In plywood industry water is mainly needed for soaking the logs. Dissolved air flotation with chemical precipitation was found to be a suitable treatment method for the soaking basin overflow of a plywood mill using birch as raw material. According to pilot treatment studies over 90% reductions of the suspended solids are possible with a hydraulic surface load of 6.5 m3/(m2h). In subsequent experience in full scale following reductions have been achieved: suspended solids 93%, BOD7 50%, CODCr 57%, P 92% and N 52%. Two-thirds of the flotation treated water is led to flue gas scrubbers and circulated back to the soaking basin. Optionally water can be led to the heat recovery, too. One-third of the flotation treated water is disposed of as the mill effluent. Concentrations of organic matter in the system have been reduced after the addition of flotation indicating the possibility of further closure. However, due to the use of aluminium sulphate in coagulation, aeration is needed for sulphate reduction prevention. Further closure of the water system is possible in the future if the heat recovery is renovated, preventing the increase of the water temperature (now 37°C) which otherwise might cause occupational safety hazards.

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