Review of the current status of the leachate treatment process: The previously used natural degradation purification method was not allowed to be used because of serious environmental pollution. At present, artificial degradation and purification methods are mainly used. It utilizes the biodegradability of leachate, and artificially installs facilities and equipment to allow the leachate to be purified by anaerobic, aerobic, standing, and precipitation methods to effectively eliminate leachate. The purpose of polluting the environment. The main treatment schemes at home and abroad are divided into: off-site treatment and on-site treatment.
Off-site treatment mainly refers to the combined treatment of landfill leachate and urban domestic sewage, which uses domestic sewage to dilute high-concentration landfill leachate and then treats it. This method can save the cost of separately setting up a landfill leachate treatment system, and can reduce Leachate treatment costs. The disadvantage is that the transportation of landfill leachate causes a relatively large economic burden, and the unique water quality characteristics of the leachate will impact the operation of urban domestic sewage treatment plants, and even damage the normal operation of urban sewage treatment plants.
On-site treatment mainly refers to the spraying of leachate into the reservoir area, or the establishment of a sewage treatment plant nearby, which is not economically suitable.
The treatment of landfill leachate is an indispensable part of sanitary landfill projects for municipal solid waste. At present, the treatment methods of landfill leachate are mainly biological treatment, physical and chemical treatment and land treatment.
Land treatment mainly removes suspended particles and dissolved components in the leachate by filtering soil particles, ion exchange adsorption and precipitation. Through the action of microorganisms in the soil, organic matter and ammonia nitrogen in the leachate are transformed, and the amount of leachate is reduced by evaporation. The current land methods for leachate treatment are mainly recharge and constructed wetlands.
Leachate reinjection, as one of the landfill leachate treatment methods, has been widely used abroad. It is estimated that 50% of landfills in the UK have been recharged with leachate. There are many researches on the re-irrigation method in China. Some people have done experimental research in China to study the influencing factors of leachate re-irrigation in detail. It was found that a certain proportion of fine sand was added to the subclay used in the experiment to improve it. Covers the water permeability and air permeability of the soil layer. When the water inlet load is 6.6 ～ 115g / (m2 • d), the COD removal rate can reach about 98% after two months of operation.
The re-irrigation method has begun to be used in some leachate treatment in China. Constructed wetland is a new treatment process that has appeared in recent years. For the treatment of landfill leachate, there are many applications abroad. TjasaBulc constructed a 450m2 constructed wetland to study the leachate treatment and found that the COD removal rate was 68%, the BOD5 removal rate was 46%, the NH3-N removal rate was 81%, and the Fe removal rate was 80%. CraigD.Martin constructed an artificial wetland with a length to width ratio of 10: 1 and a depth of 0.5m, and planted a variety of aquatic weeds, and conducted research on the treatment of nutrients.
The Ithaca Landfill in New York, USA, began using subsurface wetlands to treat landfill leachate in 1989. The Perdido landfill in Escambia, Northern Europe and Finland has been operating artificial wetland systems in cold climates for 13 years and is still functioning normally. Many countries, including Norway, Canada, the United Kingdom, Slovenia, and Poland, have successfully applied less labor.
The land treatment system is mostly used for urban sewage treatment, and has less application in the treatment of landfill leachate. After the landfill leachate is poured, the nutrient content of the soil increases, the aeration gap increases, and the soil fertility increases significantly. Heavy metals and toxic and hazardous substances cannot be processed.
Physical and chemical treatment of leachate
Physical and chemical treatment methods mainly use physical and chemical methods to remove pollutants in wastewater. The main methods used in leachate treatment are: activated carbon adsorption, chemical precipitation, density separation, chemical oxidation, chemical reduction, membrane dialysis, stripping, Various methods such as wet oxidation. Compared with the biological method, the physical and chemical method is less affected by changes in water quality and water volume, and the effluent water quality is stable, especially for landfill leachate with low BOD / COD and difficult biological treatment. Due to the high cost of physicochemical treatment, it is generally used for pretreatment or advanced treatment of leachate.
Status of activated carbon used in landfill leachate treatment
Adsorption has been used for material separation for a long time. Commonly used adsorbents are activated carbon, zeolite, fly ash, and municipal waste incinerator bottom slag. In the treatment of leachate, the adsorbent is mainly used to remove difficult-to-degrade organic matter, metal ions and color in water. With bauxite adsorption, 48.93% of the organic matter was removed. If bioactive granular charcoal is used, the organic matter in the leachate is degraded by acclimation and cultivation of the biofilm on the activated carbon. Experiments show. Bioactive carbon adsorption has great potential for treating leachate or high-concentration organic wastewater.
Application of Coagulation in Landfill Leachate Treatment
Polyaluminium chloride and polyacrylamide, composite coagulant (90% PAC + 10% PAM) and reagent A (a kind of chitosan) are used in the case of different pH and different dosage The comparative analysis of COD removal effect of landfill leachate was carried out. The CODCr concentration of the landfill leachate stock solution was 3927 mg / L. The experimental results showed that when the combined coagulant dosage was 400 mg / L at pH 5.5 to 8, the removal rates of COD were 38.63% and 37.84%, respectively; the reagent A was at pH 8 and the dosage was 100 mg At / L, the removal rate of COD reached 39.85%.
