Closing the water circuit in waste paper processing mills results in high concentrations of COD in the recirculating white water. Part of the COD is easily degradable and causes problems with microbiological growth in the water system. The load of dissolved COD to a paper mill producing moulded paper was determined: partly the incoming dissolved COD with the waste paper and partly the production of dissolved COD in the stock preparation and paper machine.

Pilot-plant experiments with biofiltration document that it is possible to remove efficiently easily degradable COD from the recycling white water. Very high loads have been applied to the filter owing to the high temperature and easy degradable carbon. Controlling the dosage of nutrients is essential to avoid growth problems in the white water system. It is shown that nutrients can be dosed to the filter with concentrations close to zero in the effluent, and still achieve significant removal capacity of the filter.

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