# Application of Ultrafiltration for Recovery of Bioactive Phenolic Compounds from Rose Wastewater
## 3 Results and Discussion  
The permeate flux is one of the main characteristics of each membrane process. Figure 1 shows the effect of VRR on permeate flux during the ultrafiltration of rose wastewater. Flux decreased with the rise in VRR in all three membranes studied (\(p<0.05\)). This was due to the increase in the concentration solution and led to an increase in the dynamic viscosity [32] and concentration polarization [33]. According to [PERSON] et al. [34], the decrease in flux is related to the formation of a cake layer, whose thickness increased during the process. The authors found a linear dependence between flux and transmembrane pressure when pomegranate juice was subjected to ultrafiltration. Similar to their results, in our study, permeate flux increased with the rise in transmembrane pressure in all membranes (\(p<0.05\)). This could be explained by the fact that an increase in recirculation velocity improves the mass transfer coefficient and the hydrodynamical conditions, leading to a decrease in the effect of concentration polarization and an increase in flux [35]. According to [PERSON] et al. [36], the decrease in permeate flux is different in various membrane types during the nanofiltration of apple juice. Our results demonstrate that permeate flux depends on the molecular weight cut-off of the membrane, as it increases at a higher molecular weight cut-off. [PERSON] et al. [37] also found a dependence between permeate flux and the molecular weight cut-off of the membrane when blackberry (_Rubus adenotrichus_ Schltdl.) juice was subjected to ultrafiltration. [PERSON] and [PERSON] [38] established no significant difference between the permeate flux of 5 and 10 kDa membranes during a 7-fold ultrafiltration of broccoli juice, while a 50 kDa membrane had 3-4 times higher flux. This observed relationship suggests that selection of an appropriate molecular weight cut-off is crucial for the optimization of the ultrafiltration process, in particular for complex matrices such as rose wastewater. This aligns with previous studies that emphasize the role of molecular weight cut-off in the determination of flux behavior and fouling propensity during the ultrafiltration of organic-rich feed solutions [39]. In our case, the lowest results for permeate flux were obtained with the 1 kDa membrane, a VRR of 8, and a transmembrane pressure of 0.2 MPa, while the highest were with the 25 kDa membrane, a VRR of 0 (beginning of the process), and a transmembrane pressure of 0.5 MPa. Energy consumed is one of the most important parameters in each industrial process. Figure 2 presents the energy demand under the same working conditions as in Figure 1. The energy demand increased with transmembrane pressure and rise in VRR (\(p<0.05\)). The effect of transmembrane pressure is related to the increase in power required of the pump, provoking an increase in energy consumption [40]. The same authors established that an increase in operational pressure from 3 to 6 bar provoked a rise in the total energy consumed from 51.57 to 62.54 kJ/dm\({}^{3}\). The effect of the VRR could be explained by the increase in fluid viscosity with the rise in VRR because of an increase in dry matter content, as well as the prolongation of the duration of the process [41]. An inverse relationship was observed between permeate flux and energy demand. This can be explained by the fact that permeate flux participates in the denominator in Equation (3), used for calculating energy demand. The effect of molecular weight cut-off was also observed, as energy demand increased with its diminution. This could be explained by the fact that pore size decreased with the decrease in the molecular weight cut-off of the membrane [42], which led to a drop in permeate flux, and thus increased the energy demand.