A method for recovering alkali from desizing wastewater of printing and dyeing

Abstract

The present application relates to the technical field of industrial wastewater treatment and resource recovery, and discloses a method for recovering alkali from desizing water of printing and dyeing wastewater, which solves the problem of low recovery efficiency caused by serious membrane pollution when the existing membrane separation technology is used to treat the printing and dyeing desizing wastewater. The method comprises the following steps: homogenizing and air flotation pretreatment of high-temperature desizing water to remove most of the grease and suspended solids; then, the clear liquid is filtered through an ultrafiltration membrane, a negative pressure environment is constructed on the membrane shell side to real-time separate the permeate, micro-bubbles are injected into the feed liquid to form a gas-liquid two-phase flow, and the net driving force on the membrane surface is controlled to be maintained in the range of 0.02-0.04 MPa micro-positive pressure, so as to reduce the membrane pollution; the primary filtered water is further purified through nanofiltration, and finally, high-purity recovered alkali is obtained. The present application effectively solves the problems of serious membrane pollution and rapid flux decay in the traditional membrane treatment process, and realizes efficient recovery of alkali and long-term stable operation of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial wastewater treatment and resource recovery, and particularly relates to a desizing water alkali recovery method for printing and dyeing wastewater. BACKGROUND

[0002] The wastewater generated in the printing and dyeing production process, especially the desizing wastewater, has been a severe challenge in the field of environmental governance and resource recovery for a long time due to its unique physicochemical properties. The desizing wastewater usually exhibits the characteristics of high alkalinity, high temperature, high viscosity and high solid content. The core pollutants of the desizing wastewater not only include a large amount of polyvinyl alcohol (PVA) sizing agent, but also contain various oils, waxes and various additives. These complex components make the treatment of desizing wastewater much more difficult than that of general industrial wastewater. The traditional water treatment methods are often difficult to efficiently and economically achieve the deep purification of wastewater and the recovery of useful substances, especially in the recycling of alkaline substances, which has a significant bottleneck.

[0003] Among various treatment technologies, membrane separation technology has gradually been introduced into the treatment of printing and dyeing wastewater due to its advantages of efficient separation, energy saving and environmental protection, and has shown great potential in the field of alkali recovery.

[0004] However, due to various pollutants in the desizing water, serious membrane fouling (concentration polarization and gel layer formation) can easily occur in the traditional membrane filtration process, resulting in a sharp decrease in membrane permeation flux, frequent cleaning and unstable system operation, thus making the alkali recovery inefficient and costly. In addition, calcium and magnesium ions in the wastewater are easy to combine with carbonate ions to form scale, which further aggravates the membrane clogging. The existing membrane separation technology is difficult to maintain high recovery rate while running stably for a long time.

[0005] Therefore, it is necessary to provide a desizing water alkali recovery method for printing and dyeing wastewater to solve the above technical problems. SUMMARY

[0006] The present application overcomes the shortcomings of the prior art and provides a desizing water alkali recovery method for printing and dyeing wastewater.

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: a desizing water alkali recovery method for printing and dyeing wastewater, comprising the following steps:

[0008] S1, after homogenizing the high-temperature desizing water, most of the oils, waxes and suspended solids in the desizing water are removed by dissolved air flotation treatment to obtain a clear liquid;

[0009] S2, the treated clear liquid is filtered through an ultrafiltration membrane, and a negative pressure environment is constructed on the membrane shell side of the ultrafiltration membrane to real-time extract the permeate liquid that penetrates through the ultrafiltration membrane to obtain primary filtered water;

[0010] S3, the primary filtration water is purified by a nanofiltration membrane, under the action of pressure, the nanofiltration membrane selectively intercepts the polyvalent ions and trace organic matters in the primary filtration water, so that water and caustic soda are penetrated;

[0011] S4, the nanofiltration permeate obtained in S3 is used as high-purity recovered lye and directly used in the printing and dyeing production process.

[0012] In a preferred embodiment of the present application, in S2, inert gas is injected into the feed liquor to form a gas-liquid two-phase flow during the ultrafiltration membrane filtration process, and the ratio of the amount of inert gas injected to the volume flow rate of the feed liquor is 0.05-0.5:1.

[0013] In a preferred embodiment of the present application, the inert gas is nitrogen, which is injected in the form of fine bubbles with a diameter of 20-100 mu m.

[0014] In a preferred embodiment of the present application, in S2, the net driving force on the surface of the ultrafiltration membrane is maintained in the micro-positive pressure range of 0.02-0.04 MPa by real-time regulation of the pressures on the feed side and the membrane shell side of the ultrafiltration membrane.

[0015] A printing and dyeing wastewater desizing water alkali recovery device, comprising: a homogenization adjusting mechanism, a dissolved air flotation mechanism, an ultrafiltration membrane separation mechanism and a nanofiltration mechanism connected in sequence;

[0016] The ultrafiltration membrane separation mechanism comprises an ultrafiltration membrane assembly, an ultrafiltration feed pump, an ultrafiltration circulating pump, a gas injection module and a permeate booster pump; the ultrafiltration feed pump is used to send the liquor to the ultrafiltration membrane separation mechanism; the ultrafiltration circulating pump is used to drive the liquor to flow at high speed in the ultrafiltration membrane assembly; the gas injection module is used to inject inert gas into the circulating liquor to form a gas-liquid two-phase flow; the permeate booster pump is connected to the membrane shell side of the ultrafiltration membrane assembly, and is used to build a negative pressure environment on the membrane shell side and to extract the permeate, while boosting the permeate;

[0017] The feed inlet of the nanofiltration mechanism is connected with the outlet of the permeate booster pump, and is used to receive the boosted primary filtration water;

[0018] And a control system, which is electrically connected with the homogenization adjusting mechanism, the dissolved air flotation mechanism, the ultrafiltration membrane separation mechanism and the nanofiltration mechanism, is used to monitor and regulate the operating parameters of each mechanism.

[0019] In a preferred embodiment of the present application, the gas injection module comprises a gas source, a flow controller and a mixer, and the mixer is a high-pressure jet pump or a static mixer.

[0020] In a preferred embodiment of the present application, the control system is configured to collect the feed inlet pressure, outlet pressure and membrane shell side pressure of the ultrafiltration membrane assembly in real time, and dynamically adjust the operating state of the ultrafiltration feed pump and the permeate booster pump, so that the net driving force on the surface of the ultrafiltration membrane is maintained within the range of 0.02MPa-0.04MPa.

