Reverse osmosis (RO) produces water of excellent purity for many applications including semiconductor manufacturing, pharmaceutical, medical, laboratory and power generation to name a few. The RO process removes over 99% of the dissolved solids from the feedwater. But this means that 1% or so remain, which is unacceptable for many applications. RO permeate is not considered of equal quality to demineralized or deionized water. Nor is it considered ultrapure water. To achieve and maintain this level of purity, the RO permeate must be ‘polished’.
An RO that processes feedwater having a specific conductance of 300 micromhos/cm, for example, would produce a permeate stream of 5 micromhos/cm or less. A conductivity of 5 is equivalent to a resistivity of 0.2 megohm-cm. Getting the permeate to within the 10 to 18 megohm-cm quality range, however, requires additional treatment, This is commonly accomplished by one of three methods.
On-site mixed bed ion exchange polisher
Portable exchange mixed bed polisher
Electrodeionization polisher (EDI)
ON-SITE MIXED BED POLISHER
A mixed bed polisher consists of a mixture of cation and anion exchange resins contained within the same vessel. As the name implies, the two resin types are mixed together by air agitation to form a homogeneous blend. In essence, it’s like taking two traditional ion exchange resin beds (cation and anion) and combining them into one vessel.
Water that passes through the mixed bed unit exchanges cations (positively charged ions) for hydrogen and anions (negatively charged ions) for hydroxide. The hydrogen and hydroxide ions combine to form pure water (H2O). Thus, the feedwater to a mixed bed demineralizer is purified to achieve demineralized or deionized water quality.
When the feedwater to the mixed bed has been pretreated by reverse osmosis, the ionic loading on the ion exchange bed is extremely low. All the heavy lifting has been done by the RO unit. The mixed bed’s job is to remove the last traces of impurity that has passed through the membrane. The salt passage through a reverse osmosis unit is typically 1% to 1.5%. For a feedwater having a total dissolved solids (TDS) content of 200 ppm, let’s say, the feedwater to the mixed bed would have a TDS of 2 to 3 ppm. In addition, the mixed bed removes residual carbon dioxide that passes through the membrane. The expected final effluent from the RO/MB train is of demineralized water quality as indicated by a resistivity of 10 to 18 megohm-cm, very low total organic carbon (TOC) levels, and near-neutral pH.
At some point, as with all ion exchange columns, the mixed bed unit depletes its exchange capacity and must be regenerated. With a polishing unit, this happens rather infrequently because of the low ionic loading on the MB. Generally, mixed bed units are regenerated based on volume thruput or days in service rather than by detection of ionic breakthrough. It’s not uncommon for a mixed bed polisher to have sufficient capacity to theoretically remain in service for 1,000,000 gallons or more, which can be equivalent to several weeks or months of service.
Regenerating a mixed bed polisher is a bit challenging. During the backwash, the two resins, which are intimately mixed while in service, must be separated by density differences into two layers; the lighter anionic resin on top and the heavier cationic resin on the bottom. Failure to get good separation will result in a poor regeneration outcome.
Once separated, the acid and caustic regeneration chemicals are prepared. Depending on the equipment manufacturer, the properly diluted acid and caustic solutions are pumped or educted into the resin at sufficient strength to accomplish the regeneration. The cation resin is thereby restored to the hydrogen form and the anion resin to the hydroxide form. The acid and caustic regeneration chemicals are collected at the interface between the two resins where they exit the vessel for disposal down the drain. In a perfect situation, the low pH acid and high pH caustic streams neutralize one another prior to discharge. However, in many cases, the waste effluent must be collected for pH-adjustment prior to disposal. The entire regeneration process requires the purchase, storage, handling and disposal of acid (sulfuric or hydrochloric) and caustic soda (sodium hydroxide), which at best is unpleasant and at worst hazardous to one’s health and the environment.
Following the regeneration procedure, the two resins are slow rinsed to remove residual acid and caustic. This step is followed by a fast rinse. The two resin beds are then air agitated to mix the cation and anion resins together once again. The mixed bed is then rinsed down to quality prior to being returned to service. If all goes well, the MB polisher is ready for another extended run time. If not, the process is repeated.
PORTABLE EXCHANGE MIXED BED
On-site mixed bed polishers do an excellent job of treating the effluent from an RO system. The major difficulty arises from having to periodically regenerate the unit, which requires the purchase, storage, handling and disposal of concentrated, corrosive acid and caustic soda chemicals. Since the regeneration is done rather infrequently, having personnel on site who know how to safely and properly step the MB through the regeneration procedure is a challenge. Even though the regeneration cycle is guided by a programmable logic controller (PLC), it still takes human intervention to verify, fine tune and adjust the process to guarantee a quality regeneration.
