**1. Introduction**

The adoption of attached mass reactors for the biological treatment of wastewater started in the late 1800s early 1900s [1]. Trickling filters have been used much earlier than the fundamentals of biochemical engineering applied to sewage purification had been established. During the early 1950's, development of plastic media resulted in the introduction of innovative packing and during the early 1960s rotating discs, included in several chemical process, have begun to be considered for oxygen transfer [1]. In the last 40 years submerged attached growth aerobic processes became established options for wastewater treatment, taking the advantage of a reduced space required with respect to activated sludge [2]. Nowadays, the combination of attached growth/activated sludge systems, also referred to as hybrid process or Integrated Fixed-Film Activated Sludge (IFAS) is an optimized alternative, as well as the Moving Bed Biofilm Reactor (MBBR), which typically ensure an increased treatment capacity, reduced sludge production and handling, low spaces and independency of a secondary clarifier. All attached mass systems have to take into account that mass

transfer is the rate-limiting-steps of the overall biodegradation process. In addition, biofilm quality, structural properties and stability greatly affect the overall process performance, as well as effluent quality and conditions for foam formation. Consequently, the characterization and the understanding of physiology of the biofilm is an essential key for a correct management of the biodegradative process.
