Professor Raphael Semiat

Critical flux detection in a silica scaling RO system Lisitsin, D., Hasson, D. and Semiat, R. Desalination 186, 311-318, 2006 Reverse osmosis desalination has achieved prominence as the most cost effective desalting technology. The objective of the present paper was to develop adequate tools for diagnosing malfunctioning of the generally adopted spiral wound module design. Efforts were undertaken to apply residence time distribution (RTD) techniques combined with concentration polarization (CP) determinations for characterizing the degree of uniformity of flow in spiral wound membrane passages and diagnosing flow mal-distribution defects. Axial mixing intensities were evaluated from experimental RTD measurements on the basis of the Exponentially Modified Gaussian Model (EMG). The levels of the axial mixing intensities, characterized by the dispersion coefficient, indicated significant deviations from ideal plug flow. Theoretical analyses showed that the presence of axial dispersion can result in significant deviations from the module performance characteristics evaluated according to the usual assumption of ideal plug flow. A striking concentration polarization result was that increase of the Reynolds number (Re) reduced the CP level as expected, but only up to a certain critical Re after which the CP level increased with Re. This result can be explained by the effect of Re on the dispersion level. Free-surface flow of concentrated suspensions Singh, A., Nir, A. and Semiat, R. Accepted to the Journal of Multi Phase Flow. International Journal of Multiphase Flow 32(7), 775-790, 2006 Free surface flows of concentrated suspensions exhibit many interesting phenomena such as particle segregation and surface corrugation. In this work the flow structures associated with free surface has been studied experimentally. The free surface velocity for neutrally buoyant suspension of uniform spheres in a gravity driven inclined channel flow was determined by particle imaging velocimetry (PIV) technique. Experiments were carried out for concentrated suspensions with particle fractions φ ranging from 0.40 to 0.50. The measured velocities show blunted profile in the channel. The blunting of the velocity profile increases with the particle concentration. The rms velocity fluctuations measured at the free surface progressively increase with particle fraction φ and are linear in shear rate γ. The surface roughness were characterized by analyzing the power spectral density of the refracted light from the free surface. The characteristics observed are in support of the earlier findings of Loimer et al. (2002). Scale control in saline and wastewater desalination Hasson, D. and Semiat, R. Israel Journal of Chemistry 46(1), 97-104, 2006 This paper briefly reviews current understanding of basic phenomena involved in scale formation, outlines current scale control techniques and indicates some promising research directions. Boron removal from water by complexation to polyol compounds Geffen. N., Semiat, R., Eisen M., Balazs Y., Katz I. and Dosoretz, C. G. Journal of Membrane Science 286(1-2), 45-51, 2006 Boron is an important micronutrient for plants, animals and humans, although the range between deficiency and excess is narrow. The use of desalinated water and treated wastewater for irrigation may result in excess boron. In aqueous environments (i.e. neutral pH) boron is mainly present as boric acid, which is mostly undissociated and therefore only partially rejected by desalination membranes. Boric acid/borate reacts with neutral polyolic compounds, generating anionic complexes. This work reports on the complexation of boron with mannitol integrated with membrane desalination. The separation of the complex was studied in a wide range of conditions by nanofiltration (NF), and partially by reverse osmosis (RO) membranes. NMR analysis and chemical equilibrium modeling system (Mineql+) were applied to provide a better understanding of experimentally observed rejection patterns. The addition of mannitol in excess formed mainly a 2,2-di-borate ester and some monoborate esters as function of the pH, improving boron rejection by NF up to 90% at pH 9. Mineql+ calculations indicate that reactants concentration has a strong influence on the ionized boron species and therefore on the rejection of boron. Sea water-RO membranes having a much higher basal rejection for boric acid, rejected almost 97% mannitol-complexated boron at pH of 9. Detection of fouling on RO modules by residence time distribution analyses Hasson, D., Drak, A., Komlos, C., Yang, Q. and Semiat, R. Desalination 204(1-3), 132-144, 2007 A simple and reliable technique for on line detection and diagnosis of scaling and fouling deposits could improve significantly operational control of RO plants. This paper presents results of an ongoing research project aiming to develop a novel method for online detection of membrane fouling by a residence time distribution (RTD) technique. The method is based on determination of flow dispersion intensities from online RTD signals that can be simply measured. The systems investigated in this paper are spiral wound membranes fouled with either Mg(OH)2 or CaCO3. Analyses of RTD data of fouled membranes show that an increasing membrane permeability loss is accompanied by a systematic increase in the dispersion coefficient, thus providing an indicator for detecting a fouling event. It is also found that the magnitude of the dispersion coefficient is affected by deposit morphology, thus indicating the possibility for diagnosing the nature of the fouling deposit. Another fouling indicator may be derived from the observation that the extent of the dead volume monitored by the RTD signal increases with the fouling level while dead volumes are absent in clean modules. © 2007 Elsevier B.V. All rights reserved. Bacterial community composition and structure