## 3 Results
### Textile WW Treatment Performance Using SD Technology Laboratory Setup
#### 3.1.2 Disc Rotational Speed Influence on the Treatment Efficiency Using SD Laboratory Setup  
To establish the disc rotational speed influence on the SD technology efficiency, each variation in time of the two investigated WW quality indicators were represented at constant WW flowrates. In Figure 6a-e, the discoloration was plotted at seven different disc rotational speeds and a constant flowrate of 10, 15, 20, 25, and 30 L/h, respectively. A rotational speed of 100 rpm rendered a maximum color removal of 45.21% for 10 L/h, after 50 min, as seen in Figure 6a. At 850 rpm, the maximum color removal was of 43.07% for 10 L/h, after 60 min (Figure 6a); 31.58% for 25 L/h, after 40 min (Figure 6d); and, finally, 30.18% at 30 L/h, after 30 min (Figure 6e). At 550 rpm and 15 L/h, the highest discoloration was of 53.50% (Figure 6b), which is the highest obtained value out of all acquired data within this study, while at 400 rpm and 20 L/h the largest color removal was 46.48%. As an additional observation, if a set of indicators, for a certain WW flowrate and rotational speed have not been graphically represented, it means they did not indicate satisfactory results. Figure 6: Influence of the disc rotational speed on discoloration at different WW flowrates: (**a**) 10 L/h, (**b**) 15 L/h, (**c**) 20 L/h, (**d**) 25 L/h, (**e**) 30 L/h. In Figure 7a-e, the suspended solids removals are plotted as the variation in time for the above-mentioned seven rotational speeds and five WW flowrate values. At 10 L/h, the 250 rpm rendered a maximum suspended solids removal of 54.29% after 60 min (Figure 7a), while at 15 L/h 550 rpm gave the highest removal of 61.13% after 50 min (Figure 7b). For both 20 L/h and 25 L/h flowrates, at a rotational speed of 850 rpm the maximum suspended solids removals of 51.88% after 50 min (Figure 7c) and 54.51% after 40 min (Figure 7d) were obtained. At the largest WW flowrate of 30 L/h, the 1200 rpm gives a removal of only 14.08% after 25 min, as seen in Figure 7d. All graphs plotted at 10, 15, and20 L/h presented increasing trends, which suggests that even higher SS removal values might be attained, provided longer working time periods. All experimental results concerning the maximal efficiency in color and suspended solids are summarized in Table 2.  
Figure 7: Influence of the disc rotational speed on the suspended solids removal at different liquid flowrates: (**a**) 10 L/h, (**b**) 15 L/h, (**c**) 20 L/h, (**d**) 25 L/h, (**e**) 30 L/h. One can observe that the highest removal for color (53.50%) and suspended solids (61.13%) was obtained at a flowrate of 15 L/h and a rotational speed of 550 rpm after 50 min. Some of the maximum color and suspended solids removal values correlated to the rotation speed and the WW flowrate can be explained in relation to the disc flowing film characteristics, as follows. Depending on the liquid flowrate and disc rotational speed, several flow regimes are found on the surface of a spinning disc. At low liquid flowrates and rotational speeds, visual observations indicated either a smooth or slightly wavy film surface, suggesting a laminar flow, similar to other literature studies [34]. In such cases, no significant micromixing is taking place, since an injected dye maintains a sharp front within the film or even within the waves formed on the film surface [35]. Additionally, at low constant rotational speed values, an increase in the flowrate is characterized by shorter residence time periods on the disc [6; 36; 37], which essentially explains the decrease in the color and suspended solids removals at 100 rpm, with the WW flowrate increases (Table 2). At higher rotational speeds, disordered ripples of finer structure are formed on the liquid film surface, indicating the start of a turbulent flow regime and, thus, an increased micromixing which