Compared with the biological treatment method, the physical and chemical method is less affected by the water quality and water quality, and the effluent water quality is relatively stable, especially the BOD5 / COD ratio is low, and it has a better treatment effect on the landfill leachate that is difficult to biologically treat. In theory, physical and chemical treatment can remove all pollutants in wastewater, so physical and chemical treatment is generally used as pretreatment and advanced treatment in landfill leachate treatment. The early physical and chemical pretreatment can remove most of the toxic metal ions and SS in landfill leachate. . Physical and chemical treatment can also remove some organic substances (humic acid, fulvic acid and halogenated hydrocarbons) that are difficult to biodegrade, so the physical and chemical treatment method is often placed in the advanced treatment of landfill leachate.
There are also many researches on physicochemical treatment methods abroad, and most of them are membrane treatment, photocatalytic oxidation, and more advanced chemical technology research. G. Baccmgarten and CFSerfriend used fiber filtration membranes instead of reverse osmosis membranes and conducted research on leachate post-treatment. The results showed that the former was more economical. Soo-M.Kim et al. Studied the leachate treatment by combining the classic Fe2 ++ H2O2 reaction with ultraviolet light. The COD removal rate was not less than 70%. When the light radiation was 80kW / m3, the oxidation rate could be increased by 6 times. When the light radiation is increased to 160kW / m3, the degradation rate will be doubled.
Biological treatment of landfill leachate mainly refers to wastewater treatment methods that rely on the metabolism of microorganisms in the treatment system and the adsorption of pollutants by microbial flocs to remove organic pollutants in the leachate, which can be divided into anaerobic and aerobic Handle both. Domestic and foreign methods for landfill leachate treatment include anaerobic treatment systems and aerobic treatment systems.
The most common leachate treatment methods include extended aeration, biological turntables, and aeration stabilization ponds. These methods have achieved certain effects in reducing BOD5, COD, and ammonia nitrogen in landfill leachate. Other pollutants such as iron , Manganese and other metal ions.
At present, domestic and foreign landfill leachate treatment is mainly a biological treatment method. The biological method can have a good removal effect on easily biodegradable wastewater, and the process is relatively mature and the operating cost is relatively low. However, for organic wastewater with high concentration and poor biodegradability, it is difficult to achieve satisfactory results with conventional biological treatment technology. The concentrations of COD, ammonia nitrogen, and metal ions in the landfill leachate are very high. These characteristics limit the application of conventional biological treatment methods in the landfill leachate treatment.
If the sludge concentration is increased in the treatment system and the sludge residence time is prolonged, the wastewater treatment effect can be improved. Increasing the sludge concentration can reduce the sludge load in the system and improve the system's removal effect of organic matter in wastewater. Prolonging the residence time of sludge will change the microbial population in the system, which is beneficial to the growth and domestication of nitrifying bacteria.
In recent years, a new type of water treatment technology, TMBR has appeared at home and abroad. TMBR is a new type of water treatment technology combining membrane separation technology and activated sludge method. The use of membrane retention allows microorganisms to be completely trapped in the bioreactor to achieve complete separation of hydraulic retention time and sludge age, thereby ensuring that The system maintains a high concentration of activated sludge for a long time. It can be seen that TMBR has great advantages in treating difficult-to-degrade organic wastewater and high-concentration ammonia nitrogen wastewater.
Membrane methods include: ultrafiltration membrane (UF), nanofiltration membrane (NF) and reverse osmosis (RO). The main advantages are:
First, the filtration accuracy is relatively high, and the pore size of the membrane is relatively small. In particular, the pore size of the reverse osmosis membrane is generally 0.1 μm to 1 μm, which can remove bacteria, microorganisms, and dissolved salts. At present, it is widely used in desalination, pure water, high-purity water, material separation and concentration. The second is that the operation is not easily affected by the environment. The major environmental factors that affect reverse osmosis are pressure, temperature, and water quality. These quantities can be measured and controlled.
But membrane technology also has its disadvantages:
First, when the concentration of influent pollutants is high, the osmotic pressure of the influent water is particularly high, and the high pressure of the influent water is needed to overcome the osmotic pressure in order to achieve material separation. Therefore, it has high operating pressure and high energy consumption when treating wastewater with high pollutant concentration. Especially for reverse osmosis, its operating pressure is relatively high and energy consumption is large. Nanofiltration systems have lower operating pressure and lower energy consumption than reverse osmosis systems.
The second and most important aspect is that the membrane method is a pure physical separation, so the membrane method itself cannot digest pollutants. It can only separate water from pollutants, it cannot degrade pollutants, and it cannot achieve pollution. Detoxification and resource utilization. The concentration of the pollutants from the separation and treatment is 3 to 5 times the concentration of the incoming water. This concentrated solution is more difficult to handle than the original solution and is more likely to cause environmental damage.
