## 3 Results and Discussion
### Operational Aspects and Judement of the Plant Managers
#### 3.2.4 Critical Operational Aspects  
Figure 7 shows the main operation troubles related to the MBR processes. Fouling of membranes and plant shutdown are the most relevant issues, even if, concerning the last aspect, the plant managers reported it as an occasional or rare event. Membrane fouling is a common problem in MBR plants, and in order to reduce its negative effects, collaboration between the membrane manufacturer and the plant manager is a key factor. The survey results highlight a significant difference between the membrane configurations: flat-sheet membranes are less affected by fouling issues with respect to the hollow-fiber configuration (Figure 8). In effect, according to [28], the advantages of the hollow-fiber configuration lie in the ability to induce movement of the membranes by mechanisms such as bubbling, and the feasibility of backwashing. Moreover, [PERSON] et al. [40] showed that the hollow-fiber membrane exhibited fouling intensity proportional to the operating flux (0.05-0.3 m/day). Conversely, the flat-sheet membrane suffered from immediate internal pore blocking beyond a critical flux of 0.2 m/day. Figure 6: Operational advantages of MBR technology. The most important operation and maintenance routine in MBRs is the membrane cleaning. If the membranes are not kept clean, the fouling problems can lead to plant shutdown, as well as cost increase, performance reduction and briefer membrane lifespan [41]. According to the opinion of MBR managers and the scientific literature, fouling preventive-control strategies are an important issue that strongly concern not only the research but the industry as well [42]. Fouling phenomena depend on many factors such as operating conditions, membrane cleaning strategies, characteristics of wastewater influent, and membrane properties. Fouling is associated with four successive steps. Initially smallest pores are blocked, followed by obscuring the inner surface of larger pores. Direct blockage and accumulation of particles on the larger pores is the next step, and finally the formation of the cake layer is followed. However, the identification of each step is not easy [26]. Fouling issue is strongly influenced by extra extracellular polymeric substances (EPSs), biological substances which largely determine properties of sludge flocs, including hydrophobicity, adhesion, flocculation, settling and dewatering properties. Several developments in fouling prevention and control strategies were identified [4; 42]: suitable feed pre-treatment, physical/chemical cleaning, flux reduction, aeration increase, ultrasound or ozone application, and membrane surface modification. Moreover, the operating conditions modification (e.g., MLSS concentration, HRT, SRT, Food/Mass ratio) affects the fouling: increasing SRT leads to a decrease in the EPS level which provides lower fouling on the membrane surface. On the other hand, the effect of the fouling on the membrane surface is not negligible. Figure 8: Operational troubles of MBR plants: comparison between flat-sheet and hollow-fiber membranes. Figure 7: Operational troubles of MBR plants: comparison between municipal and industrial facilities. hand, the increase in the MLSS concentration results in the elevation of the mixed liquor's viscosity that may worsen fouling and also require more demand for aeration. Hence, assessing optimum SRT is essential to control fouling phenomena [26]. Even the problem of plant shutdown can be avoided, or at least reduced, with regular recovery and cleaning operation. This aspect has led to the need of qualified staff: the acquisition of suitable skills to solve the several troubles and to obtain the knowledge of a new process takes time, but this aspect could also represent an opportunity for the technical growth of the plant human resources.