Crossflow sampler - modified fouling index ultrafiltration (CFS-MFIUF) as a potential fouling indicator for reverse osmosis filtration system

摘要

Membrane fouling is always a limiting factor that affects the performance of reverseosmosis (RO) filtration. In addition to other fouling problems such as scaling,biofouling and organic fouling, colloidal fouling is an important fouling type in ROprocesses. The lack of quality technique to accurately measure the colloids content ofthe feedwaters prior to the RO units leads to severe RO fouling and may trigger othertypes of fouling mechanism to occur. The available fouling test is Silt Density Index(SDI) which is empirical and potentially unreliable.The effectiveness of fouling test is greatly relied on their ability to capture criticalfactor of feed water component which may contribute significantly to the fouling inreverse osmosis (RO). This thesis developed a fouling index known as the CrossflowSampler-Modified Fouling Index Ultrafiltration (CFS-MFIUF) measured under constantflux. CFS-MFIUF utilises a crossflow cell to simulate the hydrodynamic conditions inthe actual RO units followed by a dead-end MFI assessment device to measure thefouling propensity of these foulants. CFS-MFIUF concept is based on the assumptionthat the particles that are able to permeate through the CFS surface represent theparticles that will be attached on the RO surface and could not be detached later on.CFS-MFIUF was compared against the conventional fouling index, Modified FoulingIndex-Ultrafiltration constant flux (MFI-UFconst.flux). The results indicated that theMFI-UFconst.flux tends to predict a higher fouling propensity by 10 to 40%. Thisdiscrepancy highlighted the hydrodynamic fractionation of the feed by the CFS.Factors affecting the performance of CFS-MFIUF have been thoroughly investigated,iiand it was found sensitive to both crossflow velocity and permeate fluxes. The CFSMFIUFtest has been further streamlined by means of reducing the duration of the testto less than 2 hour by scaling performance at high fluxes to typical RO fluxes througha flux correction factor. This flux correction factor was proposed to correlate the CFSMFIUFmeasured at dead-end flux of 120 LMH to CFS-MFIUF measured at dead-endflux of 20 LMH for RO fouling rate prediction at crossflow velocity of 0.37m/s.CFS-MFIUF was then applied to predict fouling in a lab scale RO and was able toaccurately predict transmembrane pressure (TMP) rise under no salt conditions.However, data from the CFS-MFIUF for high salinity suspension was found tounderestimate the TMP rise in RO possibly due to the occurrence of cake-enhancedosmotic pressure (CEOP) effect. As a result, the concept of CFS-MFIUF has beenextended to include the CEOP effect due to cake formation. The new model combiningCFS-MFIUF and CEOP effect provides a good estimation on the RO fouling profile.The results indicated that CFS-MFIUF offers numerous advantages over the existingfouling indices. CFS-MFIUF combining with CEOP may be a more realistic approachto determine fouling propensity of feed water prior to the RO units. Additional studieson the effects of cake thickness and porosity on CEOP have been conducted. Theresults indicated that the structure of the deposit can significantly influence the CEOPeffect, in particularly the cake porosity. For the same amount of foulant, CEOP appears to be more dominant with the cake with lower cake porosity.