4 (S4), respectively-to kraft pulp mill effluent. The system stability was checked by analyzing heterotrophic biomass activity and discharge quality, evaluated using Ames test and Daphnia magna. The results show that the chemical oxygen demand (COD) removal efficiency for normal conditions was 64.84%, falling to 61.68%, and 61.31% for S1 and S2, respectively, and values of 52.11% for S3 and 20.34% for S4. The biomass activity decreased after each shock was applied, but then recovered. There was no evidence of lethal toxicity (LC50) to Daphnia magna at any of the concentrations. Therefore, it is feasible to apply doses S1 and S2 to an AS system that treats kraft pulp mill effluent.
