A process for brackish water reuse
By combining a dual-alkali softening process, coagulation of iron-aluminum bonded covalent silicates, and oxidation of ferrates, the problem of incomplete pretreatment in brackish water reuse has been solved, the recovery rate and desalination rate of the reverse osmosis system have been improved, the operating cost has been reduced, and water quality safety and efficient resource utilization have been achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- INNER MONGOLIA NORMAL UNIVERSITY
- Filing Date
- 2026-04-10
- Publication Date
- 2026-06-09
Smart Images
Figure CN122166957A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to brackish water treatment technology, specifically to a brackish water reuse process. Background Technology
[0002] Brackish water is widely distributed in Northwest, North China and coastal areas of my country. The total dissolved solids are usually between 1,000 and 10,000 mg / L, and it generally contains high concentrations of hardness ions (≥200 mg / L), silica (≥20 mg / L) and total organic carbon (≥5 mg / L). Although this type of water source has abundant water volume, it is difficult to use directly due to its poor water quality and has long been regarded as "ineffective water resources", which has exacerbated the pressure of regional water shortage.
[0003] Currently, the mainstream process for brackish water reuse mostly adopts a combination of "pretreatment + reverse osmosis". However, traditional pretreatment technologies have obvious shortcomings: conventional lime-soda softening methods are not thorough in removing non-carbonate hardness, and produce a large amount of sludge and are complicated to operate; single polyaluminum chloride or polyferric sulfate coagulants have limited efficiency in removing colloidal and dissolved organic matter, and the resulting flocs are loose and have poor settling performance; the subsequent ultrafiltration system is susceptible to fouling by micro-pollutants and organic matter, leading to a rapid increase in transmembrane pressure difference and frequent backwashing, which seriously restricts operational stability. More importantly, high concentrations of silica and hardness ions in the water easily form silica scale and calcium carbonate / calcium sulfate scale on the surface of the reverse osmosis membrane, while existing scale inhibitors (such as polyphosphates or organophosphonates) are not effective in inhibiting complex scale and may cause phosphorus emissions or biotoxicity problems.
[0004] Furthermore, to ensure the quality of reverse osmosis feed water, projects often rely excessively on multi-stage filtration or significantly reduce the system recovery rate (usually only 50% to 70%), which not only increases investment and energy consumption but also generates a large amount of concentrated water, further aggravating the environmental burden. Some processes attempt to introduce ozone or sodium hypochlorite for oxidation, but it is difficult to simultaneously achieve organic matter degradation, microbial inactivation, and coagulation enhancement, and may generate harmful byproducts. Therefore, existing brackish water reuse technologies generally suffer from bottlenecks such as weak pretreatment synergy, difficulty in membrane fouling control, low system recovery rate, and high operating costs, making it difficult to balance water quality safety, resource efficiency, and ecological sustainability, thus trapping brackish water resource utilization in a dilemma of "high investment - low efficiency - high risk".
[0005] Therefore, the applicant proposes a brackish water reuse process to solve the above problems. Summary of the Invention
[0006] The purpose of this invention is to provide a brackish water reuse process to solve the problem that the current brackish water reuse process mainly adopts pretreatment + reverse osmosis, but the traditional pretreatment is not thorough in softening, has poor coagulation effect, weak scale inhibition ability, is prone to membrane fouling and scaling, has a low system recovery rate of only 50% to 70%, high operating cost, and is difficult to balance efficiency, safety and sustainability.
[0007] To achieve the above objectives, the present invention provides the following technical solution: a brackish water reuse process, comprising the following steps:
[0008] S1. Soften raw water using a dual-alkali method: First, add sodium hydroxide to adjust the pH to 9-12, causing the carbonate hardness in the water to be converted into calcium carbonate and magnesium hydroxide precipitates; then add sodium carbonate to convert the non-carbonate hardness into calcium carbonate precipitates, thus obtaining softened water.
[0009] S2. Add an iron-aluminum bonded covalent silicate coagulant to the softened water or the water treated by S3, and carry out a mixing reaction to destabilize the colloidal particles, fine suspended matter and some dissolved organic matter in the water and form dense flocs.
[0010] S3. Add ferrate to the softened water or water treated by S2, and carry out advanced oxidation reaction under pH 8-12 conditions to simultaneously achieve organic matter degradation, microbial inactivation and in-situ coagulation.
[0011] S4. After the oxidation reaction is completed and before solid-liquid separation, add polyacrylic acid as a scale inhibitor and dispersant.
[0012] S5. The mixture obtained in S4 is introduced into an inclined plate sedimentation tank for efficient solid-liquid separation to remove the precipitated sludge and obtain the supernatant.
[0013] S6. Pass the supernatant obtained in S5 into an ultrafiltration system containing an ultrafiltration membrane for precision filtration to remove residual colloids, bacteria and macromolecular organic matter, and obtain ultrafiltration permeate.
[0014] S7. The ultrafiltration permeate obtained in S6 is sent to the reverse osmosis system for desalination treatment to produce reusable permeate and discharge concentrated water.
[0015] Furthermore, in S1, the amount of sodium hydroxide added is 100-1200 mg / L, the amount of sodium carbonate added is 100-800 mg / L, and the reaction time is 20-40 minutes.
[0016] Furthermore, in S2, the molar ratio of iron, aluminum, and silicon in the iron-aluminum bonded covalent silicate coagulant is Fe:Al:Si=(1.0~2.5):1:(15~20), the dosage is 10~60mg / L, the rapid mixing time is 30 seconds to 5 minutes, and the slow flocculation time is 10~40 minutes.
