Makeup Water Treatment: Sodium Softening and Reverse Osmosis

Conscientious operation and maintenance is critical for steam-generator system reliability. Here, important details for softening and reverse osmosis systems are explained and compared

As steam generation technology and higher pressure and temperature boilers evolved in the 20th century, improved makeup-water-treatment methods became mandatory. The development of synthetic ion exchange (IX) resins represented a great step forward, with sodium softening becoming the standard for lower-pressure (<600 psi) industrial units. Softening, combined with downstream forced-draft decarbonation, removes hardness and carbon dioxide, which otherwise can generate boiler scale and corrosive condensate.

In the power industry, the maturation of reverse osmosis (RO) technology in the last third of the 20th century and continuing forward induced a move towards RO as the primary boiler-makeup demineralization process, with ion exchange for final polishing. RO’s ability to remove 99% or greater of all dissolved ions make it a logical option for makeup water treatment at many industrial facilities. This article examines important details and offers some comparisons between softening and RO, and it emphasizes that conscientious operation and maintenance of any system is critical for steam generator reliability.

Beginning boiler-water treatments

An early boiler-water-treatment method was to put sawdust or potato peels in the boiler, as these materials leached complex organic molecules that sequestered hardness ions. This method was not exceptionally scientific.

The development of synthetic ion exchange resins (Figure 1) in the 1930s revolutionized makeup water production capabilities. One of the first applications, which continues to the present, was sodium softening for lower-pressure boilers (Figure 2). These units can tolerate moderate concentrations of most dissolved ions, apart from the hardness ions, calcium and magnesium, which generate scale on boiler tubes and other internals.

Table 1 is an extract taken from the latest revision of the American Society of Mechanical Engineers (ASME) industrial boiler water guidelines [1]. The complete guidelines are available from the ASME at very reasonable cost and the author recommends that a copy should be in the library of any industrial plant with steam generators. Note the very low feedwater-hardness limits for all cases. As this author has learned from both personal observations and discussions with water treatment colleagues over four-plus decades, poor makeup-system operation and maintenance procedures at some facilities have led to large or repetitive upsets that allow deposition and scaling in boiler tubes. A common result is tube deformation and failure.

Cases are even known where plant management directed operators to bypass a malfunctioning makeup-treatment system to keep boilers online. The results were calamitous. Boiler tube failures can literally occur within days or sometimes just hours after such actions. A key takeaway from this discussion is that conscientious makeup-system operation and maintenance are critical for protecting steam generators. We will now examine developments that have significantly advanced system performance and effluent quality.

Modern developments

The fundamental exchange reactions in a sodium softener are shown in Figure 3. The “R” in R-SO₃ -Na⁺ symbolizes the organic backbone of the resin beads [3]. Note the reaction arrows that indicate greater attraction (affinity) of the dissolved cations, and especially the divalent ions — Ca²⁺ and Mg²⁺ — to the resin over sodium. Two aspects of this chemistry, one related to improved softener design and the second to advancements beyond softening, provide the foundation for much of the remaining discussion.

First, Figure 4 shows how the primary hardness ions form layers within the resin per their affinity to the exchange sites.

When the resin reaches exhaustion, magnesium will begin to break through, at which point regeneration is necessary. To reverse the reaction shown in Figure 4 and restore the resin to its sodium form, brine regeneration (normal salt concentration of 8–12% [3]) is necessary. Consider the classic softener design in Figure 5.

Both the service water and regenerant paths are from top to bottom, which represents co-current regeneration. The piping arrangement for this design is straightforward. However, during regeneration, the previously captured hardness ions cascade through the entire bed, attaching and then detaching from exchange sites as they move along. This process consumes extra regenerant, and it also may leave a “heel” of unregenerated resin at the bottom of the vessel, from which hardness ions slough off during normal operation.

Per a concept that has for years been popular in high-purity ion exchange systems, softeners designed with countercurrent regeneration features are available (Figure 6). With the addition of some extra piping and control valves, the regenerant is introduced in the opposite direction of service flow. The regeneration process is more efficient, and it does not leave the heel of exhausted resin at the outlet. Such units can easily produce water that meets the hardness limits shown in Table 1.

Regardless of whether a softener has co- or counter-current regeneration, the chemistry shown in Figure 3 is still the same. The effluent ionic concentrations basically remain unchanged, just with sodium replacing hardness, and the dissolved constituents still enter the boiler. Referring again to Table 1, lower-pressure boilers can tolerate moderate levels of most non-hardness dissolved impurities. Note, however, the low feedwater limits for iron, copper, and total organic carbon (TOC). (Impurities often enter via condensate return, which I will briefly address in the final section of this article.) The limits are in large measure designed to minimize contaminant carryover to steam. Concentrations are usually controlled through boiler blowdown, so higher impurity levels translate to increased blowdown and greater energy costs. Consequently, RO is gaining popularity for industrial makeup-water treatment. Modern RO membranes can remove 99% or more of all dissolved impurities and thus may be very attractive for some applications.

