This study evaluated the basic design principles of a sequencing batch reactor ( SBR), subject to seasonal wastewater flow fluctuations. Emphasis was placed upon highlighting the role of major process parameters, namely the cycle time, fill volume, number of reactors, the stationary volume, which holds the settled biomass and finally the sludge age in optimizing the necessary SBR configuration, which will effectively serve under these fluctuations. For this purpose, two sites with different wastewater generation schemes were selected in Erbil, a city in northern Iraq. The first one was a residential unit and the second, a luxury hotel with 250 rooms and a full capacity of 500 guests, which was attained in the summer period. The basic approach for SBR design for organic carbon chemical oxygen demand (COD) removal was summarized in the study. While the necessary methodology looks simply, it requires a detailed wastewater characterization involving the assessment of volatile and fixed solids components and COD fractionation. The adopted approach also relies on correlations between major parameters and process kinetics and stoichiometry. System optimization against seasonal fluctuations was essentially based on the utilization of spare reactor volume created under low wastewater flow conditions, without changing the selected parallel reactors, to increase the sludge age to the extent possible, which resulted in minimizing the excess sludge generation.