Nanofiltration coupling induced crystallization wastewater hardness removal system

Abstract

The utility model belongs to the field of water treatment, and particularly relates to a wastewater hardness removal system for nanofiltration coupling induced crystallization. The wastewater hardness removal system comprises a nanofiltration unit, a gypsum reactor and an ettringite reactor. Wherein the nanofiltration unit is configured to add a scale inhibitor into the wastewater, so that calcium ions and sulfate ions are supersaturated, and divalent salt is intercepted, so that the divalent salt is enriched on a nanofiltration concentrated water side; the gypsum reactor is communicated with a nanofiltration concentrated water liquid outlet of the nanofiltration unit and is configured to add an agent for destroying a scale inhibitor, so that calcium ions and sulfate ions are combined to produce calcium sulfate crystals; the ettringite reactor is communicated with the liquid outlet of the gypsum reactor, and is configured to add alkali and aluminum salt, further remove hardness of the liquid discharged from the gypsum reactor, and produce ettringite. Therefore, the consumption of the chemical agent is low, the subsequently added sodium carbonate can be reduced, meanwhile, the increase of ion concentration caused by adding the chemical softening agent is reduced, and the treatment pressure of a subsequent water treatment system is reduced.

Description

Technical Field

[0001] This disclosure pertains to the field of water treatment, and specifically relates to a wastewater hardness removal system using nanofiltration coupled with induced crystallization. Background Technology

[0002] High-salinity, high-hardness wastewater refers to wastewater containing high concentrations of dissolved salts (such as chlorides and sulfates) and hardness components (mainly calcium and magnesium ions). The generation of saline wastewater is widespread, and its volume is increasing year by year. Removing organic pollutants from saline wastewater is crucial to mitigating its environmental impact.

[0003] Existing wastewater hardness removal systems generally remove hardness through "lime-soda ash" or "caustic soda-soda ash" softening. This has problems such as high consumption of chemical agents and large amounts of solid waste sludge. Moreover, the addition of chemical softening agents increases the ion concentration in the wastewater, which increases the processing pressure on subsequent water treatment systems, especially membrane concentration and evaporation crystallization systems. Utility Model Content

[0004] The purpose of this disclosure is to overcome the shortcomings of the prior art and provide a wastewater hardness removal system using nanofiltration coupled to induce crystallization.

[0005] This disclosure provides a wastewater hardness removal system using nanofiltration coupled with induced crystallization, comprising:

[0006] A nanofiltration unit is configured to add a scale inhibitor to the wastewater to supersaturate calcium and sulfate ions and retain divalent salts, thereby enriching the divalent salts on the nanofiltration concentrate side.

[0007] A gypsum reactor is connected to the nanofiltration concentrate outlet of the nanofiltration unit and is configured to add an agent that destroys scale inhibitors, so that calcium ions combine with sulfate ions to produce calcium sulfate crystals.

[0008] An ettringite reactor, wherein the outlets of the ettringite reactor and the gypsum reactor are connected, and the reactor is configured to add alkali and aluminum salt to further deharden the effluent from the gypsum reactor and produce ettringite.

[0009] In one embodiment of this disclosure, the alkali is sodium hydroxide and the aluminum salt is aluminum sulfate.

[0010] In one embodiment of this disclosure, the scale inhibitor is a calcium sulfate scale inhibitor, and the agent that destroys the scale inhibitor is polyaluminum chloride or polyferric sulfate.

[0011] In one embodiment of this disclosure, the wastewater hardness removal system further includes a pretreatment unit configured to remove colloids, suspended solids and impurity particles from the wastewater and output the resulting clarified liquid to the nanofiltration unit.

[0012] In one embodiment of this disclosure, the wastewater hardness removal system further includes a calcium carbonate reactor, which is connected to the outlet of the ettringite reactor and is configured to add sodium carbonate to the produced clear liquid to remove small amounts of calcium ions from the liquid and produce calcium carbonate.

[0013] In one embodiment of this disclosure, the wastewater hardness removal system further includes a tubular microfiltration unit connected to the outlet of the calcium carbonate reactor and configured to further filter out the produced calcium carbonate particles.

[0014] In one embodiment of this disclosure, the wastewater hardness removal system further includes an ion exchange unit, which is connected to the outlet of the tubular microfiltration unit and is configured to remove residual calcium and magnesium ions in the liquid and output the liquid to a sodium sulfate evaporation and crystallization unit for evaporation and crystallization.