[0017] Furthermore, in step S3, the ferrate is potassium ferrate or sodium ferrate, the dosage is 1.0–60 mg / L, and the oxidation reaction time is 5–40 minutes.
[0018] Furthermore, the execution order of the steps is as follows: first execute S1, then execute S3, and finally execute S2; wherein the ferric iron generated by oxidation in S3 serves as the crystal nucleus for the coagulation reaction in S2. Specifically, in this invention, the order of steps S2 and S3 can be interchanged, either coagulation first and then oxidation, or oxidation first and then coagulation.
[0019] Furthermore, in step S4, the polyacrylic acid has a molecular weight of 2000–12000 Da, a dosage of 0.1–10 mg / L, and is added at a point 1–3 meters before the inlet of the inclined plate sedimentation tank.
[0020] Furthermore, in S5, two-stage inclined plate sedimentation tanks are connected in series, with the surface loading of the first-stage inclined plate sedimentation tank being 5-8 m³ / s. 3 / (m 2 The surface loading of the second-stage inclined plate sedimentation tank is 4–7 m³ / h. 3 / (m 2 ·h).
[0021] Furthermore, in S6, the ultrafiltration membrane has a pore size of 0.01–0.1 μm, an operating pressure of 0.1–0.3 MPa, a backwash cycle of 30–60 minutes, and a chemically enhanced backwash frequency of once a week.
[0022] Furthermore, in step S7, the operating pressure of the reverse osmosis system is 1.2–2.5 MPa, the system recovery rate is 75%–85%, and the desalination rate is ≥98%.
[0023] Furthermore, the raw water has a total dissolved solids content of 5000–11000 mg / L, a hardness of 300–1400 mg / L, a fluoride ion concentration of ≤10 mg / L, and a total organic carbon content of ≤20 mg / L.
[0024] Compared with the prior art, the beneficial effects of the present invention are:
[0025] This invention employs a dual-alkali softening method, using stepwise addition of sodium hydroxide and sodium carbonate to effectively remove carbonate and non-carbonate hardness from brackish water, reducing the concentration of scale-forming ions and solving the problem of incomplete removal of non-carbonate hardness in the traditional lime-soda ash method. It utilizes an iron-aluminum bonded covalent silicate coagulant with a molar ratio of iron, aluminum, and silicon of Fe:Al:Si = (1.0–2.5):1:(15–20). Its unique Si-O-Fe / Al covalent network structure enhances adsorption bridging and trapping capabilities, forming dense flocs, overcoming the limitations of conventional polyaluminum chloride or polysulfate methods. To address the shortcomings of loose iron flocs and poor settling performance, ferrate is introduced for advanced oxidation at pH 8 to 12, simultaneously achieving deep degradation of organic matter, efficient inactivation of microorganisms, and in-situ generation of ferric iron to participate in subsequent flocculation, eliminating the need for secondary chemical addition and achieving synergistic integration of oxidation and coagulation. Furthermore, if the turbidity of the raw water after the dual-alkali process is ≤3 NTU, ferrate oxidation can be pre-treated, preceding covalent silicate coagulation, further reducing the dosage of silicate coagulant in the coagulation stage. Precise addition of polypropylene with a molecular weight of 2000 to 12000 Da before solid-liquid separation further reduces the overall coagulant dosage. Acetic acid scale inhibitors and dispersants effectively chelate residual calcium, magnesium, and silicate ions, inhibiting the precipitation of silica and calcium scale on the membrane surface. This avoids the ecological risks and inadequate control of complex scale associated with traditional phosphorus-containing scale inhibitors. After efficient solid-liquid separation in an inclined plate sedimentation tank, the effluent turbidity is less than or equal to 1 NTU. Further precision filtration through an ultrafiltration system results in a fouling index of less than 2.5, providing highly stable feed water and significantly reducing membrane fouling load. Ultimately, a high recovery rate of 75% to 85% and a desalination rate of greater than or equal to 98% are achieved in the reverse osmosis system, superior to the 50% to 70% recovery rate of traditional processes. The concentrate... Discharge is reduced by approximately 50%, and the annual cleaning frequency of reverse osmosis membranes is reduced from 6 times to 2 times, extending membrane life and reducing operation and maintenance costs. The entire process uses only environmentally friendly reagents, with no phosphorus and no harmful byproducts. Combined with reagent savings, reduced energy consumption, and improved operational stability, the cost per ton of produced water is reduced by approximately 33%, and the TDS fluctuation of the produced water is controlled within ±10 mg / L. This effectively solves the core bottlenecks in existing brackish water reuse technologies, such as weak pretreatment synergy, severe membrane fouling, low system recovery rate, and high operating costs, achieving a balance between water quality safety, efficient resource utilization, and ecological sustainability. Attached Figure Description
[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.
[0027] Figure 1 This is a flowchart of the process steps of the present invention;
[0028] Figure 2This is a schematic diagram of the overall process of implementing the present invention;
[0029] Figure 3 Scanning electron microscope image and surface element distribution diagram of the dried iron-aluminum bonded covalent silicate coagulant;
[0030] Figure 4 Infrared spectrum of iron-aluminum bonded covalent silicate coagulant;
[0031] Figure 5 This is a schematic diagram showing the changes in the main pollutant indicators of brackish water during the process in Example 1.
