Dow Water & Process Solutions

Designing a System with Both Reverse Osmosis and Ion Exchange Resins

This section gives general information on designing a FILMTEC™ reverse osmosis system in combination with DOWEX™ ion exchange resins. Links are provided in the text to more detailed information to assist in making a specific design. Useful FILMTEC membranes Engineering Information (2.5MB PDF) is available with conversion tables, conductivity curves, pH, osmotic pressure data etc., which can be used for design purposes.

Introduction
Ion exchange resins are used in two main positions within a reverse osmosis (RO) system:

• In the pretreatment, a strong acid or weak acid cation resin softener or a weak acid cation resin dealkalizer are used to remove scale forming cations such as Ca²⁺, Ba²⁺ and Sr²⁺
• As post-treatment, a working mixed bed is used for high quality water applications.

The Reverse Osmosis System Analysis (ROSA) computer program is able to make membrane element designs with resin pretreatment and the CADIX ion exchange computer program can be used to design the softeners, dealkalizers and mixed beds.

Softening With a Strong Acid Cation Exchange Resin
In the resin softening process, scale forming cations are removed and replaced by sodium ions. The resin is regenerated with NaCl at hardness breakthrough. As the pH of the feed water is unchanged by this treatment, no degassifier is needed. Only a small amount of CO₂ from the raw water is present that can create a conductivity increase in the permeate. It is even possible to lower the permeate conductivity by adding some NaOH to the softened feed water (up to pH 8.2) in order to convert residual carbon dioxide into bicarbonate, which is then rejected by the membrane. The rejection performance of the FT30 membrane is optimal at the neutral pH range.

With DOWEX ion exchange resins, the removal efficiency for Ca²⁺, Ba²⁺ and Sr²⁺ is >99.5%, which is usually enough to eliminate any risk of carbonate or sulfate scaling. Softening with a strong acid cation exchange resin is very effective and safe, provided the regeneration is performed properly. With DOWEX uniform particle sized ion exchange resins and a counter-current regeneration technique such as an UPCORE™ System, it is possible to minimize the sodium chloride consumption to 120% of the stoichiometric value.

Strong Acid Cation Softening Resins are mainly used in small or medium size brackish water plants, but not in seawater plants. This process requires a relatively high sodium chloride consumption, which might cause environmental or economic concerns.

A DOWEX Resin Engineering Brochure (68KB PDF) is available for designing an ion exchange softener system. Membrane element performance with a softened feed is provided in the Membrane System Design Guidelines (126KB PDF).

Dealkalization With a Weak Acid Cation Exchange Resin
Dealkalization with a weak acid cation exchange resin is preferred over a strong acid cation in large brackish water plants for partial softening, due to reduced consumption of regeneration chemicals. In this process, only Ca²⁺, Ba²⁺ and Sr²⁺ linked to bicarbonate (temporary hardness) are removed and replaced by H⁺, thus lowering the pH to 4-5. As the resin contains carboxylic acidic groups, the ion exchange process stops when the pH reaches a value of 4.2, where the carboxylic groups are no longer dissociated. Only those scale-forming cations are removed which are associated with alkalinity, so this process is ideal for waters with a high bicarbonate content and maximum operating capacity is achieved when the total hardness / alkalinity > 1. The bicarbonate is converted into carbon dioxide:

In most cases, carbon dioxide is not desired in the permeate, so it can be removed by degassing either in the permeate or in the feed stream. Degassing the permeate is favored where a potential for biofouling is suspected (surface waters, high TOC, high bacteria counts). A high CO₂ concentration at the membranes helps to keep bacteria growth low. Degassing the feed is preferred when optimum salt rejection is the priority. Removing CO₂ also leads to an increase in pH (see above equation), and at pH > 6 the rejection is better than at pH < 5.

The advantages of dealkalizing with a weak acid cation exchange resin are:

• Regeneration is very efficient, (105-110% acid stoichiometry), thus minimizing the operating costs and the environmental impact.
• The TDS value of the water is reduced by the removal of bicarbonate salts, so the permeate TDS value is also lower.

The disadvantages are:

• Residual hardness: if a complete softening is required, a sodium exchange process with a strong acid cation exchange resin can be added, even in one vessel. The overall consumption of regeneration chemicals is still lower than softening with a strong acid cation exchange resin alone, but due to the higher investment costs, this combination will only be attractive for plants with high capacity. Another possibility to overcome this drawback of incomplete softening is to dose an antiscalant into the dealkalized water. Although no scaling problems with waters dealkalized by weak acid cation exchange have been encountered, it is strongly recommended to calculate the solubilities of the residual sparingly soluble salts and to take the respective measures.
• Variable pH of the treated water: the pH of the dealkalized water ranges from 3.5 to 6.5 depending on the degree of exhaustion of the resin. This cyclic pH variation makes it difficult to control the salt rejection of the plant. At pH < 4.2, the passage of mineral acid may increase the permeate TDS. It is therefore recommended to use more than one filter in parallel and to regenerate them at different times in order to level out the pH. Other possibilities to avoid extremely low pH values are CO₂ removal or pH adjustment by NaOH afterwards.

An Engineering Brochure (119KB PDF) is available for designing a dealkalizer with DOWEX MAC-3 ion exchange resin.

Water Softening Using a Weak Acid Cation Exchanger in H⁺ form
Waters with a total hardness / alkalinity < 1 can be softened very efficiently using a weak acid cation exchange resin in H⁺ form. Running the resin to the hardness breakthrough generates a large volume of water with very low level of residual hardness, typically < 0.02 meq/l (1 ppm CaCO₃) at the plateau. A higher level of hardness is released at the beginning of the service run. This level depends essentially on two parameters:

• The amount of acid used to regenerate the resin.
• The FMA (Free Mineral Acids) level in the raw water.

Both parameters are responsible for a lowering of the pH, which results in increased cation leakage. To minimize the effect of low pH, the resin should be regenerated with a minimum amount of acid, i.e. a ratio to stoichiometry not above 105%. The FMA level cannot be changed. To ensure a low level of hardness leakage in the treated water, the first 15% of the treated water should be discharged, as illustrated in the graph below:

Working Mixed Bed to Treat RO Permeate
In high purity water applications such as boiler feed, a mixed bed is added as a polishing step to improve the quality of an RO permeate from typically ~20 mS/cm to within the range 0.1-1.0 mS/cm. The sizing of the mixed bed depends on the RO permeate composition: the presence of CO₂ and silica will generally require a higher anion to cation ratio. If the permeate is degassed before the mixed bed, less anion is required. A working mixed bed can be regenerated either in-situ or externally.

General guidelines are given below for designing a working mixed bed downstream of an RO plant:

• Flow rate 20-40 bed volumes per hour, depending on the feed TDS.
• Resin volume ratio of cation to anion should be in the range 30:70 to 60:40.
• Minimum cation resin bed depth of 45-50 cm (1.5-1.7 ft).
• Regenerant levels of 100g HCl or 160g H₂SO₄ per liter cation (6.2 or 10 lbs/ft³) and 100g NaOH per liter anion (6.2 lbs/ft³). The NaOH is preferably heated to 50°C (122°F).
• The mixed bed service run length will depend on the permeate TDS, but regeneration every 1 or 2 days is typical.

For designing a specific plant, the CADIX computer design program has an option for working mixed beds treating RO permeate.