A printing and dyeing wastewater zero discharge treatment process based on low-pressure high-multiple membrane concentration and salt separation cooperation

Abstract

The application discloses a printing and dyeing wastewater zero discharge treatment process based on low-pressure high-multiple membrane concentration and salt separation cooperation. The wastewater sequentially passes through double-alkali softening pretreatment, organic pollutant treatment, forward osmosis low-pressure high-multiple concentration treatment, salt separation treatment and evaporation crystallization steps; wherein the forward osmosis low-pressure high-multiple concentration treatment is operated in sections along the flow direction, the osmotic pressure difference is gradually increased, the transmembrane pressure difference is gradually decreased, and the interface electrochemical buffer and low-frequency gas-liquid disturbance can be set to inhibit the concentration polarization and pollution. The concentrated liquid is separated into two salt water streams mainly containing Na2SO4 and NaCl through nanofiltration, and is evaporated and crystallized respectively; and the water produced by the draw solution is returned and reused. The process realizes high-multiple concentration and stable salt separation under low mechanical pressure, reduces energy consumption, improves water recovery rate and crystal purity, and realizes wastewater zero discharge.

Description

Technical Field

[0001] This invention relates to the technical field of wastewater treatment, and in particular to a zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy. Background Technology

[0002] To achieve wastewater reuse, the dyeing and printing industry typically employs a combined process of biological pretreatment, ultrafiltration, and reverse osmosis. This route can stably produce reusable permeable water, but it also generates reverse osmosis concentrate with high salinity and low biodegradability. This concentrate often contains high levels of residual organic matter and color, along with scaling ions such as calcium, magnesium, and silicon, and auxiliaries such as surfactants and dispersants, exhibiting characteristics such as high osmotic pressure, increased viscosity, and susceptibility to interfacial fouling. These factors lead to problems in subsequent deep wastewater reduction processes, including concentration polarization, scale compaction, flux decline, and increased cleaning frequency, resulting in higher operating energy consumption and maintenance costs.

[0003] To reduce evaporation load and control pollution risks, engineering practice often employs a combined softening and oxidation biological process to reduce hardness and organic load. This is followed by further concentration using nanofiltration or reverse osmosis membranes, ultimately resulting in evaporation crystallization to solidify salts and recycle the mother liquor. For example, patent CN110937754A discloses a method for zero discharge of cotton dyeing and printing wastewater, which achieves wastewater reuse through a combination of biochemical pretreatment, membrane separation, and evaporation crystallization. However, this scheme still suffers from problems such as high energy consumption and severe membrane fouling. Patent CN105585199B discloses a zero discharge treatment process for dyeing and printing wastewater, which treats high-salinity wastewater by combining multi-stage membrane filtration with evaporation crystallization. Although it can reduce the volume, concentration polarization and flux decay are prone to occur under high osmotic pressure conditions. In addition, patent CN103508602B discloses a zero discharge process for high-salinity industrial wastewater that integrates membrane and evaporation crystallization, which achieves zero discharge through membrane concentration and evaporation coupling. However, it still faces problems such as severe scaling in mixed salt systems, unstable crystal forms, and high energy consumption. It is evident that existing technologies mostly rely on single-stage or fixed-drive membrane processes, which are insufficient in controlling the unevenness of mass transfer along the process under high-concentration conditions. Compaction is prone to occur at the inlet section, and the flux at the end section decreases significantly. Pollution and scaling accumulate continuously in space. At the same time, the crystallization interference is severe after the mixed salt enters the evaporation and crystallization stage, resulting in limited operational stability and energy efficiency.

[0004] In summary, the key challenge facing the industry is how to achieve high-level stable concentration of reverse osmosis concentrate under conditions of low energy consumption and low mechanical pressure, and how to effectively connect it with subsequent salt separation and crystallization processes, thereby achieving a comprehensive balance between operational economy, system stability, and the quality of crystallized products. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems in the prior art and provide a zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy.

[0006] To achieve the above objectives, this invention provides a zero-discharge treatment process for dyeing and printing wastewater based on the synergistic effect of low-pressure high-expansion membrane concentration and salt separation, comprising the following steps in sequence: dual-alkali softening pretreatment, organic pollutant treatment, forward osmosis low-pressure high-expansion concentration treatment, salt separation treatment, and evaporation crystallization; wherein:

[0007] The forward osmosis low-pressure high-concentration treatment involves sending water treated for organic pollutants into a forward osmosis membrane concentration unit, within which at least three zones are arranged along the flow direction; adjusting the operating conditions of each zone so that the average osmotic pressure difference Δπ from the inlet to the end of the zone gradually increases to the range of 1.0–3.5 MPa, while the average transmembrane pressure difference ΔP gradually decreases to the range of 0.01–0.10 MPa; and ensuring that the average osmotic pressure difference Δπ between adjacent zones from the inlet to the end increases by 1.2–2.0 times, while the average transmembrane pressure difference ΔP decreases by 0.2–0.8 times.

[0008] Where Δπ is calculated based on the osmotic pressure of the fluid on both sides of the membrane in the partition at the membrane surface, and ΔP is calculated based on the average of the static pressure at the inlet and outlet of the partition.

[0009] The permeate obtained from the extract liquid recovery section is returned to the front-end system as makeup water for reuse.

