Biological treatment of saline wastewater in a rotating bio-disc contactor (RBC) was investigated under different operating conditions, such as A/Q ratio, feed COD, COD loading and salt concentrations. COD removal efficiencies were determined and correlated with the aforementioned variables. The constants of the proposed empirical equation were determined using experimental data. Plots of COD removal efficiency as functions of these variables were established in order to estimate the performance of the system under different operating conditions. (C) 2002 Elsevier Science Ltd. All rights reserved.