IMPROVED EFFLUENT QUALITY AT A BLEACHED KRAFT MILL AS DETERMINED BY LABORATORY BIOTESTS

A life-cycle experiment with fathead minnows and Ceriodaphnia survival /reproduction tests were used to evaluate the quality of the effluent from a bleached kraft mill after the implementation of various process modifications and effluent treatment changes. In the life-cycle exper iment, the fish were exposed in the laboratory to well water (control) and five concentrations (1.25%, 2.5%, 5%, 10%, or 20%) of effluent fr om the egg stage to sexual maturity and reproduction (similar to 190 d ). None of the effluent concentrations significantly affected the hatc hing of the eggs, the mortality, weight, length, gender balance, repro duction, and prevalence of visible morphological or histopathological abnormalities of the hatched fish, and the hatchability of the first g eneration eggs. In Ceriodaphnia tests, the IC25 of the effluent affect ing reproduction was approximately 80%. This threshold concentration i s well above the 0.7% average yearly concentration of the effluent tha t exists in the recipient near the point of discharge. The results of these biotests were compared to the results of the same biotests condu cted earlier with the effluent from the mill prior to process and trea tment modifications. The comparison indicated that since the earlier w ork, the quality of the mill's effluent improved substantially Thresho ld concentrations affecting fathead minnows in the life-cycle experime nt and the Ceriodaphnia tests increased by more than eightfold and app roximately twofold, respectively. While the most important change in t he mill operating conditions responsible for the improvement could not be identified, these results indicate that mills can undertake proces s and treatment modifications that result in the discharge of effluent s seemingly compatible with the aquatic environment.