## 3 Results and Discussion  
### Coagulation/Flocculation Results and Modeling  
Figure 1 displays the OMW turbidity and COD values obtained at 120 min using different chitosan concentrations.  
Figure 1: Coagulation/flocculation optimization tests results in terms of percentage removal (**a**) and of chemical oxygen demand/turbidity (COD/Tur) g removed/g of chitosan used (**b**) (temperature = 25 \({}^{\circ}\)C, initial pH = 4.6, initial COD = 12.24 g L\({}^{-1}\), initial turbidity = 3.89 g L\({}^{-1}\)).  
As reported in Figure 1a, the optimal chitosan concentration appears to be equal to 500 mg/L, very close to the optimal value found by other authors that treated similar OMW streams [44,45]. If one considers instead the optimal use of chitosan, it can be observed from Figure 1b that for COD removal, a better chitosan concentration would have been 100 mg/L, since a qCCD of 19.58 g/g was obtained, whereas for Tur removal, the optimal chitosan amount resulted to be again 500 mg/L (qTur = 112.93 g/g). The coagulation/flocculation step is fundamental for the subsequent AOPs technology applications in order to reduce the turbidity of the effluent. This aspect is important for the photocatalysis process, allowing the light to penetrate into the liquid volume with higher intensities. Another important result was the observed pH increase, that reached 5.6. Therefore, the first pretreatment allowed for the reduction of 90% of the initial turbidity and 33% of the initial COD, providing also a partial pH neutralization of the OMW. 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 24 h). As already observed in previous studies, a chitosan concentration above the optimum value increases the turbidity of the stream due to the restabilization mechanism typical to these processes. The results achieved can be compared with those reported in the literature for classical coagulants, such as the one reported by [PERSON] and coworkers [37] who observed a turbidity removal of about 90% on a real OMW using a commercial Nalco-77171.  
Once the optimal coagulant concentration was fixed, the subsequent kinetic experiments were performed. Figure 2 reports the kinetic data results and modeling.  
The model was able to fit the overall experimental data well, including the asymptotic trend reached after 35-40 min. The second order of reaction was found to be suitable for this kind of mechanism, as demonstrated also by the high correlation coefficient, R2, obtained (0.98). The regressed kinetic constant was equal to \(6.68\times 10^{-4}\) L/mg min, which result is close to the regressed value obtained by [PERSON] and coauthors, who modeled the coagulation kinetic of tannery wastewater by a pseudo-nth order kinetic model, obtaining a reaction order of 1.96 and a kinetic constant of \(8.62\times 10^{-4}\) L/mg min [46]. From the kinetic constant value and rate values, the Brownian diffusion coefficient trend was estimated (Figure 3).  
Figure 2: Olive mill wastewater (OMW) coagulation/flocculation kinetic results and modeling (chitosan concentration = 500 mg L\({}^{-1}\), temperature = 25 \({}^{\circ}\)C, initial turbidity 3.89 g L\({}^{-1}\)).  
The initial D value was \(2.85\times 10^{-18}\) m\({}^{2}\)/min and reached the maximum value of about \(9.2\times 10^{-17}\) m\({}^{2}\)/min at the end of the process. The reported values were characterized by the same order of magnitude as the D coefficients values calculated by [PERSON] and coworkers [47].