Water Softeners and Reverse Osmosis for Boiler Feedwater
Most boilers are fed through a softener, which removes the hardness that forms scale. Where the raw water is high in dissolved solids, reverse osmosis is added to remove most of everything else. The choice shows up directly in how much water and heat the boiler loses to blowdown.
You'll learn
- Explain how an ion exchange softener works and is regenerated
- Explain what reverse osmosis removes
- Compare their effect on hardness, TDS and blowdown
- Know when demineralization is needed
Before this: Raw water quality
Two different jobs
A softener is a vessel of resin beads that grab calcium and magnesium from the water and release sodium in their place. Sodium salts do not form scale, so softened water protects the boiler from hard scale. But the total amount of dissolved salt hardly changes.
Reverse osmosis (RO) pushes water under pressure through a membrane fine enough to hold back most dissolved salts. It produces water with a small fraction of the original dissolved solids, and a reject stream that carries the rest away.
How each works in practice
- Softener service: water flows down through the resin; hardness ions are exchanged for sodium until the resin is exhausted.
- Regeneration: brine (salt solution) is passed through the resin, which releases the hardness and takes up sodium again; the bed is rinsed and returned to service. Duplex softeners regenerate one bed while the other works.
- RO: water is filtered, often softened or dosed to protect the membranes, then pumped at high pressure through spiral membranes. Typically a quarter or more of the water is rejected to drain, depending on the system.
- A shell boiler held at 3,000 ppm TDS. Softened borewell feed at 600 ppm, or RO feed at 50 ppm.
- Blowdown as a percentage of evaporation:
- Softened feed: 25.0%. RO feed: 1.7%.
Choosing, and what can go wrong
Softening alone suits low-TDS raw water and low-pressure shell boilers. Where the raw water is high in TDS, RO after softening often pays for itself in reduced blowdown and fuel. Higher-pressure water tube boilers, and any boiler feeding a turbine, need demineralized water, produced by ion exchange that removes both cations and anions, often after RO, because silica and dissolved solids that a shell boiler tolerates deposit on high-heat-flux tubes and carry over to turbines.
- Hardness slip: a softener run past exhaustion, or regenerated with too little salt, passes hard water. Test softener outlet hardness every shift.
- Resin fouling: iron and organic matter coat resin and cut capacity. Pre-filtration and periodic resin cleaning prevent it.
- Membrane scaling and fouling: RO membranes clog if pretreatment fails; rising feed pressure and falling output are the early signs.
- RO permeate is aggressive: very pure water dissolves CO₂ and is slightly acidic. Feedwater pH has to be corrected to protect the feed system.
Key takeaways
- Softeners swap hardness for sodium: no scale, but TDS unchanged.
- RO removes most dissolved solids, with a reject stream.
- Feed TDS drives blowdown: 25% on softened 600 ppm water, about 2% on RO water, at 3,000 ppm boiler TDS.
- High-pressure and turbine boilers need demineralized water.