With the improvement of living standards, people pay more and more attention to the environment of daily life. In recent years, with the vigorous promotion of the construction of waste incineration power plant in China, more and more waste appears in the public field of vision as renewable energy, which provides convenience for people's life, and also promotes the pace of the future ecological harmonious society. However, for waste incineration power plant, how to deal with the leachate generated in the incineration process is particularly important. Therefore, this paper expounds and analyzes the characteristics of landfill leachate in waste incineration power plant, so as to explore the targeted Leachate Disposal Measures, in order to help the development of subsequent waste incineration power plant.
Introduction
With the rapid development of China's economy and urbanization, land resources are increasingly scarce, which makes the traditional sanitary landfill disposal method of domestic waste gradually show its disadvantages. As one of the common disposal methods of domestic waste, composting technology can avoid the generation of leachate, but it is limited by the lack of effective sorting of domestic waste in China. Domestic waste incineration power generation can not only effectively deal with domestic waste, but also make use of incineration heat energy, which achieves the goal of "reducing, recycling and harmlessness" of solid waste. It is most in line with China's national conditions and belongs to the industry encouraged by China. It has become a new way to solve domestic waste in recent years. However, like the sanitary landfill disposal method, domestic waste incineration power generation also faces the problem of leachate treatment. Landfill leachate has been recognized as high risk and high pollution wastewater, which has great harm to groundwater and surface water. How to treat it reasonably has become a difficult and hot topic in the field of environmental research.
1 Introduction of leachate treatment system
1.1System structure
The landfill leachate treatment system adopts the process of "coagulation sedimentation pretreatment + UASB anaerobic reactor + tmbr external membrane biochemical reactor + RO reverse osmosis". The main equipment of landfill leachate treatment system includes coagulation sedimentation tank, UASB anaerobic reactor, denitrification tank, carbonization tank, nitrification tank, external tubular ultrafiltration membrane and reverse osmosis membrane module, etc.
1.2 Characteristics of leachate quality in power plant
The leachate of MSW incineration power plant contains aromatic hydrocarbons, heterocyclic compounds, halogenated compounds, alkanes, olefins, alcohols, phenols, aldehydes, ketones, carboxylic acids, lipids, amines and amides, which are difficult to degrade. At the same time, it also contains pollutants such as ammonia nitrogen, bacteria, viruses and heavy metal ions. The water quality of leachate from waste incineration power plant has the following characteristics. a. The concentration of pollutants is high. The concentration of COD and BOD in the leachate of waste incineration power plant is tens of thousands of mg / L, which is one or two hundred times of the common municipal sewage_ 1]。 b. The content of ammonia nitrogen is high. There are a lot of nitrogen-containing substances (such as proteins) in garbage, especially in domestic garbage in China, which leads to a high content of ammonia nitrogen in landfill leachate. C. The content of phosphorus is low and the proportion of nutrition is imbalance. The phosphorus content in landfill leachate is usually low, and its phosphorus concentration is equivalent to that of municipal wastewater. For biological treatment, the appropriate proportion of nutrients in sewage is Bod: n: p-100:5:1, while the BOD: P in general landfill leachate is more than 300, which is far from the amount of phosphorus required for microbial growth. d. The water quality fluctuates greatly. Due to the influence of water quantity, weather and climate, garbage source and other factors, the water quality composition of landfill leachate fluctuates greatly. In general, the amount of water is less and the concentration of pollutants is higher in dry season in winter; In the rainy season of summer, the amount of water is more and the concentration of pollutants is lower.
2 Leachate treatment process
2.1 Anaerobic treatment system
After the leachate is pretreated, it often needs to be pumped to the anaerobic tank for the first biochemical treatment. For example, in this stage, the upflow sludge bed filter (UBF) technology, which is relatively more mature, can be selected according to the actual treatment needs. Through the application of this technology, the sewage will be sucked into the bottom of the reactor, In the anaerobic state, the sewage and mud are fully separated, and in the process of separation, the macromolecular organic matter is adsorbed or decomposed, and then cod is greatly degraded, or hydrolyzed and acidified into easily degradable small molecular organic matter, and the generated biogas is returned to the negative pressure bin of the garbage pool for combustion support of the incinerator The precipitated sludge is also sent back to the sludge thickening tank for dewatering treatment. However, anaerobic treatment is sensitive to temperature fluctuation, so the waste heat steam of incineration plant can be used to heat the anaerobic process to ensure the stability of anaerobic reaction temperature.
