The effluents from CTMP mills are on the one hand too dilute for evaporation and recovery, and on the other hand too highly polluted for conventional aerobic secondary treatment. In summer 1986 we therefore started an extensive research program in cooperation with the Ahlström Engineering division in Savonlinna, Finland, and Paques-Lavalin in Toronto, Canada, in order to investigate the anaerobic treatability of CTMP effluent, using UASB technology. This research included fundamental work in the lab, as well as on-site pilot work in Finland and in Canada.

As a result, two full-scale plants are in operation. The first plant was started up in October 1988 at Quesnel River Pulp in B.C., Canada, and is treating up to 140 tons of COD per day in two reactors of 3500 m3 each. The second plant was ready for start-up in January 1990 at the Enso-Gutzeit Kotka mill in Finland.

The cautious approach for these types of effluents was necessary due to earlier reports on the toxicity of softwood extractives, bleaching agent hydrogen peroxide, complexing agent DTPA and high sulphur levels. Besides this, it was necessary to confirm that granular seed sludge would not deteriorate but would develop normally.

The behaviour of hydrogen peroxide was especially interesting and the high redox potential caused could be resolved in a very cost-efficient way without utilising chemicals, enzymes or activated sludge.

Resin acids were indentified to be responsible for reducing methanogenic activity considerably. They were eliminated during aerobic post-treatment to very low levels. Lab studies clearly demonstrated how methanogenic activity could be increased by adding dilution water or aerobically treated effluent.

The concentration of the resin acids appeared to be associated with raw material (spruce, fir or pine), the season (summer or winter) and with fine fibrous material in the effluent. Sulphur levels in the effluent were relatively high, but resulting sulphide levels were not toxic to methanogens and COD/sulphur ratios were high enough to achieve acceptable removal efficiencies.

The paper presents the results from research as well as flow diagrams of the full-scale plants, and results from more than one year full-scale operation at Quesnel River Pulp in B.C. Canada.

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