Zero discharge process for advanced treatment and reuse of landfill leachate

Abstract

The application provides a zero discharge process for advanced treatment and reuse of landfill leachate, and relates to the technical field of sewage treatment. The process specifically comprises the following process steps: step 1: raw material pretreatment, landfill leachate is sent into a conditioning tank for homogenization and equalization, and then sequentially enters a coagulation sedimentation tank and a flotation tank to remove suspended solids, colloids and part of organic matter; step 2: two-stage A / O biochemical treatment, pretreated wastewater sequentially enters a first-stage A / O system and a second-stage A / O system to remove most of the organic matter and ammonia nitrogen, and then enters a secondary sedimentation tank for sludge-water separation. The application can completely solve the problem that concentrated water is difficult to treat in the traditional process, realize efficient reuse of water resources and resource utilization of salts, reduce the operation cost of the system and improve the stability of the system.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically a zero-discharge process for the deep treatment and reuse of leachate from waste-to-energy plants. Background Technology

[0002] Waste-to-energy incineration is currently one of the main methods of municipal solid waste treatment in my country. However, the incineration process generates a large amount of high-concentration leachate. Leachate is extremely complex, characterized by high COD concentration, high ammonia nitrogen content, high salinity, high heavy metal content, and large fluctuations in biodegradability. It is recognized worldwide as one of the most difficult wastewaters to treat. Currently, domestic waste-to-energy plants generally use a traditional process of pretreatment + biological treatment + membrane treatment for leachate. This process has some drawbacks: The challenge lies in treating concentrated wastewater. Membrane treatment systems generate approximately 20%-30% concentrated wastewater, which contains high concentrations of salt, recalcitrant organic matter, and heavy metals. This wastewater cannot be treated using conventional processes. Recirculating concentrated wastewater back into the incinerator is extremely harmful. Most power plants directly recirculate this wastewater back into the incinerator, which lowers the furnace temperature, increases the flue gas treatment load, and corrodes equipment and pipes. Furthermore, the accumulation of salts within the furnace leads to coking and slagging, severely impacting the safe and stable operation of the incineration system. Evaporation crystallization is also energy-intensive. In the few power plants that use evaporation crystallization, the high organic content of the concentrated wastewater easily generates foam and scale during evaporation, resulting in energy consumption as high as 80-120 kWh / t of water. Moreover, the crystallized salts have complex compositions, requiring complete disposal as hazardous waste, leading to extremely high disposal costs.

[0003] Therefore, this invention proposes a zero-discharge process for the deep treatment and reuse of leachate from waste-to-energy plants, thereby effectively solving the aforementioned problems and difficulties. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a zero-discharge process for the deep treatment and reuse of leachate from waste-to-energy plants. This process can completely solve the problem of difficult treatment of concentrated wastewater in traditional processes, achieve efficient reuse of water resources and resource utilization of salts, while reducing system operating costs and improving system stability.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant, the process specifically includes the following steps: Step 1: Raw material pretreatment; The leachate from the waste-to-energy plant is sent to an equalization tank for homogenization and equalization, and then sequentially enters a coagulation sedimentation tank and an air flotation tank to remove suspended solids, colloids and some organic matter. Step 2: Two-stage A / O biochemical treatment; The pretreated wastewater sequentially enters the primary A / O system and the secondary A / O system to remove most of the organic matter and ammonia nitrogen, and then enters the secondary sedimentation tank for sludge-water separation. Step 3: Three-stage membrane separation for deep treatment; The effluent from the secondary sedimentation tank enters the ultrafiltration system, nanofiltration system and reverse osmosis system in sequence. The nanofiltration permeate and reverse osmosis permeate meet the reuse standards and are reused in the waste-to-energy plant. Step 4: Advanced oxidation treatment of reverse osmosis concentrate; The reverse osmosis concentrate enters the ozone-hydrogen peroxide advanced oxidation reaction tower to degrade recalcitrant organic matter. Step 5: Electrodialysis pre-concentration; The effluent from the advanced oxidation treatment is mixed with the nanofiltration concentrate and then enters the electrodialysis system for concentration. The electrodialysis permeate is returned to the equalization tank for reprocessing. Step 6: MVR evaporation crystallization and separation of fractional salts; The electrodialysis concentrate enters the MVR evaporation crystallization system for evaporation crystallization to obtain mixed crystalline salt. The mixed crystalline salt is then processed by a stepwise salt separation system to separate industrial salt for resource utilization, while a small amount of miscellaneous salt is disposed of as hazardous waste. Step 7: Sludge treatment; The sludge generated at each stage is concentrated and dewatered before being sent to a waste incinerator for incineration.

