The high-rate activated sludge (HRAS) process is suitable for the removal of organics from wastewater. The application of this process recognizes that there are mainly three forms of substrate in wastewater – particulate, colloidal, and soluble. Particulate and colloidal substrates are removed effectively from solution by biological flocculation whereas the soluble fraction can be removed by intracellular storage, biosynthesis or biological oxidation. Experiments were devised to understand the impact of important process parameters such as solids retention time (SRT), hydraulic residence time (HRT) and dissolved oxygen (DO) levels on the performance of HRAS. The results indicated that DO concentrations of less than 0.5 mg/L had a significant impact on the removal levels of particulate and colloidal chemical oxygen demand (COD). In the case of soluble COD, the removal rates were significantly impacted at DO levels of 0.1 mg/L. Particulate and colloidal COD removal was maximized at SRT and HRT values of 2 days and 30 minutes, respectively. However, the soluble COD removal was high at SRT values of as low as 0.3 days. These results seem to indicate that the removal of soluble substrate is less affected by changes on the process parameters tested during this study than the particulate and colloidal COD removal fractions.