This study was conducted to investigate the potential of rock-bed filtration method to accomplish removal of pollutants from water/wastewater. The purpose of this investigation was to evaluate the process efficiency, optimum design parameters, and most appropriate operating conditions. Experimental setup consisted of two rectangular reactor units (named RBF I and RBF II), filter media (with equivalent diameters of 2-4 cm and 5-7 cm), aeration system (3 and 6 air diffusers). The experiments were divided into 3 runs with different operating conditions. The results of this research showed that the maximum removal efficiency was found for particulate matter (SS and VSS), ranging 60-90 % throughout the experimental period. This implied that the main removal mechanism was sedimentation of the particulate matter. In case of T-BOD, removal was not significant at the beginning but reached up to 81-82 % during the third run. This means that for effective biofilm performance, longer operation is required. An increase in HRT increased the removal efficiency in all cases except S-COD. However, maximum increase in removal efficiency was observed between 6 h and 9 h and then only a nominal increase for 9 h < HRT < 12 h. Thus, the optimum HRT seems to be 9 h. The effect of rock size was observed as about 8 % increase in particulates removal for smaller rocks compared to the bigger ones. Smaller rocks showed a bit better particulate removal initially but the removal decreased later due to clogging. The aeration had some effect on SS and VSS removal (8-9 % improvement). The average percent removal of T-BOD during the second run with 3 air diffusers in the first half period and with 6 air diffusers during the second half one, were 46 % and 76 %, respectively. More aeration (6 diffusers) helped microorganisms in attached biofilm to remove more T-BOD. Porosity in both reactors reduced from about 47-48 % to 42-43 % over a period of 5 1/2 months. The porosity reduction in RBF I was only slightly higher than RBF II.