Treatment Technologies
Thermal Desalination Companies
MED, MSF, and MVC thermal desalination providers for high-salinity, high-TDS, or waste-heat integrated projects.
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Thermal Desalination Technologies: MSF, MED, and Vapour Compression for Seawater Freshwater Production
Thermal desalination processes evaporate seawater using heat energy, producing freshwater as condensed vapour while leaving behind concentrated brine. Three principal thermal technologies are in commercial operation: Multi-Stage Flash (MSF), Multiple Effect Distillation (MED), and Mechanical/Thermal Vapour Compression (MVC/TVC). Global thermal desalination capacity (2023): approximately 35 percent of world desalination capacity (versus 65 percent for membrane-based RO); predominantly in Arabian Gulf (Saudi Arabia, UAE, Kuwait, Qatar, Bahrain) where thermal energy from co-located power stations and cheap natural gas historically made thermal desalination economical. GWI Desalination Markets: total global installed thermal desalination capacity approximately 30 million m3/day; largest MSF plant: Jebel Ali Phase 2 (UAE, 636,000 m3/day); largest MED: Ras Al Khair (Saudi Arabia, 1,000,000 m3/day MED component). Product water quality from thermal desalination: TDS typically 5 to 30 mg/L (essentially distilled water); requires post-treatment remineralisation (lime/CO2 or calcite contactor) to prevent corrosion and meet WHO minimum calcium (20 mg/L) and alkalinity guidelines.
Multi-Stage Flash (MSF) distillation: seawater (feed) is heated in the brine heater by steam to the top brine temperature (TBT, typically 90 to 120 degrees C for standard MSF; low-temperature MSF at 65 to 70 degrees C to reduce scaling); heated brine passes through a series of 15 to 25 stages of progressively lower pressure; at each stage, brine partially flashes (instantly evaporates) as pressure is reduced; vapour condenses on tubes carrying incoming feed seawater, preheating the feed and producing distillate (pure condensate). Performance ratio (PR): mass of distillate produced per unit mass of steam consumed; standard MSF: 8 to 12 (kg distillate per kg steam); advanced MSF: up to 15 to 20. Specific heat consumption: 65 to 95 kWh/m3 thermal. MSF advantages: proven technology with 50+ year track record; robust to feed water quality variation; handles high-salinity seawater (greater than 50,000 mg/L TDS). Disadvantages: high thermal energy demand; very high capital cost (USD 1,500 to 2,500 per m3/day installed capacity); declining in favour of MED and RO in new projects due to cost.
Multiple Effect Distillation (MED): steam from first effect (hot, high-pressure evaporator) condenses to drive evaporation in subsequent effects (each at progressively lower temperature and pressure); long-tube vertical or horizontal tube falling film designs; 6 to 16 effects typical; TBT 55 to 75 degrees C (low-temperature MED reduces corrosion and scaling). Thermal Vapour Compression MED (TVC-MED): steam ejector or thermocompressor recycles vapour from last effect back to first effect to improve heat economy; PR of 10 to 16 for TVC-MED. MED advantages over MSF: lower TBT reduces scale formation (less anti-scalant required) and corrosion; higher PR (better energy efficiency); lower CAPEX than MSF. MED PR: 8 to 16 depending on number of effects and whether TVC applied; specific thermal energy 35 to 65 kWh/m3. Mechanical Vapour Compression (MVC): compressor (single-stage centrifugal, 7 to 25 kW/m3 electrical energy) drives vapour from evaporator, raises pressure, returns as heating steam; no thermal steam required; fully electric process; PR equivalent not applicable; specific electrical energy 7 to 12 kWh/m3; used for smaller-scale systems (100 to 5,000 m3/day) and industrial applications where electrical energy is cheaper than heat. Leading suppliers: IDE Technologies, Doosan Enpure, SUEZ, Veolia Water Technologies, ACCIONA Agua.
Frequently Asked Questions
What is multi-stage flash (MSF) desalination?
Multi-Stage Flash (MSF) is a thermal desalination process that produces freshwater by evaporating heated seawater through a series of progressively lower-pressure chambers (stages). Process: (1) Feed seawater is pre-heated in the heat recovery section by passing through tubes in each stage, gaining heat from condensing vapour; (2) In the brine heater, steam from a power plant or dedicated boiler raises the brine temperature to the top brine temperature (TBT): 90 to 110 degrees C for standard MSF, 60 to 70 degrees C for low-temperature MSF; (3) Heated brine enters the first (highest pressure) stage through a throttling orifice; the sudden pressure reduction causes partial flash evaporation; (4) Vapour rises and condenses on the tube bundle carrying the incoming feed, producing distillate (freshwater) and heating the feed; (5) Brine passes through 15 to 25 stages of decreasing temperature and pressure, flashing a fraction in each stage; (6) Distillate and concentrated brine are collected at the last stage. Product: TDS less than 20 mg/L (essentially distilled). Energy: 65 to 95 kWh/m3 thermal, 3 to 5 kWh/m3 electrical; co-located with power stations that supply steam. Scale: individual MSF units up to 75,000 m3/day; multi-unit plants up to 1,000,000 m3/day. MSF has been the dominant Gulf desalination technology since the 1960s, now being superseded by MED and SWRO in most new projects.
