Evaluation of various pretreatment for particle and inorganic fouling control on performance of SWRO

Seawater desalination of interest is rising to overcome water scarcity. Especially, desalination using RO membranes is increasingly being used for production of drinking water for brackish and/or seawater. SWRO has operational problems that can be caused by various membranes fouling including particulate, inorganic, organic and bio-fouling. Membrane-based pretreatment for particle fouling and the integrated membrane systems with MF, NF, and RO were recently introduced to SWRO. However, little information is known for SWRO processes combined with various pretreatments. Effects of various pretreatments on performance of SWRO were evaluated in this study. The RO membrane flux was substantially improved by nanofiltration pretreatment resulting in the highest flux regardless of pretreatment for particle fouling. However, the water flux with the other pretreatments including MF with antiscalant addition is severely decreased during a few hours. The microfiltration pretreatment showed greater flux than the coagulation pretreated water. Coagulation pretreated waters was observed to have relatively high MFI resulting from residuals such as flocs and colloidal particles. The correlations of turbidity, normalized flux ratio and MFI were substantial indicating feasibility of the use of MFI for particle fouling index. The conductivity removal was significant, but the cation removal was not great by NF. The results of the inorganic ions removal indicate that pretreatment process affects properties of feed water to RO. The application of inorganic scaling control pretreatment has shown no influence on the crystal structure but there were changes in the intensity related with calcium scale inhibition in the XRD analyses.

Fig. 5. XRD spectrum of CaCO3 and CaSO4 with different pretreatment of “MF+pH” (a), “Coag+AS” and “MF+NF” (b). The goal of this work is to study performance of RO desalination processes for seawater, and to evaluate effects of various pretreatment on the performance of RO. Microfiltration or coagulation was considered for particulates fouling control and antiscalant addition or nanofiltration was evaluated for scale control. The RO membrane flux was substantially improved by nanofiltration pretreatment resulting in the highest flux regardless of pretreatment for particle fouling. It was due to not only inorganic scale matter but also colloidal particles by NF pretreatment. Compared to coagulation pretreatment, microfiltration was shown to have slightly higher flux. The coagulated feed water to RO showed the most rapid flux decline. It indicated that membrane based pretreatment could be more competitive than typical coagulation pretreatment although this interpretation could be limited since settling was not applied in this experiments. Coagulation pretreated waters was observed to have relatively high MFI, in consistent with the flux decline behaviors. The correlation of MFI with turbidity and flux was substantially great with exception of the result from coagulation pretreatment. The MFI with the result from the coagulated water was closely corresponded with turbidity but not with flux decline rates. The correlations of turbidity, normalized flux ratio and MFI were substantial indicating feasibility of use of MFI. The conductivity removal was significant, but the cation removal was not great with NF. The results of the inorganic compounds removal indicate that pretreatment process affects properties of feed water to RO. The application of inorganic scaling control pretreatment has shown no influence on the crystal structure but there were changes in the intensity related with calcium scale inhibition in the XRD analyses.