SLUDGE HANDLING AND DISPOSAL AT FINNISH ACTIVATED-SLUDGE PLANTS

Between 1984 and 1986 a total of 13 activated sludge treatment plants were built in Finland to handle waste waters from the pulp and paper mills. With just a few exceptions these plants have come up to expectations, achieving BOD7 reductions of 80 - 95 percent. These plants produce around 94,000 tonnes (dry weight) of sludge a year, of which some 27,000 tonnes is biosludge. The biosludge is separated by sedimentation in clarifiers and thickened in round sedimentation basins to a consistency of 3 - 5 percent. At one mill anaerobic sludge from an oversized thickener is returned to the aeration basin. At one mill a drum press is used to press a pretreated mixture of sludge and bark to around 42 percent consistency for burning. Six mills burn all or part of their sludge in a bark-fired boiler. On a dry weight basis the sludge burned normally accounts for 5 - 10 percent of total fuel consumption. Burning sludge has been associated with a reduction in steam generation and an increase in running problems. The other method of disposing of sludge is landfilling. Sludge is not used as a fertilizer or soil improvement agent in Finland.