Desalination Plant Equipment: SWRO vs Thermal, Materials & Selection

Quick Answer

Desalination separates fresh water from seawater or brackish water by two fundamentally different routes. Membrane desalination — seawater reverse osmosis (SWRO) — forces feed water through semi-permeable membranes at pressures of roughly 55–80 bar for seawater, and is now the dominant technology worldwide because its energy consumption (typically 3–4 kWh/m³ with energy recovery) is far below thermal alternatives. Thermal desalination — multi-stage flash (MSF) and multi-effect distillation (MED) — evaporates and recondenses water, tolerating poor feed quality and producing very high purity distillate, but consuming considerably more energy; it remains significant in the Gulf where it is coupled to power generation in cogeneration plants. The defining engineering challenge in either process is materials: seawater at elevated temperature with high chloride content attacks conventional stainless steel through pitting, crevice corrosion, and chloride stress corrosion cracking, which is why duplex 2205 and especially super duplex 2507 (PREN ≥40), 6% molybdenum super austenitics, titanium, and copper-nickel alloys dominate wetted components. Key equipment includes intake screens and pretreatment filtration, high-pressure pumps and energy recovery devices, membrane pressure vessels, thermal-side heat exchangers and evaporator tubes, and brine handling systems — with PREN ≥40 the accepted threshold for warm seawater service.

Across the Middle East, North Africa, and increasingly Central Asia and the Mediterranean, desalination has moved from a specialist technology to core water infrastructure. Egypt is building desalination capacity alongside its industrial expansion; Saudi Arabia operates some of the world's largest plants; the UAE, Qatar, Oman, and Kuwait depend on it for the majority of municipal supply; and Israel, Spain, and Algeria run major SWRO facilities.

For equipment suppliers, desalination has one characteristic that dominates everything else: it is the most aggressive corrosion environment in routine industrial service. Seawater combines high chloride concentration, dissolved oxygen, elevated temperature, and — in thermal plants — concentration of those chlorides through evaporation. Materials that perform perfectly well in most process plants fail quickly here. A 316L component in warm seawater will pit; put it under stress and it will crack.

This is why desalination projects specify duplex, super duplex, 6% moly super austenitics, titanium, and copper-nickel far more heavily than most other industries, and why material substitution to save cost is an unusually bad idea in this sector. The saving is small; the failure is expensive and often occurs in components that are difficult to isolate and replace.

For water sector engineers, EPC contractors, and procurement teams — this guide covers desalination plant equipment: the two main process routes, the equipment in each, materials selection, and the specification issues that matter.

For the materials context, see Duplex & Super Duplex Pipe.

The Two Process Routes

Seawater Reverse Osmosis (SWRO)

Feed water is pressurised above its osmotic pressure and forced through semi-permeable membranes that pass water and reject dissolved salts.

Operating pressure: roughly 55–80 bar for seawater (brackish water RO operates far lower, typically 10–25 bar)

Recovery: typically 35–50% for seawater — meaning most of the feed leaves as concentrated brine

Energy: approximately 3–4 kWh/m³ with modern energy recovery, the lowest of any desalination route

Advantages:

• Lowest energy consumption, and therefore lowest operating cost in most cases
• Modular — capacity added in trains
• Lower capital cost per m³ than thermal for most sizes
• No thermal energy required, so it does not need to be co-located with a power plant

Limitations:

• Sensitive to feed water quality — requires substantial pretreatment
• Membranes are consumables requiring periodic replacement
• Vulnerable to biofouling, scaling, and organic fouling
• Product water requires post-treatment remineralisation

Thermal Desalination: MSF and MED

Multi-Stage Flash (MSF) — heated seawater flashes to steam in a series of chambers at successively lower pressures; the vapour condenses on tube bundles as product water.

Multi-Effect Distillation (MED) — seawater is sprayed onto heated tube surfaces and evaporates; the vapour produced in each effect provides the heat for the next, at progressively lower pressure. MED generally operates at lower top brine temperature than MSF and is more energy-efficient.

Advantages:

• Tolerates poor and variable feed quality, including algal blooms and high turbidity that shut down SWRO plants
• Produces very high purity distillate
• Robust and long-lived
• Efficient when coupled with power generation using waste or low-pressure steam

Limitations:

• Much higher energy consumption than SWRO
• Higher capital cost and larger footprint
• Best economics require co-location with a power plant
• Scaling control limits top brine temperature

Where each fits: SWRO dominates new standalone capacity worldwide on energy cost. Thermal remains significant in the Gulf, where cogeneration with power plants, challenging Gulf seawater (high salinity, high temperature, periodic algal blooms), and existing infrastructure support it. Hybrid plants combining both are common.