[0010] Salt separation involves sending the forward osmosis concentrate obtained from low-pressure high-concentration forward osmosis treatment into a nanofiltration salt separation unit for selective separation to obtain brine mainly composed of sodium sulfate and brine mainly composed of sodium chloride.

[0011] This invention achieves high concentration under low mechanical pressure by setting multiple zones arranged along the flow direction within the forward osmosis membrane channel and precisely controlling the average osmotic pressure difference of each zone to gradually increase to 1.0–3.5 MPa and the average transmembrane pressure difference to gradually decrease to 0.01–0.10 MPa. This design ensures that water flux is primarily driven by osmotic pressure difference rather than mechanical pressure difference, significantly reducing membrane concentration polarization, scale compaction, and organic fouling accumulation, effectively delaying flux decay and reducing cleaning frequency.

[0012] By limiting the osmotic pressure difference increment between adjacent zones to 1.2–2.0 and the transmembrane pressure difference decrease to 0.2–0.8, a controllable segmented gradient driving force field is formed in the flow direction. This achieves a dynamic balance between osmotic pressure and transmembrane pressure in spatial distribution, fundamentally avoiding the membrane concentration polarization and scale compaction problems caused by sudden increases in local osmotic driving force or sudden drops in pressure difference under traditional single-zone operation modes. This proportional relationship is achieved by synergistically matching the nonlinear response law between osmotic pressure and transmembrane pressure, ensuring that the rate of change of the chemical potential gradient on both sides of the membrane surface remains within a stable range. This results in a more uniform water flux distribution, more sufficient boundary layer disturbance, and more controllable pollutant migration.

[0013] In multi-zone forward osmosis systems, traditional designs often focus on increasing the overall osmotic pressure differential or reducing the average transmembrane pressure, while neglecting the cumulative effects of uneven distribution of driving forces along the flow path. This is particularly true under high concentration conditions, where non-uniform mass transfer phenomena often occur within the membrane channel, such as severe compaction at the inlet and insufficient flux at the outlet, easily leading to concentration polarization, localized reverse osmosis, and scale buildup. This invention limits the osmotic pressure differential increment between adjacent zones to 1.2–2.0 and the transmembrane pressure differential decrease to 0.2–0.8, resulting in a gradual distribution of driving forces along the flow direction. This creates a coordinated relationship between osmotic pressure and transmembrane pressure, thereby achieving spatial balance and continuous transition of driving forces. When the osmotic pressure gradient increment is below 1.2, the driving force in the terminal section is insufficient, making it difficult to maintain a stable flux and causing a decrease in the concentration ratio. When it is above 2.0, the membrane gradient in the inlet region becomes too steep, easily leading to reverse osmosis and localized compaction of the scale layer. Similarly, when the transmembrane pressure gradient decreases below 0.2, the flow resistance increases significantly, and the flux decreases, while when it exceeds 0.8, the weakening effect of compaction stress is not significant. This range allows the system to maintain a relatively balanced membrane surface stress and a relatively stable water flux distribution at high concentration ratios, suppressing concentration polarization and making the migration and deposition of pollutants in the flow channel more controllable.

[0014] The low-pressure, high-efficiency membrane concentration unit, in conjunction with the upstream dual-alkali softening and organic pollutant treatment, significantly reduces the hardness and organic load of the water entering the membrane system, resulting in a cleaner membrane environment and improved concentration efficiency. The concentrate is further separated into two brine streams, primarily sodium sulfate and sodium chloride, by the nanofiltration desalination unit. This transforms the evaporation and crystallization process from a mixed-salt system to a two-stage single-salt precipitation process, reducing scaling and crystal structure disorder, improving crystallization efficiency and product purity, while simultaneously lowering evaporation energy consumption. The permeate is recycled back to the upstream system via the drawdown recovery section, forming an internal circulation, achieving high water recovery rate and zero discharge.

[0015] As a further improvement of the present invention, an interfacial electrochemical buffer is provided at the inlet end of the forward osmosis membrane concentration unit. A reversible redox pair consisting of ferrous and ferric iron is added to the buffer. The total iron concentration in the buffer is 0.2 to 3.0 mg / L, the redox potential is +150 to +350 mV, the pH is 6.8 to 7.8, and the residence time in the buffer is 10 to 60 s.

[0016] As a further improvement of the present invention, a solid-liquid separation unit is provided downstream of the interfacial electrochemical buffer zone, and the concentration of soluble iron in the water entering the membrane channel after being treated by the solid-liquid separation unit is not greater than 0.3 mg / L.

[0017] And definition ,

[0018] Where E is the redox potential (mV) of the buffer zone, and pH is the pH value of the buffer zone;

[0019] The forward osmosis membrane channel is divided into n≥3 partitions along the flow direction and satisfies

[0020] .

[0021] An interfacial electrochemical buffer zone is set at the inlet of the forward osmosis membrane concentration unit, and reversible Fe is introduced into the buffer zone. 2+ / Fe 3+ The redox pair, by controlling the redox potential, pH, and total iron concentration, creates a stable microenvironment at the membrane inlet with a regulatory effect. This microenvironment can effectively buffer transient pH fluctuations in the influent during the initial stage of concentration polarization, reduce the probability of iron ions depositing on the membrane surface in colloidal or hydroxide form, inhibit the formation of inorganic scale nuclei and their co-adhesion with organic pollutants from the source, and reduce the accumulation of interfacial fouling and scale compaction effect.