[0006] Furthermore, in the raw material pretreatment process of step 1, the hydraulic retention time in the equalization tank is 36-48 hours; the PAC dosage in the coagulation sedimentation tank is 50-150 mg / L, and the PAM dosage is 1-3 mg / L; and the flotation agent is added to the dissolved air flotation tank at a concentration of 5-10 mg / L.

[0007] Furthermore, in the two-stage A / O biochemical treatment process in step 2, the hydraulic retention time of the anaerobic tank in the primary A / O system is 18-24h, the hydraulic retention time of the anoxic tank is 12-16h, and the hydraulic retention time of the aerobic tank is 36-48h; the hydraulic retention time of the anoxic tank in the secondary A / O system is 8-12h, and the hydraulic retention time of the aerobic tank is 12-18h; the sludge concentration is controlled at 3-6g / L.

[0008] Furthermore, in the three-stage membrane separation deep treatment process of step 3, the ultrafiltration system uses a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 30-50 kDa, an operating pressure of 0.15-0.25 MPa, and a recovery rate of 90%-95%; the nanofiltration system uses an anti-fouling spiral wound nanofiltration membrane with a molecular weight cutoff of 200-300 Da, an operating pressure of 0.8-1.2 MPa, and a recovery rate of 75%-80%; and the reverse osmosis system uses a high-pressure anti-fouling spiral wound reverse osmosis membrane with an operating pressure of 2.0-2.8 MPa and a recovery rate of 80%-85%.

[0009] Furthermore, in step 4, the advanced oxidation treatment of reverse osmosis concentrate, the ozone dosage is 80-150 mg / L, the molar ratio of hydrogen peroxide to ozone is 0.3-0.5:1, the reaction time is 1.5-2 h, and the pH is controlled at 7-8.

[0010] Furthermore, in step 5, during the electrodialysis pre-concentration process, the electrodialysis system uses a homogeneous ion exchange membrane, with a current density of 15-25 mA / cm², an operating voltage of 0.8-1.2 V / membrane, a concentration factor of 5-8 times, and a salt concentration of 15%-20% in the concentrate.

[0011] Furthermore, in the MVR evaporation crystallization and stepwise salt separation process in step 6, the MVR evaporation crystallization system has an evaporation temperature of 85-90℃, a vacuum degree of -0.085 to -0.09MPa, and an energy consumption of 35-45kWh / t water. The stepwise salt separation system adopts a stepwise recrystallization process, separating sodium sulfate crystals at 20-25℃ and sodium chloride crystals at 30-35℃. The industrial salt purity is ≥98%, and the recovery rate is ≥85%.

[0012] Furthermore, in step 7, during the sludge treatment process, the sludge has a moisture content of ≤60% after dewatering by a plate and frame filter press.

[0013] This invention provides a zero-discharge process for the deep treatment and reuse of leachate from waste-to-energy plants. It offers the following advantages: 1. This invention provides a zero-discharge process for deep treatment and reuse of leachate from waste-to-energy plants. This process achieves true zero discharge and a high water reuse rate. The entire process has no external wastewater discharge, and all wastewater is treated and reused, completely solving the pollution problem of landfill leachate. The water reuse rate can reach more than 98%, which greatly saves precious water resources and reduces the water cost of waste-to-energy plants.

[0014] 2. This invention provides a zero-discharge process for the deep treatment and reuse of leachate from waste-to-energy plants. It features low operating costs and stable system operation. The combined process of electrodialysis pre-concentration and MVR evaporation crystallization consumes only 35-45 kWh / t of water, saving more than half the energy compared to traditional evaporation processes. Resource utilization of crystalline salts can offset some operating costs, resulting in an overall operating cost reduction of 40% compared to traditional processes. The staged treatment process ensures reasonable load on each unit, and advanced oxidation treatment effectively removes recalcitrant organic matter, significantly reducing pollution in the membrane and evaporation systems. Membrane lifespan is extended by more than two times, and the system can operate stably for more than a year. This not only avoids environmental pollution from wastewater discharge but also achieves resource utilization of water and salts, demonstrating significant environmental, economic, and social benefits. Attached Figure Description