Plant Equipment by Section

Intake and Screening

• Open intake (offshore or shoreline) or subsurface intake (beach wells, which provide naturally filtered feed)
• Coarse and fine screens, travelling band screens, drum screens
• Intake pumps — large low-head pumps handling seawater
• Intake piping — commonly GRP/FRP, HDPE, or coated concrete for large diameters
• Chlorination or alternative biofouling control at the intake

Pretreatment (Critical for SWRO)

Membrane life and plant availability depend on pretreatment more than on the membranes themselves.

• Coagulation and flocculation dosing systems
• Dissolved air flotation (DAF) — increasingly standard where algal blooms occur
• Media filtration — dual media or multimedia gravity/pressure filters
• Ultrafiltration (UF) membranes — now widely specified in place of, or after, media filtration
• Cartridge filters — final barrier, typically 5 µm, protecting the high-pressure pumps and membranes
• Chemical dosing — antiscalant, acid, sodium bisulphite (dechlorination before membranes), biocide
• Backwash and CIP (clean-in-place) systems

See Industrial Strainers for filtration principles.

High-Pressure System (SWRO)

• High-pressure pumps — multistage centrifugal or positive displacement, delivering 55–80 bar; the single largest energy consumer in the plant. Materials are typically duplex or super duplex — see Centrifugal Pumps
• Energy recovery devices (ERD) — isobaric pressure exchangers or turbochargers that transfer pressure energy from the reject brine to the incoming feed, recovering the great majority of that energy. ERDs are the single reason modern SWRO energy consumption fell to 3–4 kWh/m³
• Booster pumps
• High-pressure piping, valves, and fittings — super duplex is the standard here

Membrane Trains

• Pressure vessels (membrane housings) — FRP, typically holding 6–8 elements in series
• Spiral-wound SWRO membrane elements
• Manifolds, interconnectors, and racks
• Instrumentation — pressure, flow, conductivity per stage

Thermal Plant Equipment (MSF/MED)

• Evaporator shells and flash chambers
• Tube bundles — the largest materials cost; titanium, copper-nickel (90/10 and 70/30), or super austenitic depending on the stage temperature and water chemistry
• Brine heaters and heat rejection sections
• Condensers
• Ejectors and vacuum systems
• Demisters
• Brine recirculation pumps — large duty, corrosive service

See Heat Exchangers.

Post-Treatment

• Remineralisation — limestone contactors or lime and CO₂ dosing, since RO permeate is aggressive to distribution pipework
• pH adjustment
• Disinfection
• Product water storage and distribution pumps

Brine and Waste

• Brine outfall — diffusers designed to disperse concentrated brine and limit local salinity increase
• Brine handling piping and pumps
• Backwash and chemical waste treatment
• Increasingly, brine minimisation and concentration systems

Materials — The Core Engineering Decision

Seawater service is where material selection gets tested hardest.

The Corrosion Environment

• High chloride — drives pitting, crevice corrosion, and chloride stress corrosion cracking
• Dissolved oxygen — sustains the cathodic reaction
• Elevated temperature — Gulf seawater is warm year-round, and thermal plants raise it further; chloride SCC risk rises sharply with temperature
• Concentration — brine on the reject side is substantially more concentrated than feed
• Biological activity — biofilms create differential aeration and localised attack

PREN and Material Ranking

PREN = %Cr + 3.3(%Mo) + 16(%N). A PREN of at least 40 is the accepted threshold for good pitting and crevice corrosion resistance in warm seawater.

The high-pressure section is where super duplex earns its cost. Pressures of 55–80 bar combined with concentrated seawater rule out anything less.

For material detail including welding requirements, see Duplex & Super Duplex Pipe.

Fabrication Cautions

• Duplex and super duplex welding requires ER2209/ER2594 filler, nitrogen-bearing shielding gas, controlled interpass temperature, and ferrite verification — poor welding creates the weak point in an otherwise correct material selection
• Pickling and passivation after fabrication is essential; heat tint and embedded iron are pitting initiation sites in seawater
• Crevices are the enemy — flange faces, gaskets, threaded connections, and under deposits. Crevice corrosion resistance, not just pitting resistance, drives selection
• Avoid dissimilar metal contact without isolation; galvanic effects in seawater are severe

Selection and Specification

1. Feed water source and analysis — salinity, temperature range, turbidity, algal bloom history, organic content, and how they vary seasonally
2. Product capacity and quality required
3. Process route — SWRO, MED, MSF, or hybrid, based on energy cost, feed quality, and whether co-generation steam is available
4. Energy strategy — energy recovery device selection is central to SWRO economics
5. Pretreatment design — matched to the worst credible feed condition, not the average. This is where most SWRO availability problems originate
6. Materials — PREN ≥40 for warm seawater wetted parts; select by section and temperature
7. Redundancy — trains, pumps, and filtration sized so maintenance does not stop production
8. Brine discharge — diffuser design and environmental permitting
9. Post-treatment — remineralisation to protect the distribution network

Common Specification Mistakes

After 15+ years supplying materials and equipment to water and industrial projects:

Mistake 1: Using 316L in Seawater Service

316L specified for seawater piping or components to reduce cost. Pitting and crevice corrosion begin quickly, and chloride stress corrosion cracking follows under stress and temperature.

