Seawater is a complex mix, averaging 35 parts per thousand (PSU) salt. Six main ions make up most of it: chloride, sodium, sulfate, magnesium, calcium, and potassium. Knowing the exact chemical makeup of seawater is super important for building effective seawater desalination systems. Things like choosing the right membranes, how much pressure to use, what pre-treatment chemicals are needed, and how much energy the system will consume all depend on the specific ion levels in the water.

What’s Actually in Seawater, Chemically Speaking?

Seawater is a complex solution. It’s mostly dissolved inorganic salts, with smaller amounts of dissolved gases, nutrients, trace metals, and organic compounds. Even though the total saltiness can really change from one ocean to another, or even along different coasts, the relative amounts of the major ions stay surprisingly consistent worldwide. We call this the Marcet Principle or the Principle of Constant Proportions. Oceanographer William Dittmar first figured this out after the HMS Challenger expedition back in the 1870s.

This principle is a huge deal for designing seawater desalination. It means that while the exact amount of each ion changes with local salinity, you can predict its ratio to the total saltiness. So, engineers building reverse osmosis systems for seawater desalination can figure out things like how likely membranes are to scale, what the osmotic pressure will be, and what operating settings they need, all from just one salinity measurement. They use those established ion ratios to estimate the full ion profile.

According to Global Water Intelligence, over 20,000 desalination plants worldwide now make more than 95 million cubic meters of fresh water every day. Almost all of them use reverse osmosis technology, designed with this known seawater chemistry in mind. AMPAC USA, founded in 1989 and certified to ISO 9001:2015, has built seawater RO systems for marine, offshore, coastal industrial, and military uses in over 40 countries. We’ve got decades of real-world data on seawater composition to back us up.

Major Ion Makeup of Seawater (mg/L) by Global Region

The table below shows typical concentration ranges for the main ions in seawater, generally around 35 PSU. Areas with higher evaporation, like the Arabian Gulf and Red Sea, will have proportionally higher concentrations. Open-ocean regions are usually more stable.

Major Ion	West Asia & Middle East	North America	North Africa	Europe	South & Central America	Global Open Ocean
Chloride (Cl?)	19,000-21,000	18,500-19,500	18,800-20,000	18,600-19,600	18,500-19,400	~19,000
Sodium (Na?)	10,500-11,600	10,200-10,800	10,400-11,200	10,300-10,900	10,200-10,800	~10,500
Sulfate (SO?-?)	2,700-3,000	2,600-2,800	2,650-2,900	2,600-2,850	2,600-2,800	~2,700
Magnesium (Mg-?)	1,300-1,450	1,250-1,350	1,280-1,400	1,260-1,360	1,250-1,340	~1,300
Calcium (Ca-?)	380-450	380-420	380-430	380-420	380-410	~400
Potassium (K?)	360-420	360-400	360-410	360-400	360-390	~380
Bicarbonate (HCO??)	120-160	110-140	115-145	110-140	110-135	~130
Bromide (Br?)	60-75	60-70	60-72	60-70	60-68	~65
Strontium (Sr-?)	7-10	7-9	7-9	7-9	7-9	~8
Boron (as B)	4-6	4-5	4-5	4-5	4-5	~4.5

Source: Water Conditioning & Purification Magazine, January 2005; UNESCO IOC Seawater Thermodynamic Standards.

A few regional notes:

• Values for West Asia and the Middle East are usually higher because of lots of evaporation in the Arabian Gulf (salinity 38,000-45,000 mg/L TDS) and Red Sea (38,000-43,000 mg/L TDS).
• North America, Europe, and South/Central America show stable open-ocean chemistry at 33,000-35,000 mg/L TDS.
• You’ll see small changes near river mouths, where polar ice melts, and in areas with a lot of freshwater runoff, like from rain or glaciers.

Trace Elements and Smaller Stuff

Besides the six main ions, seawater has dozens of trace elements, but at much lower levels. These are vital for marine life, but they also matter for desalination system design if their concentrations get high enough to cause membrane scaling or affect the quality of the treated water.

