Optimizing Reverse Osmosis Membrane Parameters through the Use of the Solution-Diffusion Model: A Review

When designing and building an optimal reverse osmosis (RO) desalination plant, it is important that engineers select effective membrane parameters for optimal application performance. The membrane selection can determine the success or failure of the entire desalination operation. The objective of this work is to review available membrane types and design parameters that can be selected for optimal application to yield the highest potential for plant operations. Factors such as osmotic pressure, water flux values, and membrane resistance will all be evaluated as functions of membrane parameters. The optimization of these parameters will be determined through the deployment of the solution-diffusion model devolved from the Maxwell Stephan Equation. When applying the solution-diffusion model to evaluate RO membranes, the Maxwell Stephan Equation provides mathematical analysis through which the steps for mass transfer through a RO membrane may be observed and calculated. A practical study of the use of the solution-diffusion model will be discussed. This study uses the diffusion-solution model to evaluate the effectiveness of a variety of Toray RO membranes. This practical application confirms two principal hypotheses when using the diffusion-solution model for membrane evaluation. First, there is an inverse relationship between membrane and water flux rate. Second, there is a proportional linear relationship between overall water flux rate and the applied pressure across a membrane.

Experimental Investigation on Saline Water Purification Using Reverse Osmosis by a Novus Biomimetic Membrane

A substantial amount of Earth’s water is inadequate for human consumption while local demand is outstripping traditional supplies in many world regions;thereby,brackish and seawater treatment has become a prerequisite.This investigation suggested a complete design of an RO-based desalination filter with a multilayer biomimetic membrane.The study demonstrated a comprehensive method for experimentally fabricating a proprietary biomaterial-based multilayer nano-porous membrane.This analysis revealed that Silk Nano-Fibril(SNF)and Hydroxyapatite(HAP)extracted from Bombyx Mori silkworm cocoons may be utilized to manufacture highly methodical multilayer membranes by incorporating protein-self-assembly and in-situ-bio-mineralization.Membrane’s aquaporin layer containing lipid-bilayers has rapid water permeability and high efficacy at eliminating salt ions and contaminants.The 4µm thick SNF/HAP membrane showed a considerable decrease in salinity,with a salt rejection of 93.33%.The proposed membrane had a saline water permeability of 6.58 LMH/bar,almost 61.09%higher than conventional TFC membranes.Hydrophobic barrier and spiral-wrapped filter architecture of the membrane enable low fouling and self-cleaning properties.The schematic filter design and biomimetic fabrication of the SNF/HAP membrane have formulated a conceptual framework that might direct to the broad-scale,low-cost RO water purification filters,increasing the efficiency of water desalination and boosting the effectiveness of water treatment technologies to reduce potable water scarcity.

Highly hydrophobic oil−water separation membrane: reutilization of waste reverse osmosis membrane

The increasing applications of seawater desalination technology have led to the wide usage of polyamide reverse osmosis membranes,resulting in a large number of wasted reverse osmosis membranes.In this work,the base nonwoven layer of the wasted reverse osmosis membrane was successfully modified into the hydrophobic membrane via surface deposition strategy including TiO_(2) and 1H,1H,2H,2H-perfluorooctyltrichlorosilane(PFOTS),respectively.Various techniques were applied to characterize the obtained membranes,which were then used to separate the oil–water system.The optimally modified membrane displayed good hydrophobicity with a contact angle of 135.2°±0.3°,and its oil–water separation performance was as high as 97.8%.After 20 recycle tests,the oil–water separation performance remained more than 96%,which was attributed to the film adhesion of the anchored TiO_(2) and PFOTS layer on the surface.This work might provide a new avenue for recycling the wasted reverse osmosis membrane used in oily wastewater purification.

Fatty acid fouling of forward osmosis membrane:Effects of pH,calcium,membrane orientation,initial permeate flux and foulant composition

