What is a watermaker pump and how does it work?
A watermaker pump is a high-pressure positive-displacement piston pump that forces salt water against a semi-permeable membrane to separate salt from water through the reverse osmosis process. The operating pressure is 55–80 bar for seawater (TDS up to 35,000 ppm) and 15–30 bar for brackish water (TDS 1,500–8,000 ppm). Hawk Pumps produces pumps dedicated to the desalination sector with anti-corrosion materials – such as AISI 316L stainless steel and EcoBrass® – designed to resist chlorides and operate continuously, both on board vessels and in civil and industrial plants.
What is a desalinator and what is it used for
A desalinator (also known as a watermaker in the marine sector) is a system that transforms salt water (sea or brackish) into fresh drinking or technical water, through the reverse osmosis process. It is used in three main contexts:
- Marine and yachting sector: to produce fresh water on board during navigation.
- Civil applications: hotels, resorts, islands, and coastal locations without access to a reliable water network.
- Industrial applications: production of technical water for processes requiring low conductivity (cooling, boiler feed, pharmaceutical washing).
The high-pressure pump is the heart of a desalinator: without the correct pressure, the membranes cannot separate the salts from the water and the system does not produce permeate. For this reason, the choice of the right pump is the most critical factor in the design phase.
How a watermaker pump works: the process step by step
The operation of a reverse osmosis desalinator unfolds in five phases, in which the pump plays a decisive role:
- Pre-filtration: the incoming salt water passes through mechanical filters that retain sand, sediment, and microplastics, protecting both the pump and the membranes.
- High-pressure pumping: a piston pump increases the water pressure to the value needed to overcome the natural osmotic pressure of the salt fluid (about 25 bar for seawater with TDS of 35,000 ppm).
- Reverse osmosis through the membranes: the pressurized water is forced against semi-permeable membranes. Water molecules pass through, while salts and contaminants are retained.
- Separation between permeate and concentrate: two distinct streams are obtained at the outlet, permeate (fresh water, 30–45% of the inlet volume) and concentrate/brine (with salts and impurities, to be discharged).
- Energy recovery (optional but increasingly common): an Energy Recovery Device (ERD) recovers part of the hydraulic energy of the concentrate and transfers it to the incoming water, reducing pump consumption by up to 60%.
The exact pressure to be reached depends on the salinity of the treated water: the more salt, the higher the osmotic pressure to overcome and therefore the higher the operating pressure required of the pump.
Typical operating pressures of a desalinator
The correct sizing of the pump starts from the chemical composition of the inlet water. Here are the reference values:
|
Type of water |
TDS (dissolved salts) |
Pump operating pressure |
Typical permeate yield |
Recommended Hawk series |
|
Mains / fresh water |
< 500 ppm |
5 – 15 bar |
75 – 85% |
Standard pumps |
|
Light brackish water |
500 – 3,000 ppm |
10 – 20 bar |
70 – 80% |
Standard pumps |
|
Brackish water |
3,000 – 10,000 ppm |
15 – 30 bar |
60 – 75% |
XLTI Chemical Series |
|
Seawater |
30,000 – 45,000 ppm |
55 – 80 bar |
30 – 45% |
NHDP Chemical, NMT-ES Series |
Hawk pumps for watermakers
Hawk Pumps has developed dedicated series for reverse osmosis and desalination plants, each optimized for a specific range of pressure, temperature, and fluid composition:
NMT-ES Stainless Steel Series
Piston pump in AISI 316L stainless steel, with the head and all internal metal components in contact with water made of stainless steel for maximum corrosion resistance. Maximum pressure 200 bar, water temperature up to 85 °C. Special seals make it compatible with aggressive chemical agents and salt water: it is the typical choice for industrial and pharmaceutical reverse osmosis plants.
NHDP Chemical Series
Head in EcoBrass®, a lead-free alloy with very high corrosion resistance. Maximum pressure 200 bar, water temperature up to 65 °C. Suitable for marine desalination, hull washing, and plants operating with chlorides.
XLTI Chemical Series
Maximum pressure 150 bar, water temperature up to 65 °C. A versatile solution for compact watermakers on board small and medium-sized vessels, and for civil plants treating brackish water.
All these series can be viewed in the reverse osmosis pumps section of the Hawk website, together with unloader valves for pressure regulation and compatible electric motor pump units.
The materials that make the difference
Salt water is one of the most aggressive fluids a pump can handle: chlorides, oxygen, and variable temperatures can destroy standard components within a few months. For this reason, in a desalinator, materials are even more important than nominal pressure. The main ones are:
- AISI 316L stainless steel: the standard for heads in contact with seawater, thanks to its resistance to chlorides.
- EcoBrass® brass: a lead-free alloy with high resistance to dezincification and corrosion, certified for use in contact with drinking water.
- Technical ceramics (aluminum oxide): used for pistons. It guarantees high surface hardness, long seal life, and resistance to cavitation.
- Special elastomers: seals and O-rings in materials compatible with high concentrations of chlorides and any chemical agents used for membrane cleaning (CIP).
How to choose the right watermaker pump
The selection of a watermaker pump is based on four technical variables:
- Flow rate required by the system (l/min or m³/h): determined by the desired permeate production and the recovery rate of the membranes.
- Operating pressure: depends on salinity, temperature, and type of membrane.
- Construction materials: chosen according to water composition and the presence of cleaning chemicals.
- Operational continuity: in many watermakers (especially on board) the pump must work 24/7, with long maintenance cycles and quick interventions.
For more on the technical parameters of piston pumps, see the Hawk guides on how to calculate a pump's flow rate and the NPSH, a parameter that prevents cavitation phenomena that are particularly critical in reverse osmosis plants.
Why choose Hawk for desalination plants
Hawk Pumps is the brand of Leuco S.p.A., an Italian company based in Reggio Emilia, specialized for over 30 years in the design and production of high-pressure piston pumps. Since 2000 its quality system has been certified UNI EN ISO 9001; since 2004 Leuco has been part of the Kärcher Group, a world leader in the cleaning sector. Hawk pumps for reverse osmosis are used worldwide on marine, civil, and industrial desalinators, and are part of a catalog that includes over 70 models of pumps and motor-pump units.
Related questions
What is the pressure of a seawater watermaker pump?
For the desalination of seawater (TDS 30,000–45,000 ppm) the operating pressure is typically 55–80 bar. The Hawk NHDP Chemical and NMT-ES pumps, with a maximum pressure of 200 bar, operate in this range with a wide safety margin.
How much fresh water does a reverse osmosis desalinator produce?
The recovery rate (ratio between water produced and water entering) varies according to salinity: 30–45% for seawater, 60–75% for brackish water, up to 85% for water with low conductivity.
What materials are needed for a pump working with seawater?
The standard materials are AISI 316L stainless steel, EcoBrass® brass for the heads, technical ceramics for the pistons, and special elastomers for the seals, all resistant to the corrosive action of chlorides.
Are Hawk watermaker pumps suitable for continuous 24/7 use?
Yes. The NMT-ES, NHDP Chemical, and XLTI Chemical series are designed for continuous operation on board vessels and in civil and industrial plants, with extended maintenance cycles and quick access to wear-prone components. For a technical evaluation, you can contact the Hawk Pumps team.