A full-scale rotating biological contactor (RBC) was used to study the effectiveness of supplemental aeration and an enlarged first stage in improving the performance of an RBC system. These experimental studies suggest that generally the sludge production was higher in the first stage because of the higher soluble COD (SCOD) removal rates. However the sludge production in the first stage was significantly higher in the presence of supplemental air than without the air. At lower organic loadings, the first-Stage sludge production was 1 . 09 kg SS/kg (2 . 40 Ib SS/lb) and 0 . 572 kg SS/kg (1 . 26 Ib SS/lb) SCOD removed with and without supplemental ail; respectively. In addition, with supplemental air there was an increasing stabilization of volatile solids in the subsequent stages when the substrate was limiting in the succeeding stages. Overall sludge production was the same with and without supplemental aeration at low and high organic loading rates. However overall sludge production decreased substantially with the use of an enlarged first stage, particularly with supplemental aeration, which results in lower operating costs for sludge stabilization and disposal. This occured despite higher first-stage sludge production rates with the use of supplemental air. Also, the overall performance of the RBCs receiving supplemental aeration was significantly better when compared to the RBCs receiving no supplemental air, both in terms of SCOD or SBOD5 removal and ammonia nitrification.