[0032] Figure 6 This is a schematic diagram comparing the water quality of the pre-membrane flow in this technology with that of existing technologies in Example 1;
[0033] Figure 7 This is a schematic diagram showing the changes in the main pollutant indicators of brackish water during the process in Example 2;
[0034] Figure 8 This is a schematic diagram comparing the water quality of the pre-membrane flow in this technology with that of existing technologies in Example 2;
[0035] Figure 9 This is a schematic diagram showing the changes in the main pollutant indicators of brackish water during the process in Example 3;
[0036] Figure 10 This is a schematic diagram comparing the water quality of the pre-membrane flow in this technology with that of existing technologies in Example 3;
[0037] Figure 11 This is a schematic diagram of the preparation process of the iron-aluminum bonded covalent silicate coagulant in Example 4. Detailed Implementation
[0038] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.
[0039] As attached Figure 1 - Figure 11 As shown:
[0040] Example 1:
[0041] This invention provides a brackish water reuse process for raw water with a total dissolved solids (TDS) of 6500 mg / L, hardness of 320 mg / L, alkalinity of 190 mg / L, and total organic carbon (TOC) of 7.2 mg / L, comprising the following sequential steps:
[0042] S1, Double Alkali Method for Deep Softening
[0043] Raw water is introduced into the reaction tank. Sodium hydroxide (NaOH) is first added at a controlled dosage of 580 mg / L to adjust the pH of the reaction system to 10.8–11.2, causing the carbonate hardness in the water to convert into calcium carbonate and magnesium hydroxide precipitates. Subsequently, sodium carbonate is added at a controlled dosage of 135 mg / L to convert non-carbonate hardness into calcium carbonate precipitates. The reaction time is controlled at 30 minutes to ensure complete removal of scale-forming ions and provide water quality assurance for the subsequent membrane system.
[0044] S2, iron-aluminum bonded covalent silicate reinforced concrete
[0045] The softened effluent is introduced into a mixing reaction tank, where an iron-aluminum bonded covalent silicate coagulant is added. The molar ratio of iron, aluminum, and silicon in this coagulant is strictly controlled at Fe:Al:Si = 2.5:1:15, and the dosage is 30–40 mg / L.
[0046] The mixing process employs a two-stage precision stirring technique:
[0047] Rapid stirring: 250 rpm for 3 minutes to fully disperse the agent and destabilize it by contacting colloidal particles, fine suspended matter and some dissolved organic matter in the water;
[0048] Slow flocculation: 40 rpm for 30 minutes to promote the full growth of flocs and form dense flocs with good settling properties.
[0049] This coagulant enhances its adsorption bridging ability through its Si-O-Fe / Al covalent network structure, significantly improving the removal efficiency of organic matter and colloids.
[0050] S3, ferrate advanced oxidation and in-situ coagulation
[0051] The coagulated effluent is fed into an oxidation reactor, where potassium ferrate is added at a concentration of 1.5–2.0 mg / L. The oxidation reaction is carried out for 30 minutes at a pH of 9.0–10.8.
[0052] Ferrate releases highly oxidizing reactive oxygen species in an alkaline environment, simultaneously achieving deep degradation of residual organic matter in water, efficient inactivation of bacteria and viruses, and in-situ generation of ferric iron hydrolysis products. This ferric iron does not require additional coagulant addition and directly participates in the subsequent flocculation process, achieving synergistic integration of advanced oxidation and coagulation.
[0053] S4, Precise Dosing of Environmentally Friendly Scale Inhibitor
[0054] Polyacrylic acid is added as a scale inhibitor and dispersant in the effluent 2 meters before it enters the inclined plate sedimentation tank after the oxidation reaction is completed.
[0055] Molecular weight: 8000~10000 Da;
[0056] Dosage: 0.15~0.25mg / L.
[0057] Polyacrylic acid effectively chelates residual calcium, magnesium and silicate ions, inhibiting their precipitation in subsequent solid-liquid separation and on the membrane surface, thus preventing the formation of inorganic scale.
[0058] S5, High-efficiency two-stage inclined plate sedimentation solid-liquid separation
[0059] The mixture obtained from S4 is introduced into the first-stage inclined plate sedimentation tank.
[0060] Operating parameters: Inclined plate angle 60°, plate spacing 100mm, surface load controlled at 6~8m. 3 / (m 2 •h), hydraulic residence time 45 minutes.
[0061] Under these conditions, the flocs of alum and the metal hydroxide flocs generated by oxidation settle efficiently, yielding a preliminary clear supernatant.
[0062] Subsequently, the supernatant enters the second-stage inclined plate sedimentation tank for fine treatment:
[0063] Operating parameters: Surface load controlled at 5~7m 3 / (m 2 ·h).
[0064] After two stages of sedimentation, the final effluent turbidity is stably controlled below 0.8 NTU, providing the ultrafiltration system with feed water of extremely low pollution index.
[0065] S6, Ultrafiltration Precision Filtration
[0066] The supernatant obtained from S5 was fed into an ultrafiltration system using a hollow fiber ultrafiltration membrane with a pore size of 0.02 μm, and operated at an operating pressure of 0.2 MPa.
[0067] Cleaning strategy: Perform air-water backwash every 45 minutes, and perform enhanced chemical backwash containing sodium hypochlorite once a week.
[0068] The ultrafiltration permeate water has a stable SDI of less than 2.5, effectively removing residual colloids, bacteria and macromolecular organic matter, meeting the requirements for reverse osmosis feed water.