[0021] In a preferred embodiment of the present application, the ultrafiltration membrane assembly adopts a tubular membrane assembly, and the membrane material is polyvinylidene fluoride or polyether sulfone.

[0022] In a preferred embodiment of the present application, the mixer outlet is arranged at the inlet of the ultrafiltration membrane assembly, close to the edge of the membrane, and is annularly distributed, so that the generated bubble groups are close to the inner wall of the membrane.

[0023] In a preferred embodiment of the present application, the dissolved air flotation mechanism includes a high-pressure dissolved air tank, a releaser and an air flotation tank, and a slag scraping mechanism is arranged above the air flotation tank.

[0024] The present application solves the defects in the background art, and has the following beneficial effects:

[0025] (1) The present application provides a method for recovering alkali from desizing water of printing and dyeing wastewater, by injecting a specific proportion of inert gas into the feed clear liquid in the ultrafiltration membrane filtration process to form a gas-liquid two-phase flow, using the local turbulence and shear effect generated by the micro-bubbles on the membrane surface to effectively scrub and peel off the macromolecular pollutants such as polyvinyl alcohol (PVA) and gel layers attached to the membrane surface; this process significantly reduces the concentration polarization phenomenon and inhibits the deposition of pollutants on the membrane surface, thereby directly increasing the permeation flux of the ultrafiltration membrane and delaying the flux decay; compared with the existing technology which only relies on high-speed circulation or simple cross-flow filtration, the present application can maintain the efficient operation of the membrane without frequent chemical cleaning through the synergistic effect of the gas-liquid two-phase flow.

[0026] (2) The present application collects the pressure parameters of the ultrafiltration membrane feed side and the membrane shell side in real time, and dynamically estimates the membrane surface boundary layer pressure drop based on the built-in fluid mechanics model, and then accurately controls the operating state of the ultrafiltration feed pump and the permeate booster pump, so that the net driving force on the membrane surface is stably maintained within the micro-positive pressure interval of 0.02MPa-0.04MPa; effectively overcoming the reverse adsorption force generated by the macromolecular pollutants when only applying negative pressure on the shell side, while fully utilizing the negative pressure to improve the mass transfer efficiency, avoiding the rapid clogging of the membrane holes, and achieving the balance between high flux and low pollution.

[0027] (3) The application integrates a multi-stage treatment process of gas dissolved air flotation pretreatment, gas-liquid two-phase flow ultrafiltration and nanofiltration deep purification, and adopts a permeate booster pump to pressurize the ultrafiltration permeate and directly send it into the nanofiltration mechanism; the device realizes the compactness of the process flow and the energy gradient utilization, avoids the secondary lifting and energy loss of the intermediate link; compared with the traditional separate treatment unit, the application reduces the equipment investment and land area through system optimization and control integration, and improves the overall efficiency of alkali recovery and energy utilization. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments described in the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0029] Figure 1 is the overall structure diagram of the recovery device of the present application;

[0030] Figure 2 is the flow chart of the recovery method of the present application.

[0031] In the figure: 1, homogenization adjusting mechanism; 2, dissolved air flotation mechanism; 21, high-pressure dissolved air tank; 22, releaser; 23, air flotation tank; 3, ultrafiltration membrane separation mechanism; 31, ultrafiltration membrane assembly; 32, ultrafiltration circulating pump; 33, ultrafiltration feed pump; 34, gas injection module; 35, permeate booster pump; 4, nanofiltration mechanism. DETAILED DESCRIPTION

[0032] The existing membrane filtration system, whether it is to reduce membrane fouling by adopting cross-flow filtration mode, or to design a pretreatment unit to reduce the pollution load into the membrane system, when dealing with printing and dyeing desizing wastewater, it still generally faces a series of inherent problems. Specifically, PVA and other high molecular polymers, grease, wax and suspended solids in desizing water, in the traditional pressure-driven membrane filtration process, can easily form a dense gel layer on the membrane surface and cause serious concentration polarization effect. Concentration polarization refers to the phenomenon that the concentration of solutes near the membrane surface is much higher than that of the bulk solution, which significantly increases the mass transfer resistance of solutes to the membrane surface, resulting in a sharp decrease in membrane permeation flux and low system efficiency. In order to maintain the necessary flux, frequent chemical cleaning is often required, which not only increases the operating cost and shortens the service life of the membrane, but also makes it difficult for the entire system to achieve long-term stable operation. In addition, the hardness ions such as calcium and magnesium commonly present in desizing wastewater can easily combine with carbonate ions in an alkaline environment to form insoluble salts such as calcium carbonate and magnesium hydroxide. These substances are prone to scale inside or on the surface of the membrane pores, further exacerbating membrane clogging, making it difficult for membrane separation technology to achieve high recovery rate and long-term stable operation in practical application.

[0033] With the increasing demand for membrane separation efficiency and stable operation, the applicant gradually focused on a new approach to optimizing membrane flux and reducing concentration polarization while exploring more efficient membrane separation strategies, that is, by applying low-pressure suction on the permeation side to remove permeate in real time. The theoretical basis of this is to create a local low-pressure environment on the membrane permeation side, so that pure water or small molecule solutes that have just permeated the membrane can be quickly "pulled away", effectively preventing these substances from accumulating on the membrane surface, thereby eliminating or significantly reducing the concentration polarization phenomenon and maintaining a stable mass transfer driving force on both sides of the membrane. This "push-pull" synergistic effect is highly expected to significantly improve the membrane permeation flux, inhibit gel layer formation, and even to some extent, alleviate inorganic salt scaling.