[0022] This invention establishes a mathematical relationship between the interface buffer state and the driving force parameters along the membrane, creating a dynamic coupling between the two. By introducing a dimensionless coefficient reflecting the stability of the buffer zone and linking it to the ratio of the product of the osmotic pressure difference and the transmembrane pressure difference, a comprehensive threshold constraint is formed, ensuring that the driving force distribution along the flow direction is compatible with the inlet chemical steady state. This constraint, on the one hand, unifies the traditionally independent chemical water quality control and membrane process driving force design into a calculable coupled system, achieving coordinated matching between the interface state and the mass transfer field; on the other hand, the threshold setting in this relationship allows the system to automatically adjust the ratio of osmotic pressure along the membrane to the transmembrane pressure based on the actual stability of the interface buffer zone, thereby maintaining the balance between the membrane surface chemical potential gradient and mechanical stress, and preventing local driving force abrupt changes from causing reverse osmosis or flux collapse.

[0023] Through this correlation mechanism, when the interface buffer state is more stable, the system automatically strengthens the osmotic pressure dominance effect, enabling high concentration to be achieved under relatively low mechanical pressure; when the interface state deviates from the optimal range, the driving force distribution automatically converges, maintaining the safe operation of the system. This adaptive design based on mathematical constraints overcomes the limitation of the chemical environment and pressure distribution being disconnected in existing multi-zone forward osmosis systems, achieving simultaneous control of chemical steady state and hydrodynamic steady state within the same parameter system.

[0024] As a further improvement of the present invention, a gas-liquid injection component is provided in the membrane channel, the gas injection frequency of which is 0.1 to 1.0 times / second, the gas phase volume fraction is 0.5% to 5.0%, and the bubble characteristic diameter is 0.1 to 1.5 mm.

[0025] By employing the above technical solution, a low-frequency pulsating gas-liquid two-phase flow can be formed in the membrane channel. As bubbles flow across the membrane surface, they periodically burst and peel off, generating transient shear disturbances and local micro-eddies. This causes the membrane boundary layer thickness to decrease periodically, effectively suppressing concentration polarization and localized accumulation of contaminants. Compared to continuous bubbling or pure liquid-phase turbulent flow, this frequency matching with the gas phase volume fraction maintains the continuity and uniformity of micro-disturbances on the membrane surface without significantly increasing energy consumption, thus significantly improving permeate flux stability. If the injection frequency is too low, the bubble renewal rate is insufficient, and the membrane surface disturbance effect weakens; if it is too high or the gas phase volume fraction is too large, bubble coalescence leads to turbulent flow, increased pressure drop, and even interference with reverse osmosis flow. The defined range ensures that the gas and liquid phases maintain a dynamic equilibrium with uniform distribution and appropriate shear in the membrane channel, balancing enhanced mass transfer with structural stability. The characteristic bubble diameter is controlled within 0.1–1.5 mm, ensuring that the bubble rise velocity in the fluid matches the tangential velocity at the membrane surface. Tiny bubbles can adhere to and slide along the membrane surface, enhancing local turbulence. Larger bubbles, on the other hand, tend to float and detach from the membrane surface, resulting in discontinuous disturbances. This allows the bubble clusters to form a dense but not crowded disturbance zone on the membrane surface, enabling periodic peeling and interfacial regeneration of the fouling layer and significantly slowing down the fouling accumulation process.

[0026] As a further improvement of the present invention, the tangential flow velocity of the membrane surface in the membrane channel is 0.10 to 0.40 m / s.

[0027] Controlling the tangential flow rate keeps the shear force of the liquid mainstream on the film surface within a moderate range, which can maintain the stable migration of bubbles along the flow direction, and prevent premature bubble breakage due to high flow rate or bubble coalescence due to low flow rate.

[0028] As a further improvement of the present invention, the membrane channel is divided into three parts along the flow direction: an inlet section, a middle section, and a terminal section, with the length ratio of the three parts being 1.0-1.5:1:0.8-1.2.

[0029] The membrane channel is divided into three sections along the flow direction: inlet, middle, and end. The length ratio of these three sections is controlled at 1.0–1.5:1:0.8–1.2 to match the residence time of the fluid along the flow path with the mass transfer requirements. A longer inlet section delays the formation of initial concentration polarization, the middle section provides a stable osmotic pressure driving force, and a moderately shortened end section reduces the residence accumulation effect of high-concentration tail sections. This length ratio creates a natural gradient distribution along the flow path, allowing the fluid shear and osmotic pressure difference to gradually connect spatially, thereby balancing the coupled effects of velocity decay and concentration rise. If the inlet section is too short, the initial control of membrane fouling will be insufficient; if the end section is too long, the tail flow velocity will be too low, and the flux will drop sharply. The defined ratio range ensures that the hydrodynamic conditions of each section of the membrane surface are coordinated, allowing the concentration process to maintain stable flux and low fouling rate even at high ratios.

[0030] As a further improvement of the present invention, the membrane material of the forward osmosis membrane in the forward osmosis membrane concentration unit is one of polypropylene, polyester, polysulfide, polycarbonate, nano alumina, cellulose acetate or polyamide.