[0069] S7, High-Recovery Reverse Osmosis Desalination
[0070] The ultrafiltration permeate is fed into the reverse osmosis system, which operates at a pressure of 1.8 MPa. The system recovery rate is controlled at 80%, and the desalination rate reaches 98.5%.
[0071] The recycled water produced has a TDS of less than 100 mg / L and can be directly used for industrial circulating cooling or boiler feedwater; the concentrated water is collected and then evaporated and crystallized or discharged in compliance with standards.
[0072] To verify the beneficial effects of this method, scientific demonstration was conducted through theoretical experiments. Specific data comparisons are as follows:
[0073]
[0074] Data Explanation:
[0075] Excellent ultrafiltration feed water quality: This technology employs a triple synergistic pretreatment process of "double alkali softening (S1) + iron-aluminum bonded covalent silicate coagulation (S2) + ferrate advanced oxidation (S3)," and in particular, optimizes the molar ratio of coagulants (2.5:1:15) and stirring kinetics (3 min of fast stirring + 30 min of slow stirring), effectively removing colloids, organic matter, and scale-forming ions. This results in an ultrafiltration feed water fouling index that is consistently below 2.5, far superior to existing technologies (typically >5), significantly reducing the risk of membrane fouling.
[0076] Breakthrough in reverse osmosis system performance: Thanks to the synergistic effect of precise front-end softening and a new type of scale inhibitor (S4 with a molecular weight of 8000-10000Da polyacrylic acid), the precipitation of silicon / calcium is effectively suppressed. This solution can safely increase the recovery rate of the RO system to 80%, while the recovery rate of traditional processes is generally limited to below 60% due to the high risk of scaling. At the same time, the desalination rate is stable at ≥98.5%, and the TDS of the product water is <100mg / L, meeting the high-standard industrial reuse requirements.
[0077] Membrane maintenance and concentrate reduction: Due to the excellent quality of the influent (SDI<2.5), the annual chemical cleaning frequency of the RO membrane is reduced from 6 times to 2 times, extending the membrane life by more than 30%; the concentrate discharge ratio is reduced from 40% to 20%, which greatly reduces the burden of subsequent concentrate treatment and is in line with the policy orientation of efficient water resource utilization.
[0078] Green and safe chemical agents: This solution uses only polyacrylic acid as a scale inhibitor and dispersant, which is phosphorus-free and non-biotoxic; while existing technologies generally rely on polyphosphates or organophosphonates, which pose risks of phosphorus emissions and ecological safety hazards.
[0079] Economic advantages: Comprehensive savings in reagents (optimized dosage, such as oxidant only 1.5-2.0 mg / L, scale inhibitor only 0.15-0.25 mg / L), reduced energy consumption (high recovery rate reduces pump power), and lower maintenance costs (reduced cleaning frequency). The operating cost of this solution is 3.2 yuan per ton of water produced, which is 33.3% lower than the existing technology (4.8 yuan), and has extremely high promotion value.
[0080] Operational stability: Closed-loop control of all process parameters (e.g., precise control of surface load in the sedimentation tank at 6-8m in the first stage). 3 / (m 2 ·h), Level 2 5~7m 3 / (m 2·h), S3 ferrate in situ generates ferric iron to achieve oxidation-coagulation integration), so that the TDS fluctuation of the product water is controlled within ±10mg / L. However, due to the large fluctuation of the pretreatment, the TDS often fluctuates by ±50mg / L, which seriously affects the stability of the downstream process.
[0081] Example 2:
[0082] This invention provides a reuse process for brackish water with high total dissolved solids (TDS) and high hardness. For raw water with a TDS of 10300 mg / L, hardness of 1341.7 mg / L, alkalinity of 183.32 mg / L, calcium ion concentration of 281.55 mg / L, magnesium ion concentration of 329.17 mg / L, and initial turbidity of 18.7 NTU, the process includes the following sequential steps:
[0083] S1, Double Alkali Method for Deep Softening
[0084] Raw water is introduced into the reaction tank. Sodium hydroxide (NaOH) is first added at a controlled dosage of 1180 mg / L to adjust the pH of the reaction system to 11.0-11.5, causing the carbonate hardness in the water to convert into calcium carbonate precipitate, while simultaneously promoting the precipitation of most magnesium ions as magnesium hydroxide. Subsequently, sodium carbonate is added at a controlled dosage of 590 mg / L to convert non-carbonate hardness into calcium carbonate precipitate. The reaction time is controlled at 30 minutes. This high-dose dual-alkali method deeply softens the water, significantly reducing the concentration of scale-forming ions and laying the foundation for the stable operation of the subsequent membrane system.
[0085] S2, iron-aluminum bonded covalent silicate reinforced concrete
[0086] The softened effluent is introduced into a mixing reaction tank, where an iron-aluminum bonded covalent silicate coagulant is added. The molar ratio of iron, aluminum, and silicon in this coagulant is strictly controlled at Fe:Al:Si = 1:1:20, and the dosage is 40–50 mg / L. The mixing process employs a two-stage precision stirring technique.
[0087] Rapid stirring: 250 rpm for 3 minutes to fully disperse the agent and destabilize it by contacting residual colloids, fine suspended matter and some dissolved organic matter in the water;
[0088] Slow flocculation: 40 rpm for 30 minutes, to promote the full growth of flocs under a high ionic strength background, forming dense flocs with excellent settling performance.
[0089] This coagulant enhances adsorption bridging and trapping capabilities through its Fe–O–Al–O–Si covalent network structure, ensuring high-efficiency coagulation performance even in high-salinity environments.
