Seawater reverse osmosis plants: pretreatment, membranes and post-treatment

Water stations & desalination · 24 Oct 2026

Reverse osmosis (RO) is the most common way to make fresh water from seawater in the Middle East. The membranes do the separation. Most operating problems, however, start before the membranes, in pretreatment, or after them, in post-treatment. This article walks through the three stages and the checks that matter.

Stage 1: Pretreatment

RO membranes are sensitive to particles, organic matter and biological growth. The World Health Organization (WHO) guidance on desalination notes that pretreatment removes the particles and organic matter that cause scaling and fouling.

Typical pretreatment steps

• Intake screening
• Coagulation and filtration (media or membrane filtration)
• Cartridge filters just before the high-pressure pumps
• Chemical dosing (for example antiscalant), as the design requires

The Silt Density Index (SDI)

SDI is the standard way to judge whether pretreated water is clean enough for RO. The test is described in ASTM D4189. Water is passed through a 0.45 µm membrane filter at a constant 207 kPa (30 psi), and the rate at which the filter plugs gives the index.

Common practice is to aim for an RO feed SDI below 3. Higher values point to weak pretreatment and a higher fouling risk. The membrane supplier's warranty conditions usually set the limit for a given plant.

Chlorination by-products

WHO notes that chlorine is widely used in pretreatment. In bromide-rich seawater, it forms brominated by-products. The design must therefore manage where oxidants are added, and how by-products and any effect on the membranes are controlled. Follow the membrane supplier's limits.

Stage 2: The membranes

RO is a strong barrier to microorganisms, but only while membranes and seals are intact. WHO stresses that integrity monitoring is essential.

Conductivity monitoring is the common method. WHO notes that its sensitivity limits what it can prove, at about 2 logs of removal. WHO also notes that some RO membranes can pass viruses. For that reason, RO should be followed by disinfection.

Daily operating checks

• Feed and permeate conductivity (salt rejection trend)
• Differential pressure across each stage (fouling trend)
• Normalised permeate flow (performance against design)
• Feed pH, temperature and SDI
• Cartridge filter differential pressure

Trend these numbers. A slow drift is easier to act on than a sudden failure.

Stage 3: Post-treatment

RO permeate is not ready to distribute. WHO describes desalinated water as low in minerals and poorly buffered, which makes it corrosive to pipes and tanks.

Remineralisation

Post-treatment usually includes blending and remineralisation, often with calcium carbonate (limestone). This adds hardness and alkalinity, protects the network, and can improve taste.

Without conditioning, corrosion can release iron, lead, copper and nickel from pipe and fitting materials. Materials and coatings in contact with the water should be certified for drinking water use.

Boron

RO removes boron less well than most other inorganic ions. The WHO health-based guideline value for boron is 2.4 mg/L. Some crops are sensitive at much lower levels (around 0.5 mg/L), which matters where product water is also used for irrigation.

The boron target for the product water should be agreed at the design stage, because it shapes the process design.

Blending

Some plants blend a small share of other water into the permeate. WHO notes that blended water must be treated and monitored as source water in its own right. Blending with seawater above about 1% can cause taste and corrosion problems.

Use a water safety plan

WHO recommends a water safety plan for desalination systems. It maps the whole chain, from intake to tap, and covers:

• the hazards at each step;
• the control measures for each hazard;
• the monitoring that shows each control is working;
• what to do when a limit is exceeded.

Start it at the design stage for new plants, and apply it to existing plants too.

Summary

Protect the membranes with good pretreatment, and check it with SDI. Monitor integrity and performance every day. Condition the product water so it does not corrode the network, and watch boron where irrigation or tight limits apply. Tie it all together in a water safety plan.

General guidance only. The authority having jurisdiction and the official text govern.