## 3 Results and Discussion  
### Coagulation and Flocculation of OMW at Different Coagulant Dosages  
The coagulation and flocculation of OMW was initially optimized by altering the coagulant dosage. The range was mentioned in the Materials and Methods and the tests were conducted at constant pH of raw OMW. At first, based on the literature, the coagulant dosage was at a low dosage [4]. Gradually, the dosage was increased from low, mid to high and the results are reported in Table 2, and the optimal alum concentration appears to be equal to 600 mg/L, very close to the optimal value found by other authors that treated similar OMW streams [8]. The coagulation-flocculation step is fundamental for the subsequent applications (advanced oxidation processes, membrane filtration, etc.) to reduce the turbidity of the effluent. This aspect is important for the photocatalysis process, allowing the light to penetrate the liquid volume with higher intensities. Therefore, the coagulation allowed the reduction of 96.31% of the initial turbidity, 64.63% of the COD, 23.77% of TOC and 66.15% of total phenols; however, the total phenols reduction was high at 700 mg/L. The COD reduction was mainly given by the sorption of organic acids and other surface-charged contaminants onto precipitated flocks, removed in the subsequent sedimentation process (sedimentation time equal to 60 min). As already observed in previous studies, an alum concentration above the optimum value increases the turbidity of the stream due to the restabilization mechanism typical of these processes; similarly, increasing the alum dosage also led to a decrease in TOC removal in another study [8].  
The results achieved can be compared with those reported in the literature for a classical coagulant at pH 7 and 2500 mg/L dosage of FeCl\({}_{3}\), where the removal of COD, TOC, and total phenols was, respectively, 57%, 45% and 26% on real OMW [23]. Using the FeSO\({}_{4}\)-7H\({}_{2}\)O coagulant at 6.67 g/L and flocculant FLOCAN 23 at 0.287 g/L at pH (5.3) achieved 72 \(\pm\) 1.5% COD and 40 \(\pm\) 1.3% total phenols removal [24]. In this study, 700 and 500 mg/L alum dosages also show efficient results, but the 600 mg/L dosages achieved better results for three controlled parameters except for total phenols. To select the operating dosage, it is a difficult task to assure optimal performances when the treatment process involves several parameters not all pointing in the same direction. To solve this problem of choice, GRA is applied and reported in Section 3.2.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Experiment** & **Dosage (mg/L)** & **TOC\%** & **COD\%** & **Phenols\%** & **Turbidity\%** \\ \hline
1 & 100 & 14.97 & 51.96 & 30.77 & 7.44 \\
2 & 200 & 15.36 & 55.71 & 33.85 & 17.97 \\
3 & 300 & 18.31 & 59.74 & 36.54 & 37.50 \\
4 & 400 & 21.04 & 59.88 & 53.85 & 53.13 \\
5 & 500 & 21.12 & 60.21 & 62.31 & 93.81 \\
6 & 600 & 23.77 & 64.63 & 65 & 96.31 \\
7 & 700 & 22.98 & 63.85 & 66.15 & 94.44 \\
8 & 800 & 16.48 & 63.82 & 62.31 & 93.13 \\
9 & 900 & 15.60 & 62.69 & 60.77 & 91.69 \\
10 & 1000 & 12.92 & 62.56 & 54.62 & 91.25 \\
11 & 1500 & 12.54 & 62.24 & 53.08 & 87.84 \\
12 & 2000 & 11.80 & 62.22 & 50.38 & 86.88 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Percentage removal of targeted pollutants vs. coagulant dosage.