[0034] However, when the applicant applies the "permeate-side low-pressure suction" technology to the complex system of printing and dyeing desizing wastewater containing a large amount of high-molecular pollutants, an inherent non-obvious contradiction gradually appears and becomes a key bottleneck restricting the performance of the technology. The reason is that when a suction negative pressure is applied to the permeate side (i.e. the membrane shell side) of the ultrafiltration membrane, the negative pressure not only acts on the clear liquid that has permeated the membrane to promote its rapid extraction, but also is reversely transmitted to the membrane surface on the feed side through the membrane pore structure. For high-concentration and high-viscosity macromolecular pollutants such as polyvinyl alcohol (PVA) in printing and dyeing desizing wastewater, these substances are intercepted because their molecular size is much larger than the pore size of the ultrafiltration membrane. At this time, the reverse negative pressure transmitted from the permeate side will generate a strong "adsorption" or "suction" force on these impermeable pollutants in the membrane feed boundary layer, which is directed to the membrane surface. Although these macromolecular substances are ultimately blocked by the ultrafiltration membrane and cannot truly penetrate, this continuous reverse pulling force will cause them to adhere to the ultrafiltration membrane surface with a stronger tendency, and then be forced to squeeze into or firmly adsorbed at the entrance of the membrane pore. This "adsorption blocking" phenomenon is different from the traditional concentration polarization and gel layer formation mechanism. It is a physical adsorption enhancement effect driven by negative pressure, and its consequence is that the membrane pores are severely blocked, resulting in a significant reduction in the effective filtration area of the membrane, a decrease in the permeation rate, and even more rapid and severe membrane fouling than in the traditional pressure-driven mode.

[0035] Therefore, the applicant further proposes a printing and dyeing wastewater desizing water alkali recovery method, which can effectively utilize the positive mass transfer gain brought by the permeate-side low-pressure suction, while avoiding or significantly reducing the reverse adsorption force generated on the macromolecular pollutants, thereby realizing efficient and long-term stable membrane separation and alkali recovery of high-pollution and high-viscosity wastewater.

[0036] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0037] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other ways different from those described herein, therefore, the scope of protection of the present application is not limited to the specific embodiments disclosed below.

[0038] Figure 1 A printing and dyeing wastewater desizing water alkali recovery device structure diagram of the present application is shown. The printing and dyeing wastewater desizing water alkali recovery device comprises a homogeneous adjustment mechanism 1, a dissolved air flotation mechanism 2, an ultrafiltration membrane separation mechanism 3, a nanofiltration mechanism 4 and a control system interconnected in a specific way.

[0039] The homogenizing adjustment mechanism 1 of the present embodiment receives desizing wastewater from the printing and dyeing production line and controls the water quality characteristics and temperature fluctuations within a stable range acceptable to the subsequent treatment unit. Specifically, the homogenizing adjustment mechanism 1 comprises at least two homogenizing adjustment tanks, each of which is made of corrosion-resistant 304L stainless steel and has an internal volume of 20-30 m 3 The bottom of the homogenizing adjustment tank is provided with a liquid outlet connected to the dissolved air flotation mechanism 2 through a high-temperature and corrosion-resistant UPVC pipeline.

[0040] The homogenizing adjustment tank is provided with a stirring mechanism, which comprises a planetary gear reducer motor installed at the top of the homogenizing adjustment tank and a three-blade propeller stirrer arranged at the output end of the reducer motor. The diameter of the stirring blade of the three-blade propeller stirrer is 1-1.5 m, and the rotating speed can be continuously adjusted within the range of 30-80 rpm / min through a frequency converter, ensuring that the wastewater in the tank can be fully mixed and effectively avoiding the settlement of solid substances and local concentration unevenness.

[0041] Further, a jacketed temperature control mechanism is arranged around the tank body of the homogenizing adjustment tank, which maintains the temperature of the wastewater in the tank within the preset interval of 70°C-90°C by circulating the introduction of industrial waste heat steam or hot water. A temperature sensor (such as a PT100 type thermistor) monitors the temperature in the tank in real time and transmits the data to the control system, which accurately regulates the steam or hot water flow in the jacket according to the preset value.

[0042] The dissolved air flotation mechanism 2 receives the water from the homogenizing adjustment mechanism 1, which is used to remove a large amount of oil, wax and suspended solids in the desizing water after homogenization, and reduce the pressure of the subsequent ultrafiltration process. The dissolved air flotation mechanism 2 comprises a high-pressure dissolved air tank 21, a releaser 22 and a flotation tank 23. The high-pressure dissolved air tank 21 is made of Q235-B carbon steel with a corrosion-resistant coating on the inner wall, and the design pressure is 1.0 MPa, and the working pressure is usually set at 0.4-0.6 MPa. The high-pressure dissolved air tank 21 forcibly mixes part of the treated water with compressed air through a vortex pump, so that the air is saturated and dissolved in the water under high pressure. The dissolved gas water enters the releaser 22 through the pipeline, which is made of stainless steel and has a special flow channel structure inside, which can instantaneously reduce the pressure of the high-pressure dissolved gas water to normal pressure, thereby generating fine bubbles with a diameter of about 20-50 μm at the bottom of the flotation tank 23. These fine bubbles combine with the oil, wax and suspended solid particles in the wastewater through adsorption, bridging and other actions during the rising process, forming scum with a density less than water and floating to the surface of the flotation tank 23.

[0043] Further, the air flotation tank 23 adopts a reinforced concrete structure, and the tank body is built according to the requirements and actual space. In a specific embodiment, the tank body has a size of 8 m in length, 3 m in width and 3.5 m in depth, and is designed with a water inlet distribution area, a scum separation area and a water outlet collection area. A reciprocating scum scraping mechanism is arranged above the air flotation tank 23, which includes a speed-reducing motor for driving, a chain wheel and chain or gear and rack transmission system driven by the speed-reducing motor and crossing the air flotation tank 23, a scum scraping plate rigidly connected with the transmission system, and a stroke switch arranged at both ends of the tank body for controlling the stroke range of the scum scraping plate. The scum scraping plate moves along the track on the top of the tank body in a reciprocating straight line under the driving of the transmission system, so as to periodically scrape the floated scum into the scum collection groove at the edge of the tank, and then pump the scum to a sludge treatment system by a screw pump. The gas-water ratio of the dissolved air flotation process is accurately controlled by a control system, and is maintained at 1:5-1:10 (volume ratio) to ensure the best pollutant removal effect.

[0044] After the treatment of the dissolved air flotation mechanism 2, most of the oil, wax and suspended solids in the desizing water are effectively removed. The turbidity of the effluent is significantly reduced, thereby greatly reducing the pollution load of the subsequent ultrafiltration membrane separation mechanism 3. The treated clear liquid is collected from the water outlet collection area of the air flotation tank 23, and the water quality meets the requirements of the ultrafiltration system. The clear liquid is smoothly sent to the subsequent ultrafiltration membrane separation mechanism 3 by a delivery pump.