Most RO systems are of the spiral-wound design in which the membrane and spacer sheets are wrapped around a perforated central tube (Figure 7). Each assembly is known as an “element.” Purified water, also known as permeate, is produced by the process of crossflow filtration, where, as water flows along the membrane, pressure forces the water through the membrane. The permeate spirals down to the central core and exits the element. The concentrate (also called reject) flows to the next element.

RO elements are installed in series within pressure vessels (Figure 8), where the number of membranes per vessel (commonly four, five or six) and number of vessels are calculated from such variables as required throughput, feed water quality, operating temperature range and others.

The basic RO configuration is shown in Figure 9. For normal surface and ground waters, each stage will produce approximately 50% permeate and 50% reject. Thus, the overall feed-to-permeate conversion of a standard two-stage RO is 75%. Per these recovery ratios, half as many membranes are needed for the second stage as for the first stage. Membrane flux rate (gallons per square foot per day or metric equivalents) is a critical design factor for the number of membranes and pressure vessels required per application. Flux rates are influenced by feed water chemistry. Reference 4 offers additional details.

The tightly wound membrane and spacer materials in each RO element make them, and especially the lead elements, susceptible to particulate fouling. A once-common pretreatment configuration is shown in Figure 10.

Standard for RO units then and now is a set of cartridge filters (5–25µm, nominal pore size [4]) immediately upstream of the RO. The cartridge filters capture particulates that escape the pretreatment system, but they do not have the capacity to serve as stand-alone filtration for RO systems in most applications.

Another critical issue involves the feed of chlorine (or other oxidizing biocide) to plant water systems for microbiological control. Strong oxidizing agents, and in particular chlorine, will severely damage most RO membranes. The most common control method is injection of a reducing agent such as liquid sodium bisulfite (NaHSO₃) immediately upstream of the RO unit. However, some microbes can go into hibernation when sensing the oxidizer and then re-emerge and flourish when the biocide disappears. Injection of a non-oxidizing biocide downstream of the reducing agent feed has proven successful in destroying re-emergent bacteria. Alternatively, mild or selective oxidizing biocides have been developed that control microbial growth but do not attack RO membranes [2].

Relatively recently, other membrane processes have emerged, such as micro- and ultrafiltration (MF and UF) (Figure 11), which serve as an alternative to clarification and media filtration for particulate removal.

This author was directly involved with projects to replace aging clarifiers with microfilters for high-purity makeup water production. The results were immediate and impressive. For example, the frequency of cartridge filter changeouts was reduced from three weeks to three months. Unlike clarifiers, MF and UF units may be able to operate with only brief daily attention (apart from quarterly offline membrane cleanings), and that from a computer screen in a control room or laboratory. However, as with any equipment or process modifications, a thorough evaluation is necessary before changes are made. For example, if the raw water source is a river, heavy rainfall may cause a large increase in suspended solids that can potentially overwhelm MF or UF membranes.

Another increasingly important pretreatment issue involves sustainability. Either by mandate or choice, industrial plants may have to rely on alternative supplies, such as municipal wastewater treatment plant (WWTP) effluent, as the primary water source. Many WWTPs have primary and secondary treatment capabilities, but not tertiary treatment. Thus, the raw water coming to the industrial plant likely will have elevated concentrations of organic compounds, phosphate, nitrogen species and other impurities. These compounds can cause severe difficulties in all plant water systems, especially with regard to increased microbiological fouling potential. Installation of tertiary treatment systems, such as a membrane bioreactor (MBR) or a moving-bed bioreactor (MBBR), is emerging at industrial plants with WWTP effluent makeup. MBR and MBBR are much more compact than older technologies, such as activated sludge, and they produce a crystal-clear discharge.

Another critical RO issue to consider is that as the reject flows through the elements, the impurity concentration increases. For the single-pass, two-stage design with 75% recovery shown in Figure 9, the increase is four-fold at the final elements of the second stage. This effect greatly increases scaling potential. The most common deposits are carbonate, sulfate and silicate combinations with hardness ions, but others are also possible. A thorough analysis of makeup water chemistry is necessary for selection of the appropriate scale inhibitors.

Finally, various factors, and most notably temperature, can mask performance changes that might be due to fouling, scaling or membrane degradation. If undetected, the conditions will accumulate over time to cause irreversible membrane damage. Reputable RO manufacturers offer what are known as “normalization” programs, which, when RO operating data are entered, will calculate system performance relative to “normal” or start-up conditions. The program will alert operators that membrane cleaning is needed to prevent irreversible membrane damage. A rule-of-thumb suggests cleaning when normalized performance drops by 10%.

