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A combined bed refers to a system where cation and anion resins are placed in two separate devices, one a cation bed and the other an anion bed, to distinguish it from a mixed bed where both resins are mixed in one device. Because the combined bed is located earlier in the water treatment system flow and bears the majority of the desalination load, its electroregeneration has different characteristics compared to a mixed bed.
Asynchronous Regeneration of Cation and Anion Beds
In the regeneration practice of combined bed water treatment systems, the cation and anion beds are often regenerated asynchronously, requiring separate regeneration at different times. When the mixed bed resin is sent to the aforementioned external electroregenerator for regeneration, both H+ and OH- ions generated by water dissociation are utilized, resulting in neutral concentrate effluent. During the electroregeneration of the combined bed, if the exhausted cation bed resin is regenerated first, H+ ions are utilized, but OH- ions are not, resulting in slightly alkaline concentrate effluent. If the exhausted anion bed resin is regenerated at a different time, OH- ions are utilized, but H+ ions are not, resulting in slightly acidic concentrate effluent. If the slightly alkaline (or acidic) wastewater can be collected to regenerate the corresponding anion (or cation) bed, additional regeneration equipment and systems are required; if it is discharged directly, the power consumption of external electroregeneration will increase due to the separate regeneration of the cation and anion beds.
Requirement for High Regeneration Intensity of External Electroregenerator
Compared to mixed beds, multi-bed regeneration typically bears the majority of the desalination load. For example, in a series desalination system of a single-stage multi-bed regeneration system and a single-stage mixed bed regeneration system, the multi-bed regeneration system needs to bear 90% of the desalination load. The time for disconnecting the multi-bed regeneration system and stopping its operation for regeneration is usually 8–24 hours, so all operations of external electroregeneration should be completed within 8 hours. Due to the large desalination load of the multi-bed regeneration system, the electroregeneration intensity in a short time is also high; therefore, the external electroregeneration device for multi-bed regeneration should be a high-intensity electroregeneration device.
The Influence of Hardness Ions on Membrane Scaling
Mixed bed cation exchange resins, used as fine treatment equipment in water treatment systems, are mainly used to remove residual NaCl salts from the water; therefore, the exhausted cation exchange resin is in the Na form. Compound bed cation exchange resins are used to remove most of the salts in the water; therefore, the exhausted cation exchange resins contain not only Na but also Ca and Mg forms. When the exhausted cation exchange resin from the compound bed enters the external electroregenator for regeneration, the presence of a large number of OH- ions in the regeneration chamber can cause the surface of the ion exchange membrane and its ion channels to be blocked by Ca(OH)2 and Mg(OH)2 precipitates. This causes the ion exchange membrane to lose its selective migration ability for ions. Therefore, the external electroregenator used for mixed bed resin regeneration cannot be directly used for the electroregeneration of exhausted cation exchange resin from the compound bed.
The Influence of Inorganic and Organic Precipitates on the Resin Surface
The compound bed is located at the beginning of the water treatment system flow. If the pretreatment equipment for removing suspended solids and organic matter in the water is not working properly, inorganic precipitates and organic matter will form on the resin surface. During the electroregeneration of multi-bed resin, these inorganic and organic precipitates are carried along with the resin to the external electroregenerator. This can severely contaminate or clog the ion exchange membrane, affecting the regeneration effect and preventing the external electroregenerator from functioning properly. Therefore, a resin wiping process needs to be added before the resin electroregeneration to clean the resin thoroughly before sending it to the external electroregenerator.
Compared to mixed-bed resin, the difference in the external electroregenerator for multi-bed resin lies in the addition of a bipolar membrane to the membrane pair structure. This is equivalent to inserting a bipolar membrane in the middle of the mixed-bed resin electroregeneration chamber, dividing it into two: one becomes the cation exchange chamber and the other becomes the anion exchange chamber. Under the action of a DC electric field, the H+ and OH- ions generated by water dissociation enter their respective cation and anion regeneration chambers, respectively, and undergo exchange reactions with the corresponding exhausted resin, converting the exhausted resin into H-form and OH-form, thus achieving electroregeneration. Simultaneously, it avoids harmful side reactions to the resin electroregeneration process. Because the mixed bed is located at the front end of the desalination system, the exhausted cation exchange resin, besides adsorbing most of the ions in the water, also adsorbs all the Ca2+ and Mg2+ ions. If this resin is sent to the original mixed bed electroregeneration chamber, the H+ ions generated by water ionization during electroregeneration can exchange with the Ca2+, Mg2+, and Na2+ ions on the resin. The exchanged Ca2+ and Mg2+ ions may then react with the OH- ions generated by water ionization, forming Ca(OH)2 or Mg(OH)2 precipitates. These precipitates cover the surface of the resin or membrane, clogging the pores and affecting subsequent ion migration, diffusion, and exchange processes, ultimately making it difficult to sustain resin electroregeneration.
A bipolar membrane consists of an anion exchange resin layer, a cation exchange resin layer, and an intermediate hydrophilic layer. Under the action of a DC electric field, it can directly ionize water into H+ and OH- ions, forming opposing ion currents under the influence of the electric field. Therefore, by inserting a bipolar membrane into the middle of a mixed-bed resin regeneration chamber, it can be divided into two electroregeneration chambers for the separate regeneration of anion and cation bed resins. By sending the failed cation bed resin and the failed anion bed resin into their respective external electroregeneration chambers for a certain regeneration time, fresh regenerated resin with a regeneration degree comparable to that of acid-base chemical regeneration can be obtained.
Experimental Research Results
In 1992, Millipore, Inc. of the United States designed an EDI technology using bipolar membranes and applied for a patent[4]. It was reported that under the condition that the conductivity of the raw water was 1 μS/cm, the voltage drop at the interface of the bipolar membrane was greater than 1 V, the measured current efficiency was less than 30%, and the mass concentration of H+ and OH- ions generated by the hydrolysis of ions by the bipolar membrane could reach more than 104 mg/L, while the mass concentration of impurity ions in the raw water was only 10-2 to 10-5 mg/L, and the mass concentration of the two ions differed by 106 to 109 times. This ratio is 2 to 5 orders of magnitude higher than that of traditional chemical regeneration, so the regeneration degree of the resin should be higher than that of chemical regeneration.
After completing the experimental research on the electroregeneration of mixed-bed ion exchange resin, Hebei University of Architecture and Technology cooperated with Hebei Electric Power Equipment Factory and Taiyuan University of Technology to conduct an electroregeneration experiment of mixed-bed resin using bipolar membranes (Hebei Province 2000 Science and Technology Research and Development Guidance Project 00213093)[5]. They used domestically produced bipolar membranes and other materials, and manufactured an EDI device with anion and cation resins respectively filled on both sides of the bipolar membrane, following Millipore's EDI technology using bipolar membranes. The experimental results of electro-regeneration of multi-bed ion exchange resins showed that when the regeneration voltage was 60V and the regeneration time was 60min, the effect of the electro-regeneration of the resin in this experimental device was close to that of chemical regeneration, demonstrating good technical feasibility.
Cao Liancheng and Deng Yongnan of Huazhong University of Science and Technology also conducted an electro-regeneration experiment of multi-bed resins using bipolar membranes (1999 Hubei Provincial Science and Technology Commission Key Science and Technology Project 992P1202)[6], and reached the same conclusion as the above experiment.
Beijing Guodian Longyuan Environmental Protection Co., Ltd. and North China Electric Power University focused on developing multi-bed resin electro-regeneration using bipolar membranes as a key technology for implementing resin electro-regeneration. The experimental results showed that the regeneration effect on anion resins was very good, reaching or exceeding the effect of alkali regeneration, while the regeneration effect on cation resins was slightly worse[7].
Structure of the Electro-regenerator
Based on the practice of developing electro-regeneration of ion exchange resins, the author applied for a utility model patent for "Electro-regeneration device for mixed-bed ion exchange resins" [8], which was used as a supplement to the invention patent for "Electro-regeneration method and device for ion exchange resins". In addition to proposing that the bipolar membrane divides the original mixed-bed resin electro-regeneration chamber into two parts, dividing it into an electro-regeneration chamber for cation exchange resin and an electro-regeneration chamber for anion exchange resin in a mixed bed, this utility model patent also proposes to fill the concentrate chamber with conductive resin using the equal-gap method. This can reduce the resistance of the concentrate chamber, improve the current efficiency, and also save troublesome measures such as concentrate circulation or concentrate salting.
The electro-regenerator for mixed-bed ion exchange resins mainly consists of three parts: membrane stack, electrode device and end clamping device. The basic unit of the membrane stack is the membrane pair. The membrane stack is composed of several membrane pairs. Each membrane pair consists of one anion exchange membrane, one hollow partition for anion exchange resin electro-regeneration, one bipolar membrane, one hollow partition for cation exchange resin electro-regeneration, one cation exchange membrane and one hollow partition for the concentrate chamber, arranged alternately in a fixed program. The inlets of the anion exchange and cation exchange resin electroregeneration chambers are connected to the outlets of the exhausted anion and cation exchange resins, respectively. Exhausted anion and cation exchange resins are fed into the cavities of the hollow partitions in the anion and cation exchange resin electroregeneration chambers using pure water via hydraulic conveying until the regeneration chambers are completely filled. The cavity of the hollow partition in the concentrate chamber is filled with conductive resin to reduce the resistance of the concentrate chamber during resin electroregeneration. The hollow partitions in the anion exchange and cation exchange chambers are 10–20 mm thick; the hollow partitions in the cation exchange and concentrate chambers are 5 mm thick. These partitions are all made of rigid polypropylene. The anion exchange membranes and cation exchange membranes can be made of heterogeneous membranes, both of which are flexible materials. They are pressed together with the rigid partitions, and the deformation of the membranes achieves a seal, preventing leakage. The more membrane pairs arranged in parallel, the greater the amount of failed resin that a single multi-bed ion exchange resin electro-regenerator can electro-regenerate.
The electrode assembly is located at both ends of the outer side of the membrane stack, including a positive electrode separator, a positive electrode, a positive electrode chamber, a negative electrode, a negative electrode chamber, and a negative electrode chamber separator.
The clamping device is located at both ends of the outer side of the electrode assembly, including left and right clamping plates and 16 pairs of bolts. Tightening the nuts on the bolts in a specific sequence compresses several membrane pairs, electrode separators, and left and right clamping plates into a single unit.
Therefore, at the interface between the resin or membrane (especially bipolar membranes) and water in the membrane pair, water ionization occurs due to polarization. The H+ and OH- ions generated by water ionization exchange with ions on the spent resin. Simultaneously, these ions exchanged from the spent anion resin are subjected to an electric field force and pass through the ion exchange membrane into the concentrate chamber for discharge. Ultimately, the spent resin is converted to the H,OH form, achieving electroregeneration.