[0045] The treated clear liquid is delivered to the ultrafiltration membrane separation mechanism 3, which is used for solid-liquid separation of the clear liquid, efficiently intercepts macromolecular substances such as polyvinyl alcohol (PVA), and allows small molecules such as water and alkali to pass through. The ultrafiltration membrane separation mechanism 3 includes at least six parallelly operated tubular ultrafiltration membrane assemblies 31, an ultrafiltration circulating pump 32, an ultrafiltration feed pump 33, a gas injection module 34 and a permeate booster pump 35.

[0046] The tubular ultrafiltration membrane assembly 31 is made of a hydrophilic modified porous tubular membrane made of polyvinylidene fluoride (PVDF) or polyether sulfone (PES) to enhance its anti-pollution performance. Specifically, the membrane assembly selects a PVDF tubular membrane with an outer diameter of 25 mm and an inner diameter of 16 mm, and the average pore size is 20 nm, and the rejection rate of polyvinyl alcohol (PVA) is higher than 99%. Each membrane tube is arranged inside a membrane shell made of UPVC material to form an open cross-flow filtration channel. It should be noted that each membrane assembly contains 20 membrane tubes, and the effective membrane area is 5 m 2 .

[0047] The ultrafiltration circulating pump 32 is a variable frequency controlled centrifugal pump with a rated flow of 50 m 3 / h, head 20 m. The ultrafiltration circulating pump 32 forms a circulating loop with the feed side of the tubular ultrafiltration membrane module 31, while the ultrafiltration feed pump 33 is a frequency-controlled screw pump for stably feeding the pretreated clear liquid into the circulating loop, and the ultrafiltration circulating pump 32 is used to drive the clear liquid to flow at high speed in the membrane tube of the tubular ultrafiltration membrane module 31, with a linear speed adjustable in the range of 3-6 m / s, to generate strong shear force, thereby effectively inhibiting concentration polarization and membrane fouling.

[0048] The gas injection module 34 is arranged between the outlet pipeline of the ultrafiltration circulating pump and the feed port of the tubular ultrafiltration membrane module, for uniformly injecting inert gas into the clear liquid in the circulating loop to form stable gas-liquid two-phase flow. The gas injection module 34 includes a gas source, a flow controller, and a high-pressure jet pump or a static mixer. The gas source is selected from an industrial-grade liquid nitrogen storage tank or a site nitrogen generator, which provides nitrogen gas with a purity of not less than 99.9%. The flow controller is selected from a Coriolis mass flowmeter for accurate measurement and control, with a control accuracy of ±0.5% FS and a response time of less than 1 s, to ensure that the ratio of the gas injection amount to the volume flow rate of the feed clear liquid (gas-liquid ratio) is controlled in the range of 0.05-0.5:1 in volume ratio. The high-pressure jet pump or the static mixer is used to realize uniform dispersion of micron-sized gas bubbles in the clear liquid.

[0049] The gas injection module 34 utilizes the Venturi effect or a special turbulent structure design to efficiently and uniformly disperse and mix the quantified nitrogen gas into the feed clear liquid in the form of micro-bubbles with a diameter of 20-100 μm, to form stable gas-liquid two-phase flow. These micro-bubbles continuously perform "brushing" action on the membrane surface through their own rising buoyancy and convective shear force in the high-speed flowing liquid phase, effectively stripping the attached macromolecular pollutants and inhibiting the formation of concentration polarization layer.

[0050] The permeate booster pump 35 is a frequency-controlled centrifugal pump, which has the following two functions:

[0051] Function one: the permeate booster pump 35 is connected to the permeation side (i.e. the membrane shell side) of the tubular ultrafiltration membrane module 31, to maintain a preset negative pressure environment (-0.01 MPa to -0.08 MPa) on the membrane shell side, to real-time extract the permeate liquid through the membrane, to ensure the continuous progress of the permeation process;

[0052] Function two: the permeate booster pump 35 boosts the pressure of the extracted permeate liquid to the range of 0.8-1.5 MPa, to improve the pressure of the primary filtered water (water and small molecular substances such as caustic soda) filtered through the tubular ultrafiltration membrane module 31, so that the primary filtered water with high pressure has higher purification efficiency in the subsequent nanofiltration mechanism 4, to realize the optimization of system integration and energy efficiency.

[0053] The nanofiltration mechanism 4 of the embodiment is connected with the outlet of the permeate booster pump 35, and is used for further purifying the primary filtered water. Since the primary filtered water filtered by the ultrafiltration membrane still contains multivalent ions (such as calcium ions and magnesium ions) and trace organic substances in addition to water and caustic soda, the nanofiltration mechanism 4 is used to purify the primary filtered water, selectively retains the multivalent ions (such as calcium ions and magnesium ions) and trace organic substances, and recovers high-purity lye.

[0054] Specifically, the nanofiltration mechanism 4 includes at least three spiral-wound nanofiltration membrane assemblies, and a nanofiltration circulating pump and a nanofiltration feed pump connected with the nanofiltration membrane assemblies. The nanofiltration membrane assemblies, the nanofiltration circulating pump and the nanofiltration feed pump jointly constitute a nanofiltration membrane separation loop, and the feed pressure of the nanofiltration membrane separation loop is 0.8 MPa-1.5 MPa. The effective membrane area of each nanofiltration membrane assembly is 40 m 2 2, and the membrane sheet is selected from NF270 or similar composite polyamide nanofiltration membranes, and the molecular weight cut-off (MWCO) range is 150 Da to 300 Da. The rejection rate of the membrane to divalent ions (such as calcium ions and magnesium ions) is higher than 95%, and the permeation rate of the membrane to monovalent ions (such as sodium ions) is higher than 70%, which ensures the purity of the recovered lye.

[0055] Importantly, a control system associated with the operation of the entire printing and dyeing wastewater desizing water and alkali recovery device is provided. The control system coordinates the operation of each processing unit to realize the automatic operation and parameter optimization of the homogenizing mechanism 1, the dissolved air flotation mechanism 2, the ultrafiltration membrane separation mechanism 3 and the nanofiltration mechanism 4, thereby ensuring the efficiency and stability of the alkali recovery process.

