A FIELD-STUDY OF ENVIRONMENTAL IMPACTS AT A BLEACHED KRAFT PULP-MILL SITE ON THE BALTIC SEA COAST

The rate of technical development in bleaching of chemical pulp and th e upgrading of process control and wastewater treatment systems in the pulp industry have been extremely rapid over the past few years. When assessing the environmental impacts of bleached kraft pulp mill efflu ents (BKMEs), it is therefore more important than ever to carefully ch aracterize the bleaching process, the composition of the treated efflu ent, and the degree of exposure of sensitive target organisms in the r eceiving water body. These requirements have not always been fulfilled in previous reports on biological effects of BKMEs in Scandinavia. Th is work presents the results of a comprehensive field study of the imp acts of a 350,000-tonne kraft mill, bleaching softwood and hardwood pu lp in campaigns according to the sequence O(D25,C70+D5)(EOP)D(EP)D. Th e effluent is treated in an aerated lagoon with a mean retention time of 8-9 days, practically eliminating chlorate and resin acids, and red ucing the discharge of adsorbable organic halogen (AOX) to an average of 1 t/day (1.3 and 0.4 kg/ADt for softwood and hardwood, respectively ). The treated effluent is discharged through a 1.3-km-long diffuser, at a water depth of 9-12 m, into a well ventilated coastal area, givin g a 1000-fold dilution within 3-4 km from the diffuser. The actual exp osure of the coastal ecosystem to BKME components was determined by an alysis of extractable organic chlorine in suspended solids and of conj ugates of chlorophenolics in the bile of feral perch. Despite a major damage to the benthic communities that occurred about 10 years ago, an d was due to large chlorate discharges at the time, no direct detrimen tal effects on benthic flora and fauna could be ascribed to the presen t BKME discharge. Instead, a clear recovery of the Fucus community, al though not yet completed, could be demonstrated. Studies of the compos ition, abundance and biomass of the fish community, the recruitment an d survival of fish fry, and the physiological status of perch, using a set of biomarkers, revealed that even in the most BKME-exposed area, only minor effects were detected. These effects were related to eutrop hication/enrichment rather than to the action of toxic substances. The general effect picture, thus, was essentially of a different type tha n the one recorded in previous studies of mills, which used older tech nology and less effective process and effluent treatment control, and which were discharging into enclosed, shallow bays of the Baltic Sea. (C) 1994 Academic Press, Inc.