1. A membrane distillation method for treating wastewater based on gas-liquid contact acidification heat exchange is characterized in that,

the wastewater treatment method comprises the following steps:

step 1: after the wastewater is subjected to microfiltration pretreatment, most of granular solid impurities in the wastewater are separated and removed;

step 2: directly contacting the solution obtained in the step 1 with convective smoke in a heat exchange tower coated with an anticorrosive coating in a spraying manner to finish a heat exchange process to form mixed acid liquor;

and step 3: the acid liquor formed in the step 2 and the flue gas residue fall into a primary collecting tray together under the action of gravity; fog particles and slurry drops carried by the flue gas are collected by a demister at the top of the tower so as to prevent the liquid drops from being carried outside the heat exchange tower to cause the loss of circulating water and the corrosion of a flue;

and 4, step 4: 3, after the liquid in the primary collection tray is subjected to microfiltration pretreatment, intercepting most of flue gas residues, and enabling the acid liquor to enter a hot end inlet of a primary membrane distillation system for subsequent treatment;

and 5: starting a membrane distillation system to realize efficient treatment of the acidified wastewater: the hot end of the membrane distillation system intercepts pollutants to form concentrated solution which is collected in a concentrator; the permeated water vapor passing through the membrane is condensed at the cold end of the membrane distillation system and is collected in a water outlet pool;

step 6: liquid at a water outlet at the hot end of the primary membrane distillation system in the step 6 enters the heat exchange tower again in a spraying mode, is in direct contact with convective smoke, completes secondary heat exchange, and recovers the waste heat of the smoke after primary heat exchange;

and 7: carrying out corresponding treatment on the concentrated solution obtained in the step 5;

and 8: for microfiltration pretreatment systems, purging is performed when flux is less than 50% of the initial flux.

2. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the equipment for realizing the method for treating the wastewater comprises a gas-liquid direct contact heat exchange tower, a collecting disc arranged in the gas-liquid direct contact heat exchange tower, a spray disc positioned above the collecting disc and a flue gas pipeline positioned at the bottom of the gas-liquid direct contact heat exchange tower, wherein the spray disc is sequentially connected with a microfiltration pretreatment system and a wastewater feeding water tank through pipelines, the collecting disc is connected with another microfiltration pretreatment system through a pipeline, the other microfiltration pretreatment system is sequentially connected with a concentrator and a membrane component hot end, the membrane component hot end is communicated with the pipeline between the microfiltration pretreatment system and the wastewater feeding water tank, the membrane component cold end is communicated with a cooling constant temperature tank through a pipeline, the cooling constant temperature tank and the membrane component cold end are both communicated with a water outlet pool, and the outlet end of the concentrator is connected with concentrated liquid post-treatment equipment.

3. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the heat exchange mode in the heat exchange process is that low-temperature wastewater enters a heat exchange tower in a spraying mode and directly contacts flue gas flowing up in the counter-current mode for heat exchange, and the water-gas ratio is 0.1-10; the heat exchange tower is a gas-liquid direct contact heat exchange tower, and spray trays with different heights and quantities can be arranged in the heat exchange tower according to needs so as to realize multi-stage heat recovery of flue gas waste heat and fully utilize heat energy.

4. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the top of the heat exchange tower is provided with a W-shaped demister so as to effectively prevent liquid drops from being carried outside the heat exchanger by flue gas to cause circulating water loss and flue corrosion.

5. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the nozzle for atomization is a solid cone nozzle, the spraying angle is 50-70 degrees, and the spraying height is 2-4 m.

6. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the microfiltration pretreatment system is used for filtering flue gas residues and relieving the scaling problem of a subsequent membrane distillation system.

7. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the wastewater comprises industrial wastewater and domestic wastewater with various concentrations.

8. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the membrane distillation membrane material is in the form of a hollow fiber membrane or a flat membrane, is made of PVDF, PTFE and PP hydrophobic membrane materials, and has an average pore diameter of 0.01-5.00 mu m.

9. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: the membrane distillation system comprises a Direct Contact Membrane Distillation (DCMD), an Air Gap Membrane Distillation (AGMD), a Vacuum Membrane Distillation (VMD) and a Sweep Gas Membrane Distillation (SGMD) system.

10. The method for treating wastewater by membrane distillation based on gas-liquid contact acidification heat exchange according to claim 1, characterized by comprising the following steps: according to the heat recovery condition, the operation temperature of the hot end of the membrane distillation system can be 35-75 ℃; the mixed liquid formed after the direct contact heat exchange of gas and liquid is acidic, thus being beneficial to slowing down the crystal deposition on the surface of the membrane, reducing pollution and ensuring the stable operation of the membrane distillation process.

Background art:

the wastewater is the wastewater of production and living sewage, and the wastewater of production sewage is the wastewater generated in the industrial production process and contains pollutants discharged along with water in various process flows; domestic sewage and wastewater are water produced in daily life of residents; the waste water is various in types and more in pollutants, for example, the smelting waste water contains heavy metals, the petroleum refining industry contains phenols, the printing and dyeing waste water contains various dyes, the leaching liquor of a refuse landfill contains various toxic and harmful substances, and the like. The method is necessary for ensuring the water environment safety and the efficient treatment of wastewater.

The industry is rapidly developed in China, a large amount of energy is consumed in mass industrial production, and incineration capacity, such as coal-fired power generation and garbage incineration, is widely adopted in industrial production as an important energy supply mode; however, the waste heat of the flue gas generated by incineration is ignored and not well utilized, and the energy loss caused by the waste heat is as high as 70%; along with the warming of climate, under the aim of 'double carbon', energy conservation and emission reduction are particularly important, and how to improve the multi-stage utilization of low-efficiency heat is particularly critical.

The membrane distillation technology is a new membrane separation technology driven by heat, has low dependence on electric energy compared with other high-pressure driven membrane technologies, and has strong pollution resistance when running at low pressure; as the hydrophobic membrane is adopted in the membrane distillation process, only gas-phase substances are allowed to pass through the membrane pores, theoretically, ions and macromolecules can be intercepted by 100 percent, and the potential is full of in the treatment of high-pollution wastewater; more importantly, the membrane distillation can be operated at low temperature, so that the low-carbon operation can be realized by efficiently utilizing low-temperature waste heat.

The invention content is as follows:

the invention provides a membrane distillation wastewater treatment method based on gas-liquid contact acidification heat exchange, which can recover waste heat in flue gas, intercept particulate matters and avoid secondary pollution.

It was found in the study that: when the membrane distillation is used for treating wastewater with complex and various pollutant components, the composite pollution of organic matters and inorganic matters can cause the dirt deposition on the surface of the membrane, and cause the related problems of flux reduction, interception performance reduction, membrane wetting and the like; through the acidification treatment of the wastewater, the deposition of a pollution layer on the surface of the membrane in membrane distillation can be effectively reduced, and the good membrane interception performance and the stability of membrane flux are kept.

In the incineration capacity, high-temperature flue gas needs to be discharged, contains acidic gas and generally needs to be subjected to desulfurization treatment to reduce the harm to the environment, so that the acid liquor naturally formed after the flue gas and the wastewater are directly contacted in the heat exchange tower is used as the feeding liquid for membrane distillation treatment, and an acidic environment can be provided for the subsequent concentration process; the waste heat in the flue gas is recovered through gas-liquid contact heat exchange, a driving force is provided for membrane distillation treatment of the waste water, and in order to avoid the situations that granular substances in the waste water enter a heat exchange tower to block a spray head and residues in the flue gas enter a membrane distillation system along with acid liquor after heat exchange and the like, a set of microfiltration pretreatment system is respectively arranged behind a feeding water tank and in front of a concentrator of the membrane distillation system, so that particles are intercepted, and secondary pollution is avoided. In order to achieve the purpose, the invention adopts the following technical scheme:

s1: carrying out microfiltration pretreatment on the wastewater in the feed water tank, and separating to remove granular solid impurities in the percolate;

s2: directly contacting the wastewater subjected to microfiltration treatment with convective smoke in a heat exchange tower in a spraying manner to finish heat exchange, and forming acid liquor and smoke residue which fall into a primary collecting tray together;

s3: the flue gas after the heat exchange in the gas-liquid direct contact heat exchange tower rises to the top of the tower and is discharged through the demister, so that fog particles and slurry drops carried by the flue gas can be collected, and the liquid drops are effectively prevented from being carried outside the heat exchange tower to cause the loss of circulating water and the corrosion of a flue.

S4: after the liquid in the primary collection tray is subjected to microfiltration pretreatment, most of flue gas residues are intercepted, so that acid liquor enters a primary membrane distillation system for treatment;

s5: starting a membrane distillation system to realize efficient treatment of acidified wastewater, and collecting the cold-end permeate of the membrane distillation pool in a water outlet pool;

s6: the liquid passing through a hot end water outlet of the primary membrane distillation system enters a heat exchange tower again in a spraying mode, is in direct contact with convective smoke gas to complete secondary heat exchange, and the waste heat of the smoke gas after primary heat exchange is recovered to form acid liquid which falls into a secondary collection tray;

s7: repeating the steps until the temperature of the flue gas is lower than 25 ℃, and finishing the operation of the multistage membrane distillation system;

s8: and (3) correspondingly treating the concentrated solution obtained in the step (5), wherein the concentrated solution mainly comprises two types: one is a concentrated solution with high-value pollutants, and recyclable components can be extracted by crystallization, evaporation and the like, so that the aim of recycling resources is fulfilled; one is a pollutant that is difficult to degrade and can be further treated by biological, chemical and physical methods;

s9: for microfiltration pretreatment systems, cleaning should be performed when the flux is less than 50% of the initial flux.

Furthermore, the equipment of the wastewater treatment method comprises a gas-liquid direct contact heat exchange tower, a collection disc arranged in the gas-liquid direct contact heat exchange tower, a spray disc positioned above the collection disc and a flue gas pipeline positioned at the bottom of the gas-liquid direct contact heat exchange tower, wherein the spray disc is sequentially connected with a microfiltration pretreatment system and a wastewater feeding water tank through pipelines, the collection disc is connected with another microfiltration pretreatment system through a pipeline, the other microfiltration pretreatment system is sequentially connected with a concentrator and a membrane component hot end, the membrane component hot end is communicated with the pipeline between the microfiltration pretreatment system and the wastewater feeding water tank, the membrane component cold end is communicated with a cooling constant temperature tank through a pipeline, the cooling constant temperature tank and the membrane component cold end are both communicated with a water outlet pool, and the outlet end of the concentrator is connected with concentrated solution post-treatment equipment.

Further, the heat exchange mode in the heat exchange process is that low-temperature wastewater enters a heat exchange tower in a spraying mode and directly contacts flue gas flowing up in the counter current to exchange heat, and the water-gas ratio is 0.1-10; the heat exchange tower is a gas-liquid direct contact heat exchange tower, and spray trays with different heights and quantities can be arranged in the heat exchange tower according to needs so as to realize multi-stage heat recovery of flue gas waste heat and fully utilize heat energy.

Furthermore, the W-shaped demister is arranged on the top of the heat exchange tower, so that liquid drops are effectively prevented from being carried to the outside of the heat exchanger by flue gas, and circulating water loss and flue corrosion are avoided.

Further, the nozzle for atomization is a solid cone nozzle, the spraying angle is 50-70 degrees, and the spraying height is 2-4 m;

furthermore, the microfiltration pretreatment system is used for filtering flue gas residues and relieving the scaling problem of a subsequent membrane distillation system.

Further, the wastewater includes industrial wastewater and domestic wastewater with various concentrations.