The answer to this problem for many plants has been to contract with a service company to provide portable mixed bed polishers (rental units, essentially). These are skid mounted tanks or smaller portable fiberglass units that are delivered to the site in a fully-regenerated condition. Connection is made to the RO system via flexible hoses, rigid pipe or both. The installation of the portable units also includes a water quality sensor such as a conductivity indicator light or meter.
The portable mixed bed units remain in service until the sensor detects that the ion exchange resin is exhausted and needs to be regenerated. At this time the service company returns to the site, removes the exhausted mixed bed, and replaces it with a fully regenerated unit. The exhausted unit is returned to the service providers plant where it is regenerated and made ready for the next exchange.
Success with a portable exchange service requires that you monitor the water quality from the polisher and alert the service provider in ample time to obtain a replacement before the online unit exhausts. Often two MB units are provided so that you can switch from one to the other to insure a continuous supply of high purity water. The service company is then called to replace the exhausted unit at the first opportunity.
In general, the cost of water produced by portable exchange units is greater than that produced by on-site MB polishers. It’s a service, after all, and one would expect to pay more based on the additional labor, materials and transportation required. However, since the units are regenerated off-site, one does not need to contend with the handling and disposal of hazardous acid and caustic regeneration chemicals. Plus, no trained personnel are required. Finally, since you lease the units, capital is not tied up on plant equipment and you can cancel the service, if your needs change.
Electrodeionization (EDI) represents the third option for polishing RO permeate. EDI modules utilize ion exchange resins, semipermeable membranes and an electric field to remove organic and inorganic ions from RO permeate. The DC electric field continuously regenerates the ion exchange resin by a process of ‘water splitting” that produces hydrogen (H+) and caustic (OH-) regenerants in-situ on the resin surface. In this way the ion exchange resin is maintained in a fully regenerated state. This allows the EDI process to work continuously rather than in the batch process required by traditional mixed bed polishers.
The design of an EDI module consists of process compartments that are sandwiched between a series of cation and anion specific membranes. The two electrodes are located at opposite ends of the stack to produce the DC electric field. Cations (positive charged ions) are attracted toward the cathode (negative electrode) and migrate through the cation permeable membrane. Anions (negatively charged ions) are attracted to the anode (positive electrode) and selectively migrate through the anion permeable membrane. The power required by this process is low; about 1 kilowatt-hour (kW-hr) per 1000 gallons of product water.
RO permeate is fed into the diluting compartment, which contains the mixed bed resin. The resin is sandwiched between a cation-permeable membrane and an anion-permeate membrane. The purpose of the ion exchange resin is to increase the electrical conductivity and act as a pathway for ion transfer. Thus, the mixed bed resin distinguishes electrodeionization (EDI) from electrodialysis (ED).
As the ions are attracted to their respective electrodes, they pass through the permeable membranes into the concentration compartment. The concentrate stream is recirculated through the stack with a small percentage of water sent to drain to prevent over-concentration of impurities. The EDI module is thereby capable of recovering over 90% of the feedwater as high purity product.
Since RO and EDI are both continuous processes, it would seem like an ideal marriage to couple the two processes together to continuously produce high purity, deionized water. And, in fact, this process is successfully used to produce 10 to 18 megohm-cm water for many applications. However, as with all processes, some limitations exist. For one, the EDI process requires feedwater containing less than 1 ppm hardness. Higher hardness levels cause insulating calcium carbonate scales to form in the concentrating compartment and on the cathode electrode, which eventually halts the production of purified water. The EDI module is also sensitive to elevated carbon dioxide levels in the feedwater. Carbon dioxide is not removed by RO and upon entering the EDI module causes a reduction in water resistivity. These limitations do not exist for either on-site or portable mixed bed polishers.
In addition to being a continuous process, the other significant advantage of EDI is that the purchase, storing, handling and disposal of acid and caustic regeneration chemicals is eliminated. This is good for the environment and improves the health and safety of the workplace. Similar advantages can be stated for portable exchange mixed bed polishers since they are regenerated off-site, which essentially exports the hazards and wastewater discharge to another location.
Overall, polishing RO permeate is necessary to achieve the high purity water standards required by many processes. Reverse osmosis, when used in conjunction with conventional mixed bed polishers or EDI, produce water of exceptional purity with a resistivity of greater than 10 megohm-cm, very low TOC levels and near-neutral pH. EDI offers some advantages over on-site and portable MB polishers, but it is not a total solution. Each installation offers its own set of challenges and requirements that need to be carefully evaluated before making a final decision on the optimum polishing method.