Background This study evaluated the essential role of the experimental support as an integral component of process design for the nitrogen removal activated sludge process. This role was demonstrated in prescribing and implementing a relevant design procedure; a similar treatment plant at Ambarli in Istanbul was selected to provide the necessary experimental support, which was derived from the sewage, lab-scale experiments, and from the plant itself. Each step of the design included the relevant scientific and conceptual basis and was implemented using the necessary experimental data specifically generated from a selected wastewater source. Results The target was to compile experimental data that would be necessary for the study and be equally applicable to similar design or research activities elsewhere. This data included major wastewater characteristics, chemical oxygen demand (COD) fractionation, process kinetics for COD removal and nitrification, applicable sewage temperature, and details for the impact of primary settling on system performance. Conclusion This study developed a new design approach involving solely COD for both the substrate and biomass but expressing the related component per unit flow rate. This approach offered a template that is adjustable to any similar design by only requiring specific information on the sewage flow and sludge age. The entire design procedure was presented as supported by original experimental data and in a summarized algorithm format. The study also emphasized the selection of an appropriate anoxic volume. A revised empirical expression was suggested for the computation of the corresponding denitrification potential, which was also supported by process modelling; they both indicated that the current empirical approach generally defined an excessively large anoxic volume. (c) 2022 Society of Chemical Industry (SCI).