The effectiveness of physical, chemical and biological barriers to the diffusion of genetically modified microorganisms (GMMs) to prevent their release into the environment is currently under scrutiny worldwide because of the associated potential ecological impacts. An industrial discharge of a non-sterilized fermentation broth containing GMM biomass into a conventional municipal wastewater treatment plant would deliver the GMMs into the activated sludge system process (ASSP). The present work aimed to model and evaluate the containment capability of a small ASSP (part of a 20,000 people equivalent municipal plant) in the event of receiving GMM biomass from a medium-small biotechnological plant dedicated to the production of polyhydroxyalkanoates (3000 t/year of biopolymer). An actual GMM (Pseudomonas putida KTOY06) was injected into a bench-scale ASSP (ASSPLab) in a quantity proportional to the relative dimensions of the plants mentioned. The experimental and model results indicated that the ASSP of the target municipal treatment plant would not be capable of holding back such a sudden input of GMM; 6 h after the discharge, 11-15 % of injected GMM cells were released through the clarified stream of the ASSPLab, with the rest being gradually released over time. Since the GMM employed did not exhibit any growth in the ASSPLab, its concentration in the clarified water stream would not represent a substantial risk of release into the environment if appropriate tertiary treatments were integrated. This study confirmed the necessity of a thorough risk assessment of biotechnological processes prior to their implementation.