| Water pollution and shortage has become a critical and serious challenge to society progress and economy development in the world. As an important strategic solution to water shortage, seawater desalination has been playing an increasingly momentous role. The desired performance of seawater desalination systems is guaranteed by appropriately designed pretreatment to face the varying seawater quality. It is of great theoretic and practical importance to focus on Ultrafiltration (UF) as an alternative replacement of conventional pretreatment system prior to seawater desalination. Prior to perform Ultrafiltrtion as the pretreatment, laboratory studies, pilot testing and demonstration, and membrane fouling in the filtration process should be considered. Most of the work is conducted on the desalination site and combined with demonstration seawater desalination plant which includes the feasibility studies of ceramic UF membrane and pilot testing of organic UF membrane as a pretreatment prior to seawater desalination on the laboratory scale and pilot scale respectively, the long-period investigation on demonstration seawater desalination plant and optimal performance, the studies of critical flux to minimize the membrane fouling in UF process and computer simulation on cross-flow UF process. The results are shown as follows:⑴Evaluation of the primary technical feasibility of 50nm monolith ceramic UF membrane as pretreatment prior to seawater desalination on the laboratory scale by feeding raw seawater and seawater with coagulant. The results from experiments on ceramic membrane performance including permeate flux and solute removal indicate that the optimal cross-flow velocity, TMP, feed temperature and pH are 3.0～3.5m/s, 0.14～0.18MPa, 25～30℃and 8～9, respectively; the optimum coagulation conditions were 4.5～6mg/L at pH 4～6; the optimum foulant cleaning was 1% NaClO at 40~50℃.⑵The main purposes of pilot testing of organic UF membrane treating seawater on cross-flow mode are to find the optimum filtration and backwash duration, backwash flow rate, recovery and flux. Based on maximum net product volume, performance of the UF system is good with the following conditions: backwash durationτ=30 s, filtration duration t=40 min, and backwash flow rate is 1800L/h. The operation of the UF system in high recovery-low flux mode (80%-60L/m2h) might be the best and should be adopted. The UF permeate has 100% of turbidity below 0.01NTU and 95% of the SDI15 below 3.0, which satisfies the requirement of SWRO feed water.⑶Pilot testing of organic UF membrane treating seawater in critical conditions of low feed temperature and dead-end mode mainly investigates the performance of inside-out and outside-in UF modules, evaluates the stability of dual-active layer of UF membrane as seawater desalination pretreatment, and analyses the direct operational cost. In filtration process, TMP increases rapidly and membrane fouling is obvious when inside-out module A is operated on designed flux and regular backwash; performance of outside-in module B is better than A with respect to stable TMP and flux, but there is more serious membrane fouling observed when module B is operated at up to 120% each designed flux. Membrane fouling can be controlled effectively by CEB with NaClO or NaClO/HCl, and CEB period of 16h is the optimal parameters. Permeate quality of both module A and B can meet the requirement of SWRO feed despite the variation of the raw seawater quality. The intrinsic resistance of dual-active layer module C is higher than module A and B; in module C, TMP is the highest and permeability is the lowest of all the modules investigated, but the permeate quality is best of all the testing modules. According to the consumption of energy and chemical reagents, the direct operational cost of module B is the lowest.⑷By monitoring the performance and adjusting the process parameters of UF system in the large-scale demonstration seawater desalination plant for one year, the data indicate that the membrane fouling is significant. It is also observed that the feed temperature affects membrane fouling and permeability, and membrane fouling can be reduced to some extent by chemical cleaning and operating at optimum conditions. The quality of UF permeate can satisfy the requirement of SWRO feed water, with turbidity and SDI15 in the range of 0~ 0.02NTU and 2.1~3.5, respectively. When feed temperature is low (about 5℃), mechanical properties of UF membrane may become poor and weak and therefore, rupture. It might be prevented by cleaning and optimum operating condition.⑸The concept of"Critical Flux"(Jcrit) is introduced into the field of seawater desalination, and used to investigate the membrane fouling and the optimal operational mode. A natural flux method is proposed to measure Jcrit which is proved feasible, accurate and easy to operation. By analyzing type of Jcrit in UF treating seawater, it is assumed that the membrane fouling cannot be avoided despite the operating conditions, but can be minimized. The seawater main components (NOM, suspended matter and inorganic salts), cross-flow rate and MWCO of UF membrane have different effects on Jcrit and membrane fouling. Under sub-critical conditions, operating the system on different modes including constant flux, constant pressure and natural flux mode results in the stable performance of UF treating seawater and the trends of flux and TMP have nearly no change. Comparing with different operating modes, natural flux mode has a good performance with the lowest total resistance, and it is the best choice of all the investigated operating modes.⑹By using Monte Carlo, a different point of view of microcosmic to simulate the particle random displacement and force situation in the condition of cross-flow UF process can be provided. It can relate microcosmic parameters (the dimensionless center-to-center separation between two particles scaled by the particle radius, s, and volume fraction,φ) and measuring parameters (flux and TMP). The results indicate that the relative error of flux between Monte Carlo simulation and experimental measurement is in the range of -0.246~1.518%. From fitting the curve ofΔP~J, Jcrit is estimated to be 137.88L/m2h.
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