It is precisely because of the characteristics of the membrane method that it is widely used in the concentration and recovery of materials, and rarely used in the treatment of high-concentration organic wastewater alone. It can only play an auxiliary role in the treatment of high-concentration organic wastewater. And reverse osmosis accumulates salt and increases electrical conductivity.
Anaerobic + aerobic + membrane method
The anaerobic treatment method is most widely used in anaerobic reactors. At present, the ordinary anaerobic reactors, upflow anaerobic sludge beds (UASB), internal circulation anaerobic reactors (IC), Anaerobic fluidized bed reactor, anaerobic fixed bed reactor (anaerobic filter AF), anaerobic rotary contact reactor and the combination of the above-mentioned reactors such as anaerobic composite reactor (UBF), etc .;
The aerobic treatment methods mainly include A / O-TMBR biochemical reaction tank method, A / O method, TMBR method, biofilm method, etc. For the treatment of landfill leachate, the currently used aerobic method is mainly with delayed aeration function. A / O-TMBR biochemical reaction cell and TMBR method.
The membrane method, especially nanofiltration (NF), has a filtration pore size of 1 μm, which can remove impurities with smaller particle size in water, and the operating pressure is lower. Therefore, it will be used as a terminal process in the leachate treatment project, and combined with reverse osmosis membrane (RO), it can ensure that the final water output index of the leachate treatment system meets the requirements for the use of replenished water in the circulating water system.
This process makes full use of the characteristics of biochemical treatment that can completely degrade organic matter, which can minimize the degradation of pollutants and reduce them. In particular, the biogas produced by anaerobic reaction is a more environmentally friendly energy source. This is pollution. Material resources. At the same time, the membrane method, a physical and chemical treatment method with high processing accuracy, can effectively guarantee the quality of the effluent, especially for waste water with a high content of pollutants such as landfill leachate.
Therefore, using this process combination to treat high-concentration landfill leachate is currently the most feasible technical route to ensure the stability of the effluent, and the removal rates of CODcr, BOD5, ammonia nitrogen and chromaticity are all high, which can fully meet the relevant national emission standards. This process is currently the most widely used leachate treatment method in foreign countries or in China.
A / O + film method
The A / O method uses a higher sludge concentration to remove about 90% of COD and NH3-N. This method can achieve better treatment results and longer and more stable operation results in recent engineering examples.
Based on the above comments, the project intends to use the anaerobic treatment process.
In general, it is important to fully understand the characteristics of leachate, and to use advanced, stable, and efficient process technologies as much as possible in combination with local conditions.
Principles of landfill leachate process selection
It is necessary to choose a process capable of reducing pollutants, harmlessly, and recycling resources, and truly and completely reduce and eliminate the harm of pollutants to the environment.
The treatment process can not only effectively degrade organic pollutants, but also treat those pollutants that cannot be degraded by organisms, so as to avoid re-polluting the environment.
Regardless of the concentration of organic matter COD, BOD5, NH3-N, chroma, etc. in the landfill leachate, it is necessary to choose an efficient treatment combination process, shorten the process flow, reduce project investment, and save power consumption and operating costs , Reduce operating costs, and ensure that the treatment effect can meet the emission requirements.
According to the characteristics of large changes in water quality and quantity of landfill leachate, the selected process must have strong adaptability and operational flexibility, and it must be easy to adjust processing parameters to deal with the impact of changes in water quality and quantity.
Leachate treatment process
Aiming at the characteristics of landfill leachate and the geographical location of the waste disposal site, the technical and economic comparison of various treatment processes finally determines the process as follows:
The garbage leachate from the waste incineration power plant is discharged into the conditioning tank through a special collection pipe, and the leachate enters the coagulation sedimentation tank after the reaction of the conditioning tank. The sludge from the coagulation sediment is sent to the sludge tank through the pipeline, and the effluent flows to the equilibrium tank, which is lifted by the pump into the anaerobic reactor to react, and then flows into the first-stage A / O. Degradation of more than 90% of organic matter and ammonia nitrogen through primary A / O, and then enter secondary A / O processing unit.
The secondary A / O processing unit degrades the remaining organics and ammonia nitrogen. The TMBR system is externally placed in the A / O pool to achieve mud-water separation and ensure the stability of TMBR effluent. The water produced by TMBR is pressurized into the NF system through a lift pump set in the nanofiltration water tank. The NF system has a good interception effect on divalent ions. Engineering examples show that the NF membrane can intercept more than 80% of divalent ions, and at the same time, the interception efficiency for NH3-N is about 15%, which ensures the stability of system processing. It also lays the foundation for long-term normal use of RO membrane modules. After processing by the NF system, the service life of the RO system can reach more than 3 years.
The remaining sludge produced by the A / O pool enters the sludge, and the sludge after concentration and dehydration treatment will be solidified by fly ash, burned into the furnace, and shipped.