The ion exchange resin market is still being applied in water treatment. With the pollution of the environment, the water quality is increasingly showing a trend of deterioration. But at the same time, various industries have increasingly high requirements for water quality and stricter requirements for resin quality. Therefore, resin is still a good choice in improving the quality of pure water. Here is a brief introduction to several commonly used water treatment processes.
1、 Classification of ion exchange bed types:
The general classification of commonly used bed types in industry is as follows:
---Countercurrent regeneration bed - single chamber fixed bed, double layer bed, double chamber fixed bed
Convective regeneration——
Floating bed - single chamber floating bed, double chamber floating bed
Fixed bed - downstream regeneration - single chamber fixed bed
——Double flow regenerated bed sheet room fixed bed
——Double flow single chamber bed
Bed type mixed bed
Moving Bed Flowing Bed
In the classification shown in the above figure, the downstream regeneration single chamber fixed bed, the reverse flow regeneration single chamber fixed bed, and the single chamber floating bed are all types of resins used in one exchanger, belonging to the desalination process of a single resin; Double bed, double chamber fixed bed, and double chamber floating bed belong to strong and weak combined desalination processes. Here is an introduction to the main commonly used bed types mentioned above, and the requirements for the inlet water quality and regenerant quality of the raw water are specifically explained as follows:
1. Raw water inlet water quality requirements

2. Regenerant quality indicators
2.1 Quality indicators of industrial HCl for regeneration

2.2. Quality indicators of NaOH ion exchange membrane for regeneration

2、 Fixed bed for downstream regeneration
1. Advantages of downstream regeneration fixed bed: Due to the same direction of operation and regeneration, both the raw water flow and regeneration liquid flow through the resin layer from top to bottom. Therefore, its operation is convenient and the equipment production is relatively simple.
2. Disadvantages of downstream regeneration fixed bed: Due to the same direction of regeneration and operation, the resin regeneration rate of the protective layer after regeneration is much lower than that of the countercurrent regeneration method. The quality of the effluent is directly related to the regeneration rate of the protective layer resin at the outlet, so this low regeneration rate determines the poor quality of the effluent. In order to minimize the content of ineffective resin in the protective layer as much as possible, the only way is to increase the amount of regenerant to ensure the quality of the effluent, resulting in higher operating costs.
At present, this type of bed is mostly used for the preparation of softened water or production where water quality requirements are not very strict.
3. The operating steps of downstream regeneration are generally composed of five main steps: backwashing, regeneration, displacement, washing, and operation:
3.1. Backwashing: The purpose of backwashing is to remove the suspended solids intercepted by the resin after a cycle of operation, loosen the resin layer, prevent resin agglomeration, and prepare for regeneration.
3.2 Regeneration: The purpose of regeneration is to restore the exchange capacity of the failed resin. The level of regeneration efficiency is the key to affecting the quality of effluent and the amount of water produced during the cycle.
3.3. Replacement: Use replacement water to continue flushing the resin according to the process of regeneration liquid, so that the space above the resin and the regeneration liquid in the middle of the resin layer can fully play their role, without wasting the regeneration agent, and shortening the subsequent washing time. The replacement water is generally used as the water produced by this process. The displacement flow rate is the same as the regeneration flow rate.
3.4. Washing: Due to the slow flow rate and short time (usually around 60 minutes) of the replacement process, and a large amount of regenerant being utilized during the replacement process, the resin is washed with the inlet water of the bed after the replacement until the outlet water is qualified, and the washing is completed. The washing flow rate is the same as the operating flow rate.
3.5. Operation: After completing the above steps, the bed can be officially put into operation.
Usually, we refer to the accumulated time from the start of the ion exchange process to the end of the washing process as one working cycle of the ion exchanger.
4. The list of resin parameters for ion exchange downstream regeneration softening and desalination processes is as follows:

Note: The regeneration methods and related parameters of sulfuric acid will be introduced in detail in the following chapters.
5. Common faults in downstream regeneration: The following reasons may lead to substandard effluent quality and reduced cycle water production. Users can compare the following situations to find the reasons:
5.1. Treatment of downstream regenerated softened water:
5.1.1. The periodic water production gradually decreases:
a. Insufficient salt usage, not meeting the standard of 70-80g/mol;
b. If the water quality of preparing table salt is too poor or there are too many impurities in the salt, the quality of salt and water should be improved;
c. If the washing time is too long, the exchange capacity of the resin is lost, and the washing time is strictly controlled;
d. Resin is poisoned by metal contamination such as iron and aluminum. Soak in a 7% hydrochloric acid solution for recovery;
e. Resin clumping, poor backwashing effect, resulting in bias current and poor regeneration effect. Strengthen the backwashing effect, control the appropriate backwashing expansion rate and backwashing time.
5.1.2. Sudden Unqualified Effluent Hardness:
a. The anti-corrosion coating in the equipment has fallen off and iron ions have been introduced into the water, causing an illusion in analysis. It is necessary to check whether the anti-corrosion coating is normal;
b. The inlet and outlet valves are leaking in series, and the inlet and outlet valves are leaking into the water. Check the tightness of the valves;
5.1.3. Excessive consumption of regenerant exceeding normal values:
a. Resin poisoning and pollution, soak and recover with 7% hydrochloric acid;
b. The regeneration flow rate is too fast, the contact time of the resin is not enough, and the exchange is not complete. Adjust the regeneration time to the normal value;
5.1.4. Long term unqualified washing:
a. The washing flow rate is too low, adjust the flow rate to the normal range;
b. Check if the inlet and outlet water series valves are leaking;
c. Check if there is any leakage of resin and control the balance of inlet and outlet water flow;

3、 Countercurrent regeneration fixed bed
Countercurrent regeneration is a type of convective regeneration (a, one is running from top to bottom and regenerating from bottom to top, such as a double chamber fixed bed; b, the other is running from bottom to top and regenerating from top to bottom, such as a double chamber floating bed). During operation, the direction of water flow is from top to bottom, and during regeneration, the direction of regeneration liquid is from bottom to top, so it is called convective regeneration. The key to ensuring countercurrent regeneration is to prevent disordered layers of ion exchange resin during regeneration and replacement. For this reason, there are certain strict requirements for countercurrent regeneration, especially for the height of the resin layer and the regeneration flow rate. Convection exchangers are essentially the same as downstream exchangers, with the main difference being that they have a pressurized grease layer and an intermediate drainage device, which is located between the resin layer and the pressurized grease layer for the discharge of regenerated waste liquid. The middle discharge pipe is also the inlet pipe for small backwashing and the outlet pipe for small washing.
1. Advantages of countercurrent regeneration: Due to the opposite direction of the regeneration liquid fed into the countercurrent regeneration exchanger from the operating direction, the regeneration rate of the protective layer resin at the outlet is very high, and the quality of the outlet water is excellent. The Na+content in the outlet water can generally be lower than 100ug/l, and the conductivity of the first stage desalinated water can be less than 5us/cm. Due to the high regeneration rate of the protective layer, there is no need to increase the amount of additional regenerant. The amount of regenerant used is lower than that of the downstream type, and the discharge of regenerated waste liquid is also lower, resulting in relatively low operating costs. The adaptability to water quality is also strong, (during downstream regeneration, when the NaCl content of the raw water is greater than 120mg/l, the effluent is no longer qualified, but if reverse flow regeneration is used, when the NaCl content of the raw water reaches 200mg/l, the effluent is still qualified).
At present, there are four types of countercurrent regeneration fixed bed methods: no top pressure method, water top pressure method, gas top pressure method, and low flow rate method. Nowadays, there are relatively more regeneration methods without top pressure, while water top pressure and gas top pressure are becoming less and less common due to their relatively cumbersome and demanding operations. The low flow rate method is actually a non top pressure regeneration method without intermediate discharge devices, which is difficult to control and is rarely used. Practical experience has shown that as long as the opening area of the middle discharge pipe is appropriate, most water and gas top pressure devices can be replaced by non top pressure operation.
2. Disadvantages of countercurrent regeneration: During regeneration, the countercurrent regeneration exchanger must ensure that the resin layer cannot be disordered. Therefore, the requirements for the regeneration flow rate and the height of the grease layer are extremely strict. Often, regeneration failure occurs due to poor control of the regeneration flow rate and the height of the grease layer. Therefore, the operational requirements are relatively much stricter than those for downstream flow.
Experience has shown that controlling the height of the resin and the pressure layer at 30-50cm is more appropriate. Alternatively, the height of the grease layer is based on the basic principle of not moving during resin regeneration. The general design parameters set the height at a value of 20-25cm are not reliable parameters and can only be used as a reference!
The pressure grease layer is to add an additional 30-50cm of resin on the surface of the resin layer, which prevents the upward flow of regeneration liquid and water during regeneration and replacement from causing resin agitation, disrupting the normal layer distribution, and also having a certain filtering effect on the incoming water. Some materials suggest that the grease layer can be used as a white ball (inert resin) with a density lower than that of the resin, but in fact, this is not reasonable. Because the density of the white sphere in the resin layer is slightly smaller than that of the resin layer, it is always above the resin layer, which makes it impossible to backwash out the broken resin during operation, and even the intercepted suspended solids are not cleaned using the resin as the resin layer for backwashing.
3. Operating steps for convective regeneration
The operation steps of resin convection regeneration are divided into seven steps: small backwash, regeneration, replacement, small backwash, large backwash, operation, and large backwash. Among them, major backwashing is not carried out every cycle, but when the pressure difference increases to a certain value (exceeding 0.15MPa) or runs continuously for 1-2 months, the purpose of each step, except for minor backwashing and minor washing, is the same as that of downstream regeneration.
3.1. Small backwashing: It refers to backwashing the grease layer above the middle row. Due to the fact that most of the suspended solids intercepted by the bed during one cycle of operation are in the grease layer, as long as it is cleaned thoroughly without the need for major backwashing, it can reduce the pressure difference. In addition, conducting a major backwash on the resin completely disrupts the layer distribution during failure, greatly improving the resin loss efficiency in the protective layer. Therefore, the amount of regenerant used after major backwashing is 1.5 to 2 times the usual amount.
3.2. Small washing: It is to clean the grease layer, with water entering the inlet pipe and water exiting the middle discharge pipe. Due to the limited time required for replacement (usually within 60 minutes), there is often a significant amount of regeneration liquid remaining in the resin after the replacement is completed, including in the grease layer. In order to prevent the regeneration liquid in the grease layer from flowing back into the resin during the subsequent washing, a small washing is performed to achieve this effect. Generally, the grease layer can be cleaned in 10-15 minutes, greatly reducing the total washing time.
4. Process conditions and characteristics of four convective regeneration methods