[0015] In one embodiment of this disclosure, the wastewater hardness removal system further includes a sodium chloride evaporation and crystallization unit, which is connected to the outlet of the nanofiltration permeate of the nanofiltration unit and is configured to evaporate and crystallize the nanofiltration permeate.

[0016] In one embodiment of this disclosure, acid is added to the liquid after it has been treated by the tubular microfiltration unit.

[0017] In one embodiment of this disclosure, the regenerated waste liquid generated during the regeneration process of the ion exchange unit is output to the pretreatment unit for treatment together with the wastewater.

[0018] One of the beneficial effects of the nanofiltration coupled induced crystallization wastewater hardness removal system disclosed herein is that by adding a scale inhibitor to the nanofiltration unit to make calcium ions and sulfate ions supersaturated, they are enriched on the nanofiltration concentrate side. Then, a reagent is added to the produced nanofiltration concentrate to destroy the scale inhibitor and precipitate the supersaturated calcium sulfate. Afterward, alkali and aluminum salts are added to the produced clear liquid to further remove calcium ions. In this way, the consumption of chemical reagents is reduced, the amount of soda ash added later is reduced, and the increase in ion concentration caused by the addition of chemical softening agents is reduced, thereby reducing the treatment pressure on the subsequent water treatment system. Attached Figure Description

[0019] Embodiments of this disclosure are illustrated in conjunction with the accompanying drawings, which are included and form part of this specification, and together with their description serve to explain the principles of this disclosure.

[0020] Figure 1 This is a schematic diagram of a wastewater hardness removal system using nanofiltration coupled-induced crystallization provided in an embodiment of this disclosure;

[0021] Figure 2 This is a flowchart of a wastewater hardness removal system using nanofiltration coupled-induced crystallization, provided in one embodiment of this disclosure.

[0022] Figures 1 to 2 The correspondence between the component names and the reference numerals in the figures is as follows:

[0023] 1 Pretreatment unit, 111 Sludge scraper, 112 Sludge return pump, 12 Sand filter, 13 Ultrafiltration;

[0024] 2 nanofiltration units, 21 first dosing device;

[0025] 3. Gypsum reactor; 31. Second dosing device;

[0026] 4. Erythrite reactor; 41. Third dosing device; 42. Fourth dosing device;

[0027] 5. Calcium carbonate reactor; 51. Fifth dosing device;

[0028] 6-tube microfiltration unit, 61 sixth dosing device;

[0029] 7 ion exchange units;

[0030] 80 Sludge storage tank, 81 Lifting pump, 82 Sludge discharge pump, 83 Circulation pump, 84 Sodium chloride evaporation and crystallization unit, 85 Sodium sulfate evaporation and crystallization unit. Detailed Implementation

[0031] Various exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the present disclosure.

[0032] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this disclosure or its application or use.

[0033] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0034] In all the examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0035] It should be noted that similar labels and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be discussed further in subsequent figures.

[0036] In this article, terms such as "up," "down," "front," "back," "left," and "right" are used only to indicate the relative positional relationship between related parts, rather than to define the absolute position of these related parts.

[0037] In this article, "first," "second," etc., are used only to distinguish one another, and not to indicate degree of importance, order, or prerequisite for each other.

[0038] In this document, terms such as “equal” and “same” are not strict mathematical and / or geometric limitations, but also include errors that are understandable to those skilled in the art and permissible in manufacturing or use.

[0039] Existing wastewater hardness removal systems generally remove hardness through "lime-soda ash" or "caustic soda-soda ash" softening. This has problems such as high consumption of chemical agents and large amounts of solid waste sludge. Moreover, the addition of chemical softening agents increases the ion concentration, which increases the processing pressure on subsequent water treatment systems.

[0040] To this end, this disclosure provides a wastewater hardness removal system using nanofiltration coupled with induced crystallization, comprising a nanofiltration unit, a gypsum reactor, and an ettringite reactor. The nanofiltration unit is configured to add a scale inhibitor to the wastewater to supersaturate calcium and sulfate ions, preventing crystallization and retaining divalent salts, which are then enriched on the nanofiltration concentrate side. The gypsum reactor and the nanofiltration unit's nanofiltration concentrate outlets are connected, and the gypsum reactor is configured to add an agent that destroys the scale inhibitor, causing calcium ions to combine with sulfate ions to produce calcium sulfate crystals. The ettringite reactor and the gypsum reactor's outlets are connected, and the gypsum reactor is configured to add alkali and aluminum salts to further remove hardness from the gypsum reactor's effluent and produce ettringite.