How does thermal desalination compare to reverse osmosis?
Thermal desalination vs reverse osmosis comparison (2024 data): Energy consumption: MSF 70 to 95 kWh/m3 thermal + 3 to 5 kWh/m3 electrical; MED 35 to 65 kWh/m3 thermal + 1.5 to 2.5 kWh/m3 electrical; SWRO 2.2 to 4.0 kWh/m3 electrical only. For equivalent electrical equivalent energy: MSF approximately 15 to 25 kWh/m3 electrical equivalent (at 25 to 35 percent power station efficiency converting thermal to electrical); MED approximately 8 to 15 kWh/m3 electrical equivalent; SWRO 2.2 to 4.0 kWh/m3 - SWRO is 3 to 5 times more energy-efficient when comparing like-for-like electrical energy. CAPEX: MSF USD 1,500 to 2,500/m3/day; MED USD 1,000 to 1,800/m3/day; SWRO USD 800 to 1,500/m3/day. Product water quality: thermal produces TDS less than 20 mg/L; SWRO produces TDS 200 to 500 mg/L; both require post-treatment. Feed water quality sensitivity: thermal desalination is less sensitive to source water quality variation (no membranes to foul); SWRO requires extensive pre-treatment and careful operation. Footprint: thermal plants are larger per unit capacity than SWRO. Market trends: new desalination projects globally are predominantly SWRO (lower energy, lower cost); Gulf region still builds MED due to available waste heat from combined power-water plants. Thermal remains preferred for very high salinity feeds or industrial applications with available waste steam.
What is the performance ratio in thermal desalination?
Performance ratio (PR) in thermal desalination is the mass of freshwater produced per unit mass of steam consumed (kg distillate / kg steam, or equivalent). PR is the primary measure of energy efficiency for thermal processes: Higher PR means lower steam consumption and better energy efficiency. Single-stage (single-effect) distillation: PR approximately 0.8 to 0.9 (one kg steam produces 0.8 to 0.9 kg distillate); most inefficient. MSF (15 to 25 stages): PR 8 to 12; more stages generally increases PR but also CAPEX. MED (8 to 16 effects): PR 8 to 16; each additional effect increases PR but increases capital cost; TVC-MED (with thermocompressor): PR 16 to 20. The theoretical maximum PR is limited by the enthalpy difference between steam and product: approximately 20 to 25 at atmospheric pressure; higher values approach thermodynamic limits. PR relates to thermal energy consumption: steam enthalpy approximately 2,260 kJ/kg; at PR 10: thermal energy = 2,260 / 10 = 226 kJ/kg distillate = 62.8 kWh per m3 distillate. For MED at PR 14: thermal energy = 2,260 / 14 = 161 kJ/kg = 44.8 kWh/m3. Gain output ratio (GOR) is used interchangeably with PR in some literature and has the same definition. Economic significance: steam at USD 2 to 4 per tonne thermal cost; at PR 10, steam cost USD 0.18 to 0.36 per m3 distillate; at PR 16 (MED-TVC): USD 0.11 to 0.23 per m3 - significant cost reduction at higher PR.
Where is thermal desalination used globally?
Thermal desalination geographic distribution (GWI 2023 data): Arabian Gulf region: approximately 60 to 65 percent of global thermal desalination capacity; Saudi Arabia (largest MSF fleet globally, Jubail, Jeddah, Yanbu complexes), UAE (Jebel Ali DEWA, Taweelah), Kuwait (Doha, Al Zour), Qatar, Bahrain; historically driven by available waste steam from co-located oil-fired power stations (cheap energy) and need to avoid seawater quality variability issues affecting early RO systems. North Africa: Libya (Benghazi, Tripoli), Algeria (coastal MSF plants); oil revenue-funded public utilities. India: limited thermal capacity; some MVC installations for coastal industries. South Korea, Japan: some MED capacity for island municipalities. Declining market share: new thermal projects are predominantly MED (lower energy than MSF) and are increasingly being supplemented or replaced by SWRO; SWRO's specific energy (2 to 4 kWh/m3) vs thermal equivalent (10 to 25 kWh/m3 electrical equivalent) makes SWRO economically preferred in most new projects. Ongoing thermal market: co-located combined water and power (CWAP) plants in Gulf states continue to build MED as marginal cost for using waste steam is very low; world's largest MED plant is Ras Al Khair (Saudi Arabia, 1,000,000 m3/day); UAE's Taweelah (909,200 m3/day SWRO) signals the shift to membrane technology for large new plants. Industrial MVC: specialist application for small-scale industrial water recovery where electrical energy is available and thermal steam is not.
A Mediterranean island resort development with no piped water grid needed to supply 3,200 m3/day of potable water and 800 m3/day of process water from seawater. The island's intermittent power grid made SWRO operation unreliable, and a co-located diesel combined heat and power (CHP) plant generating waste steam at 115 degrees C was available, making thermal desalination economically attractive as the primary process.
