Boiler Makeup Water 2026: RO + Softener vs CEDI Polishing

Why Boiler Makeup Water Quality Is a Fuel-Economics Problem

A 1/16-inch scale layer on boiler heat-transfer surfaces raises fuel consumption by roughly 12%, and AMPAC attributes even higher penalties to silica: a 1/8-inch silica deposit can lift fuel use by 25% because silica forms a glass-like scale about ten times more insulating than calcium carbonate (ampac1.com, "Boiler Makeup Water Treatment Systems"). These numbers confirm that the makeup water train belongs in the fuel-budget discussion, not just the maintenance schedule. "Makeup water" is the treated fresh water added to the boiler and steam loop to replace blowdown, leaks and condensate losses, and its quality determines how much scale ever forms in the first place. This article compares the four realistic trains for industrial boiler feedwater: softener-only, RO with softener pretreatment, RO followed by CEDI polishing, and RO followed by mixed-bed ion-exchange polishing.

Water Quality Targets by Boiler Pressure

The American Boiler Manufacturers Association (ABMA) publishes water quality guidelines by operating pressure. AMPAC cites the 600 psig band as feedwater TDS below 150 ppm, hardness at zero, and silica below 0.3 ppm (ampac1.com). For boilers above 600 psi, ASME guidelines call for feedwater TDS below 0.1 ppm, which AMPAC notes requires RO followed by mixed-bed deionization, with CEDI as the modern equivalent. For low-pressure boilers under 150 psi, AMPAC reports many operators target feedwater TDS below 50 ppm, a band where a softener is still defensible for some sites. Above 200°F, sodium, bicarbonate and silica residuals concentrate rapidly and cause carryover, foaming, and silica scaling on turbine blades, so the target tightens with pressure and temperature (ampac1.com). The ABMA and ASME benchmarks make the rest of the article a pressure-graded decision rather than a feature comparison.

How RO, Softeners and CEDI Actually Work on a Boiler Skid

Reverse osmosis is crossflow filtration in which applied pressure exceeds the osmotic pressure of the feed, forcing purified water through a semipermeable membrane while salts travel with the reject (chemicalprocessing.com). AMPAC states that RO removes 95-99% of TDS in boiler-feed service, and Pure Aqua describes RO as removing "99+%" of dissolved salts (pureaqua.com). RO membranes reject silica at 85-95% depending on pH, with rejection rising at higher pH (ampac1.com). A typical boiler-feed arrangement is two-pass RO, with each stage producing about 50% permeate and 50% reject for an overall recovery near 75% (chemicalprocessing.com). A conductivity meter on the permeate line catches membrane degradation before it reaches the boiler (ampac1.com).

An industrial twin-alternating industrial water softener for RO pretreatment serves as the primary guard for the membrane train. The softener swaps calcium and magnesium for sodium on ion-exchange resin so the scaling minerals never reach the membrane, and twin-vessel operation keeps one unit in service while the other regenerates (complete-water.com, "15 Gallon Per Minute Reverse Osmosis System"). The same Complete Water Solutions project specifies a carbon filter ahead of the RO to strip chlorine, which would otherwise damage thin-film composite membranes irreversibly (complete-water.com).

CEDI is continuous electrodeionization: it polishes RO permeate to ultrapure water using electricity and ion-exchange resin, with no acid or caustic regeneration and no neutralization waste (pureaqua.com). Pure Aqua frames CEDI as the final polisher in a RO–CEDI boiler-feed train and notes that the U.S. market is moving toward RO-EDI because permits for the hazardous chemicals used in conventional ion-exchange regeneration are increasingly difficult to obtain (pureaqua.com). Mixed-bed ion exchange is the legacy polisher: it can reach the same resistivity but requires periodic acid and caustic regeneration, with tank-lining wear, chemical handling and neutralization waste that Complete Water Solutions documented in a boiler-feed DI replacement case study (complete-water.com, "Switch From DI to Reverse Osmosis For Boiler Feed Water").

RO + Softener vs CEDI: Head-to-Head Comparison

The phrasing "RO + softener vs CEDI" is misleading, because the softener sits before the RO as pretreatment and the CEDI sits after the RO as a polisher. The real comparison is RO + softener against RO + CEDI, and the deciding variable is the boiler's pressure rating. Below 600 psig, an RO + softener train is usually sufficient to meet the ABMA TDS, hardness and silica targets cited by AMPAC. Above 600 psi, the ASME band for feedwater TDS below 0.1 ppm forces a polisher, and the choice is between CEDI and mixed-bed IX (ampac1.com).

