Operational safety, compliance with standards, and economic efficiency are among the most important decision-making factors for asset and operations managers of heating networks, energy and process plants, as well as the HVAC trade. In these systems, water is not just a medium, but a safety-critical component: Incorrect water quality causes corrosion, scale formation, performance losses, and jeopardizes plant warranties. The VDI Guideline 2035 and AGFW Worksheet FW 510 therefore define limit values for conductivity, pH value, and hardness, and require comprehensive documentation. The choice between pure cation exchangers, anion exchangers, or mixed-bed systems is a strategic decision that depends not only on technical requirements but also on Total Cost of Ownership (TCO), sustainability, and project/emergency capability.
To facilitate this decision, this article explains the functionality of various ion exchange resins, analyzes their fields of application, and describes the relevant standards. Furthermore, the advantages of regenerable multi-use resins and mobile trailer systems, which can provide large quantities of ultrapure water at short notice during revisions or outages, are discussed. The aim is to provide technical background for investment and operational decisions while promoting a sustainable approach to water treatment.
Ion exchange is an electrochemical process in which dissolved ions are exchanged for other ions of the same charge. The basic material consists of so-called ion exchange resins – macromolecular synthetic resins with functional groups that can attract and release ions. While cation exchange resins bind positively charged ions (cations) such as calcium, magnesium, or sodium, anion exchange resins remove negatively charged ions (anions) such as chloride, sulfate, or nitrate. The resins are structured as small, porous beads and have a large active surface area that enables the exchange process.
As the name suggests, the resin of a cation exchanger attracts positively charged ions from the water and exchanges them for others. For example, during softening, calcium ions can be replaced by sodium or hydrogen ions. A typical cation exchanger consists of a strong acid resin based on polystyrene, whose functional groups (sulfonate groups) are negatively charged and can therefore bind cations. There are two main modes of operation:
Cation exchangers can be strong acid (for all pH ranges) or weak acid. In heating water treatment, strong acid cation exchange resins are used because they reliably bind all cations even at a neutral pH value. A completely exhausted cation exchange resin can be restored to its functional state by regeneration with hydrochloric acid or sodium chloride.
The counterpart to the cation exchanger is the anion exchanger, whose resin attracts negative ions and exchanges them for other anions. This macromolecular synthetic resin is capable of absorbing negative ions from the water and subsequently exchanging them for others. In demineralization, the anion exchanger is usually regenerated with a strong base (e.g., caustic soda) and converted into the hydroxide form, so that water (H₂O) is produced during the exchange process. Similar to cation exchangers, there are strong base and weak base resins; strong base types are necessary for the production of ultrapure water.
Mixed-bed ion exchangers combine the functions of cation and anion exchangers in a single vessel. The resin consists of a mixture of strong acid cation exchange resin and strong base anion exchange resin in a ratio of approximately 40% to 60%. This mixture allows for the removal of both cations and anions from the water, producing demineralized or highly pure water with very low conductivity. Mixed-bed filters often serve as "polishing filters" after reverse osmosis systems; they eliminate residual ionic impurities and ensure conductivities of < 0.2 µS/cm.
The exchange process in a mixed bed is chaotic: the resin beads are randomly distributed, so cations and anions are bound alternately. This immediately neutralizes each ion pair. This differs from two-stage demineralization, where the water first passes through a cation exchanger and then an anion exchanger.
The most important application area for cation exchangers is softening. By exchanging calcium and magnesium ions for sodium ions, the hardness content of the water is significantly reduced. Softened water prevents scale formation and extends the lifespan of pipelines, heat exchangers, and fittings. In heating systems, softened water can only be used for high-salt operation (100 – 1,500 µS/cm) according to VDI 2035, because the increase in sodium load raises electrical conductivity and necessitates additional conditioning agents.
For the production of demineralized water (DI water), a single cation exchange stage is not sufficient. As in a mixed-bed filter, cation exchangers in H⁺ mode work in conjunction with a downstream anion exchanger. This is the only way to remove anions and produce water that contains virtually no ions. In heating water make-up and district heating systems, DI water is required according to VDI 2035 and AGFW FW 510 to prevent corrosion.
The exchange capacity of a cation exchange resin is limited. It is determined by the number of functional groups that can bind cations. Over time, the binding sites become saturated; calcium or magnesium ions occupy all active sites, and the conductivity of the water flowing through increases. This state is referred to as exhaustion. Articles in the ORBEN knowledge base indicate that once saturated, a cation exchanger can no longer absorb additional ions. In practice, therefore, the conductivity at the resin outlet is monitored. If it rises to 3–5 µS/cm, the resin is considered exhausted and must be regenerated.