[0041] This disclosure involves adding a scale inhibitor to the nanofiltration unit to supersaturate calcium and sulfate ions, causing them to accumulate on the nanofiltration concentrate side. Then, a reagent is added to the produced nanofiltration concentrate to destroy the scale inhibitor and precipitate the supersaturated calcium sulfate. Subsequently, alkali and aluminum salts are added to the produced clarified liquid to further utilize the sulfate ions present in the wastewater to remove calcium ions. This reduces the consumption of chemical reagents and the subsequent addition of soda ash, while also reducing the ion concentration caused by the addition of chemical softening agents, thereby reducing the treatment pressure on the subsequent water treatment system.

[0042] For ease of understanding, please refer to the following: Figure 1 and Figure 2 The specific structure and working principle of the nanofiltration coupled induced crystallization wastewater hardness removal system disclosed herein will be described in detail with reference to the embodiments.

[0043] In one embodiment of this disclosure, the nanofiltration coupled induced crystallization wastewater hardness removal system includes a nanofiltration unit 2, a gypsum reactor 3, and an ettringite reactor 4. The nanofiltration unit 2 is configured to add a scale inhibitor to the wastewater to supersaturate calcium and sulfate ions, preventing crystallization and retaining divalent salts, which accumulate on the nanofiltration concentrate side. The gypsum reactor 3 is connected to the nanofiltration concentrate outlet of the nanofiltration unit 2 and is configured to add an agent that destroys the scale inhibitor, causing calcium ions to combine with sulfate ions to produce calcium sulfate crystals. The ettringite reactor 4 is connected to the outlet of the gypsum reactor 3 and is configured to add alkali and aluminum salts to further remove hardness from the effluent of the gypsum reactor 3 and produce ettringite.

[0044] In detail, the nanofiltration unit 2 of this disclosure uses a nanofiltration membrane to retain divalent salts in high-salt, high-hardness wastewater. Simultaneously, a scale inhibitor is added to the nanofiltration unit 2 via a first dosing device 21 to supersaturate calcium and sulfate ions, ensuring that the ion product of calcium and sulfate ions is 4-6 times the solubility product of calcium sulfate, thus preventing calcium sulfate from crystallizing. The nanofiltration membrane (NF) is a pressure-driven membrane separation technology intermediate between ultrafiltration and reverse osmosis, with a pore size typically around 1 nanometer. The main characteristic of nanofiltration membranes is their ability to effectively retain multivalent ions (such as divalent cations and anions), small organic molecules, and some monosaccharides, while allowing some monovalent ions and water molecules to pass through. When liquid passes through the nanofiltration membrane, larger molecules and multivalent ions are retained on the concentrate side, while smaller molecules and monovalent ions flow out with the permeate. Simultaneously, the added scale inhibitor, through chelation, reacts with scale-forming ions (such as calcium carbonate, calcium sulfate, and calcium sulfate). 2+ Mg 2+ They combine to form stable, soluble complexes, thereby preventing these ions from participating in the formation of calcium sulfate crystals.

[0045] Furthermore, the nanofiltration concentrate enriched with divalent salts can be pumped to a stirred tank via a booster pump 81 for thorough stirring to ensure uniform distribution of dissolved substances and prevent precipitation, thus preparing for the next treatment step. The stirred nanofiltration concentrate then enters the gypsum reactor 3, where a scale inhibitor-destroying agent is added via a second dosing device 31, rendering the scale inhibitor ineffective. Simultaneously, a stirrer is installed in the gypsum reactor 3 to ensure uniform mixing of the added agent with the liquid, accelerating the reaction rate. At this point, supersaturated calcium ions combine with sulfate ions to produce calcium sulfate crystals. Further, the calcium sulfate precipitates and separates, producing gypsum. The gypsum produced in this disclosure is pumped to a sludge storage tank 80 via a sludge discharge pump 82. In some embodiments, the produced gypsum can also be dehydrated and dried for resource utilization as building materials, etc.

