This study reviewed the conceptual basis and the application of extremely high rate activated sludge configurations with gravity settling. It evaluated the modified aeration process, as the historical pioneering development, and the A process (adsorption), which was inspired by the traditional AB process (adsorption/biological oxidation) promulgated in the seventies. For this purpose, a modelling framework was established to simulate the expected performance and the energy recovery potential of these processes using the currently applicable data base. The model evaluation was also extended to cover the Yenikapi plant in Istanbul as a current case study. The modelling results were discussed in view of the handicaps of gravity settling and the merit of high-rate membrane bioreactors where gravity settling would be replaced by membrane filtration. (c) 2021 Society of Chemical Industry (SCI).