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 experiment, the fish were exposed in the laboratory to well water (control) and five concentrations (1.25%, 2.5%, 5%, 10%, or 20%) of effluent from the egg stage to sexual maturity and reproduction (approximately 190 d). None of the effluent concentrations significantly affected the hatching of the eggs, the mortality, weight, length, gender balance, reproduction, and prevalence of visible morphological or histopathological abnormalities of the hatched fish, and the hatchability of the first generation eggs. In Ceriodaphnia tests, the IC25 of the effluent affecting reproduction was approximately 80%. This threshold concentration is well above the 0.7% average yearly concentration of the effluent that exists in the recipient near the point of discharge. The results of these biotests were compared to the results of the same biotests conducted earlier with the effluent from the mill prior to process and treatment modifications. The comparison indicated that since the earlier work, the quality of the mill's effluent improved substantially. Threshold concentrations affecting fathead minnows in the life-cycle experiment and the Ceriodaphnia tests increased by more than eightfold and approximately twofold, respectively. While the most important change in the mill operating conditions responsible for the improvement could not be identified, these results indicate that mills can undertake process and treatment modifications that result in the discharge of effluents seemingly compatible with the aquatic environment.