[0015] Figure 1This is a process flow diagram of the zero-discharge process for deep treatment and reuse of leachate from waste-to-energy plants according to the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention. Example 1:

[0017] like Figure 1 As shown, this embodiment of the invention provides a zero-discharge process for deep treatment and reuse of leachate from waste-to-energy plants, comprising the following steps: Step 1: Preprocessing unit; The leachate collected from the waste-to-energy plant is sent to the equalization tank, where the hydraulic retention time is 36-48 hours. The water quality and quantity are homogenized and equalized by a submersible mixer to regulate fluctuations in water quality and quantity. The effluent from the equalization tank is transferred to the coagulation sedimentation tank, where 50-150 mg / L of polyaluminum chloride and 1-3 mg / L of polyacrylamide are added sequentially. The mixture is stirred rapidly for 2-3 minutes, then slowly for 10-15 minutes, and allowed to settle for 30-60 minutes to remove most of the suspended solids, colloids and some large molecular organic matter. The supernatant from the coagulation sedimentation tank flows into the vortex flotation tank, where 5-10 mg / L of flotation agent is added to further remove emulsified oil, fine suspended solids, and residual colloids. The effluent from the flotation tank then enters the biological treatment unit.

[0018] Step 2: Two-stage A / O biochemical treatment unit; The effluent from the dissolved air flotation (DAF) tank first enters the primary A / O system: Anaerobic tank: hydraulic retention time 18-24h, sludge concentration 4-6g / L. Through the hydrolysis and acidification of anaerobic microorganisms, the recalcitrant large molecular organic matter is decomposed into easily degradable small molecular organic matter, thereby improving the biodegradability of wastewater. Anoxic tank: hydraulic retention time 12-16h, sludge concentration 4-6g / L. Nitrogen removal is achieved by using denitrifying bacteria to reduce nitrate nitrogen to nitrogen gas. Aerobic tank: hydraulic retention time 36-48h, sludge concentration 4-6g / L, dissolved oxygen controlled at 2-3mg / L. Most of the organic matter and ammonia nitrogen are removed through the metabolic action of aerobic microorganisms. The effluent from the primary A / O system enters the secondary A / O system; Secondary anoxic tank: hydraulic retention time 8-12h, sludge concentration 3-5g / L, further enhancing denitrification; Secondary aerobic tank: hydraulic retention time 12-18h, sludge concentration 3-5g / L, dissolved oxygen controlled at 1.5-2.5mg / L, deep removal of residual organic matter and ammonia nitrogen; The effluent from the secondary A / O system enters a vertical flow secondary sedimentation tank for sludge-water separation. The surface loading rate is 0.8-1.2 m³ / (m²・h), and the sedimentation time is 2-3 hours. The sludge from the secondary sedimentation tank is returned to the primary anaerobic tank at a return ratio of 100%-200%, and the remaining sludge is discharged into the sludge treatment unit.

[0019] Step 3: Three-stage membrane separation deep processing unit; The effluent from the secondary sedimentation tank first enters the ultrafiltration system: a hollow fiber ultrafiltration membrane is used, with a molecular weight cutoff of 30-50 kDa, an operating pressure of 0.15-0.25 MPa, and a recovery rate of 90%-95%. The ultrafiltration system can completely remove colloids, bacteria, large organic molecules, and suspended particles, effectively protecting the subsequent nanofiltration and reverse osmosis membrane systems; Ultrafiltration permeate enters the nanofiltration system: using antifouling spiral wound nanofiltration membranes, with a molecular weight cutoff of 200-300 Da, operating pressure of 0.8-1.2 MPa, and a recovery rate of 75%-80%. The nanofiltration system can selectively retain divalent ions and some recalcitrant organic matter. Permeate enters the reverse osmosis system, and concentrate enters the electrodialysis pre-concentration unit. Nanofiltration permeate enters the reverse osmosis system: Utilizing high-pressure, fouling-resistant spiral wound reverse osmosis membranes, operating at 2.0-2.8 MPa, with a recovery rate of 80%-85%. The reverse osmosis system removes almost all dissolved salts and small-molecule organic matter, producing permeate that meets standards and is directly reused as circulating cooling water, ash flushing water, landscaping water, and equipment cleaning water in waste-to-energy plants. Reverse osmosis concentrate enters the advanced oxidation treatment unit.

