External addition of nitrogenous compounds is the most common practice used in biological treatment of nitrogen deficient wastewaters. However, this method adds an extra cost to the treatment. As an alternative for effective biological treatment of nitrogen deficient wastewaters, nitrogen fixing bacteria, Azotobacter vinelandii was used in activated sludge culture in this study. COD removal performance of Azotobacter-supplemented activated sludge was compared with Azotobacter-free activated sludge culture for biological treatment of nitrogen deficient synthetic wastewater. Effects of important process variables such as N/COD ratio, sludge age, hydraulic residence time, feed COD concentration and the COD loading rate on the COD removal performance were investigated. Kinetic constants of the system were determined. Azotobacter addition to the activated sludge in biological treatment of nitrogen deficient wastewater (N/COD < 0.06) improved the COD removal performance significantly. (C) 2002 Elsevier Science Ltd. All rights reserved.