A comparison of the potential of primary- and secondary-treated pulp and paper mill effluents to induce mixed function oxygenase (MFO) activity in fish

Primary- and secondary-treated effluents from 13 mills representative of the kraft, chemi-thermomechanical and thermomechanical pulping sectors were tested for their potential to induce mixed function oxygenase (MFO) enzyme activity in rainbow trout (Oncorhynchus mykiss). Fish were exposed in the laboratory to 1, 5 and 10% (v/v) concentrations of effluents collected at the inlet and the outfall of secondary treatment plants. After 4 days of exposure, hepatic MFO enzyme activities were measured by the ethoxyresorufin-O-deethylase (EROD) assay. Primary-treated effluent from chemi-thermomechanical, thermomechanical and bleached kraft mills showed potential for MFO induction in fish at concentrations of 10% or lower. The MFO-inducing potential of these effluents could not be linked to any specific operating variable such as type of pulping, wood furnish and pulp bleaching or brightening. The ability of the secondary-treated effluents to cause MFO induction was significantly lower or absent at the concentrations tested.