[0020] Step 4: Reverse osmosis concentrate advanced oxidation treatment unit; The reverse osmosis concentrate is fed into the ozone-hydrogen peroxide advanced oxidation reaction tower. The ozone dosage is 80-150 mg / L, the molar ratio of hydrogen peroxide to ozone is 0.3-0.5:1, the reaction time is 1.5-2 hours, and the pH is controlled at 7-8. The advanced oxidation system can generate strong hydroxyl radicals, which can completely oxidize and decompose the recalcitrant organic matter in the reverse osmosis concentrate into CO2 and H2O, with a COD removal rate of ≥70%. At the same time, it destroys the chromophores of organic matter to decolorize the concentrate. The effluent from the advanced oxidation treatment is mixed with the nanofiltration concentrate and then enters the electrodialysis pre-concentration unit.

[0021] Step 5: Electrodialysis pre-concentration unit; The mixed concentrate is first filtered through a precision filter to remove particles larger than 5μm, and then enters the electrodialysis system. The electrodialysis system uses homogeneous ion exchange membranes with a current density of 15-25 mA / cm², an operating voltage of 0.8-1.2 V / membrane, and a flow ratio of 1:3 to 1:5 between the concentrate and desalination chambers. The electrodialysis system can concentrate the salt concentration of the mixed concentrate from 3%-5% to 15%-20%, with a concentration factor of 5-8 times. The water produced in the freshwater chamber is of excellent quality and is returned to the equalization tank for retreatment. The electrodialysis concentrate enters the MVR evaporation and crystallization unit; Step 6: MVR evaporation crystallization and stepwise salt separation unit; The electrodialysis concentrate is fed into the MVR evaporation crystallization system, with an evaporation temperature of 85-90℃, a vacuum degree of -0.085 to -0.09MPa, and a compressor temperature rise of 8-10℃. The MVR system adopts a forced circulation evaporation process, which can effectively prevent scaling and foaming. The secondary steam generated by evaporation is compressed by a compressor and used as a heating source. The energy consumption is only 35-45 kWh / t of water, which is more than 60% more energy-efficient than traditional multi-effect evaporation. The condensate produced by evaporation is of excellent quality and is reused after being mixed with the permeate from reverse osmosis. The concentrated evaporation liquid enters the crystallizer and is evaporated and crystallized at 50-60℃ to obtain mixed crystalline salt. The mixed crystalline salt is fed into a stepwise salt separation system: First, recrystallization is carried out at 20-25℃, and sodium sulfate crystals are separated by taking advantage of the difference in solubility of sodium chloride and sodium sulfate with temperature; then the mother liquor is further evaporated and concentrated, and sodium chloride crystals are separated by crystallization at 30-35℃. The sodium sulfate and sodium chloride crystals obtained by separation have a purity of ≥98%, which meets the standards for industrial salt and can be sold as industrial raw materials; a small amount of difficult-to-separate miscellaneous salts (accounting for 10%-15% of the total salt content) are entrusted to qualified units for disposal as hazardous waste. Step 7: Sludge Treatment Unit; All sludge produced by the coagulation sedimentation tank, flotation tank and secondary sedimentation tank is collected in the sludge thickening tank and concentrated by gravity for 24-36 hours. The concentrated sludge is sent to a plate and frame filter press for dewatering, and 2-5 kg / t of PAM is added to the dry sludge. The moisture content of the dewatered sludge is ≤60%. Dewatered sludge is directly fed into a waste incinerator for incineration, achieving sludge reduction and harmlessness.

[0022] The following points should be noted in this article: 1. The accompanying drawings of the embodiments disclosed herein only relate to the structures involved in the embodiments disclosed herein; other structures can be referred to in general design.

[0023] 2. Where there is no conflict, the embodiments of this disclosure and the features in the embodiments can be combined with each other to obtain new embodiments.

