Boiler feed water treatment: RO, ion exchange and demineralisation compared

Boiler feed water treatment protects the most expensive equipment on an industrial site. Hardness forms scale on the tubes, silica carries over to the turbine, and dissolved oxygen and carbon dioxide corrode the system. The right treatment depends mainly on one thing: the boiler pressure. The higher the pressure, the purer the water has to be.

What has to be removed, and why

• Hardness (calcium and magnesium): forms scale on heat-transfer surfaces, which insulates the tubes and leads to overheating and failure.
• Silica: forms hard scale and, at higher pressures, volatilises with the steam and deposits on turbine blades.
• Alkalinity: breaks down into carbon dioxide in the boiler, which makes the condensate acidic and corrodes the return lines.
• Dissolved solids: raise the conductivity of the boiler water, forcing more blowdown and wasting water, heat and chemicals.
• Dissolved oxygen: causes pitting corrosion. It is removed by deaeration and an oxygen scavenger, after the treatment plant.

Quality targets by boiler pressure

Feed water limits come from the boiler manufacturer and from guidelines such as the ASME consensus tables and EN 12952-12 (water-tube boilers) or EN 12953-10 (shell boilers). As a broad guide:

Boiler type	Typical feed water treatment	Typical target
Low-pressure shell boilers (up to about 20 bar)	Softening, often with dealkalisation or RO	Hardness close to zero
Medium-pressure water-tube boilers (about 20–60 bar)	RO, often followed by softening or ion exchange polishing	Low conductivity and controlled silica
High-pressure boilers, HRSGs and power plants (above about 60 bar)	Full demineralisation: RO + EDI, or RO + mixed bed	Conductivity below about 0.2 µS/cm, silica below about 10–20 µg/l

Always design to the boiler manufacturer’s limits; these figures are only a starting point.

The treatment options compared

Technology	What it removes	Chemicals	Best suited to
Softening (ion exchange, sodium form)	Hardness only	Salt for regeneration	Low-pressure boilers
Dealkalisation	Alkalinity	Acid or salt	Low-pressure boilers with high-alkalinity water
Reverse osmosis (single pass)	About 98–99% of dissolved salts, silica and organics	Antiscalant, cleaning chemicals	Medium pressure, or as the first stage of demineralisation
Two-pass RO	A further 90–98% of what passes the first pass	Antiscalant, caustic between passes	Feed to EDI, or high purity without regenerant chemicals
Ion exchange demineralisation (cation + anion)	Almost all ions, including silica	Acid and caustic for regeneration	Low-TDS feed water, or polishing after RO
Mixed bed ion exchange	Final polishing to very high purity	Acid and caustic	Polishing after RO or demineralisation
EDI (electrodeionisation)	Final polishing, continuously	None (electricity)	Polishing after RO, where regenerant chemicals are unwanted

RO or ion exchange?

For many years the standard demineralisation train was cation and anion ion exchange with a mixed bed. Today most new plants put RO first:

• RO removes most of the salt before the ion exchange, so the resin is regenerated far less often. Acid and caustic use falls roughly in proportion to the salt removed.
• RO also removes organics and colloids that foul anion resin.
• RO + EDI removes the regenerant chemicals completely, which matters on sites that do not want to store and handle acid and caustic.

Ion exchange alone can still be the cheaper option where the feed water is very low in dissolved solids, because regeneration is infrequent and there is no RO concentrate to dispose of. As a rule of thumb, the higher the feed water TDS and the higher the price of acid and caustic, the stronger the case for RO. The decision should be made on life-cycle cost, including water lost as RO concentrate.

Common problems with RO in boiler feed systems

RO is the workhorse of modern boiler feed treatment, and most of its problems are predictable:

• Silica scaling. Recovery is often pushed high to save water, and silica is usually the limit. It forms scale that is very hard to clean, so the antiscalant and recovery must be set for the worst-case feed water and temperature.
• Carbon dioxide passes the membrane. Dissolved CO₂ goes straight through RO and loads the EDI or mixed bed. Common fixes are caustic dosing between the passes (converting CO₂ to bicarbonate, which the second pass rejects) or a membrane degasser.
• Fouling from pretreatment. Surface and recycled waters bring biological and organic fouling. Watch normalized differential pressure on the first stage.
• Cleaning on a calendar. Cleaning too late lets fouling harden; cleaning too often wears the membranes. Clean on a trigger, and measure the recovery after each clean.
• Reading raw data. Permeate flow and conductivity move with temperature, which can change a lot through the year on an industrial site. Normalization separates real membrane changes from seasonal ones.

Beyond the treatment plant

The water treatment plant is only part of the system:

• Deaeration removes oxygen and carbon dioxide before the feed pump, with an oxygen scavenger to remove the rest.
• Condensate return saves water, heat and treatment, but contaminated condensate can bring oil, iron or hardness back to the boiler. High-pressure plants often use condensate polishing.
• Boiler water conditioning (for example phosphate or all-volatile treatment) controls pH and any residual hardness inside the boiler.
• Blowdown removes the solids that concentrate in the boiler water. Better feed water means less blowdown, which saves water, heat and chemicals.

Getting it right

When designing a new boiler feed system, start with a full analysis of the raw water across the seasons, the boiler manufacturer’s limits and the cost of chemicals and water on the site. Then compare options on life-cycle cost. Our industrial water treatment work covers design review, two-pass RO and demineralisation, and troubleshooting existing plants.

For RO plants already running, RO Insight™ monitors industrial RO systems, including single-stage, two-stage and two-pass plants. It normalizes performance for each stage, tracks silica and other scaling risks, and flags fouling and cleaning needs before they reach the boiler.