Prevention: Use PREN ≥40 materials — super duplex, 6% moly, or titanium — for warm seawater wetted duty. The cost difference is small relative to the failure.

Mistake 2: Pretreatment Designed for Average Feed Quality

SWRO pretreatment sized on typical seawater analysis. An algal bloom arrives, the membranes foul, and the plant shuts down for weeks.

Prevention: Design pretreatment for the worst credible feed condition, including bloom events. DAF and UF are specified precisely for this reason.

Mistake 3: Ignoring Crevice Corrosion

Material chosen on pitting resistance alone, with flange faces, gaskets, and threaded joints creating crevices where attack initiates.

Prevention: Assess crevice corrosion resistance, minimise crevices in design, and use appropriate gasket materials and joint design.

Mistake 4: Poor Duplex Welding and Finishing

Correct super duplex specified but welded with generic stainless procedures and left with heat tint. The welds pit and crack in service.

Prevention: Qualify duplex welding procedures properly, verify ferrite balance, and pickle and passivate after fabrication.

Mistake 5: No Energy Recovery on SWRO

High-pressure system designed without an energy recovery device. Energy consumption is far above modern benchmarks and operating cost is permanently uncompetitive.

Prevention: Include isobaric pressure exchangers or equivalent ERD in any seawater RO design.

Mistake 6: Omitting Post-Treatment Remineralisation

RO permeate sent to distribution without remineralisation. The aggressive low-alkalinity water corrodes the distribution network.

Prevention: Include limestone contactors or lime/CO₂ dosing to stabilise product water.

Mistake 7: Dissimilar Metal Contact Without Isolation

Super duplex components bolted directly to carbon steel supports or fittings in a seawater environment. Galvanic corrosion attacks the less noble metal rapidly.

Prevention: Use compatible materials, isolation kits at flanged joints, and non-metallic wear pads at supports. See Cathodic Protection for related protection of seawater structures.

Supply from Kasko Makine

Kasko Makine supplies materials and equipment for desalination, water treatment, and seawater service projects:

Piping and components:

• Super duplex (S32750, S32760) and duplex (S32205) pipe, fittings, and flanges — see Duplex & Super Duplex Pipe
• 6% molybdenum super austenitic (254 SMO / AL-6XN type) piping
• Titanium pipe and tube
• Copper-nickel 90/10 and 70/30 pipe and tube
• GRP/FRP and HDPE pipe for intake, outfall, and low-pressure service
• 316L for non-seawater and low-chloride duty
• High-pressure piping and fittings for SWRO trains

Equipment:

• High-pressure pumps and booster pumps in duplex and super duplex
• Intake and transfer pumps
• Heat exchangers — plate, shell and tube, titanium and copper-nickel tube bundles for thermal plants — see Heat Exchangers
• Pressure vessels and tanks
• Cartridge and media filter housings
• Strainers and screens — see Industrial Strainers
• Chemical dosing systems and tanks

Valves and instrumentation:

• Seawater-service valves in super duplex, titanium, and lined constructions — see Industrial Valves Guide
• High-pressure valves for SWRO trains
• Control valves and actuators
• Flanges, gaskets, and isolation kits for dissimilar metal joints — see Pipe Flanges

Corrosion protection:

• Cathodic protection anodes and components for intake structures and seawater systems

Engineering support:

• Material selection by plant section and temperature, using PREN and crevice corrosion criteria
• Duplex and super duplex welding procedure guidance
• Pickling and passivation specification
• Galvanic compatibility and isolation review
• Pump and heat exchanger selection for seawater duty
• Filtration and pretreatment component specification

Certification: EN 10204 Type 3.1/3.2 material certificates with full chemistry including nitrogen, ferrite count for duplex, ASTM G48 pitting corrosion testing, PMI, hydrostatic testing, heat traceability, third-party inspection

Logistics: Desalination materials and equipment shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Duplex and super duplex 10-18 weeks; titanium and copper-nickel 12-22 weeks; fabricated equipment by scope.

Need desalination plant equipment or materials? Send us the plant capacity and process type (SWRO, MED, MSF, hybrid), seawater analysis including salinity and temperature range, the plant section concerned (intake, pretreatment, high pressure, thermal, post-treatment), and required material grades or performance criteria to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll confirm material suitability for your chloride and temperature conditions, and provide a quotation with full certification within 48 hours.

