Studies were undertaken on the performance evaluation of three different types of aerobic reactors, namely, activated sludge process, fluidized bed reactor and submerged bed reactor. : t Initially synthetic wastewater was used for stabilizing the system and later domestic wastewater L | of IIT Madras was used as the feed for the biological systems. The hydraulic retention time was maintained as 24 h. The seed sludge was collected from NT Madras sewage treatment plant. The inlet COD to the reactors with synthetic wastewater was 1,000 ± 20 mg/L and with real wastewater, it was 150 to 350 mg/L The performance of the reactors was evaluated based on the soluble COD and nitrogen removal efficiency. The pH, temperature, dissolved oxygen (DO) and mixed liquid suspended solid (MLSS) concentration were measured periodically. The reactors were acclimatized at 35°C in batch mode and changed to continuous mode at 30°C. After the systems attained its steady state at a particular temperature, the temperature was reduced from 35°C to 5°C stepwise, with each step of 5°C. The start-up time for submerged bed reactor was slightly more than fluidized and conventional activated sludge process. The COD removal efficiency of the three reactors was higher with synthetic wastewaters as compared to actual domestic wastewater. Submerged bed reactor was more robust and efficient as compared to activated sludge and fluidized bed reactors. The COD removal efficiency of the reactors was relatively good until the operating temperature was maintained at 15°C or above. At 10°C, submerged bed reactor was able to achieve 40% COD removal efficiency whereas; the fluidized bed and conventional ASP reactors were showing only 20% COD removal efficiency. At 5°C, almost all the systems failed. Submerged bed reactor showed around 20% COD removal efficiency. However, this reactor was able to regain its 90% of original efficiency, once the temperature was raised to 10°C. At higher temperatures, the nitrification efficiency of the reactors was above 80-90%. As the temperature reduced the nitrification efficiency has reduced drastically. In summary, submerged bed reactors seems to be a better option for treating domestic wastewaters at low temperature regions. © IWA Publishing 2008.