Filtration-based and thermal processes dominate the field; data-driven studies cluster around RO, MSF, MED, electrodialysis (ED) and emerging solar/hybrid methods, with capacitive deionization (CDI) treated as a niche [S3]. A separate Japanese electrodialysis route that concentrates seawater for co-production of potable water and salt has been documented since the early 2000s [S2].
Process Map: Where the Five Routes Sit
RO uses semipermeable membranes to separate dissolved salts from feed water at ambient-to-moderate pressure, typically 55–80 bar for seawater, recovering 35–50% of feed as permeate in seawater plants [S5]. MSF and MED both evaporate then condense seawater, with MSF running at top brine temperatures of 90–115 °C and MED at 55–75 °C; both tolerate the high-fouling, high-TDS feed streams that choke membranes [S5].
Electrodialysis (ED) moves ions through selective membranes under an applied DC field, which makes it more competitive at low-to-moderate salinity (brackish water, 1–10 g/L) and well suited to the Japanese dual-purpose scheme where concentrate is sent to salt crystallization and diluate is partially desalinated for further polishing [S2]. Capacitive deionization (CDI) sits in the brackish niche as well, with energy draw reported to drop as feed salinity falls, but the technology remains pre-commercial for full-scale seawater service [S3].
Decision Criteria: Energy, Feed, Recovery, Footprint
Seawater RO is specified when feed TDS is 30–45 g/L, energy cost is moderate (3–6 kWh/m³ of product, including intake and brine discharge), and space is constrained; thermal plants (MSF, MED) are specified when feed is heavily fouled, co-generation steam is available, and unit water cost is dominated by fuel rather than membrane replacement [S5]. For brackish feeds at 1–5 g/L, ED and CDI become competitive on specific energy consumption, even though their membrane/electrode capex is higher per m³/d of capacity [S2][S3].
Emerging Routes: Hydrate, Solar, Dual-Purpose ED
Clathrate-hydrate desalination has been known since 1942 but remains pre-commercial: hydrate formation typically needs 2–6 °C and elevated pressure (3–10 MPa, depending on promoter), the per-cycle energy budget is high versus RO, and engineering challenges around guest-molecule selection and separation kinetics still block plant-scale rollout, as documented in the 2022 review of hydrate-based desalination [S1].
The Japanese dual-purpose ED plant concept continues to be cited as a path to lower unit cost by selling both diluate (after further polishing) and crystallized salt, though the diluate salinity is too high for direct potable use without a second pass [S2].
Instrumentation, Controls and Process Reality
RO and thermal plants both run on dense instrument networks: pressure transmitters monitor high-pressure pump discharge and inter-stage pressure, flow meters verify permeate and brine flows against mass balance, and industrial valves handle the frequent backflush, CIP and isolation cycles that membrane systems demand. Anti-fouling control, often the limiting factor in plant uptime, is largely a sensor-and-automation problem: high-pressure differential pressure feedback and conductivity on each pressure vessel drive recovery-setpoint decisions and CIP trigger thresholds [S5].
Limits, Failure Modes, Standards Anchors
RO membranes are vulnerable to free chlorine (typically <0.1 mg/L tolerance for polyamide), scaling above LSI 0 (CaCO₃) and biofouling; MSF/MED are vulnerable to tube corrosion in the evaporator section, which is why titanium or 90/10 Cu-Ni tubes are standard in high-TDS, high-temperature cells [S5]. ED stacks degrade with organic fouling and scaling on the anion-exchange membrane face, and CDI electrodes lose capacitance over 10⁴–10⁵ cycles in accelerated lab tests, which is a major reason seawater-scale CDI is still not commercial [S3].
Hydrate-based desalination is gated by the lack of a continuous-flow reactor design that can keep pressure and temperature stable while separating hydrate crystals from concentrated brine, plus the absence of an accepted design standard for hydrate promoter dosing [S1]. The Japanese ED/salt dual-purpose scheme, while elegant, only makes sense where a salt market exists and the diluate is polished — typically with a downstream RO pass — to bring it under the WHO 1,000 mg/L TDS guideline [S2].
Who It's For — and Who It Isn't
Seawater RO fits municipal supply in coastal regions with moderate energy cost and discharge-outfall capacity; MSF/MED fit large co-generation plants where low-grade waste steam is the cheapest heat source, particularly in the Gulf. ED and CDI fit brackish inland plants and industrial reuse, and dual-purpose ED fits a narrow slice of sites with both a salt market and downstream polishing train. Hydrate-based and solar-driven routes are not yet drop-in for any of these — they remain pilot or pre-commercial as of 2026, per the most recent process reviews [S1][S5].
For sourcing-side context on the membrane and packaged-plant side, see the supplier map in Desalination Suppliers 2026: RO/Marine Price Bands, MOQ and Audited Tiers and the steel/alloy tube angle in Steel Production Technology: BOF/EAF, Continuous Casting and Pipe-Line Process Map.
The process map is fairly stable through 2026: RO keeps gaining capacity share where electricity is cheap, thermal plants hold their co-gen positions, ED grows in brackish reuse, and the next verifiable shift to watch is whether any hydrate or solar-evaporation pilot crosses 10,000 m³/d of continuous nameplate capacity — that is the threshold most review papers use to mark the move from lab to commercial [S1][S5].