[0090] S3, ferrate advanced oxidation and in-situ coagulation
[0091] The coagulated effluent is fed into an oxidation reactor, and potassium ferrate is added at a dosage of 2.5–3.5 mg / L. The oxidation reaction is carried out for 30 minutes at a pH of 11.0–11.5.
[0092] Ferrate releases highly oxidizing reactive oxygen species in a strongly alkaline environment, simultaneously achieving deep degradation of residual organic matter, inactivation of microorganisms, and in-situ generation of ferric iron hydrolysis products. This ferric iron directly participates in the flocculation process, eliminating the need for additional iron salt coagulants and achieving integrated synergistic effects of advanced oxidation and coagulation.
[0093] S4, Precise Dosing of Environmentally Friendly Scale Inhibitor
[0094] Polyacrylic acid is added as a scale inhibitor and dispersant in the effluent 2 meters before it enters the inclined plate sedimentation tank after the oxidation reaction is completed.
[0095] Molecular weight: 8000~10000 Da;
[0096] Dosage: 0.2–0.3 mg / L.
[0097] Polyacrylic acid effectively chelates residual calcium, magnesium and silicate ions, inhibiting their precipitation on the sedimentation tank wall or subsequent membrane surface, and has a significant control effect on complex inorganic scale in high silica and high calcium background.
[0098] S5, High-efficiency two-stage inclined plate sedimentation solid-liquid separation
[0099] The mixture obtained from S4 is introduced into the first-stage inclined plate sedimentation tank.
[0100] Operating parameters: Inclined plate angle 60°, plate spacing 100mm, surface load controlled at 5~7m³ / (m²·h), hydraulic retention time 45 minutes.
[0101] Under these conditions, alum flocs and metal hydroxide flocs settle efficiently, yielding a preliminarily clear supernatant. The supernatant then enters a second-stage inclined plate sedimentation tank for further treatment.
[0102] Operating parameters: Surface load is controlled at 4-6 m³ / (m²·h).
[0103] After two stages of sedimentation, the final effluent turbidity is stably controlled at ≤1 NTU (with further optimization within the measured range of 3.7~4.0 NTU), providing low-pollution feed water for the ultrafiltration system.
[0104] S6, Ultrafiltration Precision Filtration
[0105] The supernatant obtained from S5 was fed into an ultrafiltration system using a hollow fiber ultrafiltration membrane with a pore size of 0.02 μm, and operated at an operating pressure of 0.2 MPa.
[0106] Cleaning strategy: Perform air-water backwash every 45 minutes, and perform enhanced chemical backwash containing sodium hypochlorite once a week.
[0107] The SDI (Smell Indwelling Discharge Index) of ultrafiltration permeate water is significantly reduced, meeting the requirements for reverse osmosis feed water.
[0108] S7, High-Recovery Reverse Osmosis Desalination
[0109] The ultrafiltration permeate is fed into the reverse osmosis system, which operates at a pressure of 1.8 MPa, with a system recovery rate of 78.5% and a desalination rate of ≥98%.
[0110] The TDS of the recycled water produced is less than 100 mg / L, and it can be directly used for high-quality industrial reuse; the concentrated water is collected and then evaporated and crystallized or discharged in compliance with standards.
[0111] To verify the effectiveness of this method in treating brackish water with high TDS and high hardness, scientific demonstration was conducted through theoretical experiments. Specific data comparisons are as follows:
[0112] index This technical solution Existing technical solutions Difference range (Integrated softening-oxidation-scale inhibition-UF / RO) (Conventional lime softening + PAC coagulation + UF / RO) SDI (Smell Indole Degradation Index) of Ultrafiltration Influent 5.4~9.86 15.6~19.8 Reduced by approximately 60% Reverse osmosis system recovery rate 78.50% 58% An increase of approximately 26% Reverse osmosis membrane chemical cleaning cycle 70-75 days / time 40-45 days / time Extended by approximately 71% Product water quality stability (TDS fluctuation) ±15mg / L ±50mg / L Stability improved by 70% Hardness before entering the dual-film stage (mg / L) 35.5~47.8 167.9~189.6 Reduced by 76.7% Calcium ion concentration (mg / L) before entering the double membrane 24.6~35.3 107.8~130.56 Reduced by 49.5% Magnesium ion concentration (mg / L) before entering the double membrane 0.3~0.5 45.7~49.6 Reduced by 99.1% Pre-membrane turbidity (NTU) 3.7~4.0 15.8~17.4 76% reduction
[0113] Data Explanation:
[0114] Significantly improved ultrafiltration influent water quality: This technology employs a triple pretreatment process—"dual alkali deep softening (S1, pH 11.0~11.5) + iron-aluminum-silicon covalent coagulation (S2, high dosage 40~50 mg / L) + ferrate oxidation (S3)"—to effectively remove high concentrations of scale-forming ions (Ca). 2 ⁺、Mg 2 ⁺) and colloidal particles. Especially under high TDS (10300 mg / L) background, the turbidity entering the dual membrane system is still reduced to 3.7–4.0 NTU, and the SDI is controlled at 5.4–9.86, which is far superior to the existing technology (turbidity >15 NTU, SDI >15), and significantly reduces membrane fouling load.
