Effective pre-treatment of the seawater is an essential step before the actual desalination, whether by reverse osmosis (RO) or thermal processes. The aim of this process is to improve the water quality in such a way that deposits (scaling), fouling and corrosion in the main plant are avoided. Well-planned pre-treatment extends the service life of the system components, increases operating efficiency and reduces maintenance costs. Pre-treatment comprises several coordinated steps that are adapted depending on the raw water quality and desalination process.

1. gravel and sand filtration

The first stage of pre-treatment is the removal of coarse impurities such as suspended matter, sediment and large particles. This is done by using gravel or sand filters based on the principle of mechanical filtration.

• How it works: The seawater flows through a layer of fine-grained filter material such as quartz sand or granulate. Particles remain trapped in the filter layer while the purified water is passed on.
• Objective: To prevent blockages and mechanical damage in downstream filtration or membrane systems.
• Regeneration: Regular backwashing of the filter system removes accumulated particles and restores full performance.

2. ultrafiltration (UF)

Ultrafiltration is often used as the second stage of pre-treatment. It is particularly effective in removing small particles, colloidal impurities, microorganisms and turbidity.

• Functionality: UF systems use semi-permeable membranes with pore sizes in the range of 0.01 to 0.1 micrometers. The water is pressed through these membranes, whereby unwanted particles are retained.
• Advantages:
  • High efficiency in removing microorganisms such as bacteria and algae, which reduces the risk of biofouling in the main system.
  • Reduction of turbidity and SDI values (Silt Density Index), which are crucial for the operation of reverse osmosis membranes.
• Regular cleaning: The membranes must be maintained by chemical cleaning or backwashing to prevent deposits and fouling.

3. dosing of chemicals

Chemical treatments are an integral part of pre-treatment in order to avoid specific problems such as scaling and biofouling. Dosing is carried out in precise quantities, tailored to the water quality and the desalination process used.

• Antiscalants: These chemicals inhibit the precipitation of calcium carbonate, barium sulphate or other mineral compounds that could be deposited on membranes or heat exchangers under pressure and temperature.
• Biocides: These prevent the growth of microorganisms, algae and biofilms, which can significantly impair the efficiency of the systems. Examples are oxidizing agents such as chlorine or non-oxidizing agents such as isothiazolinones.
• pH control: The pH value of the seawater is stabilized to improve the efficiency of the chemical treatment and prevent the formation of deposits.

4. degassing

Dissolved gases such as oxygen and carbon dioxide in seawater can cause corrosion and scaling in the system. Degassing is an important step in removing these gases.

• Oxygen removal: Oxygen promotes corrosion in metallic system parts. The oxygen content is reduced by chemical methods such as the addition of sodium sulphite or physical processes such as vacuum degassing.
• Carbon dioxide removal: Dissolved CO₂ increases water hardness through the formation of calcium carbonate. Degassing or the use of deacidification units helps to reduce the carbon dioxide content and thus prevent scaling.
• Technologies: In addition to chemical methods, physical processes such as membrane degassing or stripping columns are also used.