Previous researchers have indicated that biological activity in fixed bed adsorption systems can dramatically increase the expected capacity of granular activated carbon beds. The adsorbate species used in this work were acid dyes found in textile industry wastewater which had azo and di-azo structures. Pure bacterial cultures were selected which had the ability to degrade the dyes. Granular activated carbon (GAG F400) beds seeded with pure cultures showed increased dye removal over sterile beds. The performance of the system was described using a model which combines carbon adsorption and biological degradation of the adsorbate on the surface of the particle using the Monod equation. The model proved effective in describing the initial stages of the biologically activated carbon system.