Condensate return

Producing high-quality makeup water has much less meaning if the plant recovers, but does not treat, impurity-laden condensate return from steam-fed processes. Consider the following case history.

A number of years ago, the author and a colleague visited an organicchemicals plant equipped with four 550-psig package boilers with superheaters. The steam provided energy to multiple plant heat exchangers, with return of most of the condensate to the boilers. Each of the boiler superheaters was failing every 1.5–2 years from internal deposition and subsequent tube overheating. Inspection of a discarded superheater tube bundle in a laydown yard revealed deposits of approximately 1/8 to 1/4 inches in depth.

Additional inspection revealed foam issuing from the saturated steam sample line of every boiler. Past water/steam chemistry analyses performed by an outside vendor included data showing total organic carbon (TOC) concentrations of up to 200 mg/L in the condensate return. Contrast that with the <0.5 mg/L feedwater TOC recommendation from Table 1. No treatment processes or condensate polishing systems were in place to remove these organics (five phenol derivatives) upstream of the boilers. Based on the TOC data alone, it was understandable why foam was issuing from the steam sample lines, and why deposits accumulated in the superheaters and caused overheating failures.

Given the huge number of chemical plants and the variety of intermediate and final products at these facilities, a seemingly endless combination of condensate impurities may be possible across the industry spectrum. Contaminants may range from mineral salts to acids and bases to organic compounds, and so forth.

A careful and thorough evaluation is necessary to determine what, if any, polishing methods will produce acceptable condensate for the steam generators. Ion exchange is a proven technology for common dissolved inorganic ions. Activated carbon filtration may work well for oil and large organics, but perhaps not as well for small organic compounds. Pilot testing may be appropriate to find the best solution. In some cases, treatment may not be possible, requiring the condensate to be “dumped” to the plant’s wastewater treatment facility. This scenario requires a larger makeup system to account for the water loss.

At plants with substantial condensate-return systems, corrosion may generate a significant amount of iron oxide (and perhaps other metal oxide) particulates. If left untreated, the particles will travel to the boiler where the heat flux induces precipitation as porous deposits on boiler tubes. The deposits can serve as sites for concentration of impurities such as chloride and sulfate that may induce under-deposit corrosion (UDC). Well-known chemical treatment methods exist for condensate pH control and dissolved oxygen reduction. Particulate filtration offers another control method. A notable example of the latter is condensate filtration at those power plants with air-cooled condensers. Many iron oxide particulates are generated from the large surface area of carbon steel headers and piping; particulates that would generate many porous deposits in the steam generator.

Final thoughts

Makeup water treatment is a critical unit process at many industrial plants. Operational neglect or poor maintenance practices can lead to extremely costly upsets. Makeup treatment technology has advanced significantly over the decades, and with a conscientious effort by system operators, reliable high-quality water production is possible. Not to be forgotten, however, is the potential for impurity introduction from condensate return streams. ■

Edited by Dorothy Lozowski

Acknowledgement

All figures are courtesy of the author except where noted

Disclaimer

This article offers general information and should not serve as a design specification. Every project has unique aspects that must be individually evaluated by experts from reputable water treatment and engineering firms. Also, any issues that could potentially have an environmental influence, for example, wastewater discharge from a proposed makeup, process, or cooling water treatment system, must be presented to and approved by the proper environmental regulators during the project design phase.

References

1. The American Society of Mechanical Engineers, “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers,” ASME, New York, NY, 2021.

2. Buecker, B., and Sylvester, E., Modern Makeup Water Treatment Methods for Combined Cycle, Co-Gen, and Other Energy Industries; O&M Knowledge Hub, Presentation at Powergen25, February 12, 2025, Dallas, Texas.

3. Owens, D., “Practical Principles of Ion Exchange Water Treatment,” Tall Oaks Publishing, Littleton, Colorado, 1995.

4. Byrne, W., “Reverse Osmosis: A Practical Guide for Industrial Users,” Tall Oaks Publishing, Littleton, Colorado, 2002.

 

Author

Brad Buecker currently serves as senior technical consultant with SAMCO Technologies (Email: bueckerb@samcotech.com and beakertoo@aol.com). He is also the owner of Buecker & Associates, LLC, which provides independent technical writing and marketing services. Buecker has many years of experience in or supporting the power industry, much of it in steam generation chemistry, water treatment, air quality control and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas, station. Additionally, his background includes 11 years with two engineering firms, Burns & McDonnell and Kiewit, and he spent two years as acting water/wastewater supervisor at a chemical plant. Buecker has a B.S. in chemistry from Iowa State University, with additional course work in fluid mechanics, energy and materials balances and advanced inorganic chemistry. He has authored or co-authored over 300 articles for various technical trade magazines, and he has written three books on power-plant chemistry and air-pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, CTI, and he is active with Power-Gen International, the Electric Utility & Cogeneration Chemistry Workshop, and the International Water Conference.