Highly-loaded aerobic chemical oxygen demand (COD) removal reactors (also known as A-stage) include two main processes, COD removal by heterotrophic biomass and adsorption of COD on the activated sludge. A simple model to describe highly-loaded aerobic COD removal reactors has been developed. A one-year-set of measured data from a full scale wastewater treatment plant has been used to calibrate the efficiency of the adsorption and to evaluate the ability of the mathematical model to describe the measured data in both steady state and dynamic conditions. The approach lumped the efficiency of the settler and the adsorption with a simple but powerful approach which includes the use of the measured sludge retention time (SRT) and the settling efficiency. The effects of dynamics in terms of (i) seasonality (for water temperature, flow rate and concentration of pollutants), and (ii) daily variations in flow rate were investigated. Results showed how during winter the low water temperatures negatively impacted the efficiency of the A-stage, producing a higher COD concentration in the effluent, which eventually could impact the performance of the nitrogen removal in the B-stage. General guidelines for the application of the model to similar highly loaded A-stage reactor systems were provided.