Maintenance requirements diverge significantly between these two train configurations. A softener is mechanically simple but consumes salt and produces brine. RO membranes need an antiscalant program sized to seasonal feedwater variation, periodic clean-in-place, and replacement RO membrane elements and cartridges on a multi-year cycle (chemicalprocessing.com, ampac1.com). CEDI modules are largely chemical-free but only perform if the upstream RO permeate stays within specification. Where regenerant permits are hard to obtain, the chemical-handling penalty of mixed-bed IX pushes the decision toward CEDI, and Pure Aqua explicitly cites U.S. regulatory pressure as the structural reason RO-EDI is replacing conventional ion exchange in boiler-feed skids (pureaqua.com).

Matching the Train to the Boiler

For low-pressure boilers below 150 psi, a softener remains technically acceptable for some operators, but chemicalprocessing.com notes that even with decarbonation, softened water still carries chlorides, sulfates and silica to the boiler, making RO an increasingly standard addition. For mid-pressure boilers in the 150 to 600 psig band, the modern baseline is an industrial RO system sized for boiler makeup with softener pretreatment, permeate conductivity monitoring, and blowdown rate set against the local makeup water TDS (ampac1.com). For high-pressure boilers above 600 psi, the train is RO followed by a CEDI stack for RO permeate polishing or a mixed-bed IX polisher to reach feedwater TDS below 0.1 ppm (ampac1.com, pureaqua.com).

Multi-pressure plants can use a single treatment skid with zoned distribution: fully deionized water feeds the high-pressure boilers, while softened or RO-treated water feeds the lower-pressure and domestic hot-water loops, minimizing treatment cost. The pretreatment sizing rule AMPAC states is a 5-micron sediment filter and carbon block as a minimum, a softener ahead of the RO when hardness exceeds 7 grains per gallon, and an oxidizing or iron-removal step when iron exceeds 0.1 ppm (ampac1.com). Sizing the RO itself uses peak demand plus a 20-25% buffer, with a product storage tank absorbing swing loads, and a 10,000 GPD membrane array paired with a 5,000-gallon tank can satisfy peak draws of 20,000 GPD for short periods (ampac1.com). A multi-media pre-filter protecting the RO is the standard way to drop turbidity and SDI to a level the membranes can tolerate.

Engineering Trade-offs and Common Pitfalls

Membrane fouling is the dominant failure mode for boiler-feed RO. Scale, particulate and microbiological fouling each demand their own pretreatment, and the antiscalant program must be re-checked against seasonal raw-water variation (chemicalprocessing.com). The same article pushes back on the "hungry water" objection to RO permeate: aggressive permeate has fed power-plant boilers for nearly a century, and the real threat to a well-operated train is impurities from leaking heat exchangers, not the permeate itself. Silica is the parameter most often missed: it slips past softening, and partial RO removal varies 85-95% with pH (ampac1.com). A CEDI or mixed-bed polisher is the only practical way to push silica toward the ABMA 0.3 ppm target at 600 psig and the much tighter ASME band above 600 psi. Regenerant chemistry is the structural reason CEDI is winning new boiler-feed work in the United States; Pure Aqua notes that obtaining permits for the storage and disposal of acid and caustic used in conventional ion exchange is becoming harder, driving RO-EDI into mainstream specification (pureaqua.com).

Frequently Asked Questions

What budget range should a buyer plan for an RO + softener vs RO + CEDI boiler makeup train?

Research data does not publish a current dollar figure for either train; AMPAC and Complete Water Solutions describe equipment configuration rather than pricing. A buyer should request a sized quotation based on steam output, blowdown rate and a current feedwater analysis, and ask the vendor to break out the softener, the industrial RO system sized for boiler makeup, and any CEDI polish as separate line items, because OPEX differs more than CAPEX between trains once regenerant chemicals and brine disposal are counted (pureaqua.com, complete-water.com).

How do I size a boiler makeup train and choose between CEDI and mixed-bed IX as the polisher?

Size the train to peak demand plus a 20-25% buffer, with a product storage tank sized to absorb swing loads (ampac1.com). Choose the polisher by boiler pressure and by site permitting: above 600 psi the ASME band of feedwater TDS below 0.1 ppm requires a polisher, and CEDI is preferred where acid and caustic handling is constrained, while mixed-bed IX remains an option where regenerant chemicals are already permitted and the operator has an established resin-handling program (ampac1.com, pureaqua.com).

Why is a softener no longer adequate on its own above 200°F?

Softening removes hardness but leaves dissolved sodium, bicarbonate and silica. Above 200°F these residuals concentrate in the boiler, causing carryover, foaming, and silica scaling on turbine blades, which is why AMPAC recommends RO as the primary treatment stage and only treats a softener-only configuration as defensible for low-pressure boilers under 150