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| Scale formation mechanisms. (a) Bulk crystallization, (b) surface crystallization, and (c) both mechanisms ( Source : Lee & Lee 2000).
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Inorganic fouling is one of the critical operational issues in reverse osmosis membrane. Few researches investigated effects of membrane surface characteristics on inorganic fouling and on anti-scaling techniques although the fouling occurs on the membrane surface. The objective of this paper was to examine whether different characteristics of depo...
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... osmosis (RO) has been one of the most frequently used desalination technologies since it was fi rst introduced in the 1950s by research teams of UCLA and University of Florida. According to the statistical research by Greenlee et al . ( ), membrane and thermal distillation processes almost equally shared the desalination market with reverse osmosis dominating the membrane process. RO membrane is increasingly being used for the production of drinking water, brackish or seawater and wastewater reuse. The seawater reverse osmosis (SWRO) desalination has operational issues caused by various fouling including particulate, inorganic, organic and biological fouling. Foul- ing leads to the increase of operational energy, cleaning frequency and the reduction of life of membrane elements. Antifouling strategies on an SWRO system are therefore required to prevent fouling of membranes, maintain performance of process and extend service period of a system (Prihasto et al . ). Inorganic scales occur when concentrations of sparingly soluble salts, i.e. divalent ions, exceed their solubility limit. Inorganic salts which are the most likely causes of scaling are Ca 2 þ , Mg 2 þ , CO 32 À , SO 42 À , silica and iron. The common scale types are Ca-phosphate, CaCO 3 , CaSO 4 , SiO 2 and CaF. CaCO 3 precipitates have been the most prob- able foulants for seawater and wastewater reuse application (Tzotzi et al . ). According to Lee & Lee ( ), there are three mechanisms to explain fl ux decline due to scale formation in RO membranes, i.e. cake layer formation by bulk crystallization and surface blockage by surface crystallization and crystallization by both mechanisms (Figure 1). In the bulk crystallization, ions combine in a random order under the supersaturated concentrate condition and the combined ions deposit on membrane surface and then grow to a crystal form. On the other mechanism, surface crystallization forms precipitates directly on surface. Finally, when both mechanisms occur in a system, crystal particles formed in the bulk phase come up to deposition and growth of the crystals ’ progress on membrane surface. Presumably, both bulk and surface crystallization mechanisms affect scaling formation in a real SWRO system. Membranes from various manufacturers have their own surface characteristics, such as roughness and hydrophobi- city (Norberg et al. ). Lee et al . ( ) found that in general, less negatively charged surfaces accumulated more bacterial cells. The other researchers (Rahardianto et al . ) con fi rmed that doses of antiscalant added to control scales were different in accordance with four membranes in their study. Few researchers investigated effects of membrane surface characteristics on those inorganic fouling and on anti-scaling techniques. Due to the fact that fouling occurs on a membrane surface, membranes with different surface properties such as roughness and charges might show different aspects of inorganic fouling and thus need different strategies of scale inhibition. The common methods to prevent inorganic scales are pH adjustment by acids and addition of antiscalants (or inhibitors) (Fritzmann et al . ; Tzotzi et al . ; Greenlee et al . ). pH adjustment is an ef fi cient way to control CaCO 3 scale but requires large amounts of acid addition. Recently, use of antiscalants composed of various polymers has increased extensively (Li et al . ; Shih et al . ). Antiscalants are made from polycarboxylate, polyacrylate, polyphosphate, or polyphosphonate com- pound. They are added to feed water before the feed water enters a RO module. Antiscalants cannot completely prevent precipitation when ion concentrations become high as a salt concentration increases in the concentrate. The objective of this paper was to evaluate CaCO 3 scale deposition by both bulk and surface crystallization mechanisms on four different RO membrane surfaces. The deposition was analysed quantitatively by increases in dried weights and dissolution of calcium ions from each membrane coupon and qualitatively by XRD and SEM images of the fouled surfaces. Two feed solutions are used; concentrated synthetic seawater (SW) and concentrates from a real seawater RO plant. A cylindrical cell reactor was built to simulate cross- fl ow water stream