[0046] Subsequently, the supernatant from gypsum reactor 3 enters ettringite reactor 4 via booster pump 81. Alkali is added to the reactor 41 through a third dosing device 41 to adjust the pH of the liquid, ensuring that the pH range of the liquid in ettringite reactor 4 is 9-12. This ensures the stable generation of ettringite (3CaO·Al2O3·3CaSO4·32H2O). A pH sensor is installed in ettringite reactor 4 to monitor the pH value of the liquid. Based on the data from the pH sensor, the alkali dosage can be automatically adjusted by a PLC (Programmable Logic Controller) or other automated control system to maintain the set pH range.

[0047] Furthermore, the pH of the liquid can be controlled at 11.5, and then aluminum salt is added through the fourth dosing device 42 to produce ettringite, further removing calcium ions from the high-salt, high-hardness wastewater. Simultaneously, a stirrer is installed in the ettringite reactor 4 to ensure uniform mixing of the added reagents with the liquid, guaranteeing even distribution and full contact of all components, thus accelerating the reaction rate. Then, the ettringite is separated by precipitation and discharged to the sludge storage tank 80 via the sludge pump 82. In some embodiments, the produced ettringite can also be dehydrated and dried for resource utilization. As a concrete expansion agent, ettringite performs excellently in compensating for concrete shrinkage, effectively preventing structural cracking and improving the overall stability and durability of buildings.

[0048] Furthermore, the dosing device used in this disclosure includes a metering pump and a storage tank. The metering pump can automatically adjust the dosage of the reagent according to a set flow rate or a feedback signal for precise reagent dosing. The storage tank stores the required chemical reagent and is equipped with a stirrer to maintain uniform mixing of the reagent. In some embodiments, a proportioning controller may also be included to automatically adjust the reagent dosage ratio according to changes in influent flow rate and pH value to ensure optimal treatment results.

[0049] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the alkali is sodium hydroxide and the aluminum salt is aluminum sulfate.

[0050] Thus, in this disclosure, the alkali added to the ettringite reactor 4 is sodium hydroxide, and the aluminum salt added is aluminum sulfate. The use of sodium hydroxide helps maintain the required high pH value. This helps ensure the stability of aluminates and other intermediate products and promotes the formation of the final ettringite crystals. At the same time, the use of sodium hydroxide provides hydroxide ions, allowing magnesium ions in the liquid to react with hydroxide ions to form magnesium hydroxide precipitate, which is then discharged to the sludge storage tank 80 via sludge pump 82.

[0051] Using aluminum sulfate to provide the aluminum source for the ettringite reaction provides both aluminum and sulfate ions, avoiding the introduction of other high-valence ions and reducing the pressure on subsequent hardening treatment. Aluminum sulfate is a common industrial chemical, readily available and relatively inexpensive. It works in conjunction with calcium and sulfate ions to help form a stable ettringite crystal structure. Compared to other synthetic methods, using aluminum sulfate to generate ettringite does not require extreme temperature or pressure conditions, typically operating from room temperature to around 80°C, simplifying the process. Furthermore, when using aluminum sulfate in the treated liquid to generate ettringite, pH control is better possible; by adding an appropriate amount of alkaline substance, optimal ettringite formation conditions (pH 9-12) can be ensured. This flexibility helps optimize the reaction and reduce the formation of byproducts.

[0052] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the scale inhibitor is calcium sulfate scale inhibitor, and the agent that destroys the scale inhibitor is polyaluminum chloride or polyferric sulfate.

[0053] This disclosure uses a calcium sulfate scale inhibitor to supersaturate the liquid with calcium and sulfate ions. When the concentration of calcium and sulfate ions in the solution exceeds their solubility limits under specific temperature and pH conditions, the solution is in a supersaturated state. Under these conditions, without appropriate inhibition measures, these supersaturated ions easily form calcium sulfate precipitates. The calcium sulfate scale inhibitor can inhibit the formation of calcium sulfate by complexing or chelating calcium and sulfate ions, reducing their activity. If supersaturated calcium and sulfate ions form calcium sulfate particles, the scale inhibitor can also disperse and suspend the tiny calcium sulfate crystals in the water, preventing them from agglomerating and forming larger scale flakes.