Furthermore, the membrane distillation membrane material is in the form of a hollow fiber membrane or a flat membrane, is made of PVDF, PTFE and PP hydrophobic membrane materials, and has an average pore diameter of 0.01-5.00 mu m.

Further, the membrane distillation system includes Direct Contact Membrane Distillation (DCMD), Air Gap Membrane Distillation (AGMD), Vacuum Membrane Distillation (VMD), and Sweep Gas Membrane Distillation (SGMD) systems.

Further, according to the heat recovery condition, the operation temperature of the hot end of the membrane distillation system can be 35-75 ℃; the mixed liquid formed after the direct contact heat exchange of gas and liquid is acidic, thus being beneficial to slowing down the crystal deposition on the surface of the membrane, reducing pollution and ensuring the stable operation of the membrane distillation process.

Compared with the prior art, the invention has the beneficial effects that:

1. according to the composite pollution characteristic, the acid liquor formed after gas-liquid mixing is used as the feed liquor of the membrane distillation system, so that the cost of acidification of the solution is saved, and the reduction of the deposition of a pollution layer on the surface of the membrane is facilitated;

2. the residual heat of the flue gas is used as the driving force of the membrane distillation system, so that the sensible heat and the latent heat of water vapor in the flue gas can be recovered, and the energy consumption of membrane distillation is reduced;

3. the gas-liquid two-phase fluid directly contacts the heat exchange area, the heat transfer and mass transfer are completed instantly, the thermal resistance of the partition wall is avoided, and the heat exchange efficiency is high;

4. the multistage membrane distillation is introduced, the waste heat of different flue gases under different heights in the heat exchange tower can be used as a heat supply end of the multistage membrane distillation, the waste heat can be more fully utilized, and the heat efficiency is improved;

5. the concentrated solution containing high-value substances can be subjected to resource recovery by using technologies such as crystallization and evaporation, and the like, so that the method has good economic and environmental benefits;

6. the method is suitable for treating most of wastewater, and the post-treatment of the concentrated solution is determined according to specific conditions, so that the method has practical guiding significance due to the interest of the situation;

this effluent disposal system organic combination gas-liquid direct contact heat transfer tower and membrane distillation system, the antipollution characteristic of make full use of membrane distillation combines flue gas waste heat utilization, strengthens membrane distillation system's antipollution performance and fully realizes the benefit maximize of the energy, can realize the high-efficient processing of waste water.

Description of the drawings:

FIG. 1 is a flow diagram of a single stage membrane distillation process of the present invention;

FIG. 2 is a flow diagram of a multi-stage membrane distillation process of the present invention;

the figures represent the following meanings: 1-gas-liquid direct contact heat exchange tower; 2-flue gas pipeline; 3-spraying a disc; 4-a collection tray; 5-W type demister; 6-a microfiltration pretreatment system; 7-a concentrator; 8-hot end of membrane module; 9-film; 10-cold end of membrane module; 11-wastewater feed tank; 12-a water outlet pool; 13-cooling the thermostatic bath; 14-hot end piping; 15-cold end piping; 16-concentrated solution post-treatment equipment.

The specific implementation mode is as follows:

the present invention will be further described with reference to the accompanying drawings and examples of wastewater treatment, but is not limited to the examples.

The wastewater to be treated in this case contained a large amount of K⁺（2000~3000mg/L）、Na⁺（500~800mg/L）、Mg²⁺（100~300mg/L）、Ca²⁺Metal salt ions (100-200 mg/L), ammonia nitrogen (1650-1800 mg/L), organic matters (500-750 mg/L) and the like; in this example, the waste heat of flue gas in the production area is 112MW, and the waste water amount is 953.6m³D; the typical wastewater treatment energy consumption of the multistage membrane distillation is 100kW/m³It is shown that by combining the plant waste heat recovery, the full treatment of the waste water can be realized.

