An advanced treatment process was developed during the last five years to improve the treated effluent quality in view of stronger environmental regulations and the possibility for the reuse of treated effluent in the Pulp and Paper Industry.

The combination of ozone with fixed bed biofilm reactors is one of the most efficient tertiary effluent treatment processes to give maximum elimination of COD, colour and AOX with a minimum of ozone dosage. Several laboratory and pilot tests with effluent from a full biological treatment works confirmed the expected targets.

Ozone diffusion parameters, i. e. bubble size, and distribution, reaction time and hydrodynamic pattern are decisive for interface/film or bulk reaction. This is the key economic factor for the elimination of certain kinds of organic matter by transferring persistent “hard” COD into biodegradable compounds (BOD).

Two-stage ozonation with intermediate biodegradation proved to be a valuable tool for obtaining high COD elimination efficiencies in the tertiary treatment of effluent with high persistent COD concentrations.

A fully equipped bench scale plant using the ozonation - biofiltration process is in operation since the beginning of 1998. A separate batch ozonation system allows tests for optimisations which are difficult to perform in a continuous process. Results with papermill effluent are presented. This equipment gives results sufficient for determination of design parameters for an industrial size.

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