[0056] The control system takes a programmable logic controller (PLC) as the core, configures distributed I / O modules and a supervisory control and data acquisition (SCADA) human-machine interface, realizes real-time monitoring and visualized control of process parameters such as temperature, pressure, flow rate and liquid level. A variety of sensors are arranged in the system, including pressure sensors arranged at the inlet, outlet and membrane shell side of the ultrafiltration membrane assembly 31, electromagnetic flowmeters and mass flowmeters for liquid and gas flow rate monitoring, PT100 type temperature sensors, and liquid level sensors; and the actuating mechanisms include frequency converters (for adjusting the rotating speed of pumps and stirring devices), electric regulating valves (for flow rate and pressure control) and pneumatic valves.

[0057] In particular, the control system further integrates an intelligent control method for inhibiting the pollution of the ultrafiltration membrane, which includes the following steps:

[0058] Step A1, real-time acquisition of the inlet pressure , outlet pressure and membrane shell side pressure And transmit to the PLC controller; Synchronous acquisition of conductivity, turbidity parameters provided by online water quality analyzer.

[0059] Step A2, based on the pressure data collected in step A1, calculate the average transmembrane pressure difference: ; The average transmembrane pressure difference reflects the overall pressure driving liquid permeation within the entire membrane module range.

[0060] The control system built-in preset fluid mechanics model, dynamic estimation of local pressure drop in membrane surface boundary layer , The pressure drop is mainly caused by the fluid resistance when flowing through the membrane surface boundary layer, which is the key factor leading to the difference between the actual action pressure on the membrane surface and the main flow pressure; Among them, the fluid mechanics model is a pressure drop calculation model based on Darcy-Weisbach equation and flow state (laminar flow or turbulent flow) criterion in membrane channel. In order to realize the accuracy of estimation, the model introduces online water quality parameters for real-time calibration and correction: Specifically, the system uses the measurement value of online conductivity sensor to indirectly represent the change of solute concentration in feed liquid, and uses the measurement value of online turbidity meter to reflect the content of colloidal substances. These real-time water quality parameters as input variables of the model, dynamically adjust the friction resistance coefficient and boundary layer thickness parameters in the model, so that The estimated value can more truly reflect the actual fluid mechanics state of the membrane surface under the current water quality condition.

[0061] Based on the above calculation and estimation results, further solve the net driving force on the membrane surface: , The net driving force represents the effective pressure that actually acts on the membrane surface and can be used to drive the permeation after overcoming the membrane surface boundary layer pressure drop on the feed side. Maintaining a small positive value is the core of this control method to prevent adsorption and blockage of pollutants on the membrane surface.

[0062] Step A3, output control instructions to frequency converter and electric regulating valve through PID and fuzzy logic compound algorithm: adjust the rotation speed of ultrafiltration feed pump 33 to control the feed flow, adjust the rotation speed of permeate booster pump 35 to control the negative pressure on the membrane shell side, so that Stable maintenance in the micro positive pressure interval of 0.02-0.04 MPa, so as to effectively inhibit the reverse adsorption of macromolecular pollutants on the membrane surface, and ensure the continuous and stable operation of ultrafiltration process.

[0063] Through the above intelligent control method for ultrafiltration membrane pollution inhibition, even in the case of negative pressure suction on the membrane shell side, the local effective driving force on the membrane feed side is enough to overcome any reverse force that may cause the reverse adsorption of macromolecular pollutants (especially PVA), so as to effectively drive the permeation while preventing the excessive adsorption of macromolecular pollutants on the membrane surface under the action of negative pressure.

[0064] In addition, the control system also adjusts the nitrogen flow of the gas injection module 34 according to the preset gas-liquid ratio to maintain the stable state of the gas-liquid two-phase flow, and has the functions of fault self-diagnosis, alarm prompt and remote monitoring, so as to comprehensively improve the reliability and long-term operation ability of the device.

[0065] Figure 2 A printing and dyeing wastewater desizing water alkali recovery method flow chart provided by the application is shown. The recovery method based on the device includes the following steps: step S1, after high-temperature desizing water is homogenized, most of the oil, wax and suspended solids in the desizing water are removed through dissolved air flotation treatment to obtain a clear liquid; step S2, the treated clear liquid is filtered through an ultrafiltration membrane, a negative pressure environment is constructed on the membrane shell side of the ultrafiltration membrane, and permeate liquid that penetrates through the ultrafiltration membrane is removed in real time to obtain primary filtered water; step S3, the primary filtered water is purified through a nanofiltration membrane, under the action of pressure, the nanofiltration membrane selectively retains multivalent ions and trace organic matter in the primary filtered water, and water and caustic soda penetrate; and step S4, the nanofiltration permeate liquid obtained in the step S3 is used as high-purity recovered alkali liquor and directly returned to the printing and dyeing production process.

[0066] Through integrated membrane separation technology, efficient recovery and reuse of caustic soda in printing and dyeing desizing wastewater are realized, and the problem of membrane pollution is effectively solved. The various components of the method and their operating mechanisms will be described in detail below to ensure that those skilled in the art can fully understand and implement the application.

[0067] In step S1, after high-temperature desizing water is homogenized, most of the oil, wax and suspended solids in the desizing water are removed through dissolved air flotation treatment to obtain a clear liquid.

[0068] Firstly, the desizing process itself needs to be carried out in hot alkali liquor. High temperature can accelerate the swelling and dissolution of sizing materials such as polyvinyl alcohol (PVA) and starch, and improve the desizing efficiency. Therefore, the temperature of desizing water is relatively high, between 70°C and 95°C.

[0069] That is, the desizing water itself is high-temperature, and the wastewater after desizing is directly collected and pumped to the homogenizing adjusting mechanism 1, in which the desizing water is fully mixed by the stirring mechanism 12 to eliminate the instantaneous fluctuations of water quality, concentration and temperature. At the same time, the temperature control mechanism maintains the desizing water temperature in the range of 70°C-90°C, optimizing the efficiency of subsequent dissolved air flotation and ultrafiltration membrane separation. After about 2h of homogenization and heat preservation, the homogenized desizing water is pumped to the dissolved air flotation tank 2 at a constant flow rate (for example, 10m 3h) is transported to the dissolved air flotation mechanism 2. In the dissolved air flotation mechanism 2, clean water is saturated with air at high pressure in a high-pressure dissolved air tank 21, and the high-pressure dissolved air water is instantaneously decompressed by a release device 22 to generate a large number of fine bubbles. These bubbles combine with the grease, wax, and suspended solid particles in the desizing water in the air flotation tank 23 and float to form a scum layer. A scum scraping mechanism 24 arranged above the air flotation tank 23 periodically scrapes off the scum.

