As global water stress intensifies, seawater desalination has become one of the cornerstones of water security in arid and semi-arid regions. From the Gulf countries to the Mediterranean basin, Australia, and parts of North America, desalination plants now produce tens of millions of cubic meters of drinking water every day. Reverse osmosis (RO) has emerged as the dominant desalination technology due to its increasingly competitive energy efficiency and the continuous improvement of membrane performance [1].
However, beyond traditional operational parameters such as salinity, pressure, and recovery rate, one critical factor often remains underestimated: seawater temperature. Its influence extends far beyond simple variations in water production. In reality, every temperature change profoundly alters seawater chemistry by affecting dissolved gases, ionic equilibria, salt precipitation, and ultimately the overall performance of reverse osmosis systems.
Seawater is far more than a saline solution. It is a highly complex chemical system containing dozens of major and trace ions, as well as several dissolved gases that remain in dynamic equilibrium with the atmosphere. The average composition of seawater is relatively stable worldwide, being dominated by chloride and sodium ions, followed by sulfate, magnesium, calcium, potassium, and bicarbonate ions [2]. Nevertheless, the interactions among these species are strongly influenced by temperature.
As temperature rises, one of the first observable consequences is a decrease in water viscosity. Warmer water flows more easily through membrane channels and encounters less hydraulic resistance when passing through RO membranes. In industrial facilities, this phenomenon generally translates into a permeate flux increase of approximately 2–3% for every additional degree Celsius [3]. Plant operators often observe improved production performance during summer months, with higher freshwater output at the same operating pressure.
This apparent performance enhancement, however, conceals a far more complex reality. Alongside increased flux, higher temperatures modify osmotic pressure, accelerate chemical reactions, and promote salt precipitation processes that may compromise membrane performance over time. The hydraulic gains observed during warm periods are frequently accompanied by a greater risk of scaling and fouling.
One of the most significant effects concerns dissolved gases. As with most liquids, the solubility of gases in seawater decreases as temperature rises. Dissolved oxygen, for example, is considerably more abundant in cold waters than in warm waters. This reduction affects biological activity, corrosion mechanisms, and oxidation reactions occurring in pretreatment systems. While these impacts are important, it is the behavior of dissolved carbon dioxide that plays the most critical role in reverse osmosis desalination.
Dissolved CO₂ participates in a series of chemical equilibria known collectively as the carbonate system. This system largely governs the natural pH of seawater and the distribution of dissolved inorganic carbon species. As temperature increases, part of the dissolved CO₂ escapes into the atmosphere. This decrease in CO₂ concentration shifts the chemical equilibrium toward the formation of carbonate ions. The phenomenon becomes particularly significant in the highly concentrated brine streams that develop near RO membrane surfaces.
The increase in carbonate ions directly promotes calcium carbonate formation. Calcium carbonate precipitation remains one of the most common scaling mechanisms encountered in desalination plants. Studies conducted on seawater reverse osmosis (SWRO) systems have demonstrated that temperature influences both the thermodynamics and kinetics of precipitation. Not only does supersaturation increase, but crystal formation rates also become significantly faster [4].
Calcium naturally present in seawater therefore plays a central role in scaling phenomena. As water passes through membrane modules, dissolved salt concentrations progressively increase in the concentrate stream. In the final membrane elements, concentrations may become sufficiently high to exceed solubility limits. Elevated temperatures further intensify this process. Induction times become shorter, nucleation accelerates, and crystal growth occurs more rapidly on membrane surfaces.
Magnesium also contributes to these mechanisms. Although its behavior is more complex, magnesium influences calcium carbonate crystal growth and modifies the characteristics of scale deposits. Several studies have shown that magnesium ions can alter crystal morphology and affect nucleation processes [4]. These interactions help explain why scaling predictions in seawater are often more challenging than in freshwater systems.
Sulfates represent another major concern for desalination plant operators. Deposits of gypsum, barium sulfate, and strontium sulfate are among the most problematic forms of scaling encountered in high-recovery desalination systems. Contrary to some common assumptions, temperature also plays a decisive role in these processes. Recent investigations have demonstrated that increasing temperature significantly promotes calcium sulfate scaling on reverse osmosis membranes [5]. Microscopic analyses reveal that crystal morphology changes with temperature, evolving from relatively compact structures to larger and more complex formations capable of obstructing membrane channels more rapidly.
