Activated sludge plants in Finland

The performance of Finnish activated sludge plants (primary clarifier, equalization basin, buffer basin, aeration basin, secondary clarifier and sludge handling) was surveyed. Performance was generally found to be excellent. BOD7 removal averaged 94% at paper mills (21mg/l effluent concentration, 0.43 kg/t discharge) and 95% at chemical pulp mills (18 mg/l, 0.83 kg/t). CODCr removal averaged 82% at paper mills (158 mg/l effluent concentration, 3.52 kg/t discharge) and 60% at pulp mills (531 mg/l, 20.7 kg/t). Treated effluent from paper mills contained 0.57 mg/l (13.3 g/t) of total phosphorus and from pulp mills 1.31 mg/l (47.3 g/t). AOX removal was 45-65% and discharge 0.15-0.90 kg/t. Suspended solids removal averaged 91% at paper mills and 76% at pulp mills, the average overflow concentration in primary clarifier being about 150 mg/l at both types of mill. All activated sludge plants are designed according to either the low load or extended aeration principle; high load plants are no longer in use. The average sludge load was 0.15 kg BOD/(kg MLSS d), the sludge age at the extended aeration plants ranging from 20 to 50 d. Low load appearance to be the main reason for the good performance of the plants. Other factors included the introduction of equalization and buffer basins at many plants, and particularly the fact that aeration factors and nutrients were under control. All paper mill activated sludge plants use nitrogen and phosphorus additions, whereas pulp mill plants manage without phosphorus. Optimization of nutrient addition is vital because almost every mill in Finland has a limit for its phosphorus discharge. Total sludge production at the paper mill plants averaged 40.9 t/d, 15% of which (5.9 t/d, 6.0 kg/t) was biosludge. At the pulp mills the figures were 27.2 t/d (11.5 t/d biosludge, 9.5 kg/tp). Belt filter presses are used for dewatering at most plants and dry solids contents are usually 25-35%. New-generation screw presses have been introduced, mainly at new pulp mill activated sludge plants, yielding dry solids contents of 40%. Combined and dewatered sludge is normally burned in bark-fired boilers, although some mills still landfill their sludge.