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Majid Hashemi_ Boron Removal by RO

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  • 1. Boron  Boron is a chemical element with symbol B and atomic number 5. Boron, an inorganic compound, is a non-volatile metalloid that is ubiquitous in the environment in compounds called borates. Common borates include boron oxide, boric acid, and borax. 
  • 2. Boron properties
  • 3. Boron properties
  • 4. Boron properties
  • 5. Boron properties
  • 6. Natural weathering processes are largely responsible for the presence of boron in seawater. Also, boron can be found naturally in ground water, but its presence in surface water is a consequence of the discharge of treated sewage effluent, in which arises from use in some detergents, to surface waters. Boron Sources
  • 7. Anthropogenic Sources  Glass and ceramics and porcelain,  detergents and soaps , cosmetics, bleaching agents  coal burning power plants  Insecticides  High-hardness and abrasive compounds  Shielding in nuclear reactors  Pharmaceutical and biological applications  fire retardants, glazes and agricultural products  food preservatives
  • 8.  For humans boron can represent reproductive dangers and has suspected teratogenetic properties.  A major limiting factor is the possible damage to plants and crops. Excess boron also reduces fruit yield and induces premature ripening on other species such as kiwi two predominant reasons for limiting boron in water:
  • 9.  Intakes of more than 0.5 grams per day for 50 days cause minor digestive and Boron toxicity symptoms in humans include diarrhea, nausea, vomiting, lethargy, dermatitis, poor appetite, weight loss, and decreased sexual activity Health issues and toxicity
  • 10.  In seawater sources, the typical boron concentration in the raw water is in the range of 4.6 mg/l while in confined ocean bodies boron concentration can deviate substantially from this average value, for example, boron concentration in the Mediterranean Sea can be as high as 9.6 mg/l Depending on location and seasonal effects, the boron concentration can exceed 7 mg/l, e. g. in the persian Gulf Boron
  • 11. Boron removal processes
  • 12. Unlike most of the elements in seawater, boron is not ionized (i.e. it has no charge) Boron takes two forms in drinking water (or seawater): Boric Acid: H3BO3 Borate Ion: H3BO2- RO is much better at removing charged ions. Hence the removal of borate ion is much better than the removal of boric acid.The dominant form (borate or boric acid) depends on the pH:H3BO3=H++H3BO2- Why is Boron Hard to Remove?
  • 13. Boric acid is uncharged and has trigonal structure. Therefore, boric acid is nonpolar, which causes it to interact very differently with membrane materials relative to charged salt ions and polar water molecules. RO membranes that permit high boron rejection along with high salt rejection and high water permeation might be manufactured through careful consideration of physical structure and chemical composition. Why is Boron Hard to Remove?
  • 14.  .
  • 15. RO  RO processes have been widely used for seawater desalination. Despite high removal (>99%) of other ionic species from seawater, the removal of boron by RO has proven challenging. Due to the recent improvement of membrane performance, seawater RO (SWRO) membranes, which achieve up to 95% rejection of boron in the manufacturer’s testing condition, have been commercialized.
  • 16.  It is difficult for a single-stage RO process to achieve an average rejection over 90% and to produce permeate that meets the provisional WHO boron guideline. RO
  • 17.  Generally, the rejection of boron has been lower than 90% and has been reported to be as low as 40% with low-pressure brackish water RO membranes.  Previous studies have shown that boron rejection by RO membranes improves as pH increases (i.e., as major species shift toward increasingly deprotonated forms), as operating temperature decreases, and as transmembrane pressure increases RO
  • 18. Chemistry of boron  Boron is usually present in water as boric acid, a weak acid which dissociates according to:In the usual pH operating range of reverse osmosis elements, Eq. (1) is the one with the highest importance. We thus have a presence of both dissociated and non-dissociated boric acid  species in the water 
  • 19.  There are several methods applied in seawater desalination and they can also be implemented for removal of boron  1. Use of improved RO membranes with higher B- rejection  2. Increasing the pH of the water to be treated by caustic soda (or other base) prior to RO membrane, and reacidifying the treated water after the membrane to bring it to the desired acidity  3. Passing the desalinated water through two extra passages of RO treatment  4. Adding an electrodialysis stage after RO treatment  5. decreasing the tempreture AMENDMENT
  • 20.   3233 BOHHBOH Not rejected Well rejected
  • 21.  ln practice, RO seawater desalination consist of two or more passes with natural pH (pH 6 to 7) at the first pass and elevated pH up to 11 at the second pass to effectively remove boron to acceptable levels (usually less than 0.5 mg/L) Boron Removal
  • 22.  Furthermore, already at pH higher than 9, calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) salts can crystallize on the membrane surface, leading to fouling problems. For these reasons, several RO desalination plants have been designed with more stages in series operating at different pH values. In order to obtain boron concentrations 0.4 ppm often boron selective resins are coupled to the RO units Solve Problem!