[0070] In step S1, the dissolved air pressure is between 0.4 MPa and 0.6 MPa, and the air-water ratio is between 1:5 and 1:10 (volume ratio). After this step, the main grease and suspended solids in the desizing water are effectively removed, and the turbidity of the effluent is significantly reduced, thereby greatly reducing the pollution load of the subsequent step of ultrafiltration membrane separation.

[0071] In step S2, the treated clear liquid is filtered through an ultrafiltration membrane, and a negative pressure environment is established on the membrane shell side of the ultrafiltration membrane, and the permeate liquid that has passed through the ultrafiltration membrane is immediately removed, obtaining primary filtered water.

[0072] The filtration principle of the ultrafiltration membrane is based on the screening effect and the pressure difference between the two sides of the membrane. Specifically, the clear liquid treated in step S1 flows through the surface of the ultrafiltration membrane under the pressure of the feed side, and the ultrafiltration membrane has a specific pore size distribution (preferably nanoscale), which allows water molecules, dissolved alkali (NaOH), and other small molecule substances to pass through the membrane pores to become permeate, while pollutants with molecular weights much larger than the membrane pore size, such as polyvinyl alcohol (PVA), colloids, and residual grease, are selectively retained on the feed side.

[0073] That is, the clear liquid treated in step S1 is sent to the tubular ultrafiltration membrane assembly 31 by the ultrafiltration feed pump 33 at a constant pressure of 0.3 MPa to 0.8 MPa. Before entering the tubular ultrafiltration membrane assembly 31, the gas injection module 34 starts to work: high-purity nitrogen (purity not less than 99.9%) is accurately metered by a flow controller, and is uniformly injected into the clear liquid in the form of micron-sized bubbles with a diameter of 20 μm to 100 μm via a high-pressure jet pump or a static mixer, and the stable gas-liquid two-phase flow formed circulates inside the tubular ultrafiltration membrane at a linear velocity of 3 to 6 m / s under the drive of the ultrafiltration circulating pump 32.

[0074] In this gas-liquid two-phase flow ultrafiltration process, a preset negative pressure environment (-0.01 MPa to -0.08 MPa) is applied to the membrane shell side, forming a component of the transmembrane pressure difference (TMP). This negative pressure not only enhances the permeate extraction efficiency and increases membrane flux but also helps mitigate concentration polarization. However, for large molecular pollutants that are trapped, this negative pressure generates a reverse force pointing towards the membrane surface through the membrane pores, posing a risk of exacerbating membrane surface adsorption and fouling. Therefore, this invention dynamically adjusts the feed-side pressure and the membrane shell-side negative pressure through a control system to achieve a net driving force on the membrane surface. The pressure is consistently maintained within a slightly positive range of 0.02MPa-0.04MPa, thereby effectively suppressing the reverse adsorption and deposition of large molecular pollutants on the membrane surface while utilizing negative pressure to improve mass transfer efficiency, ensuring the stable and efficient operation of the ultrafiltration process. Simultaneously, the local turbulence and vortex effects generated by the high-speed flow of bubbles within the membrane tube significantly enhance the shear force on the membrane surface, creating a continuous "scrubbing" and peeling effect on large molecular pollutants such as polyvinyl alcohol (PVA) that are being applied to or have already been adsorbed onto the membrane surface. This effectively inhibits the formation of concentration polarization layers and gel layers, ensuring high permeate flux. The initial filtered water (permeate) is extracted and pressurized by the permeate booster pump 35 and enters the subsequent nanofiltration unit 4.

[0075] In step S3, the pre-filtered water is purified by nanofiltration membrane. Under pressure, the nanofiltration membrane selectively retains polyvalent ions and trace organic matter in the pre-filtered water, allowing water and caustic soda to pass through.

[0076] In other words, the pressure of the pre-filtered water obtained in step S2 has been increased to 0.8MPa-1.5MPa by the permeate booster pump 35, and then sent to the nanofiltration unit 4 for further purification. The nanofiltration unit 4 operates at a constant operating pressure of 0.8MPa-1.5MPa provided by the nanofiltration feed pump. Its nanofiltration membrane selectively retains polyvalent ions (such as calcium and magnesium ions) and trace organic matter in the pre-filtered water, while allowing monovalent ions (such as sodium ions) in water and high-purity alkaline solution to pass through efficiently, thereby ensuring the acquisition of high-purity alkaline solution.

[0077] In step S4, the nanofiltration permeate obtained in S3 is used as a high-purity recycled alkali solution and directly reused in the printing and dyeing production process.

[0078] That is, the nanofiltration permeate obtained through step S3, i.e. the high-purity recovered lye, has an electrical conductivity lower than 2000 μS / cm and a pH value stabilized at about 12.5, and can be directly reused in the desizing or scouring step in the printing and dyeing production process, significantly reducing the consumption of fresh caustic soda by the enterprise. The nanofiltration concentrate, in which multivalent ions and a small amount of difficult-to-remove organic matter are enriched, is further treated (e.g. evaporation crystallization or biochemical treatment) or discharged according to standard according to its water quality characteristics. This recovery and reuse process not only reduces production costs, but also significantly reduces the total amount of printing and dyeing wastewater discharged and the pollutant load, reflecting significant economic and environmental benefits.

[0079] In order to further illustrate the technical solutions of the present application and its beneficial effects, the following will be a detailed comparative analysis through specific examples and comparative examples. All examples and comparative examples use desizing wastewater of the same source and initial composition, and the key water quality indicators are as follows: pH value is 12.8, chemical oxygen demand (COD) is 8500 mg / L, polyvinyl alcohol (PVA) concentration is 1250 mg / L, suspended solid (SS) concentration is 850 mg / L, oil content is 320 mg / L, calcium ion (Ca 2+ ) concentration is 95 mg / L, magnesium ion (Mg 2+ ) concentration is 35 mg / L, and temperature is 85±5℃.

