Properties improvement of paper mill sludge-based granular activated carbon fillers for fluidized-bed bioreactor by bentonite (Na) added and acid washing

► Paper mill sludge was used as the main material to produce granular activated carbon fillers especially for fluidized-bed bioreactor. ► The effects of prior to and post activation acid washing treatment were investigated and the mechanism was discussed. ► The effects of HCl, HNO3 and H2SO4 washing treatment were investigated and the mechanism was discussed. ► EDX was used to investigate the elemental changes of fillers after acid washing treatment to explain the mechanism.