The enhancement effect of polyethylene (PE) surfaces modified with poly(lactic acid) (PLA) on formation of nitrifying biofilms in biological aerated filter (BAF) was investigated in this study. X-ray photoelectron spectroscopy, scanning electron microscope, and protein absorption analysis revealed that modified PE surfaces produced active groups, including carboxyl (O═C─O), hydroxyl (C─OH), and carbonyl (C═O), increased surface roughness, and enhanced the adsorption of both the bovine serum albumin and fibrinogen. During the startup period of 33 days, the average removal rates of ammonia nitrogen (NH3-H) were 68 and 72% before and after modification which were 36 and 43% for total nitrogen, 47 and 45% for total organic carbon. The results of denaturing gradient gel electrophoresis experiments demonstrated that modified carriers adsorbed and gathered more species of bacteria on surfaces. Thus, surface modification of PE suspended carrier by PLA improved the efficiency of reactor startup for drinking water treatment.