Octanoic acid（OA） was selected to represent fatty acids in effluent organic matter（EOM）. The effects of feed solution（FS） properties, membrane orientation and initial permeate flux on OA fouling in forward osmosis（FO） were investigated. The undissociated OA formed a cake layer quickly and caused the water flux to decline significantly in the initial 0.5 hr at unadjusted p H 3.56; while the fully dissociated OA behaved as an anionic surfactant and promoted the water permeation at an elevated p H of 9.00. Moreover, except at the initial stage, the sudden decline of water flux（meaning the occurrence of severe membrane fouling） occurred in two conditions： 1.0.5 mmol/L Ca2＋, active layer facing draw solution（AL-DS） and 1.5 mol/L Na Cl（DS）; 2. No Ca2＋,active layer-facing FS（AL-FS） and 4 mol/L Na Cl（DS）. This demonstrated that cake layer compaction or pore blocking occurred only when enough foulants were absorbed into the membrane surface, and the water permeation was high enough to compact the deposit inside the porous substrate. Furthermore, bovine serum albumin（BSA） was selected as a co-foulant.The water flux of both co-foulants was between the fluxes obtained separately for the two foulants at p H 3.56, and larger than the two values at p H 9.00. This manifested that, at p H 3.56,BSA alleviated the effect of the cake layer caused by OA, and OA enhanced BSA fouling simultaneously; while at p H 9.00, the mutual effects of OA and BSA eased the membrane fouling.

Fabrication and performance of PET mesh enhanced cellulose acetate membranes for forward osmosis

Polyethylene terephthalate mesh（PET） enhanced cellulose acetate membranes were fabricated via a phase inversion process. The membrane fabrication parameters that may affect the membrane performance were systematically evaluated including the concentration and temperature of the casting polymer solution and the temperature and time of the evaporation, coagulation and annealing processes. The water permeability and reverse salt flux were measured in forward osmosis（FO） mode for determination of the optimal membrane fabrication conditions. The optimal FO membrane shows a typical asymmetric sandwich structure with a mean thickness of about 148.2 μm. The performance of the optimal FO membrane was tested using 0.2 mol/L Na Cl as the feed solution and 1.5 mol/L glucose as the draw solution. The membrane displayed a water flux of 3.47 L/（m2·hr） and salt rejection of95.48% in FO mode. While in pressure retarded osmosis（PRO） mode, the water flux was4.74 L/（m2·hr） and salt rejection 96.03%. The high ratio of water flux in FO mode to that in PRO mode indicates that the fabricated membrane has a lower degree of internal concentration polarization than comparable membranes.

Fouling distribution in forward osmosis membrane process

Fouling behavior along the length of membrane module was systematically investigated by performing simple modeling and lab-scale experiments of forward osmosis （FO） membrane process. The flux distribution model developed in this study showed a good agreement with experimental results, validating the robustness of the model. This model demonstrated, as expected, that the permeate flux decreased along the membrane channel due to decreasing osmotic pressure differential across the FO membrane. A series of fouling experiments were conducted under the draw and feed solutions at various recoveries simulated by the model. The simulated fouling experiments revealed that higher organic （alginate） fouling and thus more flux decline were observed at the last section of a membrane channel, as foulants in feed solution became more concentrated. Furthermore, the water flux in FO process declined more severely as the recovery increased due to more foulants transported to membrane surface with elevated solute concentrations at higher recovery, which created favorable solution environments for organic adsorption. The fouling reversibility also decreased at the last section of the membrane channel, suggesting that fouling distribution on FO membrane along the module should be carefully examined to improve overall cleaning efficiency. Lastly, it was found that such fouling distribution observed with co-current flow operation became less pronounced in counter- current flow operation of FO membrane process.

Experimental Evidence for Hyperfiltration of Saline Water through Compacted Clay Aquitard in the Hebei Plain

It becomes an increasing concern that groundwater quality in exploited deep confined aquifer may deteriorate due to brackish water leakage from its overlying saline aquifer in Hebei Plain. However, the monitoring data show that the TDS does not significantly change in the exploited aquifer. Some physics or chemistry processes must have taken place in aquitards during brackish leakage. The semi-permeable membrane function of clay aquitard during the process of hyperfiltration（reverse osmosis） should be one of the most important processes. To confirm and test this hyperfiltration mechanism, a series of experiments were performed in which Na Cl solutions were hydraulically forced through different clay sampled from aquitard. The solution 7 g/L in NaC l was forced through at 20 °C by a fluid pressure of 0.5 kN. The results show that hyperfiltration indeed happens in caly aquitard. Semi-permeability is quantified by the reflection coefficient σ. The mean rejection coefficients（σ） for clay samples #1, #2 and #3 were estimated to be 0.063, 0.164 and 0.040, respectively. This behavior of clay was well explained with the theory of the diffuse double layer. The hyperfiltration effect is to the great extent responsible for the chemical process in the aquitard.