Continue Reading: Related Guides

• Duplex & Super Duplex Pipe — PREN, chloride SCC, and welding
• Centrifugal Pumps — Pump selection and NPSH
• Heat Exchangers — Thermal plant heat transfer
• Industrial Strainers — Filtration and screening
• Industrial Valves Guide — Seawater service valves
• Cathodic Protection — Protecting seawater structures

Frequently Asked Questions

Q: What is the difference between SWRO and thermal desalination?
A: Seawater reverse osmosis (SWRO) forces feed water through semi-permeable membranes at pressures of roughly 55–80 bar, physically separating water from dissolved salts. Thermal desalination — multi-stage flash (MSF) and multi-effect distillation (MED) — evaporates water and recondenses it as distillate. SWRO is now dominant worldwide because its energy consumption, typically 3–4 kWh/m³ with modern energy recovery, is far below thermal alternatives, and because it is modular and does not require co-location with a power plant. Thermal processes tolerate poor and variable feed quality including algal blooms, produce very high purity distillate, and are robust and long-lived, but consume considerably more energy and are most economic when coupled with power generation using low-pressure steam.

Q: What materials are used in desalination plants?
A: Seawater's combination of high chloride content, dissolved oxygen, and elevated temperature makes it the most aggressive routine industrial corrosion environment, so material selection is dominated by chloride resistance measured through PREN (Pitting Resistance Equivalent Number). A PREN of at least 40 is the accepted threshold for warm seawater. Super duplex grades S32750 and S32760 (PREN ≥40) are the workhorse of SWRO high-pressure systems including pumps, piping, and valves. Duplex 2205 (PREN ~35) suits feed-side piping and structures. Six percent molybdenum super austenitics, titanium, and copper-nickel alloys are used for heat exchanger tubes and thermal plant duty. GRP/FRP and HDPE handle low-pressure intake and outfall piping. Standard 316L is inadequate for seawater service.

Q: What is an energy recovery device in SWRO?
A: An energy recovery device (ERD) captures the pressure energy in the reject brine stream leaving the membranes and transfers it to the incoming feed water, dramatically reducing the work the high-pressure pump must do. Because seawater RO recovers only around 35–50% of the feed as product water, the majority leaves as brine still at close to operating pressure — energy that would otherwise be thrown away across a control valve. Isobaric pressure exchangers, which transfer pressure directly between the two streams, recover the great majority of it. ERDs are the single main reason modern SWRO energy consumption fell to approximately 3–4 kWh/m³, and any seawater RO design without one will have permanently uncompetitive operating costs.

Q: Why is pretreatment so important in seawater reverse osmosis?
A: Membrane life and plant availability depend more on pretreatment than on the membranes themselves. SWRO membranes are sensitive to particulate, organic, and biological fouling as well as scaling, and fouled membranes mean higher pressure, lower output, more frequent cleaning, and shortened life. Pretreatment typically includes coagulation and flocculation, dissolved air flotation where algal blooms occur, media filtration or ultrafiltration membranes, cartridge filters as a final 5 µm barrier protecting the high-pressure pumps, and chemical dosing including antiscalant and dechlorination. The critical design point is that pretreatment must be sized for the worst credible feed condition — including algal bloom events — not the average analysis, since bloom events are what shut plants down.

Q: What is PREN and why does it matter for desalination?
A: PREN, the Pitting Resistance Equivalent Number, is calculated as %Cr + 3.3(%Mo) + 16(%N) and predicts an alloy's resistance to pitting and crevice corrosion in chloride environments. In desalination it is the primary material selection criterion, because seawater's chloride content attacks conventional stainless steel through pitting, crevice corrosion, and chloride stress corrosion cracking. A PREN of at least 40 is the accepted threshold for good performance in warm seawater — which is why super duplex grades S32750 and S32760 dominate high-pressure SWRO systems while 316L (PREN around 24–26) is inadequate. PREN compares chemistry only, however, so actual performance also depends on temperature, crevice geometry, welding quality, and surface condition including removal of heat tint.

Q: Why does desalination equipment need pickling and passivation?
A: Welding and fabrication leave heat tint (oxide discolouration) on stainless, duplex, and super duplex surfaces, and can embed iron particles from carbon steel tools, brushes, or contact. Both are pitting initiation sites: heat tint represents a chromium-depleted layer with reduced corrosion resistance, and embedded iron rusts and breaks down the passive film. In most industrial service this matters modestly; in seawater it matters enormously, because chloride attack exploits exactly those weak points. Pickling removes the oxide and contamination, and passivation restores the protective chromium oxide film. Fabrication discipline — dedicated stainless tools, segregated work areas, and controlled handling — prevents contamination in the first place.