Regeneration restores the original ion exchange capacity. In the sodium form, regeneration is carried out with concentrated brine, while in the hydrogen form, acid is used. ORBEN technical articles describe how, during the regeneration of mixed-bed resins, the resin fractions are separated from each other and then treated separately with acid (cation resin) and caustic (anion resin). For pure cation exchangers, the process is identical, only the separation of the resins is omitted. The chemicals should be precisely dosed, because insufficient quantities cause incomplete regeneration, and excessive quantities increase environmental requirements and costs.
Sustainably operating companies rely on reusable resin. Exhausted resins are returned to specialized regeneration stations, professionally reprocessed, and then reused. ORBEN regenerates up to 40,000 liters of resin per day and returns the grade-specific reprocessed resin to the respective customer using batch numbers. This circular system reduces raw material consumption, minimizes waste, and lowers the CO₂ footprint. The use of reusable resin is therefore a central component of sustainable water treatment and minimizes the total cost of ownership.
Cation exchangers remove only cations. Dissolved anions such as chloride, sulfate, or silicate remain in the water and can promote corrosion or scale formation. In district heating networks, where conductivities <20 µS/cm are required, mere softening is not sufficient. Another risk is the counter-ion effect: With a high sodium concentration in the raw water, a cation exchanger cannot absorb additional sodium ions and may even release sodium by exchanging calcium ions back. Therefore, in addition to softening, anion exchangers or mixed-bed filters must be used when very low conductivities are required.
Anion exchangers bind negatively charged ions such as chloride, sulfate, nitrate, silicic acid, and organic acids. They are used in combination with cation exchangers for the production of deionized water. Strongly basic anion exchangers are particularly important because they can remove both mineral anions and weak acids (e.g., silicic acid). In district heating networks, a silicic acid concentration of < 0.5 mg/l is required; these limits can only be achieved through complete demineralization with anion exchangers.
Another application is the selective removal of specific ions, such as nitrate in drinking water treatment or chromate in industrial wastewater treatment. Weakly basic or chelating resins are used for this purpose, which preferentially bind certain anions. For heating water make-up according to VDI 2035, anion exchangers alone are not advisable, as cations would remain in the water and affect the pH value. Only the combination with cation exchangers or mixed-bed filters yields standard-compliant results.
Anion exchangers are converted into the hydroxide form using caustic solutions (e.g., sodium hydroxide). Regeneration follows the counter-current principle: The caustic solution flows from bottom to top through the resin bed, displacing the bound anions. A subsequent rinse with deionized water is important to remove excess caustic solution and stabilize the resin's pH value. In industrial plants, the rinse water must be neutralized before disposal.
The use of reusable resin is also possible with anion exchangers. Professional regeneration reduces operating costs and conserves resources. Companies that adhere to strict environmental regulations should therefore avoid single-use resins and have their resins regenerated by type.
Anion exchangers alone cannot remove cations. If only anions are removed from the water, the relative proportion of cations increases, causing the pH value to drop and conductivity to rise. Furthermore, segregation can occur if different grain sizes are mixed in a cartridge, which reduces exchange efficiency. Finally, strongly basic resins require careful handling of alkalis; improper dosing can lead to safety risks and increased environmental impact.
Mixed-bed ion exchangers combine the functionality of cation and anion exchangers in a single vessel. The resin mixture typically consists of about 40% cation resin and 60% anion resin. During operation, the resins alternate in ion uptake: first, cations are exchanged for H⁺ ions, then anions for OH⁻ ions. The bound H⁺ and OH⁻ ions react to form neutral water. This allows mixed-bed filters to remove all ionic components from the water and achieve conductivities below 0.2 µS/cm.
In conventional ion exchange systems, water is passed sequentially through a cation exchanger and an anion exchanger. This two-column process is robust, but the purity achieved is typically 1–5 µS/cm because the counter-ion effect can occur in the first step. Mixed-bed filters overcome this limitation by repeating the exchange process until no ions remain. A mixed-bed filter can be considered as an infinite number of cation/anion stages connected in series. Therefore, a mixed-bed filter is essential for applications requiring the highest purity (e.g., in semiconductor manufacturing or battery fabrication).
The regeneration of mixed-bed filters is more complex than that of single-bed filters because the two types of resin must be treated separately. In professional regeneration stations, the exhausted resin is first hydraulically separated by backwashing: the heavier cation resin sinks, while the lighter anion resin floats. Subsequently, the resin fractions are regenerated separately with hydrochloric acid and caustic soda, respectively. After thorough rinsing, the resins are remixed in the correct proportion and filled into the cartridge. For small cartridges, ORBEN offers mobile on-site regeneration, while larger quantities are processed at the regeneration station. This effort explains why mixed-bed filters are often operated as reusable resin systems; disposable cartridges would not be ecologically or economically viable.