[0031] The forward osmosis membrane concentration unit uses membrane materials such as polypropylene, polyester, polysulfide, polycarbonate, nano alumina, cellulose acetate, or polyamide to ensure the membrane has good antifouling and chemical resistance while maintaining mechanical stability.

[0032] As a further improvement of the present invention, the dual-alkali softening pretreatment includes sequentially adding sodium carbonate and sodium hydroxide solutions to the reverse osmosis concentrate of dyeing and printing wastewater, and removing hardness ions such as calcium and magnesium from the water through reaction precipitation, thereby reducing the total hardness of the resulting water to below 150 mg / L.

[0033] This dual-alkali system exhibits higher buffering capacity and selective precipitation effect compared to the single-alkali method, effectively controlling Ca2+ precipitation. 2+ Mg 2+ The co-deposition ratio is adjusted to prevent a sudden increase in solution pH or the formation of resoluble basic salts due to excessive addition of alkaline substances. Pretreatment significantly reduces water hardness and weakens the tendency for scaling into the membrane system. Combined with a subsequent electrochemical buffer zone, this further stabilizes the membrane surface chemical environment, creating clean feed water conditions for the long-term operation of the forward osmosis membrane.

[0034] As a further improvement of the present invention, the organic pollutant treatment includes passing water pretreated by dual alkali softening sequentially through a pre-ozone oxidation unit, a membrane bioreactor unit, and a post-ozone fine treatment unit to obtain treated water with reduced organic matter content and weakened color.

[0035] The ratio of biochemical oxygen demand (BOD) to chemical oxygen demand (COD) in the treated water is increased to no less than 0.3.

[0036] As a further improvement of the present invention, the ozone dosage in the pre-ozone unit is 5-15 mg / L, the sludge concentration in the MBR unit is 8000-10000 mg / L, and the membrane material of the MBR unit is a hollow fiber membrane; the ozone dosage in the post-ozone unit is 3-8 mg / L. Both the pre-ozone unit and the post-ozone unit require the use of a catalyst, and the catalyst is a silicon-aluminum catalytic oxidant.

[0037] The organic pollutant treatment unit achieves synergistic decomposition and enhanced biodegradability of recalcitrant organic matter through a combined process of pre-ozone oxidation, membrane bioreactor (MBR), and post-ozone purification. Pre-ozone oxidation breaks down dye molecules and aromatic ring structures, improving biodegradability; the MBR unit utilizes high-concentration sludge (8000–10000 mg / L) to form a biofilm layer on a hollow fiber membrane, deeply removing degradable organic matter and achieving solid-liquid separation; post-ozone purification further oxidizes residual small molecules and chromogenic substances, increasing the biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratio of the treated water to no less than 0.3, significantly improving the antifouling performance of subsequent membrane processes.

[0038] The present invention, by adopting the above technical solution, has the following beneficial effects:

[0039] This invention constructs a multi-zone, low-pressure, high-concentration system within a forward osmosis membrane channel. Through the coordinated control of progressively increasing osmotic pressure differentials and progressively decreasing transmembrane pressure differentials, a continuous and controllable distribution of membrane surface forces and osmotic driving forces in the flow direction is achieved. This design significantly improves the non-uniform mass transfer problems of severe inlet compaction and terminal flux decline in traditional single-zone forward osmosis systems, resulting in more stable water flux and effective suppression of concentration polarization. By limiting the increment and decrement factors within a specific window, the ratio of osmotic pressure to transmembrane pressure is kept within the optimal matching range, enabling high-concentration operation under low mechanical pressure conditions, significantly reducing energy consumption and extending membrane lifespan.

[0040] Secondly, this invention introduces an interfacial electrochemical buffer at the front end of a low-pressure, high-concentration membrane system, and uses reversible Fe... 2+ / Fe 3+ Redox reactions facilitate the construction of a stable electrochemical microenvironment, achieving a dynamic balance between pH and redox potential at the membrane inlet. This buffer not only suppresses the instantaneous deposition and colloid formation of Fe, Ca, and Mg ions in the feed water, but also establishes a mathematical correlation between the chemical steady-state parameter Ω and the distribution of driving forces along the flow path. This allows for the coordinated regulation of chemical and hydrodynamic steady-state states, forming an adaptively adjustable concentration process. Even under fluctuating water quality, it can maintain stable flux and low fouling rate, significantly improving the reliability of system operation.

[0041] Furthermore, by introducing low-frequency pulsating gas-liquid two-phase disturbance flow into the membrane channel and combining it with the optimized design of the ratio of tangential flow velocity to partition length, the boundary layer on the membrane surface is effectively reduced and pollutant deposits are periodically stripped away, so that the flux remains stable and the pollution recovery cycle is extended.

[0042] The dual-alkali softening and pre- and post-ozone-MBR combined organic pollutant treatment units of this invention work synergistically to significantly reduce influent hardness and organic load, increasing the B / C ratio to above 0.3, thus ensuring the stability of water quality entering the membrane system. The nanofiltration desalination unit separates the high-concentration solution into sodium sulfate brine and sodium chloride brine, achieving a transformation from mixed salt crystallization to single salt crystallization, reducing evaporation energy consumption and improving crystal purity. The permeate is recycled through the draw liquor recovery section to form an internal circulation, significantly improving the overall water recovery rate of the system, ultimately achieving efficient volume reduction and zero discharge of reverse osmosis concentrate from dyeing and printing wastewater. Detailed Implementation

[0043] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0044] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0045] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0046] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0047] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0048] Example 1

[0049] This embodiment discloses a zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-expansion membrane concentration and salt separation synergy, which includes, in sequence, dual-alkali softening pretreatment, organic pollutant treatment, forward osmosis low-pressure high-expansion concentration, salt separation, and evaporation crystallization steps. The membrane channel of the forward osmosis membrane concentration unit is divided into three parts along the flow direction: inlet section, middle section, and end section, i.e., n=3 sections.