A 4,000 m3/day low-temperature MED unit (Veolia SIDEM, 12 effects, TBT 70 degrees C) was selected to utilise waste steam from the CHP exhaust heat recovery boiler. The MED product water (TDS 18 mg/L) was remineralised through a calcite contactor (pH adjustment to 7.8, calcium hardness 80 mg/L) before blending with a small BWRO permeate stream (1,200 m3/day at TDS 150 mg/L) for final distribution. An MSF anti-scalant (polyphosphonate, 2 mg/L in seawater feed) and an acid dosing system (H2SO4 to pH 8.2) controlled carbonate scaling on the evaporator tubes.
Combined specific energy: thermal 40 kWh/m3 from waste steam (zero fuel cost since steam was available from CHP exhaust) plus 1.8 kWh/m3 electrical. Effective energy cost USD 0.18 per m3 at USD 0.10 per kWh electrical. Product water quality consistently TDS 22 to 35 mg/L after remineralisation. The MED plant achieved 94.2 percent availability over three years of operation, significantly above the 90 percent contractual guarantee.
Questions to Ask Shortlisted Providers
- 1
What is the steam or heat source specification (pressure, temperature, and reliability) for the thermal desalination system, and what happens to plant output if steam supply drops?
Thermal desalination output is directly proportional to steam supply; a steam outage from a co-located power plant shuts the desalination unit immediately; understanding the heat source reliability and backup options is critical for water security.
- 2
What anti-scaling treatment is required for the evaporator heat transfer surfaces at our seawater composition, and what is the consequence of scaling the heat transfer tubes?
Calcium carbonate and calcium sulphate scaling on evaporator tubes is the primary operational challenge; 1 mm of scale reduces heat transfer efficiency by 10 to 20 percent; uncontrolled scaling can require expensive acid cleaning and even tube replacement within 2 to 3 years.
- 3
What is the product water TDS and remineralisation specification required to meet WHO corrosivity guidelines after distillation?
Thermal desalination produces essentially distilled water (TDS 5 to 30 mg/L) that is corrosive to distribution pipework; all thermal desalination plants require post-treatment remineralisation to achieve a Langelier Saturation Index above minus 0.3.
- 4
How does the thermal plant's performance ratio compare with SWRO at our site's energy cost, and what is the whole-life cost comparison?
At electricity prices above USD 0.12 per kWh, SWRO with ERD is typically cheaper than MED or MSF on a whole-life cost basis; only where waste steam is available at negligible marginal cost does thermal desalination remain economically competitive.
- 5
What corrosion-resistant materials are used for the evaporator tubes and structure in contact with seawater, and what is the expected tube replacement interval?
Evaporator tubes in thermal desalination must resist seawater corrosion at elevated temperature; aluminium brass (SAE 72), titanium Grade 2, or duplex stainless steel are standard; tube life depends on seawater chloride concentration, temperature, and velocity.
What Drives Cost in This Category
Waste steam from a co-located power plant at zero marginal cost makes MED OPEX GBP 0.30 to 0.70 per m3 (electrical only); dedicated boiler steam at GBP 30 to 50 per tonne makes thermal OPEX GBP 1.50 to 3.00 per m3; the energy source is the dominant OPEX variable for thermal desalination.
MSF and MED capital cost scales as capacity to the power 0.6 to 0.7; a 10,000 m3/day MED plant costs approximately USD 1,200 to 1,800 per m3/day capacity; a 100,000 m3/day plant costs approximately USD 800 to 1,200 per m3/day, reflecting economies of scale.
High-turbidity or high-biofouling seawater requires coagulation, sand filtration, and biocide treatment before the thermal plant; in extreme cases (algal bloom-prone coastlines) a media filtration pre-treatment system adds USD 80 to 200 per m3/day capacity.
Concentrated brine (50,000 to 100,000 mg/L TDS from MSF/MED) must be diluted and dispersed to prevent benthic ecological damage; marine outfall design and EA/SEPA marine licence or equivalent adds GBP 200,000 to 2 million in infrastructure and environmental assessment costs.
Key Regulations & Standards
Marine and Coastal Access Act 2009 (UK): brine discharge from desalination plants to UK coastal waters requires a Marine Licence from the MMO; OSPAR Commission recommendations on brine management apply to North Sea and North Atlantic discharge areas; environmental impact assessment of brine plume required.
Water Supply (Water Quality) Regulations 2016: thermal desalination product water must be remineralised to meet all PCVs (including calcium, alkalinity, and pH parameters) before entering public supply; DWI approval required for remineralisation chemicals (lime, calcite, carbon dioxide) under Regulation 31.
Pressure Systems Safety Regulations 2000: MSF brine heaters and MED evaporator vessels operating above 0.5 bar must be included in a written scheme of examination; periodic inspection by a competent person; operating log maintained for HSE inspection.
International Desalination Association Best Practices for desalination plants (IDA 2015 updated 2022) and WHO drinking water quality guidelines for remineralised desalinated water (calcium greater than 30 mg/L, alkalinity greater than 60 mg/L, pH 6.5 to 8.5) define operational and product quality standards where no national standard exists.