The installation of in-mill biological treatment for reducing the concentration of dissolved organic matter and microbial growth in the whitewater system h as been shown to be an interesting alternative for facilitating further closure of paper mill water circuits. In the present study the possibility of operating biological treatment of whitewater from different types of paper production under the environmental conditions prevailing in respective whitewater system, i.e. temperatures between 37 and 52°C and pH-values from 3 to 8. It was shown that high removals of soluble COD, 70-90%, could be achieved even at high temperatures and non-optimal pH-levels. Laboratory scale whitewater systems operated with and without a bioprocess were used for studies of the effect of a bioprocess on key parameters such as slime growth, spore formers and odorous compounds. The introduction of a bioprocess in the whitewater circuit effectively lowered the amount of soluble organic matter and eliminated odorous VFA. However, the operation strategy and control of the bioprocess showed to be crucial, especially the dosing of nutrients, in order to avoid increased rather than decreased biofilm growth in the whitewater system.

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