[0080] Example 1

[0081] This example uses the printing and dyeing wastewater desizing water alkali recovery device and method described in the present application to treat desizing wastewater.

[0082] The specific configuration of the ultrafiltration membrane separation mechanism in the device is as follows: PVDF tubular ultrafiltration membrane is used, the membrane area is 30 m 2 , and the average pore size is 20 nm. The operating parameters are as follows: ultrafiltration feed pressure 0.35 MPa, ultrafiltration circulating linear velocity 5 m / s, membrane shell side permeate pressure -0.03 MPa, transmembrane pressure difference 0.22 MPa, membrane surface net driving force 0.025 MPa (controlled by the system), gas injection amount (nitrogen) 10% of the feed liquid volume flow rate, and operating temperature 80℃.

[0083] The device was continuously operated for 120 h (5 days), during which the membrane flux was recorded every 24 h, and the recovered lye was sampled and analyzed at the end of the operation. The results showed that the initial membrane flux was 85 L / (m 2 ·h), and after 120 h of operation, the membrane flux was stabilized at 78 L / (m 2·h) was only 8.2%. The recovered alkali liquor was clear and transparent, and the key indicators were: PVA was not detected (<5 mg / L), COD was 185 mg / L, conductivity was 1850 μS / cm, and pH value was 12.6. This shows that the method of the application can maintain a high flux in long-term operation and the purity of the recovered alkali liquor is extremely high, meeting the reuse requirements.

[0084] Example 2:

[0085] This example uses the same composition of desizing water and device core configuration as Example 1, the difference is that some parameters in the gas-liquid two-phase flow ultrafiltration process are adjusted to verify the effectiveness of the parameter range of the application. The operation parameters are adjusted as follows: ultrafiltration feed pressure 0.45 MPa, ultrafiltration circulating linear velocity 4 m / s, membrane shell side permeate pressure -0.05 MPa, transmembrane pressure difference 0.28 MPa, membrane surface net driving force 0.038 MPa (controlled by the system), and gas injection amount (nitrogen) accounts for 20% of the feed liquid volume flow rate.

[0086] In 120 h of continuous operation, the initial membrane flux was 88 L / (m 2 ·h), and the final membrane flux was maintained at 75 L / (m 2 ·h), with a flux decay rate of 14.8%. The recovered alkali liquor indicators were: PVA was 8 mg / L, COD was 225 mg / L, and conductivity was 1950 μS / cm. Although the flux decay after parameter adjustment is slightly higher than that of Example 1, it is still within an excellent range, and the alkali liquor quality also meets the standards, fully proving the effectiveness and operability of the parameter range (such as maintained at 0.02 MPa-0.04 MPa) of the application.

[0087] Comparative Example 1:

[0088] This comparative example uses the same composition of desizing water as Example 1, but the ultrafiltration membrane separation mechanism of the device has no gas injection module and permeate booster pump, i.e. it uses the traditional one-way pressure driven ultrafiltration mode without shell side negative pressure suction and gas phase assisted washing. The operation parameters are: feed pressure 0.35 MPa, no shell side negative pressure, so the transmembrane pressure difference is 0.35 MPa, and no gas injection.

[0089] The running results show that the initial membrane flux is relatively high, reaching 92 L / (m 2 ·h), but the membrane fouling is extremely rapid. By 24 h, the membrane flux has dropped sharply to 45 L / (m 2 ·h), with a flux decay rate of 51.1%. By 120 h, the membrane flux has dropped to 22 L / (m 2·h), the system could not run stably and needed chemical cleaning. The quality of the recovered alkali was also poor, with PVA concentration of 65 mg / L and COD of 680 mg / L. This shows that although the initial flux is high in the traditional mode, membrane fouling is serious, which cannot run stably for a long time and the quality of the recovered water is poor.

[0090] Comparative Example 2:

[0091] This comparative example uses the same composition of desizing water as Example 1, and the device configuration is only missing the gas injection module, but retains the permeate booster pump to build a shell-side negative pressure of -0.03 MPa. Operating parameters: feed pressure is 0.35 MPa, transmembrane pressure difference is 0.22 MPa, and the membrane surface net driving force is maintained at 0.025 MPa by the control system without gas injection.

[0092] The running results show that the shell-side negative pressure has a certain effect on maintaining the flux. The initial membrane flux is 83 L / (m 2 ·h), and the flux decreases to 58 L / (m 2 ·h) after 120 h, with a flux decay rate of 30.1%. However, compared with Example 1, the decay is significantly intensified, and the PVA concentration (28 mg / L) and COD (320 mg / L) in the recovered alkali are higher than those in Example 1. This proves that relying only on the shell-side negative pressure without gas-phase washing still cannot effectively inhibit membrane surface fouling and adsorption of pollutants.

[0093] Comparative Example 3:

[0094] This comparative example uses the same composition of desizing water as Example 1, and the device configuration is complete (including gas injection and booster pump), but the control system does not perform the intelligent control method for ultrafiltration membrane fouling inhibition. The feed pressure is fixed at 0.35 MPa, the membrane shell-side pressure is fixed at -0.03 MPa, the transmembrane pressure difference is fixed at 0.22 MPa, and the gas injection amount is fixed at 10%. The system does not dynamically calculate or control the membrane surface net driving force.

[0095] Running for 120 h, the membrane flux decreases from the initial 85 L / (m 2 ·h) to 65 L / (m 2 ·h), with a flux decay rate of 23.5%. The recovered alkali indicators are PVA: 20 mg / L, and COD: 280 mg / L. The results show that even with the same hardware, the lack of intelligent control to maintain a constant membrane surface net driving force will result in a higher pollution rate and a poorer water production quality.

[0096] Comparative Example 4:

[0097] The comparative example is based on the same conditions as Example 3 (no intelligent control), and further sets the operating parameters outside the scope of the application: the fixed feed pressure is 0.5 MPa, the fixed membrane shell side pressure is -0.08 MPa, and the transmembrane pressure difference is as high as 0.54 MPa, in an attempt to obtain high flux by increasing the driving force. The gas injection amount is fixed at 10%.

