A packed cage rotating biological contactor (RBC) system was applied to treat wastewater containing Cl, residue with concentration even up to 20 mg/L. However, Cl, exhibited a negative effect on the efficiency of the system as evidenced by the decrease in the growth of bio-film. It could be concluded that the removal efficiency of the system decreased with the increase of Cl, concentration or Cl, loading. Due to inhibition of bio-film growth by the effects of Cl, residue. the effluent suspended solids (SS) of the system was decreased. The bio-film was easily detached from the media tinder high growth rate conditions resulting in an increase of effluent SS. The COD and BOD5 removal efficiencies of the System under the highest organic and Cl-2 loadings of 4.07 g BOD5/m(2) d and 203.6 mg Cl-2/m(2) d, respectively, were 58.0 +/- 3.2% and 60.7 +/- 3.9%, respectively. while they were up to 83.3 +/- 1.8% and 85.8 +/- 2.0%, respectively, under the lowest organic and Cl-2 loading of 2.04 g BOD5/m(2) d and 25.5 mg Cl-2/m(2) d. However, the effluent SS of the system under above operating conditions was lower than 20 mg/L. (c) 2005 Elsevier Ltd. All rights reserved.