The performance and reliability of the CWM1-RETRASO model for simulating processes in horizontal subsurface flow constructed wetlands (HSSF CWs) and the relative contribution of different microbial reactions to organic matter (COD) removal in a HSSF CW treating urban wastewater were evaluated. Various different approaches with diverse influent configurations were simulated. According to the simulations, anaerobic processes were more widespread in the simulated wetland and contributed to a higher COD removal rate [72–79%] than anoxic [0–1%] and aerobic reactions [20–27%] did. In all the cases tested, the reaction that most contributed to COD removal was methanogenesis [58–73%]. All results provided by the model were in consonance with literature and experimental field observations, suggesting a good performance and reliability of CWM1-RETRASO. According to the good simulation predictions, CWM1- RETRASO is the first mechanistic model able to successfully simulate the processes described by the CWM1 model in HSSF CWs.