Evaporation for Internal Treatment of Debarking Effluents from the Forest Industry, 1995 Environmental Conference Proceedings

Debarking effluents are likely to become a big environmental issue for the forest industry. One solution to the environmental problems of the effluents would be to treat them internally and recirculate the purified water to the mill. In this report comprehensive laboratory and pilot studies form a basis for evaluating the possibility to treat debarking effluents internally with evaporation. Evaporation is shown to be an outstanding technique for treating debarking effluents, since most of the dissolved bark substances are not volatile. A possible accumulation of volatile compounds, mainly low molecular alcohols and carboxylic acids, has to be considered at system-closure. Condensate segregation, connected to steam stripping, was shown to separate easily volatile compounds effec-tively from the purified water. Carboxylic acids are prevented from volatilising by increasing pH before evaporation. Other organic com-pounds, including dissolved phenolic substances, were almost totally concentrated. Therefore, the water produced in evaporation is even purer than freshwater and no process problems are expected to occur in the mill when freshwater is replaced with the purified debarking effluent. In the evaporation trials, dissolved compounds were concentrated into a small fraction, accounting for only 1 - 3% of the original volume. The generally low molecular weight and the phenolic nature of dissolved bark compounds seem to control the viscosity of the concentrate. Solid contents of 35 - 45% is achievable in evaporation. At these solids contents, final treatment of the concentrated effluent in a bark boiler or in black liquor evaporation can be feasible. The operational costs of evaporation have been calculated to be held at a reasonable level if waste steam from different processes can be taken into use. Evaporation is, therefore, not an extremely costly way treat debarking effluents internally.