Constituent	Typical Concentration	Desalination Relevance
Fluoride (F?)	1.3 mg/L	RO systems reject about 90% of it
Silica (SiO?)	0.5-3 mg/L	Risk of concentrate scaling if you recover a lot of water
Barium (Ba-?)	0.02-0.05 mg/L	BaSO? scaling risk in the concentrated waste stream
Iron (Fe)	0.001-0.01 mg/L	Can foul membranes if it oxidizes (colloidal Fe)
Dissolved Oxygen (DO)	5-10 mg/L	Causes carbon steel parts of the system to corrode
Total Organic Carbon (TOC)	0.5-2 mg/L (open ocean)	Can lead to biological fouling; higher closer to shore
Dissolved CO?	Variable (0.1-2 mg/L)	Lowers pH; important for controlling carbonate scaling

These trace elements are why we make many pre-treatment decisions for seawater reverse osmosis. Water taken from near the shore often has much higher TOC, suspended solids, algae, and biological material, especially during harmful algal blooms. This means it needs more intense pre-treatment before it hits the RO membranes.

AMPAC USA’s seawater desalination systems include specific pre-treatment designs. We base them on a detailed analysis of the source water to keep membranes from getting fouled by these constituents.

How Seawater Composition Affects Desalination System Design

Osmotic Pressure Calculations

Osmotic pressure is the main thing that dictates how much energy an SWRO system needs. For typical seawater at 35,000 mg/L TDS, the osmotic pressure is about 27 bar (390 PSI). To push water through RO membranes against this pressure, operating pressures of 55-80 bar (800-1,160 PSI) are necessary. This depends on how much water the system recovers and the water’s temperature.

If you’re dealing with saltier feed water, like from the Arabian Gulf at 42,000-45,000 mg/L TDS, you’ll need higher operating pressures. This means bigger pumps and tougher high-pressure pipes. Less salty water, say from the Mediterranean at 38,000-39,000 mg/L TDS, allows for slightly lower operating pressures. These differences really impact the initial cost, how much energy the system uses (typically 3-5 kWh per cubic meter of treated water for SWRO with energy recovery), and which membranes you pick.

Assessing Scaling Risk

The Langelier Saturation Index (LSI) and Stiff-Davis Saturation Index (SDSI) tell us how likely the RO concentrate stream is to cause scaling. As water gets more concentrated in the RO system, usually 1.5-2 times the feed TDS in the concentrate, things like calcium carbonate, calcium sulfate, barium sulfate, and silica can build up on membrane surfaces. This causes irreversible drops in water flow.

Seawater’s calcium levels (380-450 mg/L) combined with bicarbonate alkalinity (110-160 mg/L) create a big risk of calcium carbonate scaling in the concentrate stream, especially when temperatures are high. We design antiscalant dosing programs based on the actual ion concentrations in the feed water. So, a full ion analysis is a crucial first step for any seawater RO system design.

Boron Removal Is Tricky

Boron, found in seawater at 4-6 mg/L, is a specific challenge for SWRO systems that make water for farming. WHO guidelines often set a maximum of 0.5 mg/L boron for irrigation. At seawater’s usual operating pH of 7.8-8.2, boron is mostly un-ionized boric acid (H?BO?). Standard RO membranes don’t reject it very well, only about 40-70%.

To meet WHO boron limits in the treated water, you usually need to raise the pH above 10. This changes boric acid into its ionized borate form, which RO membranes reject over 99%. Another option is a two-pass RO design, adjusting pH between passes. AMPAC USA’s SWRO systems for irrigation and city water supply always consider boron removal in their design.

Biological Fouling from Organic Stuff in Seawater

Open-ocean seawater generally has 0.5-2 mg/L TOC. But near-shore coastal intakes, especially in tropical areas or during algal blooms, can have 5-15 mg/L TOC and 10?-10? bacterial counts per milliliter. This biological load seriously risks biofouling on RO membranes. Biofilm forms, which reduces how much water can pass through the membrane and increases the pressure drop across the whole membrane system.

