Seawater Desalination Routes Compared: Thermal, Membrane and the Freeze Option
Three Mainstream Processes, Three Different Failure Modes
More than 400 years ago the British royal family offered a reward for an economical and practical seawater desalination method. The prize went unclaimed for centuries, and the reason is easy to state: separating salt from water is thermodynamically cheap and engineering-wise expensive. Worldwide there are more than 20 desalination technologies in commercial use, including reverse osmosis, low-temperature multi-effect distillation, multi-stage flash distillation, electrodialysis, vapour compression distillation, dew point evaporation, combined water and power production, integrated thermal-membrane systems, and nuclear, solar, wind or tidal-energy variants, plus microfiltration, ultrafiltration and nanofiltration as pre- and post-treatment. Broadly, though, the field divides into distillation and membranes, with low-temperature multi-effect distillation (MED), multi-stage flash (MSF) and reverse osmosis (RO) carrying the load.
Each has a characteristic weakness. MED saves energy, tolerates poor feedwater and delivers high product quality. RO needs low investment and low energy but demands stringent seawater pretreatment. MSF is mature, reliable and large in output, and it is relatively energy-intensive. The standard reading is that MED and RO represent the future direction, while MSF keeps its place wherever scale and fouling tolerance matter more than specific energy.
Where MSF Still Wins and Why It Will Keep Winning for a While
Flash evaporation is the phenomenon in which part of the seawater at a given temperature evaporates rapidly when the pressure suddenly drops. In an MSF plant, heated seawater flashes successively across multiple chambers at progressively lower pressures, and the vapour is condensed as freshwater. MSF still accounts for the largest output among global desalination plants, with the most mature technology, high operational safety and real flexibility. It is usually built alongside thermal power stations and suits large and super-large plants, which is why the Gulf states have stayed with it. Single units already produce 40,000-50,000 tons of freshwater per day.
The development trends are predictable: larger single-unit output, lower unit power consumption and better heat transfer. Any of those three is a materials problem before it is a process problem.
The Membrane Route and Its Real Constraint
RO project costs and operating expenses keep falling, and the direction of travel is consistent: lower operating pressure, higher system recovery, cheap and efficient pretreatment, and better fouling resistance. Note where the effort goes. Pretreatment, not the membrane itself, is where most plant availability is won or lost, and the same holds for the standards that govern it.
Electrodialysis follows a different logic and depends on new ion-exchange membranes. An ion-exchange membrane is a functional sheet 0.5-1.0 mm thick, classified by selective permeability as a cation-exchange membrane or an anion-exchange membrane. In a stack, cation and anion membranes alternate to form independent compartments: seawater in alternating compartments is desalinated while adjacent compartments concentrate, so freshwater and brine separate. The same stack concentrates, separates and purifies in the chemical, pharmaceutical and food industries, which is often what makes the capital case work.
Distillation, Dew Point Evaporation and the Long Tail of Methods
Distillation heats seawater to boiling and condenses the vapour. It was the first desalination technology put into industrial application, it works even in heavily polluted, highly bioactive seawater, and it produces very pure water. Against membranes it can absorb low-grade heat from power plants and other factories, tolerates poor feedwater and offers large unit capacity.
Dew point evaporation is newer and aimed at brackish water as well as seawater. It humidifies and dehumidifies a carrier gas while recovering the heat of condensation, and its heat transfer efficiency is controlled on the gas-mixture side. With air as the carrier, the dehumidification and humidification steps are coupled through heat transfer, so the latent heat of condensation passes directly to the evaporation chamber and supplies the latent heat of vaporisation for the brine. Two humidification/dehumidification-coupled units with effective heat transfer areas of 9.6 m2 and 2.75 m2 were built, together with the experimental apparatus and computer data acquisition systems, and the basic process, parameter correlation and enhanced heat and mass transfer experiments were completed.
Freeze Desalination: Physics That Pays Off
Seawater has a triple point at which vapour, liquid and solid coexist in equilibrium. Push pressure or temperature away from it and the equilibrium breaks, with the phases tending back toward one or two. Vacuum freeze desalination exploits exactly this: water itself acts as the refrigerant, seawater evaporates and freezes at the same time, and the ice crystals are separated and washed to give desalinated water. Against distillation and membranes, freezing consumes less energy, causes less corrosion and scaling, needs simple pretreatment and small equipment investment, and can handle high-salinity seawater. RSE technology, as one commercial variant is called, reports recovery efficiency up to 95% against 35% for conventional processes, investment at one quarter of distillation or reverse osmosis, and greatly reduced operation and maintenance cost.
What the Chinese Build-Out Numbers Show
China's cumulative desalination capacity, counting projects built and under construction, is about 600,000 tons/day, and policy planning assumes the market will grow more than fivefold within ten years. Before the 21st century, reverse osmosis membrane technology was monopolised abroad, and domestic production was not mastered until the late 1990s — which is why the early programme was built around imported composite membranes and spiral-wound elements. The cost of desalinated seawater has fallen to 4-5 CNY/ton, which is what turned the economics from theoretical to bankable. Research began in 1958 on electrodialysis; in 1967-1969 the State Science and Technology Commission and the State Oceanic Administration (SOA) ran a national campaign whose main force established the country's first desalination research office in Hangzhou. Shandong College of Oceanology studied CA asymmetric RO membranes first in 1965, and moved to hollow-fibre and spiral-wound elements in the 1960s and 1970s, industrialising them early.
The equipment record is instructive. In the 1960s the Shanghai 704 Research Institute built 5 m3/d vapour compression units and flash units using diesel jacket-water waste heat for shipboard duty. In the 1970s and early 1980s, pilot work on 100-ton/day MSF produced design parameters that later projects reused. After the 1980s the Tianjin institute developed 30 m3/d vapour compression units, including a vertical-tube atmospheric unit, a horizontal-tube negative-pressure unit running at 72 C, and a 30 m3/d OTE/VC unit.
Two demonstration plants explain the cost curve. In 2000, with support from the Ministry of Science and Technology, 1,000-ton/day RO plants were built simultaneously on Changdao Island and at Shengsi, pioneering work-exchange energy recovery that cut production energy to 4 kWh/ton. In 2002, under an NDRC industrialisation project, a 10,000-ton/day RO plant followed at Rongcheng, with equipment investment substantially reduced. The R&D line continued through the Five-Year Plans, and salt rejection improved from 99.2% to over 99.7% during the 11th Plan period, when designed capacity reached 600,000 cubic meters/day.
The gap that remains is not in physics. It is in equipment manufacturing, system integration and the ability to build large plants independently, which is why policy documents keep returning to membranes, membrane materials and key equipment rather than to new process concepts.