Measures against water pollution in mechanical pulp and paper mills.

Abstract

Large sums have been and are being invested in the forest industry for water preservation but the result of the investments has not been as efficient as hoped. The basic mechanism in flotation, flocculation and sedimentation is still partly unknown, despite the research carried out in the field. Research should be more systematically organized globally and carried out at an intense and escalated pace. Large amounts of money should not be invested on half-measures, without the necessary knowledge of the return in efficiency. The flow of information regarding the progress of research should be intensified. Organized cooperation of all parties concerned should be established with the aim of achieving the most effective and economical use of the collective resources of the industry and the government. The first questions to be asked are: What is discharged with the effluent water from mechanical pulp and paper mills and what influence does this have on th.e recipient? What problems occur in the treatment of waste water? It should be pointed out that as far as waste water problems are concerned the mechanical pulp and paper mill is not one of the most troublesome within the forest industry. Our main problem is to reduce effectively the discharge of fibre suspensions to an absolute minimum. These suspensions contain bark residue, fines of groundwood pulp, fibres of chemical puips being used and in some cases a certain amount of clay. From a biological point of view the influence of these suspensions is relatively low but noticeable. The waste water from a mechanical pulp and paper mill can have the following characteristics: pH around 5.0—5.5 Dry substance before treatment of about 500—700 ppm Dry substance after treatment of about 100—200 ppm, i.e. a cleaning effect of about 70 per cent. BOD7 (20°C) about 12 kg/ton daily. All mechanical pulp and paper mills have been fighting with slime problems. Owing to the extended use of closed water systems, these problems have