BACKGROUND: The aim of this study was to evaluate the ammonium nitrogen removal performance of algae culture Chlorella vulgaris in a novel immobilized photobioreactor system under different operating conditions and to determine the biokinetic coefficients using the Stover-Kincannon model. RESULTS: The photobioreactor was continuously operated at different initial ammonium nitrogen concentrations (NH4-N-0 = 10-48 mg L-1), hydraulic retention times (HRT = 1.7-5.5 days) and nitrogen/phosphorus ratios (N/P = 4/1-13/1). Effluent NH4-N concentrations varied between 2.1 +/- 0.5 mg L-1 and 26 +/- 1.2 mg L-1 with increasing initial NH4-N concentrations from 10 +/- 0.6 mg L-1 to 48 +/- 1.8 mg L-1 at theta(H) = 2.7 days. The maximum removal efficiency was obtained as 79 +/- 4.5% at 10 mg L-1 NH4-N concentration. Operating the system for longer HRT improved the effluent quality, and the percentage removal increased from 35 +/- 2.4% to 93 +/- 0.2% for 20 mg L-1 initial NH4-N concentration. The NIP ratio had a substantial effect on removal and the optimum ratio was determined as NIP = 8/1. Saturation value constant, and maximum substrate utilization rate constant of the Stover-Kincannon model for ammonium nitrogen removal by C. vulgaris were determined as K-B = 10.3 mg L-1 d(-1), U-max = 13.0 mg L-1 day(-1), respectively. CONCLUSION: Results indicated that the algae-immobilized photobioreactor system had an effective nitrogen removal capacity when the operating conditions were optimized. The optimal conditions for the immobilized photobioreactor system used in this study can be summarized as HRT = 5.5 days, NIP = 8 and NH4-N-0 = 20 mg L-1 initial nitrogen concentration to obtain removal efficiency greater than 90%. (C) 2008 Society of Chemical Industry.