Effluent-Free Drum Debarking, 1998 Environmental Conference Proceedings

Reijo Saunamäki

Martti Savolainen

Department of Environmental Protection

The Finnish Pulp and Paper Research Institute

Modern woodhandling plants in Scandinavian countries are very similar from environmental point of view. The logs are debarked in rotating drums. Water is not normally fed into the drum itself. In summer at some pulp mills debarking plants can be run totally dry. In winter, frozen logs have to be thawed in a chamber located before the debarking drum. Debarking waste water discharges may account for a substantial proportion of the mill’s total load (BOD, COD, nutrients) and water is also very toxic.

The aim of the study was to examine ways of reducing and eventually eliminating waste water from modern debarking plants. Water consumption and effluent loadings at two pulp and paper mill debarking plants in both winter and summer were monitored. Chemical and biological treatment - (batch reactors, an aerated lagoon and activated sludge) in lab- pilot- and full-scale were tested for debarking waste waters. It was estimated how the treated waters can be recycled to the debarking plant, taking into account the quality requirements in the plant.

During winter at the paper mill logs (spruce) were debarked at the rate of 2670 solid m 3 /d and total water consumption was high, 0.78 m 3 /solid m 3 (2080 m 3 /d) (0.29 m 3 /solid m 3 was fresh water and 0.49 m 3 /solid m 3 from the mill’s activated sludge plant for the combined effluent). The paper mill debarking plant waste water was concentrated and the discharges were high. The BOD load was 2.0 kg/solid m 3 (5.2 t/d or 2600 mg/L) and the COD load 5.2 kg/solid m 3 (14.0 t/d, 7000 mg/L). Effluent loadings were higher in winter than in summer. The load from the bark press had a 50 - 80 % COD contribution to the total debarking plant waste water. At the pulp mill debarking plant during the winter test 8050 solid m 3 /d was debarked (pine, birch). Hot (c. 65°C) wash water for the recovery boiler scrubber was used as well as small amounts of fresh water. The pulp mill debarking plant discharges were smaller than from the paper mill thanks to the clarifier and the lower water consumption (waste water flow 0.50 m 3 /solid m 3 , COD load 1.6 kg/solid m 3 in winter, 0.28 m 3 /solid m 3 and 0.7 kgCOD/solid m 3 in summer).

Resin and fatty acids were the main reason for the high toxicity of the debarking waste waters. At the paper mill debarking plant the waste water LC50 value was in the range 2.5-40% for Daphnia magna.

Activated sludge treatment gave good results for both debarking plants provided the loading was kept low. Chemical treatment of the purified water from the activated sludge plant resulted in very clean water.The findings from the study were used to draft out processes for a completely effluent-free debarking plant. The main finding of the TMP- study, which was conducted on pilot refiner, was that highly contaminated water can be used in debarking without affecting pulp quality. This was an encouraging finding in terms of debarking plant water cycle closure.