Mixed-bed ion exchangers are used wherever very low conductivities are required. Typical applications include:
The advantages of mixed-bed filters are clear: they achieve the highest water purity, have compact designs, and can be easily integrated into existing systems. However, acquisition costs are higher, and regeneration is more complex and expensive than with single-bed filters. Regeneration requires specialized service providers who separate and reprocess the resins by type. Disposable mixed-bed cartridges generate large amounts of waste and are therefore problematic from a sustainability perspective. Reusable resin solutions combined with professional regeneration offer the best balance of economic efficiency and environmental compatibility.
The choice between cation exchangers, anion exchangers, and mixed-bed systems depends on the required water quality, system size, application area, and normative specifications. The following decision-making logic can be helpful for asset and operations managers, as well as HVAC professionals:
The VDI 2035 guideline addresses the prevention of scale formation (Sheet 1) and water-side corrosion (Sheet 2). It defines limit values for water hardness, pH value, and conductivity, and prescribes water treatment methods. Key points include:
For industrial district heating and local heating systems, the AGFW FW 510 worksheet sets stricter limit values and distinguishes three operating modes: low-salt, saline (medium salt load), and high-salt. The guideline values are:
The regulations require continuous monitoring. For low-salt operation, oxygen content up to 0.1 mg/l can be tolerated as long as conductivity remains below 100 µS/cm. For high-salt operation, oxygen ingress must be largely prevented to avoid corrosion. Combined with the inspection intervals of VDI 2035, this creates a precise monitoring requirement: First inspection 48 hours after filling, second after three months, and then at least annually.
In addition to VDI 2035 and AGFW FW 510, other regulations play a role depending on the application:
These standards may impose additional requirements on water quality, for example, regarding microbiological parameters or chemical use. For projects in other European countries, the applicable national regulations should be checked.
The regeneration of ion exchangers is not only chemically complex but also requires careful documentation. According to technical articles by ORBEN a conductivity of <1 µS/cm at the resin outlet is considered ideal; if the value rises to 3–5 µS/cm, the resin must be replaced or regenerated. The following steps are typical:
The regeneration results and measured values should be documented in a plant logbook. This serves not only as proof of compliance with standards for regulatory authorities and manufacturers but also for internal quality assurance. For audits, operators must provide complete proof that the resin used was regenerated or replaced in accordance with regulations, and that all measurements were carried out at the prescribed intervals.
Stationary water treatment plants are the backbone of reliable operation, but emergencies and project peaks require flexible solutions. Mobile trailer systems can deliver large quantities of demineralized water in a short time and are therefore indispensable when:
Trailer systems are equipped with reverse osmosis, EDI, and mixed-bed filters and deliver flow rates of 10 to 120 m³/h. They meet the limit values of VDI 2035 and AGFW FW 510 and offer digital measurement and documentation systems. Through the use of regenerable multi-use resin and refillable cartridges, environmental impact and operating costs remain low.
The decision for a specific ion exchanger or mixed-bed system should not be made solely based on acquisition costs. Instead, the entire life cycle must be considered:
Companies that take a strategic view of their water treatment focus on TCO. This should also include CO₂ costs and future levies. Overall, it becomes clear that regenerable mixed-bed systems often represent the most economical and sustainable solution, despite higher acquisition costs.
The energy transition is opening up new application areas for ion exchangers and mixed-bed systems. In electrolyzers for green hydrogen production, ultrapure water is used as a raw material. Impurities impair electrolysis efficiency, lead to increased power consumption, and shorten membrane lifespan. For PEM electrolyzers, water with a conductivity of <0.2 µS/cm and free of silicic acid is required, which can only be achieved through a combination of reverse osmosis, EDI, and mixed-bed filtration. In battery cell production, electrolytes and additives must also be free of trace metals and anions; ultrapure water is needed for this as a process medium and for rinsing the systems.
In addition to the energy transition, the electronics and pharmaceutical industries also benefit from mixed-bed systems. Chip and display manufacturers require ultrapure water (UPW) with sub-µS conductivity, which is purified in several stages. Mixed-bed filters often serve as a safety stage behind membrane degassers and EDI. Hospitals and laboratories use mixed-bed cartridges for sterilizers to ensure no residues remain in instruments and samples. These applications demonstrate that ion exchange technology is key to industrial transformation.
Choosing between cation exchangers, anion exchangers, and mixed-bed systems is a complex task that combines technical expertise, regulatory requirements, and economic considerations. Asset and operations managers, as well as HVAC and plumbing professionals, should consider the following approach:
By taking a holistic view of water treatment, companies can combine operational safety, regulatory compliance, and economic efficiency. Ion exchange technology remains a central element – whether as a cation exchanger, anion exchanger, or mixed bed.
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