2.2 Analysis of leachate zero discharge technology
The process of reusing leachate in MSW incineration power plant mainly includes recharge of circulating cooling water, cooling of incinerator slag, allocation of lime emulsion flue gas desulphurization and plant greening. Among them, the quality of recycled water is relatively low for slag cooling and flue gas treatment of incinerator, but the amount of recycled water is limited; A certain amount of plant greening can be reused, but it is greatly affected by seasons. Due to the regional differences and the different processes of garbage collection and storage, there are differences in the amount of leachate produced in various domestic waste incineration power plants. Therefore, the slag cooling of incinerator, flue gas treatment and plant greening are insufficient to ensure the full reuse of leachate reclaimed water. The supply quantity of circulating cooling water in domestic waste incineration power plant is about 10% of the circulating cooling water quantity, and its consumption is very large, which can fully accept the production quantity of leachate. The key of circulating cooling water replenishing leachate reclaimed water is that the circulating cooling water system needs regular descaling and cleaning, and the concentrated water enters the leachate treatment system for reprocessing. Taking the domestic waste incineration power plant as an example, the supply quantity of circulating cooling water is about 750 T / D, that of slag cooling water of incinerator is about 100 t / D, that of flue gas treatment is about 80 t / D, and that of landfill leachate is about 150 t / d. after treatment, it is preferentially reused for slag cooling and flue gas treatment of incinerator with low water quality requirements, and the surplus is supplied to the circulating cooling water system, It can ensure the zero discharge of leachate in the domestic waste incineration power plant.
2.3 Advanced treatment system
After the above three stages of leachate treatment, most of BOD, ammonia nitrogen, total nitrogen, heavy metals, suspended solids, etc. have been greatly reduced, but cod, sodium and magnesium ions still exceed the standard. On this basis, if we want to achieve the expected goal of leachate treatment, and the COD value is less than 500 mg / ml, In order to meet the requirement of three-stage discharge with BOD value less than 300 mg / ml, it is necessary to add advanced treatment nanofiltration (NF), softening and reverse osmosis (RO) system after aerobic treatment to remove most of the scaling ions in ultrafiltration effluent, intercept inorganic salt and soluble organic matter, so as to achieve the purpose of purification desalination and reuse of clean water. To achieve the goal of zero discharge of concentrated leachate treatment and realize the environmental benefits of recycling.
2.4 MBR external membrane bioreactor
MBR membrane biochemical reactor is an organic combination of biological treatment technology and membrane technology. It is mainly composed of bioreactor and membrane module. Membrane filtration is used to replace the secondary sedimentation tank in traditional activated sludge process. Due to the high efficiency of membrane interception and the abundance of organisms in the bioreactor, the sludge concentration in the bioreactor can be greatly increased, which has the advantages of high pollutant removal efficiency, good effluent quality, strong adaptability to load changes, small sludge discharge, simple and compact system equipment, and less land occupation. The removal rates of COD, BOD, ammonia nitrogen and SS in MBR were 95, 90, 99 and 99, respectively. The external membrane bioreactor consists of biological denitrification system (A / O / N) and tubular ultrafiltration membrane (UF) system. A / O / N biochemical nitrogen removal system is to improve and optimize the traditional A / O process. A / O / N (anoxic denitrification aerobic carbon oxidation aerobic nitrification) process is to separate cod oxidation and nitrification, to create suitable living conditions for denitrifying bacteria, COD oxidizing bacteria and nitrifying bacteria, so that they can carry out denitrification, COD oxidation and nitrification in the best state, so as to make cod, COD and nitrification Ammonia nitrogen was effectively removed.
2.5 Pretreatment system
Pretreatment stage is aimed at the initial stage of leachate treatment. The treatment personnel can adjust the pH of leachate by adding alkaline substances into the dosing system, precipitate and remove a large amount of suspended substances in the sedimentation tank. The precipitates and suspended substances enter the sludge concentration tank through the sludge pump, and the sludge cake is mixed with waste incineration after dewatering by centrifuge or plate pressure filtration. The liquid phase is pumped to the anaerobic reactor through the effluent from the regulating tank, so as to realize the initial treatment of leachate. It should be noted that in this link, the suspended matter in the pretreated liquid can be treated by adding drugs containing ferric chloride polymer, so as to achieve the initial treatment goal.
Epilogue
China has a large population and rapid economic development. The daily output of domestic waste is about 6 million tons, accompanied by about 1.5 million tons of leachate per day, and the annual growth rate is 5% - 6%. If not treated reasonably, there is a huge environmental risk, which is a severe challenge to environmental protection. At present, the country is vigorously advocating and implementing energy conservation and emission reduction policies, and domestic waste incineration power generation is a good measure. It has great potential for energy conservation and emission reduction, which is of great practical significance for China's environmental protection and ecological civilization construction.
Source: Industrial Wastewater Circle