[0054] Subsequently, this disclosure utilizes polyaluminum chloride (PAC) to disrupt the calcium sulfate scale inhibitor, causing the supersaturated calcium sulfate to spontaneously crystallize and precipitate. This allows for the removal of calcium ions from the sulfate ions naturally present in the high-salt, high-hardness wastewater, thereby significantly reducing the amount of subsequent chemical agents such as aluminum sulfate and sodium carbonate used. Alternatively, polyferric sulfate (PFS) can also be added to disrupt the calcium sulfate scale inhibitor.

[0055] Furthermore, this disclosure does not limit the agents used to destroy scale inhibitors; those skilled in the art can determine the appropriate agents based on the specific circumstances that enable the scale inhibitors to be destroyed.

[0056] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the wastewater hardness removal system further includes a pretreatment unit 1, which is configured to remove colloids, suspended solids and impurity particles from high-salt, high-hardness wastewater and output the resulting clear liquid to a nanofiltration unit 2.

[0057] The pretreatment unit 1 of this disclosure includes coagulation sedimentation, sand filtration 12, and ultrafiltration 13. Suspended solids, colloidal particles, and dissolved organic matter in the water are removed through coagulation sedimentation.

[0058] During coagulation and sedimentation, high-salt, high-hardness wastewater first enters the coagulation tank. Coagulants are then rapidly added to the reaction tank, allowing the chemicals to fully contact the water and complete charge neutralization and initial coagulation within a short time. This process can disrupt the stability of colloids in the water by adding chemical agents (coagulants and flocculants), causing tiny particles to aggregate into larger flocs, making them easier to separate from the water through gravity sedimentation.

[0059] The water then flows into the flocculation tank, where it undergoes a longer period of gentle agitation, prompting the coagulated small particles to further aggregate into larger flocs. Through slow stirring or mixing, the already coagulated small particles further collide and combine to form even larger flocs. During this process, coagulant aids (such as polyacrylamide PAM) may be added to enhance the floc structure, making it more stable and easier to settle.

[0060] After coagulation and flocculation treatment, the water enters a sedimentation tank, where larger flocs gradually sink to the bottom due to their own weight, while the clarified supernatant is collected as the treated effluent. Simultaneously, the water is slowly stirred or mixed using a mixer to further promote the sedimentation of suspended solids.

[0061] The settled sludge needs to be removed regularly. The sludge deposited at the bottom of the sedimentation tank is collected and removed by the sludge scraper 111, and then discharged to the sludge storage tank 80 by the sludge discharge pump 82. The system is also equipped with a sludge return pump 112 to reintroduce the activated sludge at the bottom of the sedimentation tank to the previous treatment tank and the agitator of the sedimentation tank, so as to maintain the microbial concentration in the system and promote the degradation of organic matter. Reasonable control of the sludge return flow can effectively manage the amount of residual sludge generated and reduce the cost of sludge treatment.

[0062] Sand filters are primarily used to remove suspended solids, colloidal substances, and some dissolved pollutants from water. They achieve purification by allowing water to flow through one or more layers of filter media composed of sand of varying particle sizes. As water flows through the sand layer, larger suspended particles are directly blocked on the surface of the filter media or between its pores. Some smaller particles, due to inertia, deviate from their streamlines and collide with and adhere to the surface of the sand particles. Even finer particles gradually approach the sand particles under the influence of Brownian motion and eventually settle. The charged surface of the sand particles has a certain adsorption capacity for some organic matter or metal ions, promoting their coagulation and deposition.

[0063] Ultrafiltration is a membrane separation technology that effectively removes suspended solids, colloids, microorganisms (such as bacteria and viruses), large organic molecules, and other fine particles from water. It works by selectively retaining particles larger than the membrane pore size, preventing them from passing through.

[0064] Thus, through the pretreatment unit 1, the high-salt and high-hardness wastewater of this disclosure can be effectively pretreated to remove suspended solids and fine impurity particles, so that the liquid does not affect the operation of the nanofiltration membrane when it enters the nanofiltration unit 2.

[0065] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the wastewater hardness removal system further includes a calcium carbonate reactor 5, which is connected to the outlet of the ettringite reactor 4 and is configured to add sodium carbonate to the produced clear liquid to remove a small amount of calcium ions in the liquid and produce calcium carbonate.

[0066] In detail, the clarified liquid produced by the ettringite reactor 4 of this disclosure is fed into the calcium carbonate reactor 5. Sodium carbonate is then added to the calcium carbonate reactor 5 via a fifth dosing device 51 to further remove calcium ions from the liquid. A stirrer ensures that the sodium carbonate is fully dissolved and uniformly mixed with the water, allowing all calcium ions to react with it. In some embodiments, a small amount of coagulant aid is sometimes added to accelerate the calcium carbonate precipitation process and improve the floc structure. After precipitation, the calcium carbonate produced in this disclosure is discharged to the sludge storage tank 80 via a sludge pump 82. Alternatively, the produced calcium carbonate can be collected and properly treated for resource utilization.

[0067] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the wastewater hardness removal system further includes a tubular microfiltration unit 6, which is connected to the outlet of the calcium carbonate reactor 5 and is configured to further filter the produced calcium carbonate particles.

[0068] After removing calcium ions from the water using sodium carbonate and forming calcium carbonate precipitate, this disclosure uses a circulating pump 83 to output the liquid to a tubular microfiltration unit 6 for further treatment. The tubular microfiltration membrane provides highly efficient filtration performance, ensuring the removal of calcium carbonate particles from the liquid while maintaining high flux and a long operating cycle. The tubular microfiltration membrane effectively removes suspended solids and colloidal particles from the water, has a larger flow area, and better antifouling ability. The circulating pump 83 uses mechanical energy to drive the liquid to circulate within the pipeline system, providing a certain amount of power and heat. Since the solubility of calcium carbonate decreases with increasing temperature, the circulating pump 83 allows calcium carbonate in the liquid to further precipitate. This precipitated calcium carbonate is then filtered in the next step, the tubular microfiltration unit 6, and subsequently transported to the sludge storage tank 80.

[0069] Furthermore, the sludge storage tank 80 is an important component of the system, used for temporary storage of sludge after preliminary treatment or concentration. A stirring device can be used in the sludge storage tank 80 to maintain uniform sludge distribution, prevent stratification, and improve the effectiveness of subsequent treatment. This disclosure describes the dewatering treatment of the sludge in the sludge storage tank 80 before its disposal.

[0070] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the wastewater hardness removal system further includes an ion exchange unit 7, which is connected to the outlet of the tubular microfiltration unit 6 and is configured to remove residual calcium and magnesium ions in the liquid and output the liquid to the sodium sulfate evaporation and crystallization unit 85 for evaporation and crystallization.

[0071] This disclosure includes an ion exchange unit 7 in the later stage of the hardening treatment. A booster pump 81 outputs liquid to the ion exchange unit 7 to remove residual calcium and magnesium ions. This ensures that the unit is not subjected to excessively high loads for a long time while hardening is being removed, thereby extending its service life and ensuring exchange efficiency.

[0072] The ion exchange resin used in ion exchange unit 7 is a functional material widely used in water treatment, industrial separation, and purification processes. It selectively removes specific ions from a solution through physical adsorption or chemical reaction, thereby achieving water softening, desalination, and heavy metal removal. This disclosure uses a cation exchange resin, which typically contains acidic groups such as sulfonic acid groups (-SO3H) and carboxyl groups (-COOH), which can release hydrogen ions or sodium ions and react with other cations in the solution (such as Ca2+). 2+ Mg 2+ Fe 3+ The resin undergoes exchange (e.g., through various processes). Furthermore, necessary monitoring instruments (such as conductivity meters and pH meters) can be installed in the ion exchange unit 7 to track resin status changes in real time and maintain detailed operational records, enabling timely detection and resolution of problems.

[0073] Furthermore, this disclosure describes the process of supplying the liquid treated by the ion exchange unit 7 to the sodium sulfate evaporation and crystallization unit 85 via a booster pump 81. This removes water to increase the concentration of sodium sulfate in the solution until it reaches a supersaturated state, thereby promoting the crystallization of sodium sulfate. In some embodiments, a small amount of pre-prepared sodium sulfate crystals can be introduced as seed crystals to guide the molecules in the solution to arrange themselves in a specific pattern, promoting the formation of ideal crystals. Alternatively, appropriate additives (such as dispersants, stabilizers, etc.) can be added to improve the crystal structure and enhance the purity and quality of the product.

[0074] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, the wastewater hardness removal system further includes a sodium chloride evaporation and crystallization unit 84, which is connected to the nanofiltration permeate outlet of the nanofiltration unit 2 and is configured to evaporate and crystallize the nanofiltration permeate.

