Stringent discharge requirements call for advanced methods of wastewater treatment to take on where biological treatment fails to succeed. Here, the adsorption potential of fly ash, an on-site available and cheap material, was tested in batch and continuous flow fixed bed experiments using bleaching effluent from an integrated mill producing mechanical pulp. Various models were fitted to the experimental data to find the best description of the adsorption system and to obtain important model parameters: the Freundlich model yielded the highest correlation and indicated that the process was favorable. The bed depth service time model suggested that the adsorption in the column setup involved more than one rate limiting step, and the Thomas and Clark models generated similar curves which satisfactorily described adsorption at short bed depth. The fly ash showed good adsorptive properties of wood derived substances: both lignin and extractives were effectively separated from the effluent.