Membrane scaling

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Figure 1: SEM image of a virgin (new) RO membrane that has not been scaled Virgin RO membrane.png
Figure 1: SEM image of a virgin (new) RO membrane that has not been scaled
Figure 2: SEM image of a RO membrane that has been scaled RO membrane scaled.jpg
Figure 2: SEM image of a RO membrane that has been scaled

Membrane scaling is when one or more sparingly soluble salts (e.g., calcium carbonate, calcium phosphate, etc.) precipitate and form a dense layer on the membrane surface in reverse osmosis (RO) applications. [1] Figures 1 and 2 show scanning electron microscopy (SEM) images of the RO membrane surface without and with scaling, respectively. Membrane scaling, like other types of membrane fouling, increases energy costs due to higher operating pressure, and reduces permeate water production. [2] Furthermore, scaling may damage and shorten the lifetime of membranes due to frequent membrane cleanings [3] and therefore it is a major operational challenge in RO applications.

Contents

Membrane scaling can occur when sparingly soluble salts in RO concentrate become supersaturated, meaning their concentrations exceed their equilibrium (solubility) levels. In RO processes, the increased concentration of sparingly soluble salts in the concentrate is primarily caused by the withdrawal of permeate water from the feedwater. The ratio of permeate water to feedwater is known as recovery which is directly related to membrane scaling. Recovery needs to be as high as possible in RO installations to minimize specific energy consumption. However, at high recovery rates, the concentration of sparingly soluble salts in the concentrate can increase dramatically. For example, for 80% and 90% recovery, the concentration of salts in the concentrate can reach 5 and 10 times their concentration in the feedwater, respectively. If the calcium and phosphate concentrations in the RO feedwater are 200 mg/L and 5 mg/L, respectively, the concentrations in the RO concentrate will be 1000 mg/L and 50 mg/L at 90% recovery, exceeding the calcium phosphate solubility limit and resulting in calcium phosphate scaling.

It is important to note that membrane scaling is not only dependent on supersaturation but also on crystallization kinetics, i.e., nucleation and crystal growth.

Scaling compounds encountered in RO

The most common salts that cause scaling in RO processes are:

Scaling prediction methods

There are a number of indices available to determine the scaling tendency of sparingly soluble salts in a water solution. These indices provide information if a given scale-forming specie is undersaturated, saturated, or supersaturated. Scaling does not occur when a compound is undersaturated, while it will take place sooner or later when a compound is supersaturated.

The most commonly used indices to predict scaling in RO applications are:

where, IAP and Ksp are ion activity product and solubility product of the sparingly soluble salt, respectively. For instance, SI for calcium sulphate can be calculated as follows:

where, γ is activity coefficient. [Ca2+] and [SO42−] are calcium and sulphate concentrations in mol/L, respectively.

where IAP and Ksp are ion activity product and solubility product of the sparingly soluble salt, respectively. For instance, Sr for calcium sulphate can be calculated as follows:

where, γ is activity coefficient. [Ca2+] and [SO42−] are calcium and sulphate concentrations in mol/L, respectively.

LSI is used only for calcium carbonate scaling. On the other hand, SI and Sr are applicable for all compounds.

A positive value for each SI and LSI indicates that scaling may occur in RO, whereas a negative value implies that scaling will not occur. Similarly, scaling may occur when Sr>1, but not when Sr<1.

Scaling control in RO applications

There are several methods for preventing scaling in RO applications, including acidification of RO feed, lowering RO system recovery, and antiscalant addition. [4] Acidification of RO feedwater was one of the first methods for tackling calcium carbonate scaling in RO processes. [5] However, due to the risks associated with the use of acid, this method is becoming less common. Furthermore, acidification may not be effective for all types of scales; for example, it is very effective in preventing calcium carbonate scaling but not calcium sulphate scaling. [6]

Another method of preventing scaling is to operate RO at low recovery (ratio of permeate water to the feedwater). The recovery of the RO application is reduced in this approach to reduce the supersaturation level of the concentrate water to undersaturated conditions. Low recovery reduces the adverse effect of concentration polarization because there is less solute concentration on the membrane surface, reducing the potential for scale formation. This approach, however, is not very appealing or economical because it results in high specific energy consumption. Furthermore, the large amount of concentrate disposal is a problem.

