Biofilm system is suitable for the pretreatment of water because biofilm can act as niches where slow-growing degraders can survive at low organic concentrations. In order to cope with stringent environmental standards, it became essential to improve the understanding of the phenomena governing biological degradation of organics. In this study, effects of hydraulic retention time (HRT) and temperature on the efficiency of submerged biofilm reactors were investigated. Removal efficiency total organic carbon (TOC), turbidity, and algae increased with the HRT. It is considered that over 1 hr HRT is needed to ensure the removal of higher than 30% TOC. In comparison to honey comb type media, fibrous media were more favorable to the attachment and growth of active microorganisms. The close relationship with chlorophyll-a concentration and particulate organic carbon (POC) concentration make POC a surrogate parameter for estimating the removal efficiency of algae.