An enhanced lab-scale biofilm pretreatment process treating raw water obtained from eutrophicated water bodies was established and started up with a novel strategy of low-level nutrients addition and effluent recirculation. Results showed that the startup strategy was useful for biofilm formation and pollutants removal, but it had the risks of increasing substrate affinity constant (K-s) and biofilm decay in treating raw water. Fortunately, the increased Ks value did not affected the NH4+-N removal performance via keeping the NH4+-N loading rate larger than 6.29 mg L-1 d(-1). In addition, lower hydraulic retention time (HRT) favored the removal of organic matters, and the maximum TOC removal rate of 76.5 mg L-1 d(-1) were achieved at HRT of 2 h. After long-term acclimatization at oligotrophic niche, the decrease of Ks value and increase of biomass, extracellular polymeric substances, bioactivity were achieved. Finally, the stable operation of biofilm pretreatment process was realized in treating polluted raw water. (C) 2015 Elsevier Ltd. All rights reserved.