Solutions for deteriorated water quality from ion exchange equipment in softened water

The effluent water quality in water softening equipment is the main indicator for evaluating the operating conditions of chemical demineralization equipment. Deterioration of effluent water quality refers to the situation where, during the operating cycle, the conductivity and SiO2 content of the demineralized water are significantly higher than the commissioning results. Regardless of whether the water quality indicators meet the standards, it can be considered that effluent water quality deterioration has occurred.

Ion exchange is the process of separating electrolyte-containing liquid mixtures using ion exchange agents. The ion exchange process is a mass transfer and chemical reaction process between liquid and solid phases. Usually, ion exchange reactions proceed very quickly, and the process rate is mainly determined by the mass transfer rate.

Ion exchange reactions are generally reversible. Under certain conditions, the exchanged ions can be desorbed, restoring the ion exchange agent to its original state. That is, ion exchange agents can be used repeatedly through exchange and regeneration. At the same time, ion exchange reactions are quantitative, so the exchange capacity of ion exchange agents is limited.

Weak acid cation bed:
1. The effluent alkalinity leakage is higher than the specified value. This is because the regeneration is not appropriate. The regenerant should be 110% of the theoretical exchange capacity. If series regeneration is used, the amount of acid after regenerating the strong acid resin should be checked to ensure it is sufficient to regenerate the weak acid resin.
2. The effluent hardness is higher than the specified value. If sulfuric acid is used for regeneration, calcium sulfate precipitation may occur. In this case, calcium sulfate should be gradually hydrolyzed, which will produce calcium hardness. When using sulfuric acid for regeneration, a stepwise regeneration method should be adopted, using a low concentration and high flow rate first, followed by a high concentration and low flow rate. If series regeneration is used, it is necessary to check whether the regeneration waste liquid of the strong acid cation resin has been diluted.

Strong acid cation bed:
1. Effluent sodium leakage is higher than the specified value. The regeneration steps need to be checked. Sometimes, the cation bed is regenerated in series using the mixed bed regeneration waste liquid. In this case, attention should be paid to the initial 15-30% of the mixed bed waste liquid, otherwise sodium ions will enter the cation bed. In addition, the amount of acid in the mixed bed waste liquid should be checked to ensure it is sufficient.
2. Effluent hardness leakage. If sulfuric acid is used for regeneration, due to calcium sulfate precipitation, the acid concentration and regeneration flow rate should be checked. If the amount of calcium ions in the water exceeds 50% of the total ions, graded regeneration should be adopted, and the initial concentration should not exceed 2%.

Ion exchange softening is a water treatment method that uses ion exchange agents to reduce the hardness of water. Commonly used ion exchange agents include sodium-type resin (RNa), strong acid H (RH)-type resin, and weak acid H resin. The ion exchange softening system adopts a Na ion exchange softening system and an H-Na ion exchange dealkalization softening system.

Weak base anion bed:
1. Increased mineral acid leakage in the effluent. This problem can be divided into a real increase in mineral acid leakage and an apparent increase in mineral acid leakage.
　　a. Real increase in mineral acid leakage. Generally, the effluent conductivity should be 50μs/cm or less. If the regeneration is insufficient, the conductivity curve will slowly rise, which means that the effluent acidity will gradually increase.
b. Apparent increase in mineral acid leakage. Weak base resin is used as a neutralizer for mineral acids. Real weak base resin will not decompose neutral salts such as sodium chloride or sodium sulfate. Therefore, the cation bed must operate normally, with very little sodium leakage in the effluent, and a certain pH must be maintained. If the pH is greater than 3.5, it means that the cation bed has not completely removed the cations, and these neutral salts will increase the conductivity when passing through the weak base anion bed.
2. High pH, sodium leakage, and increased conductivity. This is due to the mixing of cation resin in the anion resin bed. During alkali regeneration, the cation resin is in the sodium form, and sodium is gradually released during operation. The presence of sodium in the anion bed effluent is due to sodium leakage in the strong acid cation bed effluent.
3. Silica problem. If the anion bed is regenerated in series, this problem is easily produced. The alkali solution after regeneration of the strong base anion bed contains silica. After passing through the weak base anion bed, alkaline neutralization occurs, causing the pH to decrease. When the isoelectric point of silica in the alkali solution is reached, silica precipitates on the resin. During subsequent operation, hydrolysis increases the silica in the effluent.
The solution to this problem is to first remove 15-30% of the alkali solution after regenerating the strong base anion bed, or dilute the alkali solution to 2%, and ensure that NaOH has 130% of the theoretical working exchange capacity.

Strong base anion bed:

Whether it is type I or type II, the key issue is silica leakage. Unlike strong acid cation resin and weak base anion resin, the thermal stability of strong base anion resin is lower, only 60℃ and 40℃,

Otherwise, the resin will degrade. If the strong base groups are lost due to heat and oxidation, this will cause silica leakage. Therefore, the temperature must be kept within the limit during operation.

In addition, strong base anion resin is easily contaminated by organic matter, resulting in the following consequences:
1. pH decrease;

2. Increased conductivity;

3. Increased silica leakage;

4. Increased backwash water volume.

Among them: 1 and 2 are caused by the partial hydrolysis of organic matter on the resin after regeneration, 3 is due to the steric hindrance effect of pollutants, making NaOH regeneration incomplete, and 4 is due to the amphoteric effect of pollutants.

Since the Na ion exchange softening method can only remove hardness from water but not alkalinity, both hardness and alkalinity need to be removed in some water treatment applications. Therefore, H ion exchange and Na ion exchange are set up simultaneously in the water treatment system, which can achieve the purpose of removing both hardness and alkalinity. This method is called the H-Na ion exchange softening method. The H-type ion exchange agent RH in the H-Na ion exchange softening method reacts with carbonate and non-carbonate hardness in water, exhibiting a salt removal and softening effect.

Ion exchange is a primary method for water softening and desalination. In wastewater treatment, it mainly removes metal ions. Ion exchange is essentially an exchange reaction between exchangeable ions on an insoluble ionic compound (ion exchanger) and other ions of the same nature in the solution; it is a special adsorption process (reversible chemical adsorption). Its reaction expression is: RH (exchange resin) + M+ (exchange ion) <=> RM (saturated resin) + H+. In equilibrium,

the concentrations of the reactants conform to the following relationship: [RM][H+]/([RH][M+]) = k,

where k is the equilibrium constant. If k > 1, the reaction proceeds to the right; the larger k is, the more favorable the exchange reaction is; the magnitude of k indicates the ion exchange capacity of the ion exchanger for a certain ion.

Properties of ion exchange resins: effective pH range; exchange capacity; crosslinking degree; exchange potential (the ease with which exchange ions replace exchangeable ions on the resin).
Ion exchange equipment can be divided into fixed-bed and continuous-bed types.
Ion exchange operations involve four steps: exchange, backwashing, regeneration, and rinsing. Exchange: The exchange process is mainly related to the resin bed height, water flow rate, raw water concentration, resin properties, and regeneration degree. Regeneration should be performed when the ion concentration in the water reaches the limit.
Backwashing: Its purpose is to loosen the resin bed for the next regeneration step, ensuring uniform distribution of the regenerant; it also removes impurities, broken particles, and air bubbles from the resin bed.
Regeneration: This is the reverse of the exchange process. A high-concentration regenerant flows through the resin bed, displacing the adsorbed ions and restoring its exchange capacity (important in fixed beds).
Rinsing: The residual regenerant in the resin bed is rinsed away until the effluent water quality meets the requirements.