[0024] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A zero-discharge process for deep treatment and reuse of leachate from waste-to-energy plants, characterized in that: The process specifically includes the following steps: Step 1: Raw material pretreatment; The leachate from the waste-to-energy plant is sent to an equalization tank for homogenization and equalization, and then sequentially enters a coagulation sedimentation tank and an air flotation tank to remove suspended solids, colloids and some organic matter. Step 2: Two-stage A / O biochemical treatment; The pretreated wastewater sequentially enters the primary A / O system and the secondary A / O system to remove most of the organic matter and ammonia nitrogen, and then enters the secondary sedimentation tank for sludge-water separation. Step 3: Three-stage membrane separation for deep treatment; The effluent from the secondary sedimentation tank enters the ultrafiltration system, nanofiltration system and reverse osmosis system in sequence. The nanofiltration permeate and reverse osmosis permeate meet the reuse standards and are reused in the waste-to-energy plant. Step 4: Advanced oxidation treatment of reverse osmosis concentrate; The reverse osmosis concentrate enters the ozone-hydrogen peroxide advanced oxidation reaction tower to degrade recalcitrant organic matter. Step 5: Electrodialysis pre-concentration; The effluent from the advanced oxidation treatment is mixed with the nanofiltration concentrate and then enters the electrodialysis system for concentration. The electrodialysis permeate is returned to the equalization tank for reprocessing. Step 6: MVR evaporation crystallization and separation of fractional salts; The electrodialysis concentrate enters the MVR evaporation crystallization system for evaporation crystallization to obtain mixed crystalline salt. The mixed crystalline salt is then processed by a stepwise salt separation system to separate industrial salt for resource utilization, while a small amount of miscellaneous salt is disposed of as hazardous waste. Step 7: Sludge treatment; The sludge generated at each stage is concentrated and dewatered before being sent to a waste incinerator for incineration.

2. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In the raw material pretreatment process of step 1, the hydraulic retention time in the equalization tank is 36-48h; the dosage of PAC in the coagulation sedimentation tank is 50-150mg / L, and the dosage of PAM is 1-3mg / L; the flotation agent is added to the air flotation tank at 5-10mg / L.

3. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In the two-stage A / O biological treatment process in step 2, the hydraulic retention time of the anaerobic tank in the primary A / O system is 18-24h, the hydraulic retention time of the anoxic tank is 12-16h, and the hydraulic retention time of the aerobic tank is 36-48h; the hydraulic retention time of the anoxic tank in the secondary A / O system is 8-12h, and the hydraulic retention time of the aerobic tank is 12-18h; the sludge concentration is controlled at 3-6g / L.

4. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In the three-stage membrane separation deep treatment process in step 3, the ultrafiltration system uses hollow fiber ultrafiltration membranes with a molecular weight cutoff of 30-50 kDa, an operating pressure of 0.15-0.25 MPa, and a recovery rate of 90%-95%; the nanofiltration system uses anti-fouling spiral wound nanofiltration membranes with a molecular weight cutoff of 200-300 Da, an operating pressure of 0.8-1.2 MPa, and a recovery rate of 75%-80%; and the reverse osmosis system uses high-pressure anti-fouling spiral wound reverse osmosis membranes with an operating pressure of 2.0-2.8 MPa and a recovery rate of 80%-85%.

5. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In step 4, the advanced oxidation treatment of reverse osmosis concentrate, the ozone dosage is 80-150 mg / L, the molar ratio of hydrogen peroxide to ozone is 0.3-0.5:1, the reaction time is 1.5-2 h, and the pH is controlled at 7-8.

6. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In step 5, during the electrodialysis pre-concentration process, the electrodialysis system uses a homogeneous ion exchange membrane, with a current density of 15-25 mA / cm², an operating voltage of 0.8-1.2 V / membrane, a concentration factor of 5-8 times, and a salt concentration of 15%-20% in the concentrate.

7. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In step 6, during the MVR evaporation crystallization and stepwise salt separation process, the MVR evaporation crystallization system operates at an evaporation temperature of 85-90℃, a vacuum degree of -0.085 to -0.09MPa, and an energy consumption of 35-45kWh / t of water. The stepwise salt separation system employs a stepwise recrystallization process, separating sodium sulfate crystals at 20-25℃ and sodium chloride crystals at 30-35℃. The industrial salt purity is ≥98%, and the recovery rate is ≥85%.

8. The zero-discharge process for deep treatment and reuse of leachate from a waste-to-energy plant according to claim 1, characterized in that: In step 7, the sludge treatment process, the sludge has a moisture content of ≤60% after dewatering by a plate and frame filter press.

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