Attached mass bioreactors have extensively been adopted in the last decades when specific needs have suggested this choice. Benefits and advantages of this multi-faceted technology in wastewater treatment processing are well known, along with the kinetic and mass transfer aspects regarding their operation, essentially belonging to the mass transfer with chemical reaction theory applied to enzymatic catalysis, referred to as Languimur-Hinshelwood kinetics, notably Monod/Michaelis Menten equations. On the other hand, a consolidated literature has dealt with many aspects of the development of strain colonies forming a biofilm. However, a few works have been devoted to the systematic analysis of its physiology, within the framework of the wastewater management of complex substrates and high-loads effluents. This article presents the experimental findings of a research activity covering the junction area between microbiology and bioreactor engineering, against a multifaceted set of operating parameters directly affecting health and stability of the attached biomass. In this respect, important results have been obtained, providing guidance on the attached mass reactor start-up, steady- state operation, impact of xenobiotic substrates, role of nutrients, filaments and foam formation, as well as qualitative aspects of the post-treatment effluent.