  • 23.  Single-Pass RO System  Double-Pass RO System  Single-Pass RO with Boron Specific Ion Exchange Resin  Multistage RO Systems Configurations of different process options for boron removal.
  • 24.  For the treatment of seawaters, a typical single-pass process would operate at a recovery from 40 to 50% and a permeate flux of 7 to 9 gfd (12 to 15 L/m2-hr).Typical feed pH for these systems ranges from 6.0 to7.5 (acidified) or 7.8 to 8.2. Under these conditions, the single-pass SWRO membrane unit generally produced permeate with salinity within the potable limits (i.e., less than 500 mg/L TDS) from the simulation data. However, boron concentrations in the permeate were likely to be much higher than 0.5 mg/L. Single-Pass RO System
  • 25.  the rejection of boron in a single-pass configuration can be significantly enhanced by increasing the feed pH. Figure (b) shows a single-pass RO process with feed pH adjustment. In this configuration, an antiscalant must be used if the pH is increased above 9.5. Single-Pass RO System
  • 26.  The double-pass process typically consists of a leading SWRO unit (RO1) operating at a recovery of 40 to 50% followed by a brackish RO unit (RO2) operating at a recovery of 85 to 90%. Since the feed to the RO2 process is the RO1 permeate (i.e., RO2 feed has low salinity), the RO2 unit operates at a relatively high flux (typically 20 gfd). Therefore, the number of elements required in the RO2 unit would be relatively small, thereby lowering marginal capital costs. Double-Pass RO System
  • 27. Capacity: 136,000m3/d Membrane Type 1st Pass: TM820H-400B 2nd Pass: TM720-430 Boron regulation : <0.5mg/l Recovery Rate 1st Pass: 45%, 2nd Pass: 90%, Pass 1 Permeate Tank Post Treatment Energy Recovery (DWEER : Calder) Pass 1 : SWRO Recovery : 45% Pass 2 : BWRO Recovery : 90% pH : 10.0 – 10.4 RO Feed Water Tank Bypass Scale inhibitor NaOH RO section Detail Low Pressure Pump (VFD) High Pressure Pump Booster Pump (VFD) Low Pressure Pump 2-pass system with alkaline dosing is applied for Boron removal High Boron Rejection Seawater Desalination Plant in Singapore
  • 28.  does not require a pH adjustment of the RO permeate. Recovery rates for ion exchange systems are typically very high (~98%). However, O&M costs of ion exchange systems tend to be high due to the expense of specialty resins required for removing boron and the need for resin regeneration.. For the cost analysis, it was assumed that the ion exchange unit treated 16% of RO permeate. Single-Pass RO with Boron Specific Ion Exchange Resin
  • 29. Dual-Pass RO with Boron Specific Ion Exchange Resin
  • 30.  the low recovery and scale formation potential problems associated with double-pass systems might be effectively avoided by multistage configurations without requiring the costly ion exchange process. In both configurations, additional RO units are employed to further treat the concentrate produced from the second pass RO unit. in figure 5.1-(e), the concentrate from the second-pass RO is treated by an ion exchange softening process to remove divalent cations. The effluent from the softener is further treated by RO (RO3). Since there is little calcium and magnesium present in the effluent from the softener, risk of scale formation in the RO3 unit is minimal, regardless of pH. In addition, the concentrate from the RO3 unit is essentially a pure NaCl solution. Multistage RO Systems
  • 31. In this configuration, the concentrate from the second pass RO (RO2) is directly treated with another RO unit (RO3). To prevent scale formation, pH of the RO2 concentrate is reduced by acidification, prior to processing by the RO3 unit. The permeate from RO3, which has a very low concentration of divalent cations, is further processed with an additional RO unit (RO4) at an elevated pH. Multistage RO Systems
  • 32. Estimated water production cost for each configuration.
  • 33. Feed water: pH, temperature, TDS Membrane element: membrane chemistry, element efficiency  System design and operation: average permeate flux (APF), system recovery, concentration polarization, cleanings FACTORS INFLUENCING BORON REJECTION
  • 34.  . In the case of seawater desalination by reverse osmosis, (RO) the boron rejection is usually insufficient to obtain desalinated water (RO permeate) that can meet drinking water quality requirements.  Multi-step RO systems or RO-IE (ionic exchange) combinations are then applied. Conclusions
  • 35.  Surface analyses showed that all membranes tested had a negative surface charge and a ridge and valley structure. The negative charge of the membrane played an important role in boron removal, since charge repulsion is one of the important mechanisms of boron rejection.. Conclusions