[0098] Serious problems occurred during operation. The initial membrane flux was indeed high (105 L / (m 2 ·h)), but due to the excessively high negative pressure and transmembrane pressure difference, macromolecular pollutants such as PVA were quickly and strongly adsorbed and clogged the membrane pores. After only 12 h of operation, the membrane flux dropped sharply to 30 L / (m 2 ·h). After 48 h of operation, the membrane flux was less than 15 L / (m 2 ·h), and the system was completely clogged, necessitating shutdown for chemical cleaning. This comparative example demonstrates that blindly setting high negative pressure and high TMP not only does not help, but on the contrary, can lead to catastrophic membrane pollution and system failure due to the exacerbation of the "adsorption clogging" effect.

[0099] The experimental data of the above examples and comparative examples are summarized in Table 1.

[0100] Table 1. Summary of experimental data

[0101]

[0102] Through comparative analysis of the experimental data of the above examples and comparative examples, the following conclusions can be drawn:

[0103] The experimental data show that the desizing wastewater alkali recovery method and device provided by the application has significant advantages over traditional processes. By synergistically using membrane shell side negative pressure suction, gas-liquid two-phase flow washing, and intelligent control strategy based on membrane surface net driving force, long-term stable operation of the ultrafiltration process and efficient separation of pollutants are successfully achieved. In 120 h of continuous operation, the membrane flux decay rate of the application example can be as low as 8.2%, which is significantly better than the comparative examples without gas injection and intelligent control (with a decay rate of up to 76.1% or more), and the PVA concentration in the recovered alkali solution is less than 5 mg / L, and the COD value is less than 185 mg / L, with significantly improved purity. In particular, by accurately maintaining the membrane surface net driving force in the micro-positive pressure range of 0.02 MPa-0.04 MPa, the technical problem of easy reverse adsorption of macromolecular pollutants to the membrane pores under high negative pressure suction environment is effectively overcome, and rapid clogging of the membrane pores is avoided. Comparative experiments have proven that the lack of any of the above technical elements, or improper control of key parameters, will lead to increased membrane pollution, rapid flux decline, and deterioration of the quality of the recovered alkali solution. In summary, the application effectively solves the contradiction between high flux maintenance and membrane pollution control, achieves efficient recovery of high-purity alkali solution and long-term stable operation of the system, and has outstanding industrial application value.

[0104] The above is according to the ideal embodiment of the present application for inspiration, through the above description, the relevant personnel can fully change and modify in the range without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the content of the specification, and the technical scope must be determined according to the scope of claims.

Claims

1. A method for recovering soda from desizing wastewater of dyeing and printing, characterized by, The method comprises the following steps: S1, after high-temperature desizing water homogenization, most of the oil, wax and suspended solids in the desizing water are removed by dissolved air flotation treatment to obtain clear liquid; S2, the treated clear liquid is filtered through an ultrafiltration membrane, a negative pressure environment is constructed on the membrane shell side of the ultrafiltration membrane, and the permeate liquid that penetrates through the ultrafiltration membrane is extracted in real time to obtain primary filtered water; wherein, during the ultrafiltration membrane filtration process, inert gas is injected into the feed clear liquid to form a gas-liquid two-phase flow, the ratio of the inert gas injection amount to the volume flow rate of the feed clear liquid is 0.05-0.5; and the pressure on the feed side of the ultrafiltration membrane and the pressure on the membrane shell side are adjusted in real time to maintain the net driving force on the surface of the ultrafiltration membrane in the range of 0.02-0.04 MPa. S3, the primary filtered water is purified by nanofiltration membrane, under the action of pressure, the nanofiltration membrane selectively retains multivalent ions and trace organic matter in the primary filtered water, and water and caustic soda penetrate through; S4, the nanofiltration permeate obtained in S3 is used as high-purity recovered lye and directly used in the dyeing and printing production process.

2. The method for recovering caustic from desizing wastewater of printing and dyeing according to claim 1, characterized in that: The inert gas is nitrogen, which is injected in the form of fine bubbles with a diameter of 20-100 μm.

3. A desizing wastewater and alkali recovery device for printing and dyeing wastewater, characterized in that, It comprises: sequentially connected homogenization adjusting mechanism, dissolved air flotation mechanism, ultrafiltration membrane separation mechanism and nanofiltration mechanism; The ultrafiltration membrane separation mechanism comprises an ultrafiltration membrane assembly, an ultrafiltration feed pump, an ultrafiltration circulating pump, a gas injection module and a permeate booster pump; the ultrafiltration feed pump is used to send the clear liquid into the ultrafiltration membrane separation mechanism; the ultrafiltration circulating pump is used to drive the clear liquid to flow at high speed in the ultrafiltration membrane assembly; the gas injection module is used to inject inert gas into the circulating clear liquid to form a gas-liquid two-phase flow; the permeate booster pump is connected to the membrane shell side of the ultrafiltration membrane assembly and used to construct a negative pressure environment on the membrane shell side and extract the permeate liquid, while boosting the permeate liquid; The feed inlet of the nanofiltration mechanism is connected with the outlet of the permeate booster pump to receive the pressurized primary filtered water; and a control system electrically connected with the homogenization adjusting mechanism, the dissolved air flotation mechanism, the ultrafiltration membrane separation mechanism and the nanofiltration mechanism, used to monitor and control the operating parameters of each mechanism; The control system is configured to collect the feed inlet pressure, outlet pressure and membrane shell side pressure of the ultrafiltration membrane assembly in real time and dynamically adjust the operating states of the ultrafiltration feed pump and the permeate booster pump to maintain the net driving force on the surface of the ultrafiltration membrane in the range of 0.02-0.04 MPa.

4. The desizing wastewater and alkali recovery device according to claim 3, characterized in that: The gas injection module comprises a gas source, a flow controller and a mixer, and the mixer is a high-pressure jet pump or a static mixer.

5. A desizing and effluent water alkali recovery unit according to claim 3, characterized in that: The ultrafiltration membrane assembly adopts a tubular membrane assembly, and the membrane material is polyvinylidene fluoride or polyether sulfone.

6. A desizing and alkaline recovery device for printing and dyeing wastewater according to claim 4, characterized in that: The mixer outlet is arranged at the inlet of the ultrafiltration membrane assembly, close to the membrane edge, and is annularly distributed.

7. A desizing and alkaline recovery device for printing and dyeing wastewater according to claim 3, characterized in that: The dissolved air flotation mechanism comprises a high-pressure dissolved air tank, a releaser and an air flotation tank, and a slag scraping mechanism is arranged above the air flotation tank.

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