Antiscalants addition to the RO feed is one of the most widely applied strategies in term of scale control. [7] Antiscalants can be used to increase the recovery of RO process and are primarily contains organic compounds such as sulphonate, phosphonate, or carboxylic acid functional groups [8] [9] . The addition of antiscalants hinder the crystallization process, i.e., nucleation and/or growth phase of scaling compounds. Antiscalant prevent scale formation by three mechanisms, namely threshold inhibition, crystal modification and dispersion. [10] Threshold inhibition is when antiscalant molecules adsorb on crystal nuclei and halt their nucleation process, whereas crystal modification and dispersion are the ability of antiscalants to stop the growth and/or agglomeration of crystals and particles. [11] For silica scale, there is also an additional function where it prevents polymerisation of silica monomers, hence preventing the growth of silica polymers [12] [13] . There are various commercial antiscalants on the market such as Kurita, Avista, BASF etc [14] [15] [16] . In RO applications, antiscalants are chosen based on the composition of the feedwater, and their doses are usually calculated using computer programs created by antiscalant manufacturers. For example, Avista has a chemical dosing software called AdvisorCI™ [17] , that is used to compute accurate dosing of chemicals in RO systems.

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Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison.

<span class="mw-page-title-main">Calcium carbonate</span> Chemical compound

Calcium carbonate is a chemical compound with the chemical formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.

<span class="mw-page-title-main">Brine</span> Concentrated solution of salt in water

Brine is water with a high-concentration solution of salt. In diverse contexts, brine may refer to the salt solutions ranging from about 3.5% up to about 26%. Brine forms naturally due to evaporation of ground saline water but it is also generated in the mining of sodium chloride. Brine is used for food processing and cooking, for de-icing of roads and other structures, and in a number of technological processes. It is also a by-product of many industrial processes, such as desalination, so it requires wastewater treatment for proper disposal or further utilization.

<span class="mw-page-title-main">Desalination</span> Removal of salts from water

Desalination is a process that takes away mineral components from saline water. More generally, desalination is the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture. Saltwater is desalinated to produce water suitable for human consumption or irrigation. The by-product of the desalination process is brine. Desalination is used on many seagoing ships and submarines. Most of the modern interest in desalination is focused on cost-effective provision of fresh water for human use. Along with recycled wastewater, it is one of the few rainfall-independent water resources.

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In chemistry, the common-ion effect refers to the decrease in solubility of an ionic precipitate by the addition to the solution of a soluble compound with an ion in common with the precipitate. This behaviour is a consequence of Le Chatelier's principle for the equilibrium reaction of the ionic association/dissociation. The effect is commonly seen as an effect on the solubility of salts and other weak electrolytes. Adding an additional amount of one of the ions of the salt generally leads to increased precipitation of the salt, which reduces the concentration of both ions of the salt until the solubility equilibrium is reached. The effect is based on the fact that both the original salt and the other added chemical have one ion in common with each other.

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Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.

<span class="mw-page-title-main">Forward osmosis</span> Water purification process

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Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also extends the lifetime of plumbing by reducing or eliminating scale build-up in pipes and fittings. Water softening is usually achieved using lime softening or ion-exchange resins, but is increasingly being accomplished using nanofiltration or reverse osmosis membranes.

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<span class="mw-page-title-main">Membrane</span> Thin, film-like structure separating two fluids, acting as a selective barrier