[0050] The specific steps are as follows:

[0051] The object to be treated is reverse osmosis concentrate from a dyeing and printing factory. The total dissolved solids in the influent are approximately 10,000 mg / L, the total hardness is approximately 600 mg / L (calculated as CaCO3), and the pH is 7.2.

[0052] S1 dual-alkali softening pretreatment: Sodium carbonate (250 mg / L) is first added to the reaction tank and stirred for 20 min; then sodium hydroxide solution (100 mg / L) is added to adjust the pH to 9.5±0.1, and the reaction continues for 15 min. After sedimentation, sand filtration and 5 μm security filtration, the effluent hardness is reduced to below 120 mg / L and the turbidity is less than 1 NTU, before entering the subsequent organic pollutant treatment unit.

[0053] S2 Organic Pollutant Treatment: This step uses a combination of pre-ozone oxidation, membrane bioreactor (MBR), and post-ozone purification.

[0054] (1) Pre-ozone oxidation unit: ozone dosage 10 mg / L, contact time 15 min, using a packed silica-alumina catalyst bed (space velocity 6 h⁻¹) -1 After oxidation, large organic molecules are broken down, improving the biodegradability of wastewater.

[0055] (2) MBR unit: Hollow fiber membrane is used, sludge concentration is 9000 mg / L, hydraulic retention time is 3h, and transmembrane pressure difference is maintained at 10-20 kPa. Through this step, organic matter is significantly removed.

[0056] (3) Post-ozone treatment unit: ozone dosage 5 mg / L, reaction time 1 h, also using silicon-aluminum catalytic oxidant. After treatment, the color was significantly reduced, and the ratio of biochemical oxygen demand to chemical oxygen demand (B / C) in the effluent increased to 0.32.

[0057] S3 Interfacial Electrochemical Buffer Zone and Solid-Liquid Separation: An interfacial electrochemical buffer tank with a volume of 30 L is set at the inlet of the forward osmosis membrane concentration unit.

[0058] FeSO4 and FeCl3 solutions were continuously added, with the total iron concentration controlled at 1.0 mg / L, redox potential E = +250 mV, pH = 7.3, and hydraulic retention time at 20 s.

[0059] The effluent from the buffer zone undergoes high-efficiency sedimentation and 0.1 μm hollow fiber microfiltration, and the soluble iron content is ≤0.20 mg / L when it enters the membrane system.

[0060] According to the formula: The calculated value is Ω = 1.0.

[0061] S4 forward osmosis low-pressure high-concentration (n=3): Uses cellulose acetate forward osmosis flat sheet membrane, with a channel height of 1.2 mm and a total channel length of 1.0 m. The membrane channel is divided into three parts along the flow direction: inlet section, middle section, and end section, with a length ratio of 1.3:1.0:0.9.

[0062] Determination of the pressure difference Δπ:

[0063] In the forward osmosis membrane concentration unit, sampling ports (no more than 0.5 mm from the membrane surface) are installed at the inlet and outlet of each section to collect fluid samples from both sides of the membrane under stable operating conditions. The osmotic pressure is determined using the conductivity conversion method.

[0064] At 25 °C, a conductivity-osmotic pressure calibration curve for this system was established using a NaCl / Na2SO4 mixed salt standard solution. The conductivity values ​​on the feed water side and the draw water side were measured using an online conductivity meter, and converted to the corresponding osmotic pressure π according to the calibration curve. f (Osmotic pressure on the inlet side membrane, unit MPa) and π d (Osmotic pressure on the membrane surface of the draw liquid side, in MPa).

[0065] The average osmotic pressure difference Δπ in each zone is calculated using the following formula:

[0066] ,

[0067] After running for 2 hours, the values ​​were measured three times consecutively, and the average value was taken as the Δπ for that partition.

[0068] Determination of transmembrane pressure difference ΔP:

[0069] Precision pressure sensors were installed on the water inlet and liquid drawer channels at the inlet and outlet of each zone to measure the static pressure on the water inlet and liquid drawer sides, respectively, denoted as P. f (Inlet side static pressure, unit MPa) and P d (Phase static pressure of the liquid being drawn, unit MPa).

[0070] The average transmembrane pressure difference ΔP in each zone is calculated using the following formula:

[0071] ,

[0072] To eliminate the impact of flow fluctuations, pressure data were continuously recorded for 10 minutes after the system had been running stably for 2 hours, and the average value was taken as ΔP.

[0073] The operating conditions for each partition are as follows:

[0074]

[0075] To maintain system stability, the draw solution is controlled online by conductivity and partially diluted by reflux to keep its outlet concentration below the design limit (approximately 3.0 mol / L). After treatment in the nanofiltration-reverse osmosis recovery section, the draw solution salt concentration is restored to approximately 2.0 mol / L and is reused as the inlet draw solution, achieving closed-loop operation.

