Pilot plant experiments were carried out to study the steady and unsteady state behaviors of the upflow biological aerated filter(BAF) to remove organics and suspended solids from paper industry wastewater. Expanded polypropylene beads were used as the floating media for biomass attachment and filtration. The BAF pilot plant with a treatment capacity of 100 ton/day comprised two aerated filters which have cross sectional area of 1.5 ㎠ each and total height of 6 m. The COD and SS concentrations of the effluent and the head loss buildup of aerated filters were monitored to evaluate the treatment efficiency and to optimize backwash condition of the BAF system. The removal efficiencies of 76-80% for COD and 90-92% for SS were kept stable throughout the experiment. There was no significant difference in treatment efficiencies when the organic load increased up to 0.85 hour of empty bed contact time. The analysis of 55 concentration of the effluent after periodical backwash showed sharp increase right after backwash and gradual decrease to steady state while soluble COD was maintained nearly constant. The effluent concentrations and the required time intervals to reach steady state after backwashes were dependent on backwash conditions, like superficial linear velocities of backwash water. The higher the linear velocity of backwash water, the larger the deviation of SS concentration from steady state was obtained.