Study on different ultrafiltration-based hybrid pretreatment systems for reverse osmosis desalination

Performance of different pretreatment systems with ultrafiltration (UF) prior to reverse osmosis (RO) for desalting seawater was assessed in this paper. The pilot trial was operated on seawater from Bohai Bay in Tianjin using polyvinylidene fluoride (PVDF) hollow fiber UF membrane. During these experiments, membrane specific flux (SF) was calculated to evaluate the performance of UF. Results of the research showed that in a chemical enhanced backwash (CEB) period, the combination of UF with FeCl3 flocculation pretreatment was helpful in controlling the decrease of SF during the operation. Besides, the result also showed that FeCl3 flocculation/dual-stage sand filtration/UF performed effectively in terms of the stable of SF in a CEB period than FeCl3 flocculation/UF alone. In addition, water analysis was regularly performed to quantify pollutants in the effluents of different hybrid processes. According to the experiment, UF can provide product water with consistently low turbidity (0.07-0.12 NTU) and low silt density index (SDI15 less than 2) levels regardless of seawater quality. Sand filtration showed good performance for turbidity removal (less than 0.5 NTU in effluent), but limited effect on SDI removal (not below 4). Total organic carbon (TOC) removal up to 28% was obtained in FeCl3 flocculation/dual-stage sand filtration/UF hybrid system.