[0076] According to the formula: Calculations yielded 6.4.

[0077] The forward osmosis operating pressure was controlled at 4 bar. The feed COD was approximately 160 mg / L. After concentration, the TDS of the concentrate increased to 48 g / L, a concentration ratio of approximately 8 times. No obvious compaction or sudden drop in flux was observed on the membrane surface, and the flux decay was less than 5% after 48 hours of continuous operation.

[0078] S5 salt separation treatment: The forward osmosis concentrate is introduced into the nanofiltration salt separation unit, which uses a polyamide composite nanofiltration membrane, operates at a pressure of 35 bar, and has a recovery rate of 70%.

[0079] Nanofiltration products are divided into two streams:

[0080] The main components of the retentate are Na2SO4 and SO42-. 2- / Cl - Mass ratio 2.8;

[0081] The main components of the permeate are NaCl and SO4. 2- / Cl - Mass ratio 0.4.

[0082] S7 Evaporation Crystallization and Circulation: The evaporation crystallization unit using retentate as raw material produces Na2SO4·10H2O crystals, while the evaporation crystallization unit using permeate as raw material produces NaCl crystals. Both mother liquors are refluxed to the front end of the salt separation unit to adjust the salt balance.

[0083] The two streams of brine enter the subsequent evaporation and crystallization unit respectively.

[0084] Example 2

[0085] This embodiment discloses a zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-expansion membrane concentration and salt separation synergy. The process flow is the same as in Embodiment 1, sequentially including dual-alkali softening pretreatment, organic pollutant treatment, forward osmosis low-pressure high-expansion concentration, salt separation, and evaporation crystallization steps. The difference lies in that the forward osmosis membrane concentration unit adopts an n=5 partition structure.

[0086] The specific steps for S4 forward osmosis low-pressure high-concentration are as follows:

[0087] A cellulose acetate flat sheet membrane was used, with a total channel length of 1.5 m. The channel was divided into five sections along the flow direction, with a length ratio of 1.5:1.2:1.0:0.8:0.8. The tangential flow velocity at the membrane surface was 0.20 m / s.

[0088] A gas-liquid injection device is installed in the membrane channel, with an injection frequency of 0.2 times / s, a gas phase volume fraction of 0.8%, and a bubble characteristic diameter of 0.4 mm.

[0089] The operating conditions for each partition are as follows:

[0090]

[0091] Calculate R n / R1=10.8.

[0092] The operating pressure is 4 bar, and the flux decay rate is 4.5% after 48 hours of operation, with a concentration ratio of approximately 8.3 times.

[0093] Example 3

[0094] This embodiment discloses a zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-expansion membrane concentration and salt separation synergy. The process flow is the same as in Example 1, sequentially including dual-alkali softening pretreatment, organic pollutant treatment, forward osmosis low-pressure high-expansion concentration, salt separation, and evaporation crystallization steps. The difference lies in the reduced stability (Ω) of the interfacial electrochemical buffer zone to 0.5.

[0095] S3 interface electrochemical buffer zone and solid-liquid separation:

[0096] Keeping the pH constant at 7.3, E was adjusted to +300 mV, the total iron concentration was 0.8 mg / L, and the residence time was 15 s. After hollow fiber microfiltration, the soluble iron concentration was ≤0.25 mg / L. Ω was calculated to be 0.5 according to the formula.

[0097] S4 forward osmosis low-pressure high-concentration (n=3):

[0098] The membrane channel length, structure, and partitioning method are the same as in Example 1.

[0099] The tangential flow velocity at the membrane surface was 0.30 m / s, the gas injection frequency was 0.8 times / s, the gas phase volume fraction was 3%, and the characteristic diameter of the bubble was 1.0 mm.

[0100] The operating parameters for each partition are as follows:

[0101]

[0102] Calculated R n / R1=5.58≥1.6+0.6Ω=1.9.

[0103] The operating pressure is 4 bar, and the flux decreases by about 7% after 48 hours of operation, with a concentration ratio of 7.6 times.

[0104] Compared with Example 1, the flux fluctuated slightly and there was slight deposition on the membrane surface, indicating that the chemical steady state of the membrane surface was weakened after the decrease of Ω, but it can still operate stably.

[0105] Example 4

[0106] This embodiment discloses a zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-expansion membrane concentration and salt separation synergy. The process flow is the same as in Example 1, sequentially including dual-alkali softening pretreatment, organic pollutant treatment, forward osmosis low-pressure high-expansion concentration, salt separation, and evaporation crystallization steps. The difference lies in the lowest interfacial electrochemical buffer stability (Ω=0).

[0107] S3 interface electrochemical buffer zone and solid-liquid separation:

[0108] The parameters were controlled as follows: E = +150 mV, pH = 7.8, total iron concentration 0.5 mg / L, and residence time 12 s. After precipitation and microfiltration, soluble iron ≤ 0.30 mg / L. Substituting these values ​​into the formula, Ω = 0.

[0109] S4 forward osmosis low-pressure high-concentration (n=3):

[0110] The membrane channel division and materials are the same as in Example 1.

[0111] The tangential flow velocity at the membrane surface was 0.22 m / s; the gas injection frequency was 0.3 times / s; the gas phase volume fraction was 1.0%; and the characteristic bubble diameter was 0.5 mm.