[0115] Breakthrough in reverse osmosis system performance: Thanks to the extreme removal of hardness (reduced by 76.7%) and magnesium ions (reduced by 99.1%) in the S1 stage, and the dispersion effect of high molecular weight polyacrylic acid (8000-10000Da) on residual scaling ions in the S4 stage, this solution can increase the RO recovery rate to 78.5% under challenging raw water conditions, while traditional processes can only maintain a recovery rate of 58% due to severe scaling risks; at the same time, the TDS fluctuation of the product water is controlled within ±15mg / L, and the stability is improved by 70%.
[0116] Membrane maintenance cycle significantly extended: Due to the fundamental improvement in feed water quality, the chemical cleaning cycle of reverse osmosis membranes has been extended from 40–45 days to 70–75 days (an extension of approximately 71%), effectively reducing the frequency of downtime maintenance, extending the service life of membrane modules, and significantly reducing operation and maintenance costs.
[0117] Excellent scale-forming ion control: The dual-alkali method combined with high pH control (11.0~11.5) has a targeted advantage for high-magnesium brackish water, with magnesium ions almost completely removed (reduced to 0.3–0.5 mg / L), fundamentally inhibiting the formation of Mg(OH)2 scale; at the same time, the calcium ion concentration is reduced by nearly 50%, and with the action of scale inhibitors, it effectively prevents and controls CaCO3 and silicate complex scale, ensuring long-term stable operation of the system.
[0118] Economic efficiency and environmental friendliness of the chemicals: Despite the poor quality of the raw water, the chemical efficacy was maximized by optimizing the reaction kinetics (3 min of fast stirring + 30 min of slow stirring) and precise dosing (only 2.5-3.5 mg / L of oxidant and only 0.2-0.3 mg / L of scale inhibitor), thus avoiding secondary pollution and cost waste caused by excessive dosing in traditional processes.
[0119] Example 3:
[0120] This invention provides a reuse process for brackish water containing fluoride and high total organic carbon (TOC). For raw water with a total dissolved solids (TDS) of 5200 mg / L, hardness of 674.3 mg / L, alkalinity of 103.7 mg / L, fluoride ion concentration of 7.8 mg / L, total organic carbon (TOC) of 15.0 mg / L, and initial turbidity <2.74 NTU, the process includes the following sequential steps:
[0121] S1, Double Alkali Method for Deep Softening
[0122] Raw water is introduced into the reaction tank. Sodium hydroxide (NaOH) is first added at a controlled dosage of 660 mg / L to adjust the pH of the reaction system to 10.8–11.2. This converts carbonate hardness in the water into calcium carbonate precipitate and promotes the precipitation of magnesium ions as magnesium hydroxide, while also partially removing fluoride ions. Subsequently, sodium carbonate is added at a controlled dosage of 345 mg / L to convert non-carbonate hardness into calcium carbonate precipitate. The reaction time is controlled at 30 minutes. This dual-alkali softening method effectively reduces hardness and the concentration of characteristic pollutants, creating conditions for subsequent treatment.
[0123] S3, ferrate pre-oxidation advanced oxidation (oxidation-coagulation synergy)
[0124] The softened effluent from S1 is fed into the oxidation reaction tank, and potassium ferrate is added at a dosage of 2.0–2.5 mg / L. The oxidation reaction is carried out for 30 minutes at a pH of 10.8–11.2 (connected to the subsequent coagulation and flocculation process).
[0125] Ferrate efficiently degrades high-concentration organic matter (TOC=15.0mg / L), destroys the fluorine complex structure, and transforms it into an easily precipitable form; at the same time, it generates trivalent iron colloids in situ, which serve as crystal nuclei for subsequent coagulation, achieving "oxidation-coagulation" to reduce volume and increase efficiency, without the need for additional iron salts.
[0126] S2, iron-aluminum bonded covalent silicate reinforced concrete
[0127] The oxidized effluent is introduced into a mixing reaction tank, where an iron-aluminum bonded covalent silicate coagulant is added. The molar ratio of iron, aluminum, and silicon in this coagulant is strictly controlled at Fe:Al:Si = 1.5:1:15, and the dosage is 35–45 mg / L.
[0128] The mixing process employs a two-stage precision stirring technique:
[0129] Rapid stirring: 250 rpm for 3 minutes to fully disperse the agent and utilize the Fe(OH)3 crystal nuclei generated by pre-oxidation to quickly contact and destabilize colloids, organic matter and fluorides in the water;
[0130] Slow flocculation: 40 rpm for 30 minutes, to promote the full growth of flocs in a low turbidity background, forming dense flocs with excellent settling properties.
[0131] This process utilizes the synergistic effect of oxidation and coagulation to significantly reduce the total dosage of reagents and improve the removal rate of micro-pollutants.
[0132] S4, Precise Dosing of Environmentally Friendly Scale Inhibitor
[0133] Polyacrylic acid is added as a scale inhibitor and dispersant 2 meters before the effluent enters the inclined plate sedimentation tank after coagulation and flocculation are completed.
[0134] Molecular weight: 8000~10000 Da;
[0135] Dosage: 0.15~0.25mg / L.
[0136] Polyacrylic acid effectively chelates residual calcium, magnesium and silicate ions, preventing them from precipitating on the surface of the membrane or precipitate, thus improving the system's anti-scaling ability. It has an excellent inhibitory effect on complex scale in fluoride-containing wastewater.
[0137] S5, High-efficiency two-stage inclined plate sedimentation solid-liquid separation
[0138] The mixture obtained from S4 is introduced into the first-stage inclined plate sedimentation tank.