Good biological pre-treatment for SWRO includes coagulation, flocculation, dissolved air flotation (DAF), dual media filtration, and ultrafiltration (UF) membranes. We use chlorination followed by dechlorination (with sodium bisulfite) to control biological growth. However, we have to be careful to prevent chlorine from reaching polyamide RO membranes, as they are very sensitive to oxidation.

Seawater vs. Brackish Water: Key Differences in What’s Inside

It’s vital to know the difference between seawater and brackish water when picking and designing a system. Brackish water, usually 1,000-15,000 mg/L TDS, comes from coastal aquifers, inland lakes, and industrial processes. What’s in it varies a lot by location, and you can’t predict it using the Principle of Constant Proportions that works for seawater.

Parameter	Seawater	Brackish Water
TDS	32,000-55,000 mg/L	1,000-15,000 mg/L
Osmotic Pressure	25-45 bar	1-12 bar
RO Operating Pressure	55-80 bar	10-20 bar
Energy Consumption	3-5 kWh/m- (with ERD)	0.5-2.0 kWh/m-
Water Recovery	35-50%	65-85%
Ion Profile	Predictable (Constant Proportions)	Highly variable; site analysis required

AMPAC USA designs systems for both seawater and brackish water applications, with system specifications derived from site-specific water analysis rather than generic assumptions.

How AMPAC USA Uses Seawater Composition Data in System Design

Every AMPAC USA seawater desalination project begins with a thorough analysis of the source water chemistry. Feed water data drives all key engineering decisions:

• Membrane selection: High-rejection SWRO membranes (Dow FILMTEC SW30XLE or equivalent) are specified based on TDS, temperature, and boron requirements
• Operating pressure design: System pumps are sized to overcome feed water osmotic pressure plus hydraulic resistance at the design recovery rate
• Pre-treatment design: Coagulant type and dose, filtration media, and UF specifications are based on feed turbidity, TOC, and SDI
• Antiscalant program: Specific antiscalant formulations are selected based on the scaling potential of calcium, magnesium, sulfate, silica, and barium in the concentrate at design recovery
• Energy recovery: Pressure exchanger or turbocharger ERDs are sized to recover energy from the high-pressure concentrate for large systems

This chemistry-driven design approach, backed by AMPAC USA’s 35+ years of seawater desalination experience, delivers systems that operate at design recovery and membrane life without unexpected fouling or scaling failures. Explore our full water treatment systems product range or learn about seawater desalination applications.

Frequently Asked Questions: Seawater Composition and Desalination

What is the average salt content of seawater?

The average salinity of the world’s oceans is approximately 35 PSU (practical salinity units), equivalent to 35,000 mg/L TDS. This means that every liter of seawater contains approximately 35 grams of dissolved salts. The dominant constituents are sodium chloride (~85% of total salts by mass), followed by magnesium sulfate, calcium chloride, and potassium chloride. Regional salinity varies from as low as 30-32 PSU in sub-polar regions with significant freshwater input to 38-44 PSU in high-evaporation enclosed basins like the Arabian Gulf and Red Sea.

Why does seawater composition matter for desalination?

Seawater composition directly determines the energy requirements, membrane specifications, pre-treatment design, and scaling risk management for any desalination system. The osmotic pressure of seawater – approximately 27 bar at 35,000 mg/L TDS – sets the minimum energy requirement for reverse osmosis desalination. Ion-specific concentrations determine which antiscalant chemistry is needed, whether boron removal is required for the intended water use, and how to design the pre-treatment system to prevent membrane fouling. Every AMPAC USA seawater desalination project begins with a complete source water ion analysis to correctly engineer these parameters.

What is the Principle of Constant Proportions in seawater?

