**2. Background**

Historically, Van Leeuwenhoek is credited as the first scientist who observed microbial biofilm formed on a surface [3]. Important contributions to the understanding of the mechanisms and circumstances according to which adhesion takes place were taken by Characklis [4] and Costerton et al. [5]. In the last decades, kinetic and mass transfer aspects of attached biomass have been well studied and parameterised in the scientific and technologic literature [2, 6]. Diaco and Eramo [7, 8] carried out one of the most complete studies combining diffusional and kinetic aspects with the structure of the attached mass. Oxygen transfer from the bulk liquid to the bacterial colony surface and subsequent kinetics follows the general laws of absorption with chemical reaction on a surface layer, as described in the early works of Sherwood [9] and Astarita [10]. On the other hand, kinetics taking place in a biofilm has extensively been studied according to Michaelis- Menten-Monod scenario and successfully framed within Langmuir-Hinshelwood and Hougen - Watson reaction schemes, according to the formalism of heterogeneous catalysis [11]. Williamson and McCarty [12] provided one of the first approach specifically related to the biofilm, showing the correlation between Fick diffusional phenomena and Monod kinetic theory. Authors such as O<sup>0</sup> Toole et al. [13] and many others have studied the biofilm structure with the aim to identify how diverse was the mass microbiological behavior with respect to the suspended structure, as to the metabolic, pathogenic and clinical scenarios. More recently, several authors, such as Muslu [14] and Feng et al. [15] have worked on modeling further biofilm as a chemical reaction site, bringing specific contributions to the definition of the biodegradation rate and of the related conditional factors, within the theoretical frame built up by early authors such as Atkinson and Davies [16] and La Motta [17]. Naz et al. [18] evaluated the biofilm succession on stone media and compared the biochemical changes of sludge in attached and suspended biological reactors operated under aerobic and anaerobic conditions. Ercan et al. [19] have studied the biofilm development conditions on different surfaces, predominantly from the point of view of the relationship between the structural and geometrical features of the supports and the process variables, such as oxygen transfer and chemical reaction rate for production purposes. As a matter of fact, a large and consolidated literature has been produced about the characterization of biofilm and attachment mechanism, predominantly either analyzing the process engineering aspects of the bioreactor schemes or studying the purely microbiological and clinical topics per se. Relatively few efforts have been devoted to the analysis of the physiology of the attached biomass, notably dealing with the effects of nutrients, of substrate origin, of its transient behavior in terms of formation rate and stabilization framed within the biodegradation of complex pollutants contained in the wastewaters.

#### **3. Unknown variables and uncertainties**

Jenkinson and Lappin-Scott [20] define biofilm as *the microorganism consortium which develops at the interface solid–liquid or liquid–gas*. Gottenbos et al. [21] have

*Experimental Investigation of Biomass Attachment to Wastewater Reactors DOI: http://dx.doi.org/10.5772/intechopen.94426*

pointed out that *the biofilm is a specific micro-ecosystem inside which different microbial strains effectively cooperate to get protection from ambient stress and to promote nutrients absorption*.

Formation and development of biofilm is assumed to take place according to the following phases, [22–24]:


All these phases are mediated by and are related to specific metabolic factors, which have a specific role in the progression of the process. The definition and characterization of the related parameters are of paramount importance to establish the conditions for a biofilm to be formed and stably persist in an aqueous medium. Parameters and factors affecting biofilm formation, development and effectiveness are very numerous, ranging from metabolic chemistry to genetics, hydrodynamics and transport properties. In the experimental research programme underpinning the present article, a specific focus has been made on those which were expected to have a direct, significant and macroscopic effect on the effectiveness of the wastewater treatment process.

#### **3.1 Nutrients and thermochemical environment**

Nutrients, temperature and pH significantly affect the biofilm formation and its behavior. Specifically, effect of temperature and pH is substantially known, whereas, even if a basic understanding of nutrients role has been achieved, often monitored in terms of C/N and C/P ratios, the assessment of biofilm behavior with time and with regards to changing nutrients respectively is important [25].

#### **3.2 Surface and hydrodynamics**

Surface and hydrodynamics play an important role. In order for the attachment process to start, contact of cell with solid surface is required, followed by a rapid bond. Strengthen and rapidity of these bonds are rather known, depending on a series of chemical–physical interactions and biological process which lead to a reversible adhesion [26]. This reversibility is to be considered as purely theoretical, as, if a minimum set of bonds has been achieved, the cumulative effect is sufficient to make the adhesion permanent [27]. Once again, nutrients and gram reactivity as well were expected to play an important role both in terms of adjusting the attachment capability to the specific surface and in terms of the transport properties in the aqueous medium, such as viscosity, which, in turn, belongs to the hydrodynamic part of the system.