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References

  1. ↑ Lee, S.; Lee, C.H. (2005-03-01). "Scale formation in NF/RO: mechanism and control". Water Science and Technology. 51 (6–7): 267–275. doi:10.2166/wst.2005.0646. ISSN   0273-1223. PMID   16003986.
  2. ↑ Alshami, Ali; Taylor, Trevor; Ismail, Nadhem; Buelke, Chris; Schultz, Ligia (2021-12-15). "RO system scaling with focus on the concentrate line: Current challenges and potential solutions". Desalination. 520: 115370. doi:10.1016/j.desal.2021.115370. ISSN   0011-9164. S2CID   244199653.
  3. ↑ Kucera, Jane (2010-04-05), Amjad, Zahid (ed.), "Reverse Osmosis Membrane Fouling Control", The Science and Technology of Industrial Water Treatment, CRC Press, pp. 247–270, doi:10.1201/9781420071450-c13, ISBN   978-1-4200-7144-3 , retrieved 2022-02-12
  4. ↑ Antony, Alice; Low, Jor How; Gray, Stephen; Childress, Amy E.; Le-Clech, Pierre; Leslie, Greg (2011-11-01). "Scale formation and control in high pressure membrane water treatment systems: A review". Journal of Membrane Science. 383 (1): 1–16. doi:10.1016/j.memsci.2011.08.054. ISSN   0376-7388.
  5. ↑ Lee, Hye-Jin; Halali, Mohamad Amin; Baker, Thomas; Sarathy, Siva; de Lannoy, Charles-François (2020-06-01). "A comparative study of RO membrane scale inhibitors in wastewater reclamation: Antiscalants versus pH adjustment". Separation and Purification Technology. 240: 116549. doi:10.1016/j.seppur.2020.116549. ISSN   1383-5866. S2CID   214513158.
  6. ↑ Chen, Si; Su, Jincai; Fu, Feng-Jiang; Mi, Baoxia; Chung, Tai-Shung (2013-11-08). "Gypsum (CaSO4·2H2O) Scaling on Polybenzimidazole and Cellulose Acetate Hollow Fiber Membranes under Forward Osmosis". Membranes. 3 (4): 354–374. doi: 10.3390/membranes3040354 . ISSN   2077-0375. PMC   4021950 . PMID   24957062.
  7. ↑ Pervov, Alexei G. (1991-09-01). "Scale formation prognosis and cleaning procedure schedules in reverse osmosis systems operation". Desalination. Proceedings of the Twelfth International Symposium on Desalination and Water Re-use. 83 (1): 77–118. doi:10.1016/0011-9164(91)85087-B. ISSN   0011-9164.
  8. ↑ Singh, Rajindar (2005-01-01), Singh, Rajindar (ed.), "Chapter 2 - Water and membrane treatment", Hybrid Membrane Systems for Water Purification, Amsterdam: Elsevier Science, pp. 57–130, doi:10.1016/b978-185617442-8/50003-8, ISBN   978-1-85617-442-8 , retrieved 2022-02-13
  9. ↑ Chuan Yee Lee, Brandon; Tan, Eileen; Lu, Yinghong; Komori, Hideyuki; Pietsch, Sara; Goodlett, Robb; James, Matt (2023-10-01). "Antiscalant and its deactivation in zero/minimized liquid discharge (ZLD/MLD) application in the mining sector – Opportunities, challenges and prospective". Minerals Engineering. 201: 108238. doi:10.1016/j.mineng.2023.108238. ISSN   0892-6875.
  10. ↑ Abdel-Aal, N.; Sawada, K. (2003). "Inhibition of adhesion and precipitation of CaCO3 by aminopolyphosphonate". Journal of Crystal Growth. 256 (1–2): 188–200. Bibcode:2003JCrGr.256..188A. doi:10.1016/S0022-0248(03)01354-X.
  11. ↑ Fink, Johannes (2021-01-01), Fink, Johannes (ed.), "Chapter 7 - Scale inhibitors", Petroleum Engineer's Guide to Oil Field Chemicals and Fluids (Third Edition), Gulf Professional Publishing, pp. 351–391, doi:10.1016/b978-0-323-85438-2.00007-4, ISBN   978-0-323-85438-2, S2CID   241834717 , retrieved 2022-02-13
  12. ↑ Chuan Yee Lee, Brandon; Tan, Eileen; Lu, Yinghong; Komori, Hideyuki; Pietsch, Sara; Goodlett, Robb; James, Matt (2023-10-01). "Antiscalant and its deactivation in zero/minimized liquid discharge (ZLD/MLD) application in the mining sector – Opportunities, challenges and prospective". Minerals Engineering. 201: 108238. doi:10.1016/j.mineng.2023.108238. ISSN   0892-6875.
  13. ↑ Milne, Nicholas A.; O'Reilly, Tom; Sanciolo, Peter; Ostarcevic, Eddy; Beighton, Mark; Taylor, Kelvin; Mullett, Mark; Tarquin, Anthony J.; Gray, Stephen R. (2014-11-15). "Chemistry of silica scale mitigation for RO desalination with particular reference to remote operations". Water Research. 65: 107–133. doi:10.1016/j.watres.2014.07.010. ISSN   0043-1354.
  14. ↑ "Kurita (Singapore) Pte Ltd". www.kurita.com.sg. Retrieved 2024-03-02.
  15. ↑ "Avista Membrane Treatment Solutions - Avista Technologies, Inc". Avista Membrane Treatment Solutions. Retrieved 2024-03-02.
  16. ↑ "BASF features its desalination portfolio for sustainable water treatment at IDA World Congress 2015". www.basf.com. Retrieved 2024-03-02.
  17. ↑ "Advisor™Ci Chemical Dosing System Water Treatment - Avista™". Avista Membrane Treatment Solutions. Retrieved 2024-03-02.
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