of biofilms developing on nanofiltration membranes applied to wastewater treatment Hanan Ivnitsky, Ilan Katza, Dror Minz, Galit Volvovic, Eyal Shimoni, Elina Kesselman, Raphael Semiat and Carlos G. Dosoretz Water Research 41, 17, 3924-3935, September (2007) The structure of biofilms and microbial communities developing on cross-flow nanofiltration (NF) membranes at different temperatures (20°C, 25°C, or 34°C) and operation lengths (8 h-24 d) were studied. Feedwater comprised tertiary quality wastewater effluent or synthetic media mimicking effluents of intermediate quality. After each run the membranes were autopsied for bacterial enumeration, bacterial community composition and microscopy visualization (SEM, CLSM and AFM/NSOM). Community composition was analyzed by polymerase chain reactiondenaturing gradient gel electrophoresis (PCRDGGE) coupled with sequence analysis of 16S rRNA gene fragments from dominant bands. Deposition of polysaccharides and initial bacterial colonization were observed within 8 hours, whereas developed biofilms markedly affecting membrane permeability were evident from day 23 onwards. Regardless of applied conditions, the heterotrophic plate counts in the biofilm were 34×10 CFU/cm and the thickness of biofouling layer was 20-30 μm. From a total of 22 sequences obtained from 14 independent experiments most species identified were Gram-negative (19 of 22 sequences). Proteobacteria were found to be a prevalent group in all cases (16 of 22 sequences) and among it, the β-subclass was the most predominant (8 sequences) followed by the γ-subclass (5 sequences). Pseudomonas/Burkholderia, Ralstonia, Bacteroidetes and Sphingomonas, were the dominant groups found in most cases. Even though the microbial population might be important with respect to biofouling patterns, membrane permeability decline seems to be more substantially influenced by the formation and accumulation of exopolymeric substances (EPS). Desupersaturation of RO concentrates by addition of coagulant and surfactant Yang, Q., Lisitsin, D., Liu, Y., David, H., Semiat, R. Journal of Chemical Engineering of Japan 40 (9), pp. 730-735, 2007 One way for the reuse of reverse osmosis (RO) concentrates and thus increasing the water recovery is to recycle part of the brine concentrate to the feed, after precipitating the scaling salts such as CaSO4/CaCO3 held in solution by antiscalants. However, antiscalants impart considerable stability to the super-saturated solution. In the present study, desupersaturation of RO CaSO4 concentrate in the presence of antiscalants was studied by addition of coagulant polyaluminum chloride (PACI) and surfactant sodium dodecyl sulfate (SDS). Simulated experiments for testing the changes in antiscalant concentration were also conducted. The results indicated that under certain conditions, the above methods were effective for desupersaturation of RO concentrate. After the removal precipitation reactions, the calculated super-saturation ratio of the test solution deceased to about 1.0, which demonstrated that the test solution reached thermodynamic equilibrium. With respect to the removal of scaling salt, it was found to follow the second order kinetics. The removal of scaling salt was always accompanied by the adsorption precipitation of antiscalants. The ecomonic analyses show the proposed methods were feasible. RO module RTD analyses based on directly processing conductivity signals Yang, Q., Drak, A., Hasson, D., Semiat, R. Journal of Membrane Science 306, 1-2, 355-364, 2007 Residence time distribution (RTD) techniques can be used to diagnose the flow characteristics in spiral wound reverse osmosis (RO) modules. However, the methods of processing tracer response conductivity signals and mathematically modeling of RTD curves often involve complicated steps including conductivity-concentration transformation, baseline selection and the use of exit age distribution function of Et, or dimensionless exit age distribution function of Eθ. In this paper, a simple and direct method for processing RTD signals from conductivity data was developed for spiral wound membrane RO system. Two models were tested: axial dispersion (AD) model and exponentially modified Gaussian (EMG) model. The results show that the present method provides a simple, fast and accurate RTD data reduction. The levels of the axial mixing intensities, characterized by the dispersion number D/uL, indicated significant deviations from ideal plug flow in both the laboratory and the industrial size modules. In both the modules, the dispersion coefficient D increased roughly linearly with the Reynolds number. Membrane fouling and wornout led to an increase in D. Moreover, the values of mean residence time over(t, ̄) and D/uL obtained from the EMG model were more stable against the change of the curve tail length, especially for the parameter D/uL. Furthermore, RTD analysis also indicated that the membrane wearing-out could lead to dead zones. © 2007 Elsevier B.V. All rights reserved. Membranes in desalination and water treatment Vainrot, Nataly; Eisen, Moris S.; Semiat, Raphael MRS Bulletin 33(1), 16-20, 2008 Membrane techniques for water treatment have been growing significantly for the last decade. Reverse osmosis, for example, is the leading seawater desalination technique. Nanofiltration membranes are used more and more for hardness removal and even to desalinate slightly polluted waters. Ultrafiltration and microfiltration membranes are used extensively mainly as membrane bioreactors in wastewater recovery. While this trend is growing, the membranes still may be improved significantly, based on new materials designed to increase the flux of water through the membranes at reduced pressures while maintaining or even improving the rejection of dissolved matter or suspended matter. Better membranes will reduce energy consumption while maintaining affordable separation properties.