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 %. These plants produce around 94,000 tonnes (dry weight) of sludge a year, of which some 27,000 tonnes is biosludge. On a mill basis, biosludge accounts for 10-50 % of total sludge. Generally speaking, the higher the loading at the plant, the more biosludge is produced. The yields of biosludge were 0.3-1.2 kgTS/(kgBOD removed) for a sludge load of 0.1-1.2 kgBOD₇/(kgMLVSS^*d). The biosludge is separated by sedimentation in clarifiers and thickened in round sedimentation basins to a consistency of 3-5 %. At one mill anaerobic sludge from an oversized thickener is returned to the aeration basin. The aim of this is to minimize the need for nutrient addition. Belt filter presses are normally used for sludge dewatering, the solids contents achieved being about 25-37 %. The difficulty of handling biosludge and the low solids contents achieved were considered by many mills to be the main problems of sludge handling. At one mill a drum press is used to press a pretreated mixture of sludge and bark to around 42 % 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 % 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.