The Principle of Constant Proportions (Marcet Principle) states that despite variation in total ocean salinity from place to place, the relative proportions of the major dissolved ions remain virtually constant throughout the world’s oceans. This means that if you know the total salinity of a seawater sample, you can accurately predict the concentration of each major ion (chloride, sodium, sulfate, magnesium, calcium, potassium) using fixed ratio constants established by oceanographic research. This principle was confirmed by William Dittmar’s analysis of 77 seawater samples collected during the HMS Challenger expedition in the 1870s and remains foundational to both oceanography and desalination engineering.

How does seawater salinity affect RO membrane pressure requirements?

Osmotic pressure increases roughly linearly with TDS concentration. Standard open-ocean seawater at 35,000 mg/L TDS has an osmotic pressure of approximately 27 bar (390 PSI). To achieve net water flux through the RO membrane, operating pressure must exceed osmotic pressure – SWRO systems typically operate at 55-80 bar (800-1,160 PSI). High-salinity Arabian Gulf seawater at 42,000-45,000 mg/L TDS requires operating pressures toward the upper end of this range, increasing pump energy consumption and system capital cost compared to lower-salinity Atlantic or Pacific seawater feeds.

What pre-treatment is required for seawater reverse osmosis?

Seawater reverse osmosis requires thorough pre-treatment to achieve the Silt Density Index (SDI) below 3 and turbidity below 0.1 NTU at the membrane inlet. Typical SWRO pre-treatment includes: coagulation with ferric chloride (to aggregate fine particles and colloids), dissolved air flotation or sedimentation, dual-media pressure filtration (anthracite and silica sand), cartridge filtration (5-10 micron nominal), antiscalant injection, and dechlorination with sodium bisulfite after biocide treatment. Open intake systems with high biological load often incorporate ultrafiltration membranes as a final pre-treatment step. Proper pre-treatment is the single most important factor in achieving the 3-7 year membrane life that SWRO systems are designed for.

Does seawater composition vary in different parts of the world?

Yes, total salinity varies significantly by region, though the relative proportions of major ions remain nearly constant (Principle of Constant Proportions). The Atlantic Ocean averages 35,000-37,000 mg/L TDS, the Pacific 33,000-36,000 mg/L TDS, the Mediterranean 38,000-40,000 mg/L TDS, the Red Sea 38,000-43,000 mg/L TDS, and the Arabian Gulf 38,000-45,000 mg/L TDS. These regional differences matter significantly for SWRO system design – a system designed for Mediterranean seawater requires higher operating pressure than one designed for lower-salinity Pacific feeds, affecting energy consumption, pump sizing, and long-term operating costs.

Can reverse osmosis remove all ions from seawater?

Standard thin-film composite SWRO membranes reject 99-99.7% of monovalent ions (Na?, Cl?, K?) and 99.5-99.9% of divalent ions (Ca-?, Mg-?, SO?-?) under standard operating conditions. However, some constituents require special attention. Boron, present in seawater as neutral boric acid at typical seawater pH, is only 40-70% rejected by standard membranes – achieving WHO drinking water and irrigation limits for boron (<0.5 mg/L) requires pH adjustment or two-pass RO design. Carbon dioxide passes freely through RO membranes and must be removed by degassing or pH adjustment in the post-treatment train. AMPAC USA engineers post-treatment specifically for the end-use requirements of each seawater desalination project.

Design Your Seawater Desalination System with AMPAC USA

AMPAC USA, founded in 1989 with ISO 9001:2015 certified manufacturing, has designed and deployed seawater desalination systems for coastal municipalities, industrial facilities, offshore platforms, marine vessels, and US military operations in 40+ countries. Our engineering approach begins with complete seawater chemistry analysis and ends with a fully optimized system – from intake screening through product water post-treatment.

Whether you need a 5,000 GPD shipboard watermaker or a 500,000 GPD coastal desalination plant, AMPAC USA’s team provides complete engineering, manufacturing, installation support, and ongoing service. Contact our team for a complimentary feasibility review, or browse our water treatment product catalog and application-specific solutions.

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