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Membrane scaling

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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

Membrane scaling is when one or more dissolved substances, i.e., sparingly soluble salts exceed their maximum solubility limits in the concentrate and precipitate onto the membrane surface in reverse osmosis (RO) applications. The increased concentration of sparingly soluble salts in the concentrate is primarily caused by the withdrawal of permeate water from the feedwater, which increases concentration polarization in the region next to the membrane surface and causes scaling.[1] 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. Figures 1 and 2 show scanning electron microscopy (SEM) images of the RO membrane surface without and with scaling, respectively.

Scaling compounds encountered in RO

In RO applications, the most common types of scales are:

Scaling prediction methods

There are a number of indices available to determine the scaling tendency of the 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 in RO applications are:

LSI and S&DSI are used only for calcium carbonate scaling for brackish water and seawater applications, respectively. On the other hand, SI and Sr are applicable for all compounds in both brackish and seawater applications.

A positive value for each SI, LSI, and S&DSI 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 are primarily organic compounds containing sulphonate, phosphonate, or carboxylic acid functional groups[8] that 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[9]. 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[10]. There are several commercial antiscalants on the market. 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.

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.
  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.
  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.
  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.{{cite journal}}: CS1 maint: PMC format (link) CS1 maint: unflagged free DOI (link)
  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. ^ Abdel-Aal, N.; Sawada, K. "Inhibition of adhesion and precipitation of CaCO3 by aminopolyphosphonate". Journal of Crystal Growth. 256 (1–2): 188–200. doi:10.1016/S0022-0248(03)01354-X.
  10. ^ 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, retrieved 2022-02-13
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