[0075] In detail, water pretreated by nanofiltration has had most of its multivalent ions and other impurities removed, significantly reducing the calcium and magnesium ion content in the influent, decreasing the likelihood of scaling during evaporation, and lowering the processing difficulty and energy consumption of the evaporation system. Evaporation of nanofiltration permeate not only yields high-quality freshwater but also recovers valuable sodium chloride crystals, achieving dual utilization of both water and salt resources.

[0076] During evaporation, the concentration of sodium chloride continuously increases. When it reaches its solubility limit, the supersaturated sodium chloride will form crystals and precipitate. Heating causes water to change from a liquid to a gaseous state, while non-volatile substances such as sodium chloride remain in the solution. As water continues to evaporate, the concentration of sodium chloride in the solution gradually increases, eventually reaching saturation and beginning to crystallize.

[0077] like Figure 1 and Figure 2 As shown, in one embodiment of this disclosure, acid is added to the liquid after it has been treated by the tubular microfiltration unit 6.

[0078] In detail, acid is added to the liquid treated by the tubular microfiltration unit 6 via the sixth dosing device 61. In this disclosure, sulfuric acid is added to the liquid to adjust its pH without introducing other ions. At the same time, a pH sensor is set to keep the pH of the liquid neutral or slightly alkaline (pH 7-9). Based on the data from the pH sensor, the dosage of the reagent can be automatically adjusted by a PLC (programmable logic controller) or other automated control system to maintain the set pH range. In this way, the next ion exchange unit 7 can exhibit optimal exchange capacity and stability.

[0079] like Figure 2 As shown, in one embodiment of this disclosure, the regenerated waste liquid generated during the regeneration process of the ion exchange unit 7 is output to the pretreatment unit 1 for treatment together with the high-salt, high-hardness wastewater.

[0080] The regeneration process restores the exchange capacity of ion exchange resins and removes impurities adsorbed or bound during use. The resulting regeneration wastewater typically contains high concentrations of salts (such as calcium chloride CaCl2 and magnesium chloride MgCl2) and sometimes residual acids (such as HCl or H2SO4). Because it contains calcium ions, magnesium ions, and sulfate ions, the regeneration wastewater can be treated together with the high-salt, high-hardness wastewater described in this disclosure.

[0081] For better understanding, please refer to the following... Figure 1 and Figure 2 The working principle of the nanofiltration coupled induced crystallization wastewater hardness removal system provided in this disclosure will be explained in detail with an application scenario.

[0082] High-salt, high-hardness wastewater enters pretreatment unit 1, where suspended solids are removed through coagulation and sedimentation. Then, it passes through sand filter 12 and ultrafiltration 13 to further remove fine impurities, colloidal substances, and some dissolved pollutants from the water.

[0083] Then, the divalent salt is retained to the concentrate side in nanofiltration unit 2, and the nanofiltration permeate is output to sodium chloride evaporation and crystallization unit 84 to produce sodium chloride crystals. At the same time, calcium sulfate scale inhibitor is added to the nanofiltration concentrate to make calcium ions and sulfate ions supersaturated and prevent precipitation.

[0084] Subsequently, polyaluminum chloride (PAC) is added to gypsum reactor 3 to disrupt the form of the calcium sulfate scale inhibitor, allowing calcium ions to combine with sulfate ions to produce calcium sulfate crystals. Furthermore, the flocculation effect of PAC causes the calcium sulfate to precipitate and separate, producing gypsum. The supernatant from gypsum reactor 3 enters ettringite reactor 4, where sodium hydroxide is added until the solution pH reaches 11.5, followed by the addition of aluminum sulfate. The reaction produces ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) and magnesium hydroxide, which are separated from the solution through precipitation. The supernatant from ettringite reactor 4 enters calcium carbonate reactor 5, where sodium carbonate is added to further remove calcium ions, generating calcium carbonate precipitate. The liquid then passes through a tubular microfiltration unit 6 to further remove calcium carbonate particles.

[0085] The liquid output from the tubular microfiltration unit 6, after pH adjustment, enters the ion exchange unit 7 to remove all the remaining trace amounts of calcium and magnesium ions in the high-salt, high-hardness wastewater. The liquid then enters the sodium sulfate evaporation and crystallization system to produce sodium sulfate crystals.

