Design and operation of saline wastewater treatment systems are difficult because of adverse effects of salt on microbial flora. Quantification and modeling of salt inhibition effects are essential in designing biological treatment processes for saline wastewater. Synthetic wastewater containing 0-10% salt (NaCl) was treated in a rotating biodisc contactor (RBC) unit operating in a continuous mode. Effects of important process variables such as the A/Q ratio, COD loading rate, and salt concentration on COD removal rate and efficiency were investigated. The system's performance improved with an increasing A/Q ratio; however, performance decreased with an increasing COD loading rate and salt content. The liquid phase was aerated to keep suspended cells active at high feed COD concentrations such as S-o = 5,000 mg/L. A mathematical model was developed to describe the system's behavior. Model parameters were determined by using the experimental data. Salt inhibition was found to be significant for salt concentrations larger than 2% NaCl. The experimental results and mathematical model may be used in design of RBC units heating saline wastewater.