Silica presents an additional operational challenge. Found in many seawaters as dissolved silicic acid, silica can undergo polymerization reactions under favorable physicochemical conditions. Higher temperatures accelerate these reactions and promote the formation of colloidal particles that are difficult to remove. Unlike carbonate or sulfate scales, silica deposits often resist conventional chemical cleaning procedures, making their management particularly challenging in industrial desalination plants.
In Mediterranean regions, these phenomena become especially important. The Mediterranean Sea is characterized by relatively high salinity, typically ranging between 36 and 39 g/L, and seasonal temperature fluctuations that may exceed fifteen degrees Celsius between winter and summer. Large desalination plants operating along the coasts of Algeria, Spain, and other Mediterranean countries must therefore cope with significantly different operating conditions throughout the year.
Algeria provides an excellent example of this challenge. The new desalination plants developed under the country’s water security strategy operate in an environment where seawater temperatures vary substantially throughout the year. Operators generally observe higher production rates during summer months but also experience increased risks of carbonate and sulfate scaling. This situation requires continuous adjustment of operating parameters to maintain optimal plant performance.
International experience has demonstrated that effective management of these phenomena relies primarily on dynamic operational strategies. Antiscalant dosing programs should be adjusted according to the actual seawater temperature rather than relying on annual average values. A fixed dosing strategy throughout the year frequently results in underdosing during warm periods and overdosing during colder seasons.
pH control also remains one of the most effective operational tools. Moderate acidification of feedwater limits the conversion of bicarbonate ions into carbonate ions and significantly reduces the risk of calcium carbonate precipitation. This approach continues to be one of the most efficient methods for preventing scaling in large-scale desalination facilities.
Continuous monitoring of saturation indices constitutes another widely adopted best practice in modern desalination plants. Langelier, Stiff-Davis, and advanced thermodynamic indices can help predict precipitation risks before they become critical. Specialized software packages such as PHREEQC, ROSA, and IMSDesign now allow operators to simulate the evolution of brine chemistry in real time and optimize operating conditions accordingly.
Seasonal optimization of plant operation also offers promising opportunities. During the hottest periods, a slight reduction in recovery rate can significantly decrease scaling risks by limiting maximum salt concentrations in the final membrane elements. Although this strategy may slightly reduce overall water production efficiency, it often lowers maintenance costs and extends membrane lifespan.
Recent advances in digitalization are opening new perspectives for the desalination industry. Online monitoring systems now enable the simultaneous measurement of temperature, conductivity, pH, dissolved oxygen, alkalinity, and saturation indices. Combined with artificial intelligence tools, these technologies could eventually support predictive scaling and fouling management, fundamentally transforming traditional desalination plant operation.
Conclusion
At a time when climate change is progressively altering the physical and chemical characteristics of the world’s oceans, understanding the interactions between temperature, dissolved gases, and ionic composition has become more important than ever. Future generations of desalination plants will not only need to produce more freshwater using less energy but also adapt to seawater whose properties will continue to evolve over time. In this context, mastering these physicochemical processes represents one of the key scientific and operational challenges facing modern desalination.
References
[1] Qasim M., Badrelzaman M., Darwish N.N., Darwish N.A., Hilal N. Reverse osmosis desalination: A state-of-the-art review. Desalination, 459 (2019), 59–104.
[2] Millero F.J. Chemical Oceanography. Fourth Edition. CRC Press, Boca Raton, 2013.
[3] Franks R., Chilekar S., Bartels C.R. The Unexpected Performance of Highly Permeable SWRO Membranes at High Temperatures. IDA Journal of Desalination and Water Reuse, 4(1) (2012), 52–56.
[4] Waly T.K.A., Kennedy M.D., Witkamp G.J., Amy G., Schippers J.C. The Role of Inorganic Ions in the Calcium Carbonate Scaling of Seawater Reverse Osmosis Systems. Desalination, 284 (2012), 279–287.
[5] Ashfaq M.Y., Al-Ghouti M.A., Da’na D.A., Qiblawey H., Zouari N. Investigating the Effect of Temperature on Calcium Sulfate Scaling of Reverse Osmosis Membranes Using FTIR, SEM-EDX and Multivariate Analysis. Science of the Total Environment, 703 (2020), 134726.
