Subsurface flow constructed wetlands (SSF CWs) are nowadays one of the most common types of eco-technologies for wastewater treatment used throughout the world. In SSF CWs three relevant elements contribute to wastewater treatment: granular medium, plants and bacterial communities. However, from these three, it is widely accepted that the activity of diverse bacterial communities growing in the form of biofilms is the most important element in pollutant transformation and removal