Changes in water quality in Lake Päijänne following decrease of effluent load from the pulp and paper industry in 1969–1989

Abstract

Lake Päijänne, the second largest lake in Finland, has been seriously polluted since the 1960s due to the effluents from pulp and paper mills, notably the Äänekoski sulphate and sulphite pulp mills situated about 50 km north of the lake, and the sulphite pulp mill and paper mills of Jämsänkoski and Kaipola on Central Lake Päijänne.

A sulphite lye evaporation and combustion plant installed at the Jämsänkoski sulphite pulp mill in 1969 reduced the organic pollution of Central Päijänne. Lignin concentration decreased and the oxygen balance improved. The sulphite pulp mill was closed in 1981 and replaced by a thermo-mechanical pulp mill. Eutrophication is the main threat to water quality in Central Lake Päijänne at present.

A significant improvement in the water quality, especially in oxygen balance, was achieved in the watercourse of Äänekoski and in Northern Päijänne after replacement of the old sulphite and sulphate pulp mills at Äänekoski by a large sulphate pulp mill with a biological purification plant employing the activated sludge method. The BOD₇-loading dropped from 46 to 3–41 d ^-1, but the nutrient loading has not decreased sufficiently, and the Äänekoski watercourse and Northern Päijänne are still eutrophic.