5. Operation steps and process parameters of convective regeneration

4、 Single chamber floating bed
A single chamber floating bed is a type of operation in which the water flow direction is from bottom to top during operation and the regenerated liquid is from top to bottom during regeneration. Contrary to countercurrent regeneration, it is another form of convective regeneration.
The single chamber floating bed process has two situations: resin filled and resin not filled. A fully filled bed, also known as a full chamber bed (in the regenerated form, the resin fills the entire chamber), and cannot float during operation. During the early and middle stages of operation of the full chamber bed, the resin layer will not be disordered, and the effluent quality is excellent. Occasional shutdown has little impact on water quality and quantity. But in the later stage of operation, as the failure rate of the resin increases, the volume of the resin shrinks, and a certain water cushion layer will appear at the bottom. In addition, the resin protection layer in the later stage is already very thin, so it cannot be stopped, otherwise it will cause the entire bed to fail earlier.
A floating bed that is not fully filled has a certain amount of backwashing space and can be backwashed (due to the obstruction of a water cap, this backwashing effect is not good and can only be referred to as resin loosening). Due to the presence of this space, there will be some resin floating during operation. The amount of resin floating depends on the size of the space and the flow rate of the bed. Experience has shown that at least 50% of the resin is in a compacted state to ensure the quality of the effluent. And after the bed is formed, it cannot be stopped, otherwise serious disorder will occur, which will cause the resin to fail prematurely.
1. Advantages of a single chamber floating bed:
1.1. Due to the top-down regeneration method, there will be no disordered layers and the regeneration operation is simple. Moreover, there are no easily damaged middle exhaust devices, which improves the reliability of operation;
1.2. During operation, the direction of water flow and gravity are reversed, and the degree of resin compaction is much smaller than that of downward flow operation, so the water flow resistance is smaller;
1.3. Due to the resin filling being relatively full, the space utilization rate is high, reaching over 90%, which is much higher than the 60% space utilization rate of downstream and countercurrent regeneration equipment;
1.4. Due to the inability to backwash frequently and the small water cushion inside the bed, the self use water consumption is very low, only about 5%.
1.5. High equipment output. Due to the small pressure difference during operation, the flow rate can reach 40m/h, which is much higher than other single chamber beds.
2. Disadvantages of a single chamber floating bed:
2.1. Due to the lack of or small backwashing space, additional in vitro cleaning tanks are generally required, which increases investment and complexity of in vitro backwashing;
2.2. It is not allowed to shut down in the middle and later stages, and continuous operation is required, otherwise it will affect the water quality and periodic production capacity;
2.3. Due to the inability to backwash inside the bed, strict requirements are placed on the turbidity of the incoming water, which should be less than 2mg/l;
2.4. Due to the concentration of fine resin in the bed at the outlet, it is easy to drain from the gap of the water cap. The addition of cation resin into the anion resin will increase the conductivity of the anion bed, and the anion resin will enter the thermal system, causing equipment corrosion. For this reason, a resin trap must be installed on the outlet pipe of the floating bed.
3. Operation and regeneration parameters of a single chamber floating bed
The operation process of a floating bed is divided into the following steps: water production, bed falling, regeneration, replacement, bed formation, washing, and operation. The parameters are shown in the table below:

Explanation: After external cleaning of the floating bed, it should be regenerated in multiple doses.
5、 Related calculations for single room beds (applicable to the three bed types mentioned above):
1. Work exchange capacity
1.1. The calculation formula for cation resin production is as follows:
Qa=(A+S)V/ VR（5-1）
In the formula: Qa: working exchange capacity of cation resin, mol/m3;
A: Average inlet alkalinity of cation bed, mmol/l;
S: Average effluent acidity of cation bed, mmol/l;
V: Total amount of periodic water production, m3;
VR: Volume of resin in the bed (excluding the volume of pressure grease layer for countercurrent regeneration), m3;
1.2. The calculation formula for the production of anion resin is as follows:
Qk=（S+〔CO2〕+〔SiO2〕）V/ VR（5-2）
In the formula: Qk: working exchange capacity of anion resin,
[CO2]: Average CO2 concentration of influent water in the anion bed, mmol/l;
[SiO2]: Average SiO2 concentration in the influent of the anion bed, mmol/l;
S. V, VR as above (5-1);
2. Acid and alkali consumption
2.1. The calculation formula for acid consumption is as follows:
Kac=Mac/(A+S)V(5-3)
In the formula: Kac: Acid consumption, g/mol;
Mac: Amount of acid used for regeneration based on pure acid, g;
S. A and V are the same as equations (5-1);
2.2. The calculation formula for alkali consumption is as follows:
Kal= Mal/（S+〔CO2〕+〔SiO2〕）V(5-4)
In the formula: Kal: Alkali consumption, g/mol;
Mal: Amount of alkali used for regeneration based on pure alkali, g;
S. [CO2], [SiO2], V have the same meanings as (5-1) and (5-2);
3. Calculation of regenerant dosage:
M=Qg × VR × k/1000a
In the formula: M: Amount of regenerant (industrial acid and alkali), Kg;
Qg: resin working exchange capacity, mmol/l;
VR: Volume of resin in the bed (excluding the volume of pressure grease layer for countercurrent regeneration), m3;
K: Actual consumption of regenerant (referring to acid or alkali consumption), g/mol;
a: The content of industrial acid or alkali,%;
4. Calculation of self use water consumption:
F=(V1+V2+V3)/V × 100%（5-5）
In the formula: F: self use water consumption,%
V1: Backwashing water consumption, m3;
V2: Water consumption for regeneration (including replacement water), m3;
V3: Washing water, m3;
V: Total water production per cycle, m3; 6、 Double bed
A double bed ion exchanger is a fixed bed ion exchanger equipped with a combination of strong and weak resins in the same exchanger. Generally, the upper layer is a weak resin, and the lower layer is a corresponding strong resin. By utilizing the difference in wet true density and different particle sizes of the two resins, the natural layering of the two resins is achieved through backwashing. A double bed containing strong acid resin and weak acid resin is called a cation double bed; A double bed containing both strong and weak alkali resins is called a negative double bed; During operation, water flows through the entire resin layer from top to bottom; During regeneration, the regeneration liquid undergoes countercurrent regeneration from bottom to top; The structure of a double bed ion exchanger is the same as that of a fixed bed for countercurrent regeneration, and the operating steps are also the same.
1. The advantages of double bed: good effluent quality, low dosage of regenerant, large amount of periodic production water, and high average resin production efficiency; Due to the use of weakly alkaline resins, it has certain advantages in preventing organic pollutants from polluting strongly alkaline resins.
2. Disadvantages of double bed: As the resin usage time increases and is contaminated by organic matter and silicon, the specific gravity of the resin will change, resulting in poor layering effect of strong and weak resins. Strong and weak resins will mix with each other, affecting the exchange ability. The experimental results show that for a cation double layer bed, when the resin content of the mixed layer reaches 25%, the average production time will decrease by 5-10%. When the mixed fat of the double layer bed reaches 30%, it will also have a significant impact on the work schedule. In addition, due to the finer particle size of the weak resin, the operating pressure difference is relatively large. Due to the countercurrent regeneration of the double-layer bed, the regeneration flow rate should not be too high, so it is necessary to prevent the deposition of colloidal silicon in the anion double-layer bed. Because during operation, the strong alkali resin absorbs a large amount of silicic acid, during regeneration, the alkali solution first contacts this layer of resin, resulting in a large amount of silicon compounds in the waste alkali solution. When this type of waste liquid comes into contact with weakly alkaline anion resin, OH - in the waste liquid is quickly neutralized by the acid adsorbed by the weakly alkaline resin, causing the pH value of the waste liquid to immediately drop below 8, reducing the solubility of silicic acid, and forming some colloidal silicon to deposit in the resin layer, contaminating the weakly alkaline anion resin, and prolonging the subsequent washing time and reducing the periodic water production.
3. Scope of application of double bed: The positive double bed is suitable for water with high carbonate hardness, while the negative double bed is suitable for water with high strong acid and anion content. General experience suggests that the height of weak resins should be at least 40% or more of the total height of strong and weak resins in order for a double bed to have design value. The double bed has poor adaptability to changes in raw water quality. Because the ratio of strong to weak resin in the bed is fixed, it cannot automatically adapt when the water quality changes. Therefore, the water quality of the water source used must be stable.
4. Common faults of double bed:
4.1. Reduction in periodic water production: The water quality composition of the raw water has changed. Analyze the water quality of the raw water to understand the changes in composition. A slight change in water quality can increase the amount of water produced during the cycle by appropriately changing the strength ratio of the resin while the total height of the resin remains unchanged.
4.2. Long washing time: Weak alkali resin has silicon pollution, and the resin should be properly recovered.
5. Operation and process parameters of double bed

