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. © 2015 Elsevier B.V. All rights reserved.

The UF pretreatment provided consistent good quality water to the RO membranes, with the turbidity of UF effluent stayed stable from 0.07 to 0.12 NTU, and SDI15 around 2. However, UF showed very limited removal effect for organic matter, but after combined with flocculation/dual-stage sand filtration, the TOC retention rate of UF can be effectively increased. Sand filtration showed good performance for turbidity removal (less than 0.5 NTU in effluent). Nonetheless, the SDI removal of sand filtration was very limited, even after the flocculation/dualstage sand filtration, the SDI remains in the range of 3 to 5. After dual-stage sand filtration, the peak value of particle size in the filtrate ranged from 0.1 to 0.3 μm, while in UF effluent, the figure was less than 10 nm, demonstrated that sub-micron particles had high impact on SDI in seawater. The attenuation rate of SF of UF was significantly controlled by the combination of flocculation. Coarse sand filtration showed very little improvement for UF, while fine sand filtration and dual-stage filtration showed a better effect. The results also showed that all of the sand filtration/UF systems can achieve more stable operation with the addition of flocculation, especially the flocculation/dual-stage filtration/UF system showed a distinct good performance, with the attenuation rate of SF reduced to 0.008 L/m5 h m. Based on all the results presented, it can be concluded that the best treatment effect appears in flocculation/ dual-stage sand filtration/UF system, which can effectively reduce the attenuation rate of SF to 0.008 L/m5 h m. However, the complex process will necessarily need more investment and operating costs. In the subsequent research, more qualitative optimization should be done to make the answer to be more persuasive.