[0139] Operating parameters: Inclined plate angle 60°, plate spacing 100mm, surface load controlled at 5~7m³ / (m²·h), hydraulic retention time 45 minutes.
[0140] Under these conditions, fluorine-containing flocs and metal hydroxides settle efficiently, yielding a preliminarily clear supernatant. The supernatant then enters a second-stage inclined plate sedimentation tank for further treatment.
[0141] Operating parameters: Surface load controlled at 4~6m 3 / (m 2 ·h).
[0142] After two stages of sedimentation, the final effluent turbidity is stably controlled at ≤1 NTU, and the fluoride ion concentration is significantly reduced, providing extremely low-pollution influent for the ultrafiltration system.
[0143] S6, Ultrafiltration Precision Filtration
[0144] The supernatant obtained from S5 was fed into an ultrafiltration system using a hollow fiber ultrafiltration membrane with a pore size of 0.02 μm, and operated at an operating pressure of 0.2 MPa.
[0145] Cleaning strategy: Perform air-water backwash every 45 minutes, and perform enhanced chemical backwash containing sodium hypochlorite once a week.
[0146] Ultrafiltration produces water with an extremely low SDI (Smell Indole Discharge Index), effectively trapping residual colloids, bacteria, and large molecular organic matter.
[0147] S7, High-Recovery Reverse Osmosis Desalination
[0148] The ultrafiltration permeate is fed into the reverse osmosis system, which operates at a pressure of 1.8 MPa, with a system recovery rate controlled at 82% and a desalination rate ≥98%. The produced reusable permeate has a TDS of less than 100 mg / L and a fluoride ion concentration reduced to 0.9–1.5 mg / L; the concentrate is collected and then evaporated and crystallized or discharged in compliance with standards.
[0149] To verify the effectiveness of this method in treating brackish water containing fluoride and high organic matter, scientific demonstration was conducted through theoretical experiments. Specific data comparisons are as follows:
[0150]
[0151] Data Explanation:
[0152] Excellent ultrafiltration influent quality: This solution is designed for brackish water with low turbidity, high organic content and fluoride. It adopts the strategy of "pre-oxidation (S3, 2.0~2.5 mg / L potassium ferrate) + dual alkali softening (S1, pH 10.8~11.2) + synergistic coagulation (S2, 35~45 mg / L)" to make the ultrafiltration influent SDI≤3.7, which is far superior to the existing technology (8.6–10.3), effectively controlling the risk of organic and colloidal pollution.
[0153] Synergistic removal of characteristic pollutants: Potassium ferrate has extremely high oxidation efficiency under alkaline conditions, reducing TOC from 15.0 mg / L to 4.8 mg / L (a reduction of 66.2%); at the same time, it destroys the complexed form of fluoride, and combined with co-precipitation under high pH, reduces the fluoride ion concentration from 7.8 mg / L to 0.9–1.5 mg / L (before entering the double membrane), and the final produced water meets agricultural irrigation standards, greatly expanding the reuse scenarios.
[0154] Reduced dosage and process flexibility: Because the S3 pre-oxidation generates Fe(OH)3 as a coagulation nucleus, it has a synergistic effect with the S2 iron-aluminum-silicon coagulant. Although the total coagulation dosage is adjusted to 35-45 mg / L to adapt to high organic load, the overall agent efficiency is greatly improved, and the scale inhibitor dosage is extremely low (0.15-0.25 mg / L), which reflects the intelligent design advantages of the present invention of "adjustable process and synergistic agent".
[0155] System performance: Dual alkali softening reduces hardness by 78%, magnesium ions are completely undetectable, and with the help of high molecular weight scale inhibitors (8000-10000Da), RO recovery rate is increased to 82% (36% higher than the conventional 60%); the annual cleaning frequency is reduced from 6 times to 3 times, and the TDS fluctuation of the produced water is only ±10mg / L (stability improved by 80%). The overall performance of the system is significantly better than that of traditional processes, and it is especially suitable for the treatment of brackish water containing special pollutants (such as fluoride and high TOC).
[0156] Example 4:
[0157] This embodiment provides a preparation process for an iron-aluminum bonded covalent silicate coagulant:
[0158] 1. Preparation process
[0159] The specific preparation process is as follows: Figure 11 As shown, it includes the following steps:
[0160] Preparation of solution A: Mix 3.38g of ferric chloride and 1.67g of aluminum chloride and add to 20mL of water, then stir until homogeneous.
[0161] Solution B preparation: Mix 56 mL of tetraethyl silicate and 30 mL of anhydrous ethanol and stir slowly and evenly for 20 min to partially hydrolyze the tetraethyl silicate, which will facilitate subsequent condensation with metal ions.
[0162] Mixing and Reaction: Mix solutions A and B. Slowly add 25 mL of 0.3 mol / L NaOH solution dropwise to the A / B mixture while rapidly stirring mechanically at a controlled temperature of 60°C.
[0163] Maturation: After the addition is complete, continue stirring for 30 minutes and let stand at room temperature for 1-3 days to finally obtain the iron-aluminum bonded covalent silicate coagulant CAM.
[0164] 2. Characterization of microstructure and elemental distribution
[0165] The characteristics of the obtained iron-aluminum bonded covalent silicate coagulant are as follows: Figure 3 As shown, after drying, the iron-aluminum bonded covalent silicate coagulant was observed by scanning electron microscopy. The surface had a certain granular texture, which is closely related to the iron and aluminum salts doped during the preparation process.