7、 Double chamber fixed bed
A double chamber fixed bed is also known as a double chamber bed. It is an improved bed type based on a double bed. To avoid unclear layering of strong and weak resins in a double bed, a multi empty plate with a bidirectional water cap is installed between the strong and weak resins, dividing the exchanger into upper and lower chambers, hence the name double chamber bed. The weak resin is in the upper chamber, while the strong resin is in the lower chamber, and adopts the same process of downward flow operation and upward flow regeneration as the double layer bed. To prevent a small amount of fine particles in the lower chamber strong resin from blocking the gap of the water cap, an inert resin of 25-30cm is filled between the lower chamber strong resin and the water cap, and the inert resin can also play a spatial buffering role in the volume expansion and contraction of the strong resin during transformation, which is conducive to the regeneration effect of the resin. The lower chamber is usually filled with strong resin and inert resin after regeneration, otherwise, there will be disordered layers of strong resin during regeneration, which will affect the effluent quality. The disordered layer during the regeneration of weak resin in the upper chamber has little impact on its operating effect.
1. The advantage of a dual chamber fixed bed: Since the two resins are completely separated, there are no special requirements for the true density and particle size of the resin. Since there is no need to worry about the mixing of strong and weak resins, the backwashing operation is relatively simple and reassuring. The suspended solids intercepted during operation are mainly intercepted by weak resin, and can be cleaned by backwashing before each regeneration.
2. Disadvantages of dual chamber fixed bed: Due to the resin in the lower chamber being filled, strong resin cannot be backwashed in vivo, and a separate external cleaning tank must be installed for regular external cleaning. When the strong resin fails, a certain height of water cushion will appear, so if you do not pay attention during regeneration, there will be disordered layers, which will affect the water quality of the effluent, and even regeneration failure.
3. The water quality conditions suitable for a double chamber fixed bed: Due to its strong weak combination process, the adaptation conditions for a double chamber bed are similar to those for a double bed.
4. Operating process parameters of dual chamber fixed bed: