Long-term changes in the effluent loads from pulp and paper mills, the introduction of BAT technology, and their effects on extractable organic halogens concentrations in sediment in part of the Bothnian Bay

The Bothnian Bay at Kemi, which is the northernmost basin of the Gulf of Bothnia in the Baltic Sea, is affected by effluents from point sources such as pulp and paper mills and municipal sewage plant. During the period 1988-2008, the accumulated values of effluent volume, BOD7, CODCr, Tot-P, Tot-N, TSS and AOX from these point sources fell considerably as follows: 1) effluent volume by 24%, BOD7 by 92%, CODCr by 74%, Tot-P by 79%, Tot-N by 38%, TSS by 76% and AOX by 93%. The decrease in the effluent loadings is due to the introduction of Best Available Techniques (BAT) for the production processes and wastewater treatment at the pulp and paper mills. According to the long-term monitoring program that has been carried out every third year between 1997-2009, the decreasing trend in EOX-concentrations in the top 2 cm of the bottom sediment reflects the decrease in organochlorine discharges (AOX) from the mills. In 1997, the EOX concentrations in bottom sediment varied between 3-70 μg of Cl g(-1) (dry weight), and in 2009 between 1.2-31 μg of Cl g(-1) (dry weight).