The specific implementation is shown in figure 1 of the drawings,

the device comprises a gas-liquid direct contact heat exchange tower 1, a collecting disc 4 arranged in the gas-liquid direct contact heat exchange tower, a spray disc positioned above the collecting disc and a flue gas pipeline positioned at the bottom of the gas-liquid direct contact heat exchange tower, wherein the spray disc is sequentially connected with a first microfiltration pretreatment system 6 and a wastewater feeding water tank 11 through pipelines, the collecting disc is connected with a second microfiltration pretreatment system through a pipeline, the second microfiltration pretreatment system 6 is sequentially connected with a concentrator 7 and a membrane module hot end 8, the hot end 8 is communicated with the pipelines between the first microfiltration pretreatment system and the wastewater feeding water tank 11, the cold end of the membrane module is communicated with a cooling constant temperature tank 13 through a pipeline, the cold ends of the cooling constant temperature tank and the membrane module are both communicated with a water outlet tank, and the outlet end of the concentrator is connected with a concentrated solution post-treatment device 16.

The membrane distillation system may include Direct Contact Membrane Distillation (DCMD), Air Gap Membrane Distillation (AGMD), Vacuum Membrane Distillation (VMD), Sweep Gas Membrane Distillation (SGMD), etc. systems having a membrane module hot end 8, a membrane 9, and a membrane module cold end 10; the aperture of a filter membrane of the microfiltration pretreatment system is 0.1-1 mu m, and the operating pressure is 0.3-1 bar; the concentrator is a concentrating tank and is used for storing concentrated solution for subsequent treatment; the concentrated solution post-treatment equipment comprises but is not limited to an evaporator, various biological and chemical treatment processes.

During operation, the waste liquid in the waste water feeding water tank (11) is treated by the microfiltration pretreatment system (6) to remove granular solid impurities, and then enters the gas-liquid direct contact heat exchange tower (1) in a spraying mode. In a heat exchange tower (1), mist-shaped waste water sprayed from a spray tray (3) is in convection with flue gas from a flue (2) in a counter-current manner, the mist-shaped waste water is in direct contact with the flue gas and completes heat exchange, acid mixed liquor (pH = 5.0-6.0) after heat exchange falls into a collection tray (4) under the action of gravity, and the acid mixed liquor enters a membrane distillation hot end (8) after being filtered by another microfiltration pretreatment system (6); the effluent at the hot end of the membrane distillation returns to the wastewater feed pipeline 15 to complete primary circulating water return; feeding liquid entering the hot end (8) of the membrane component is continuously concentrated in a concentrator (7) in advance, and outlet water at the cold end (10) of the membrane component enters a water outlet pool (12); the operating temperature of the membrane distillation system is 35-75 ℃ by utilizing the form of gas-liquid direct contact heat exchange, and the sensible heat of the flue gas and the latent heat of water vapor are effectively utilized; the temperature of the flue gas discharged from the tower can be reduced to 25 ℃, so that harmless emission is realized; the membrane distillation system can realize the removal rate of more than 99 percent of organic matters and metal ions in the wastewater, and effectively purify the wastewater; through acidification, the removal rate of the membrane distillation on ammonia nitrogen can be improved to 90%, and after the wastewater is concentrated by 5 times, the membrane flux is reduced by less than 25%, compared with the membrane pollution reduced by 20-40%.

Example two: the difference between the embodiment and the first embodiment is that a plurality of groups of collecting discs and spraying discs positioned above the collecting discs are additionally arranged in the gas-liquid direct contact heat exchange tower, the plurality of groups of collecting discs are respectively connected with the hot ends of the membrane components, the spraying discs are respectively connected with the hot ends of the membrane components of other groups, and the cold ends of the membrane components of each group are respectively connected with the cooling constant temperature tank and the water outlet tank.

By adopting the scheme of the second embodiment, the multi-stage utilization of the waste heat of the flue gas can be realized, and the higher wastewater treatment recovery rate can be realized. The connection sequence of the plurality of groups of spraying disks and the collecting disks is not limited to the form of the figure 2.