[0086] The generated gypsum, ettringite, and calcium carbonate can be directly transported to sludge storage tank 80 for disposal along with the pretreated sludge, or they can be dehydrated and dried separately for resource utilization. When the ion exchange unit 7 is regenerated, its regeneration wastewater can be treated together with the high-salt, high-hardness wastewater of this disclosure.

[0087] In this way, the present disclosure adds a scale inhibitor to nanofiltration unit 2 to make calcium ions and sulfate ions supersaturated, so that they are enriched on the nanofiltration concentrate side. Then, a chemical agent is added to the produced nanofiltration concentrate to destroy the scale inhibitor and precipitate the supersaturated calcium sulfate. After that, alkali and aluminum salt are added to the produced clear liquid to further remove calcium ions. In this way, the consumption of chemical agents is less, and the amount of soda ash added later is reduced. At the same time, the ion concentration caused by the addition of chemical softening agents is reduced, thereby reducing the treatment pressure on the subsequent water treatment system.

[0088] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, and are not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein. The scope of this disclosure is defined by the appended claims.

Claims

1. A wastewater hardness removal system using nanofiltration coupled with induced crystallization, characterized in that, include: Nanofiltration unit (2), the nanofiltration unit (2) is configured to add scale inhibitor to wastewater to make calcium ions and sulfate ions supersaturated and to retain divalent salts, so that the divalent salts are enriched on the nanofiltration concentrate side; The gypsum reactor (3) is connected to the nanofiltration concentrate outlet of the nanofiltration unit (2) and is configured to add an agent that destroys the scale inhibitor, so that calcium ions combine with sulfate ions to produce calcium sulfate crystals. The ettringite reactor (4) is connected to the outlet of the gypsum reactor (3) and is configured to add alkali and aluminum salt to further harden the effluent from the gypsum reactor (3) and produce ettringite.

2. The wastewater hardness removal system using nanofiltration coupled with induced crystallization according to claim 1, characterized in that, The alkali is sodium hydroxide, and the aluminum salt is aluminum sulfate.

3. The wastewater hardness removal system using nanofiltration coupled with induced crystallization according to claim 2, characterized in that, The scale inhibitor is a calcium sulfate scale inhibitor, and the agent that destroys the scale inhibitor is polyaluminum chloride or polyferric sulfate.

4. The wastewater hardness removal system using nanofiltration coupled with induced crystallization according to claim 3, characterized in that, The wastewater hardness removal system also includes a pretreatment unit (1), which is configured to remove colloids, suspended solids and impurity particles from the wastewater and output the resulting clear liquid to the nanofiltration unit (2).

5. The wastewater hardness removal system using nanofiltration coupled with induced crystallization according to claim 4, characterized in that, The wastewater hardness removal system also includes a calcium carbonate reactor (5), which is connected to the outlet of the ettringite reactor (4) and is configured to add sodium carbonate to the produced clear liquid to remove a small amount of calcium ions in the liquid and produce calcium carbonate.

6. The wastewater hardness removal system using nanofiltration coupled-induced crystallization according to claim 5, characterized in that, The wastewater hardness removal system also includes a tubular microfiltration unit (6), which is connected to the outlet of the calcium carbonate reactor (5) and is configured to further filter out the calcium carbonate particles produced.

7. The wastewater hardness removal system using nanofiltration coupled with induced crystallization according to claim 6, characterized in that, The wastewater hardness removal system also includes an ion exchange unit (7), which is connected to the outlet of the tubular microfiltration unit (6) and is configured to remove residual calcium and magnesium ions in the liquid and output the liquid to the sodium sulfate evaporation and crystallization unit (85) for evaporation and crystallization.

8. The wastewater hardness removal system using nanofiltration coupled-induced crystallization according to any one of claims 2 to 7, characterized in that, The wastewater hardness removal system also includes a sodium chloride evaporation and crystallization unit (84), which is connected to the outlet of the nanofiltration product water of the nanofiltration unit (2) and is configured to evaporate and crystallize the nanofiltration product water.

9. The wastewater hardness removal system using nanofiltration coupled-induced crystallization according to claim 6, characterized in that, Acid is added to the liquid after it has been treated by the tubular microfiltration unit (6).

10. The wastewater hardness removal system using nanofiltration coupled with induced crystallization according to claim 7, characterized in that, The regenerated waste liquid generated during the regeneration process of the ion exchange unit (7) is output to the pretreatment unit (1) for treatment together with the wastewater.

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