to each membrane in parallel. The outer diam- eter and the length of the reactor were 2 and 32 cm, respectively. In the reactor, eight acrylic coupons were inserted as shown in Figure 2. The individual membrane area was cut to the same size, i.e., 19.6 cm 2 (4 cm × 4.9 cm), and attached to a coupon. Two coupons were used for one type of membranes for duplicated samples. The cross fl ow velocity was maintained at approximately 0.14 m/s. The membrane coupons were taken out at a designated time one by one for further surface analyses. Four membranes from Filmtec SW-30HR, Toray TM820-400, Hydranautics SWC-5 and Woongjin Chemical RE 2540-SR respectively were selected to elucidate effects of different surface characteristics such as surface charge and roughness on scale formation. Table 1 summarizes the membrane surface characteristics. Over 50 h of operation were conducted for concentrated synthetic SW and 15 days of operation for the concentrates from Changwon seawater RO (CSWRO) plant. The experiment was terminated when turbidity of the feed reservoir started to increase rapidly, which indicated the occurrence of inorganic precipitation. Various factors in the real seawater interfered to form CaCO 3 scale and yielded the long operation. The concentrated synthetic seawater was formulated followed by Standard Method 8.11. To simulate the condition in the concentrate part of SWRO, 30% of recovery was selected. The rejection was decided to be 99.6%. The concentration factor (CF) in a RO system can be expressed by Equation ...
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Citations
... Previous studies on RO membrane mineral scaling have shown that membrane surface characteristics (e.g., surface charge, surface roughness/topography, and surface chemistry) can affect the rate of mineral crystal nucleation and growth, as well as scale surface adhesion [3,7,11,12]. Thus, different approaches for modifying the active layer of RO membranes have been explored with the aim of creating RO membranes of low scaling/fouling propensity, including, but not limited to: (i) physical coating with graphene oxide [13][14][15]; (ii) membrane surface topological microstructure patterning via imprint lithography [16][17][18]; (iii) the deposition of electrically conducting carbon nanotubes during polyamide (PA) active layer casting [19]; (iv) graft polymerization ("grafting-to") [20]; and (v) polymer grafting ("grafting-from") [13,[21][22][23]. ...
... Table 2) and the calcium sulfate solution was just saturated (SIg = 1.0, Table 2), crystallization of the respective mineral salts was expected to occur both in the bulk of the solution and directly on the membrane surfaces [6,12]. Crystal scales (due to both surface crystallization and the deposition of bulk-formed crystals onto the membrane surface) can block the membrane surface, thereby reducing membrane water permeability [12]. ...
... Table 2), crystallization of the respective mineral salts was expected to occur both in the bulk of the solution and directly on the membrane surfaces [6,12]. Crystal scales (due to both surface crystallization and the deposition of bulk-formed crystals onto the membrane surface) can block the membrane surface, thereby reducing membrane water permeability [12]. Gypsum crystals of platelet structure (i.e., orthorhombic or hexagonal prismatic) were observed, as illustrated in Figure 2; such crystal structures have been reported to occur primarily due to deposition of bulk-formed crystals [36,37]. ...
The gypsum and calcite scaling propensities of the thin-film composite polyamide (PA-TFC) reverse osmosis (RO) membrane, modified with a tethered surface layer of polyacrylic acid (PAA) chains, was evaluated and compared to the scaling of selected commercial RO membranes. The tethered PAA layer was synthesized onto a commercial polyamide membrane (i.e., base-PA) via atmospheric pressure plasma-induced graft polymerization (APPIGP). The PAA nano-struc-tured (SNS) base-PA membrane (SNS-PAA-PA) was scaled to a lesser degree, as quantified by a lower permeate flux decline and surface imaging, relative to the tested commercial membranes (Dow SW30, Toray SWRO, and BWRO). The cleaning of gypsum-scaled membranes with D.I. water flushing achieved 100% water permeability recovery for both the SNS-PAA-PA and Dow SW30 membranes, relative to 92-98% permeability restoration for the Toray membranes. The calcium car-bonate scaling of SNS-PAA-PA membranes was also lower relative to the commercial membranes, but permeability recovery after D.I. water cleaning was somewhat lower (94%) but consistent with the level of surface scale coverage. In contrast, the calcite and gypsum-scaled membrane areas of the commercial membranes post-cleaning were significantly higher than for the SNS-PAA-PA membrane but with 100% permeability recovery, suggesting the potential for membrane damage when mineral scaling is severe.
... Struvite formation was thus observed in two ways to find if the method of struvite collection had any effect on the nature of struvite crystal formed. Surface spread method is unidirectional in crystal formation whereas plain sedimentation method experienced bulk crystallization wherein the ions combined randomly, nucleated, and grew multidirectionally [46] as shown in Fig. 1b. Various operating conditions are shown in Table 2. Parameters such as pH, electrical conductivity, total nitrogen, total phosphorus, potassium, magnesium, and phosphate were determined before and after struvite precipitation and are presented in Table 3. ...
Phosphorus recovered from cow urine rich in nutrients serves as an alternative form of fertilizer when recovered as magnesium ammonium phosphate termed as struvite (MgNH4PO4·6H2O). Extraction of struvite was investigated through bench scale experiments, and its yield was measured by surface spread method and plain sedimentation method. Magnesium (Mg): phosphorus (P) ratio of 5: 1 yielded 77.5 mg/L and 10 mg/L of struvite by surface spread method and plain sedimentation method, respectively. A 15-fold increase in phosphorus recovery was obtained through plain sedimentation method. The mean width and length of struvite crystals obtained from plain sedimentation method were 4.4 µm and 18.4 µm, respectively. Struvite crystal size decreased by 77.78% when the rate of mixing was increased from 40 to 120 rpm. SEM analysis depicted the needle-like and lath-shaped morphology of struvite crystals. EDS analysis revealed the presence of Ca²⁺ and Na⁺ ions, and the XRD results confirmed the co-precipitation of other magnesium phosphates such as bobierrite and newberyite and sequential precipitation of K-struvite.
... Wenzel coefficient (À) Relative membrane permittivity (À) IEMs), the evidence that membrane roughness increases the extent of calcium carbonate scaling on Reverse Osmosis membranes was given by Kang et al. [44]. Moreover, in a previous study, Curcio et al. [45] demonstrated the enhancement of precipitation rate of calcium carbonate on polypropylene membranes as a function of the Gibbs free energy barrier for heterogenous nucleation. ...
The impact of fouling on the performance of Reverse Electrodialysis operated with highly concentrated brine is a poorly investigated area. In this work, the fouling propensity and stability of Ion Exchange Membranes (IEMs), developed by Fujifilm Manufacturing Europe BV (The Netherlands), is investigated under the condition of seawater and brine. The fouling propensity of the IEMs was depicted by the determination of the Gibbs energy barrier of based-on the Classical Nucleation along with the Theoretical modeling of heterogeneous nucleation as a function of electrochemical (contact angle, permittivity, charge density) and morphological (roughness) membrane properties validated by CaCO3 precipitation. Results indicate that Cation Exchange Membranes (CEM) are more susceptible to the scaling due to the reduced energy barrier of heterogeneous nucleation. FTIR-ATR analysis on six months-aged membranes samples indicated a partial modification in the chemical structure of Anion Exchange Membranes (AEM) induced by the organic fouling associated with humic substances. The tensile tests demonstrated substantial mechanical stability of IEMs. Lab-scale RED tests operated with artificial brine over 30 days showed a significant increase in pressure drop through feed channels due to significant colloidal fouling along with a 23% reduction of maximum gross power density with consequent decrease of net power density.
... Owing to its low energy consumption and efficiency, reverse osmosis (RO) has become the most popular purification methods to provide for clean water needs through membrane-based processes such as desalination, water reclamation, and water detoxification, etc. [1][2][3][4]. The RO membrane technology still faces the problems of membrane fouling and scaling in the practical application, which seriously lower membrane performance and restricts further development [5][6][7]. The recently established polyamide (PA)-based thin film nanocomposite reverse osmosis (TFN-RO) membranes have proven to be promising by both research and industrial efforts over the past decade [8][9][10][11][12][13][14]. ...
The advantages of thin film nanocomposite reverse osmosis (TFN-RO) membranes have been demonstrated by numerous studies within the last decade. This study proposes a facile and novel method to tune the microscale and nanoscale structures, which has good potential to fabricate high-performance TFN-RO membranes. This method involves the addition of alkyl capped silica nanoparticles (alkyl-silica NPs) into the organic phase during interfacial polymerization (IP). We discovered for the first time that the high concentration alkyl-silica NPs in organic solvent isopar-G can limit the diffusion of MPD molecules at the interface, therefore shaping the intrinsic thickness and microstructures of the PA layer. Moreover, the alkyl group modification greatly reduces the NPs agglomeration and increases the compatibility between the NPs and the PA matrix. We further demonstrate that the doping of alkyl-silica NPs impacts the performance of the TFN-RO membrane by affecting intrinsic thickness, higher surface area, hydrophobic plugging effect, and higher surface charge by a series of characterization. At brackish water desalination conditions (2000 ppm NaCl, 1.55 MPa), the optimal brackish water flux was 55.3 L/m2∙h, and the rejection was maintained at 99.6%, or even exceeded this baseline. At seawater desalination conditions (32,000 ppm NaCl, 5.5 MPa), the optimized seawater flux reached 67.7 L/m2∙h, and the rejection was sustained at 99.4%. Moreover, the boron rejection was elevated by 11%, which benefits from a hydrophobic plugging effect of the alkyl groups.
... 105 technology and which increase the maintenance costs of the plant. Since the membrane technology is widely adopted, there are several studies that analyse this complication with the aim of limiting its effects: the analysis conducted by (Nam, Seockheon, Dooil, Seungkwan, & Ji, 2011, p. 1573-1579 studies the influence that the different materials constituting the membranes have on the phenomenon of dirtying them, while (Cervinia, Masaki, Tetsuji, Satoshi, & Wataru, 2016, p. 308-318) studies the effect of biofilm formation on the inorganic suspended solids accumulated inside the membranes, highlighting how a very thorough pre-treatment of the feed water is fundamental in membrane systems, to avoid further aggravating the membrane fouling. ...
... Different studies on the scaling of RO membranes have shown the presence of these phases albeit in different ratios. In one of the few studies carried out to explore the influence of membrane surface properties on inorganic fouling, Kang et al. [77] analyzed calcium carbonate deposition on four different commercial membranes. They observed that calcite and vaterite were present on all the membrane surfaces with calcite the dominant phase in 3 of the membranes and vaterite in one of them. ...
... In an equipment which is operated in contact with flowing seawater (e.g. heat exchanger [1] and filter [2] ), it is well known that adhesion of calcium-based scale precipitated from the seawater causes to reduce the performance. To inhibit and/or remove the scale, the addition of chemicals to the system is generally conducted, but there is recently a demand for scale countermeasures that are environmentally more benign. ...
... (1) There was a strong correlationship between the total volume of irregular shape crystals and CaCO3 adhesion amount. (2) The correlationships between crystal number of all polymorphic crystal forms and CaCO3 adhesion amount was weak. The reason why CaCO3 adhesion amount is suppressed in Ni-P coated steels is not that the crystal nucleus number decreased. ...
The ion eluted from the metal surface is expected to utilize as the scale inhibitor. In previous study, we found that the eluted phosphate ion from Ni-P coated steel prevents the adhesion amount of CaCO3 scale. However, its scale inhibiting mechanism is unclear. The scale formation process should be explained as the nucleation (the generation of crystal on the surface), its growth and transformation. In present study, we investigated the effect of nucleation and crystal growth on the adhesion amount of CaCO3 to understand the mechanism of inhibiting the adhesion by the eluted phosphate ion. Test materials are Ni, Ni-5%P, Ni-8%P, Ni-12%P, and Ni-14%P coated steels. CaCO3 scale adhesion testing was conducted to the Ni-P coated steels. The adhesion morphology of calcium carbonate was observed by scanning electron microscopy. There were four kinds of polymorphic crystal forms on the test materials: calcite, aragonite, vaterite, and irregular shape crystal. The adhesion amounts, the crystal numbers of their polymorphs, and their sizes were measured. There was almost no correlation between the adhesion amount and the crystal number of polymorphs. Inhibiting nucleation does not affect on the inhibiting adhesion amount. On the other hand, there was a strong correlationship between total adhesion amount and the size of irregular shape crystal. The reason why the scale adhesion amount became small may be that the crystal growth by the eluted phosphate ion has been suppressed.
... In addition, other inorganic contaminants are dissolved salts such as calcium carbonate, calcium sulfate, and calcium phosphate [28]. These inorganic contaminants deposited on the membrane surface can be extensively characterized by scanning electron microscopy-energy dispersive X-ray diffraction (SEM-EDX) [28] and X-ray diffraction (XRD) [29]. Microorganisms are mainly found in activated sludge in membrane bioreactors (MBR) as biofoulants [28]. ...
... An EDX revealed that the chemical composition of these crystals were mainly carbon, oxygen, strontium and calcium (Fig. 3b). Thus, the compounds that precipitate on the top surface of the membrane might be calcium salts like calcite, vaterite or aragonite according to the literature (Kang et al., 2011) and strontium salts like strontianite as explained in a recent study on the desalination of geothermal water by RO membrane (Tomaszewska and Bodzek, 2013). Fouling and/or scaling phenomenon were also involved in the flux ratio decrease as widely described in the literature (Guo et al., 2012;Shirazi et al., 2010). ...
The recent use of the reverse osmosis (RO) process at the damaged Fukushima-Daiichi nuclear power plant generated a growing interest in the application of this process for decontamination purposes. This study focused on the development of a robust RO process for decontamination of two kinds of liquid effluents: a contaminated groundwater after a nuclear disaster and a contaminated seawater during a nuclear accident. The SW30 HR membrane was selected among other in this study due to higher retentions (96% for Cs and 98% for Sr) in a true groundwater. Significant fouling and scaling phenomenon, attributed to calcium and strontium precipitation, were evidenced in this work: this underscored the importance of the lab scale experiment in the process. Validation of the separation performances on trace radionuclides concentration was performed with similar retention around 96% between surrogates Cs (inactive) and ¹³⁷Cs (radioactive). The scale up to a 2.6 m² spiral wound membrane led to equivalent retentions (around 96% for Cs and 99% for Sr) but lower flux values: this underlined that the hydrodynamic parameters (flowrate/cross-flow velocity) should be optimized. This methodology was also applied on the reconstituted seawater effluent: retentions were slightly lower than for the groundwater and the same hydrodynamic effects were observed on the pilot scale. Then, ageing of the membrane through irradiation experiments were performed. Results showed that the membrane active layer composition influenced the membrane resistance towards γ irradiation: the SW30 HR membrane performances (retention and permeability) were better than the Osmonics SE at 1 MGy. Finally, to supplement the scale up approach, the irradiation of a spiral wound membrane revealed a limited effect on the permeability and retention. This indicated that irradiation conditions need to be controlled for a further development of the process.
... Flux decline in RO filtration is mainly due to the increase in the membrane resistance. Membrane used in this study has been reported as negatively charged [34]. From the 13 C NMR analysis, the isolated bacterial TEP/TEP precursor substances contain a substantial amount of carboxyl groups, which are negatively charged. ...