[0112] The operating parameters for each partition are as follows:

[0113]

[0114] Calculated R n / R1=1.7≥1.6+0.6Ω=1.6.

[0115] After 48 hours of operation, the flux decreased by approximately 10%, and a slight fouling layer appeared on the membrane surface at the end, with a concentration ratio of approximately 6.5 times. This indicates that even with a weak buffer, the system can still operate stably, although flux stability is slightly reduced.

[0116] Comparative Example 1

[0117] This comparative example uses a single-zone forward osmosis membrane concentration process, eliminating the multi-zone structure and Δπ / ΔP gradient control, and only performs concentration operations under constant operating pressure conditions.

[0118] The dual-alkali softening and pre-ozone-MBR-post-ozone process is the same as in Example 1, ensuring consistent feedwater conditions. The membrane module still uses cellulose acetate membranes, but only a single zone (n=1) is set up, without any zoning design along the flow path. The system operating pressure is 45 bar, and the tangential flow velocity at the membrane surface is 0.25 m / s. The draw solution is a 2.0 mol / L NaCl / Na2SO4 mixed salt, and its concentration is not adjusted along the flow path. No interfacial electrochemical buffer zone is set up; the feedwater pH is approximately 7.1, and the redox potential is approximately +200 mV.

[0119] After running for 8 hours, the concentration ratio was approximately 5.1, and the system recovery rate was 87%.

[0120] Because the osmotic pressure and transmembrane pressure difference are fixed along the process, the inlet section is significantly compacted, the membrane fouling rate is relatively fast, and the flux decreases by about 12% after 48 hours.

[0121] The membrane cleaning cycle is approximately once every 5 days.

[0122] There is no gas-liquid disturbance within the membrane channel; only single-phase liquid flows.

[0123] Comparative Example 2

[0124] This comparative example uses a traditional reverse osmosis (RO)-evaporation crystallization process, without a forward osmosis membrane section or salt separation unit.

[0125] The dual-alkali softening and pre-ozone-MBR-post-ozone process is the same as in Example 1, ensuring consistent influent conditions. The RO system operates at a pressure of 50 bar, with a single-stage recovery rate of approximately 70%. Due to limited high-salinity concentrate reflux, the concentrate TDS is approximately 40 g / L.

[0126] The RO concentrate is directly fed into the evaporation unit for mixed salt crystallization. The crystals contain a mixture of Na₂SO₄ and NaCl, resulting in high mother liquor viscosity and a slow crystallization rate. The evaporator operates at a power of 25 kW and runs for 8 hours per cycle.

[0127] The system has a total water recovery rate of approximately 82% and an energy consumption of approximately 3.6 kWh·m³ per unit of water produced. -3 The energy consumption per unit crystal is approximately 0.74 kWh·kg. -1 .

[0128] The crystalline product contains impurities and has a purity of only 91% to 93%, requiring secondary washing.

[0129] Performance testing

[0130] To verify the overall energy-saving effect and water recovery performance of the process of this invention, the main engineering indicators during system operation were tested and calculated, including concentration ratio, water recovery rate, system energy consumption, evaporation energy consumption, and comprehensive energy saving rate. All tests were conducted after the system had been running stably for 30 minutes, with the duration being 30 minutes and the temperature maintained at 25±1 ℃.

[0131] (1) Concentration ratio

[0132] The concentration ratio is used to reflect the membrane process's ability to enrich dissolved salts in wastewater.

[0133] During the test, samples of feed water and forward osmosis concentrate were collected separately, and their total dissolved solids (TDS) content was determined. The TDS test was conducted according to GB / T 5750.4—2021 standard, and the conductivity method was used for conversion.

[0134] Concentration ratio β is calculated using the following formula: ,in TDS (mg / L) of concentrated solution. The TDS (mg / L) of the influent.

[0135] If β > 6, it indicates that the system achieves a high concentration effect.

[0136] (2) System water recovery rate

[0137] Water recovery rate reflects the proportion of freshwater reused by the system relative to the influent.

[0138] During the test, an electromagnetic flow meter was used to record the total amount of water entering the system. and water production (i.e., the water produced in the extract liquid recovery section).

[0139] Water recovery rate Calculate using the following formula:

[0140] The higher the value, the more efficient the system is in utilizing water resources.

[0141] (3) System energy consumption

[0142] System energy consumption is used to measure the total energy consumption of the membrane concentration unit and auxiliary equipment when producing a unit volume of fresh water.

[0143] During the test, the total power of the inlet pump, circulation pump, gas-liquid injection device, and control system was read. The running time t (h) is accumulated through smart meters. The actual water production volume is recorded within the same time period. (m³).

[0144] Energy consumption per unit of water production, E, is calculated using the following formula:

[0145]

[0146] The result is expressed in kWh·m -3 This indicates that the lower the E value, the higher the system energy efficiency.

[0147] (4) Evaporation energy consumption

[0148] Evaporation energy consumption is used to evaluate the energy-saving effect of the evaporation and crystallization process after salt separation.

[0149] During testing, the operating power of the evaporation unit was recorded. (kW) and running time t (h), and weigh the crystal mass m (kg) obtained within the corresponding time.

[0150] Energy consumption per unit crystal Calculate using the following formula:

[0151]

[0152] The results were expressed in kWh·kg -1 Crystal representation.