[0166] Surface elemental analysis revealed that silicon, iron, and aluminum were evenly distributed on the surface of the coagulant, indicating that the iron salts, aluminum salts, and tetraethyl silicate in the material had undergone a thorough mixing and reaction.
[0167] 3. Infrared spectral structural analysis
[0168] The present invention also conducted infrared spectroscopy tests on the iron-aluminum bonded covalent silicate coagulant prepared in the preparation example, and the test results are shown in [Figure number missing]. Figure 4 ,Depend on Figure 4 It can be known that:
[0169] At 3420cm -1 and 1630cm -1 Stretching vibration peaks of hydroxyl groups (-OH) appeared at the position, and these peaks are related to the hydrolysis of the material.
[0170] At 2973cm -1 and 2891cm -1 The stretching vibration peak of CH appeared at 1390 cm⁻¹. -1 The bending vibration peaks of CH appeared at the location, and these peaks were carried by tetraethyl orthosilicate.
[0171] The most crucial point is between 950 and 1080 cm. -1 A series of sharp peaks appear, which are related to Si-O-Al and Si-O-Fe chemical bonds.
[0172] 560cm -1 and 810cm -1 The peaks are related to the stretching vibrations of Fe-O and Al-O, respectively.
[0173] The aforementioned infrared spectral characteristics indicate that the iron-aluminum covalent bond structure successfully bonded with silicate, and the iron-aluminum monomers fully participated in the polymerization reaction of silicate, confirming the successful synthesis of the target product, the iron-aluminum covalent silicate coagulant.
[0174] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.
Claims
1. A brackish water reuse process, characterized in that, Includes the following steps: S1. Soften raw water using a dual-alkali method: First, add sodium hydroxide to adjust the pH to 9-12, causing the carbonate hardness in the water to be converted into calcium carbonate and magnesium hydroxide precipitates; then add sodium carbonate to convert the non-carbonate hardness into calcium carbonate precipitates, thus obtaining softened water. S2. Add an iron-aluminum bonded covalent silicate coagulant to the softened water or the water treated by S3, and carry out a mixing reaction to destabilize the colloidal particles, fine suspended matter and some dissolved organic matter in the water and form dense flocs. S3. Add ferrate to the softened water or water treated by S2, and carry out advanced oxidation reaction under pH 8-12 conditions to simultaneously achieve organic matter degradation, microbial inactivation and in-situ coagulation. S4. After the oxidation reaction is completed and before solid-liquid separation, add polyacrylic acid as a scale inhibitor and dispersant. S5. The mixture obtained in S4 is introduced into an inclined plate sedimentation tank for efficient solid-liquid separation to remove the precipitated sludge and obtain the supernatant. S6. Pass the supernatant obtained in S5 into an ultrafiltration system containing an ultrafiltration membrane for precision filtration to remove residual colloids, bacteria and macromolecular organic matter, and obtain ultrafiltration permeate. S7. The ultrafiltration permeate obtained in S6 is sent to the reverse osmosis system for desalination treatment to produce reusable permeate and discharge concentrated water.
2. The brackish water reuse process according to claim 1, characterized in that, In step S1, the dosage of sodium hydroxide is 100–1200 mg / L, the dosage of sodium carbonate is 100–800 mg / L, and the reaction time is 20–40 minutes.
3. The brackish water reuse process according to claim 1, characterized in that, In S2, the molar ratio of iron, aluminum, and silicon in the iron-aluminum bonded covalent silicate coagulant is Fe:Al:Si=(1.0~2.5):1:(15~20), the dosage is 10~60mg / L, the rapid mixing time is 30 seconds to 5 minutes, and the slow flocculation time is 10~40 minutes.
4. The brackish water reuse process according to claim 1, characterized in that, In step S3, the ferrate is potassium ferrate or sodium ferrate, the dosage is 1.0–60 mg / L, and the oxidation reaction time is 5–40 minutes.
5. The brackish water reuse process according to claim 1, characterized in that, The execution order of the steps is as follows: first execute S1, then execute S3, and finally execute S2; wherein the ferric iron generated by the oxidation of S3 serves as the crystal nucleus for the coagulation reaction of S2.
6. The brackish water reuse process according to claim 1, characterized in that, In step S4, the molecular weight of the polyacrylic acid is 2000-12000 Da, the dosage is 0.1-10 mg / L, and the addition point is located 1-3 meters before the inlet of the inclined plate sedimentation tank.
7. The brackish water reuse process according to claim 1, characterized in that, In S5, two-stage inclined plate sedimentation tanks are connected in series, with the surface loading of the first-stage inclined plate sedimentation tank being 5-8 m³. 3 / (m 2 The surface loading of the second-stage inclined plate sedimentation tank is 4–7 m³ / h. 3 / (m 2 ·h).
8. The brackish water reuse process according to claim 1, characterized in that, In S6, the ultrafiltration membrane has a pore size of 0.01–0.1 μm, an operating pressure of 0.1–0.3 MPa, a backwashing cycle of 30–60 minutes, and a chemically enhanced backwashing frequency of once a week.
9. A brackish water reuse process according to claim 1, characterized in that, In step S7, the operating pressure of the reverse osmosis system is 1.2–2.5 MPa, the system recovery rate is 75%–85%, and the desalination rate is ≥98%.
10. A brackish water reuse process according to claim 1, characterized in that, The raw water has a total dissolved solids content of 5000–11000 mg / L, a hardness of 300–1400 mg / L, a fluoride ion concentration of ≤10 mg / L, and a total organic carbon content of ≤20 mg / L.