[0153] By comparing the conditions under which salts were separated and those under which salts were not separated. This allows us to determine the extent to which salt separation synergy reduces evaporation energy consumption.

[0154] (5) Overall energy saving rate

[0155] The overall energy saving rate is used to compare the energy efficiency advantages of the process of this invention compared with the traditional reverse osmosis-evaporation route.

[0156] In the calculation, the unit water production energy consumption of Comparative Example 2 (conventional RO + evaporation process) is used. Based on this, the energy consumption corresponding to the process of this invention is .

[0157] The overall energy saving rate η is calculated using the following formula:

[0158]

[0159] When η≥50%, it indicates that the process of the present invention has a significant energy-saving effect.

[0160] The test results of Example 1, Comparative Examples 1 and 2 are shown in the table below:

[0161]

[0162] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy, characterized in that, The treatment process includes, in sequence, dual-alkali softening pretreatment, organic contaminant treatment, forward osmosis low-pressure high-concentration treatment, salt separation treatment, and evaporation crystallization steps; among which: The forward osmosis low-pressure high-concentration treatment involves sending water treated for organic contaminants into a forward osmosis membrane concentration unit, setting up at least three zones arranged along the flow direction within the membrane channel; adjusting the operating conditions of each zone so that the average osmotic pressure difference Δπ from the inlet to the end of the zone gradually increases to the range of 1.0 to 3.5 MPa, while the average transmembrane pressure difference ΔP gradually decreases to the range of 0.01 to 0.10 MPa; and ensuring that the average osmotic pressure difference Δπ between adjacent zones from the inlet to the end increases by 1.2 to 2.0 times, while the average transmembrane pressure difference ΔP decreases by 0.2 to 0.8 times. Among them, Δπ is the osmotic pressure measured at the membrane surface of the fluid on both sides of the membrane in the partition, and ΔP is the average static pressure at the inlet and outlet of the partition. The permeate obtained from the extract liquid recovery section is returned to the front-end system as makeup water for reuse. Salt separation involves sending the forward osmosis concentrate obtained from low-pressure high-concentration forward osmosis treatment into a nanofiltration salt separation unit for selective separation to obtain brine mainly composed of sodium sulfate and brine mainly composed of sodium chloride.

2. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 1, characterized in that, The inlet of the forward osmosis membrane concentration unit is equipped with an interfacial electrochemical buffer zone. A reversible redox pair consisting of ferrous and ferric iron is added to the interfacial electrochemical buffer zone. The total iron concentration in the buffer zone is 0.2–3.0 mg / L, the redox potential is +150–+350 mV, the pH is 6.8–7.8, and the residence time in the buffer zone is 10–60 s.

3. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 2, is characterized in that, A solid-liquid separation unit is provided downstream of the interface electrochemical buffer zone, and the concentration of soluble iron in the water entering the membrane channel after treatment by the solid-liquid separation unit is no greater than 0.3 mg / L. And definition , Where E is the redox potential of the buffer zone, expressed in mV, and pH is the pH value of the buffer zone; The forward osmosis membrane channel is divided into n≥3 partitions along the flow direction and satisfies 。 4. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 1, characterized in that, A gas-liquid injection component is provided in the membrane channel. The injection frequency of the component is 0.1 to 1.0 times / second, the gas phase volume fraction is 0.5% to 5.0%, and the bubble characteristic diameter is 0.1 to 1.5 mm.

5. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 4, is characterized in that... The tangential flow velocity within the membrane channel is 0.10–0.40 m / s.

6. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 1, characterized in that, The membrane channel is divided into three parts along the flow direction: the inlet section, the middle section, and the end section. The length ratio of the three parts is 1.0-1.5:1:0.8-1.

2.

7. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 1, characterized in that, The membrane material of the forward osmosis membrane in the forward osmosis membrane concentration unit is one of polypropylene, polyester, polysulfide, polycarbonate, nano alumina, cellulose acetate or polyamide.

8. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 1, characterized in that, The dual-alkali softening pretreatment involves sequentially adding sodium carbonate and sodium hydroxide solutions to the reverse osmosis concentrate of dyeing and printing wastewater. Calcium and magnesium hardness ions in the water are removed through reaction precipitation, resulting in a total water hardness reduced to below 150 mg / L.

9. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 1, characterized in that, The organic pollutant treatment includes passing water that has undergone dual-alkali softening pretreatment sequentially through a pre-ozone oxidation unit, a membrane bioreactor unit, and a post-ozone fine treatment unit to obtain treated water with reduced organic matter content and weakened color. The ratio of biochemical oxygen demand (BOD) to chemical oxygen demand (COD) in the treated water is increased to no less than 0.

3.

10. The zero-discharge treatment process for dyeing and printing wastewater based on low-pressure high-concentration membrane concentration and salt separation synergy as described in claim 9, characterized in that, The ozone dosage in the pre-ozone oxidation unit is 5–15 mg / L, the sludge concentration in the membrane bioreactor unit is 8000–10000 mg / L, and the membrane material of the membrane bioreactor unit is a hollow fiber membrane; the ozone dosage in the post-ozone purification unit is 3–8 mg / L. Both the post-ozone purification unit and the post-ozone purification unit require the use of a catalyst, which is a silicon-aluminum based catalytic oxidant.

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