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The emissions of urban domestic waste and industrial waste in China are particularly huge. Beijing's daily emissions reach 6.5438+0.5 million tons. Wutaimen garbage dump in Wuhan has a daily processing capacity of 2,000 tons, and there are 6~7 similar garbage dumps in Wuhan. The annual output of garbage in China is about 65.438+0.5 billion t, and the growth rate is about 9% [654.38+00]. At present, the main treatment method of garbage and industrial waste in China is landfill. Because of garbage disposal, a lot of cultivated land is occupied every year. Using underground open space for landfill is a good way to alleviate the contradiction between the shortage of cultivated land and the large amount of cultivated land occupied by landfill.
Burying domestic garbage and industrial garbage near cities or mines with serious shortage of filling materials can not only economically fill mined-out areas, but also bury garbage, which can kill two birds with one stone.
Although China is developing waste incineration technology at present, the composition of waste in China is different from that in foreign countries, and non-combustible components such as ash and soil account for more than 60% of the waste content. Therefore, in the future, landfill will remain the most important way of garbage disposal in China. In order to save land, there is a broad prospect of landfill in open pit. Especially the mined-out areas with good burial conditions, undeveloped joints and fissures or poor connectivity will be one of the ideal landfills for radioactive industrial wastes-nuclear waste and chemical waste after proper enclosure and sealing.
Before 1996, Russia had accumulated 640 million m3 of nuclear waste, some of which were solid, including about 1.5GCi( 1Ci=37G decay /s). The radiation leakage value of Chernobyl nuclear power plant is only 50~250MCi). In order to build underground caves to bury nuclear waste and prevent radiation leakage, the Russian Academy of Sciences has done a lot of hard research and spent a huge amount of research and construction funds. Nowadays, the trend of burying nuclear waste in underground caves is on the rise in the world, such as Sweden, Germany, the United States, Britain, Spain, Belgium and other countries have built one or more underground caves [3].
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Solid waste treatment technology [transfer]
The general principle of solid waste treatment and utilization is to first consider reduction and recycling to reduce the generation and discharge of solid waste, and then consider appropriate treatment to speed up the material cycle. No matter how perfect the previous treatment is, there will still be some substances left, so the final treatment is essential.
1, reduction method
According to rough statistics, the utilization rate of mineral resources in China is only 50-60%, and the energy utilization rate is only 30%. About 40-50% will become waste without bringing production benefits into play, which will not only pollute the environment, but also waste a lot of valuable resources, as well as other industries. Therefore, it is promising to strengthen technical transformation, improve the utilization rate of resources and reduce the generation of solid waste. There are usually three reduction methods:
1) By changing the product design, new products with less consumption of raw materials and packaging materials are developed, and the process is reformed to strengthen management and reduce waste, thus reducing product unit consumption.
2) Improve product quality, prolong product life, and minimize product scrap probability and replacement times.
3) Develop products that can be reused for many times, so that the finished products can be recycled to replace the finished products that can only be used once, such as containers and bottles for packaging food.
2. Resource Law
Resource recovery method is a method to recover or prepare substances and energy from solid waste through various methods, and to convert waste into resources, that is, to convert it into new production factors in the same industrial sector or other industrial sectors, while protecting the environment. Its specific utilization mode is as follows:
1) as industrial raw materials, such as recovering metal elements from tailings and scrap metal slag. Nanjing Mining Bureau and other units use coal gangue with high aluminum content and low iron content to produce products such as aluminum ammonium vanadium, aluminum trioxide, polyaluminum and silicon dioxide, and extract rare metals such as molybdenum, gallium, uranium, vanadium and germanium from the remaining filtrate.
2) Energy recovery: China discharges more than 30 million tons of coal gangue every year, and its calorific value exceeds 6276 kJ/kg, which can be used as fuel for power generation in fluidized beds. There are more than 2000 fluidized beds in China, which can save a lot of high-quality coal every year. Hegang, Benxi and other places also use coal gangue to make gas and recover energy. In addition, there are ways to recover energy, such as landfill, incineration and decomposition of organic waste, to recover fuel oil, natural gas and biogas.
3) As soil improver and fertilizer: Practice has proved that using fly ash to improve soil has good effects on acid soil, cohesive soil and weak saline-alkali soil, and can increase grain yield 10-30%. It also has the function of increasing the output of fruits and vegetables. In Germany, copper slag powder is used as fertilizer for potted plants and field experiments. The results show that the yield of copper slag powder has been improved. Many experiments and practices show that pyrite cinder contains a variety of non-ferrous metals, which can be used as a comprehensive trace element fertilizer with obvious effect.
4) Direct utilization: such as direct utilization of various packaging materials.
5) Building materials: Slag, slag and fly ash can be made into various building materials such as cement, bricks and thermal insulation materials, and can also be used as cushion materials for roads and foundations.
The traditional wall material in China is clay brick. To produce 654.38 billion bricks every year, it is necessary to dig 65.438 million mu of fertile land and use 65.438 million tons of coal. China produces hundreds of billions of bricks every year. This is a great threat to China's precious cultivated land, and most of all kinds of solid wastes can find a way out in building materials production, which is of great significance to protecting land resources and improving the environment.
3. Treatment method
Solid waste is reduced, harmless, stable and safe by physical, chemical and biochemical methods, so as to accelerate the circulation of substances in the environment and reduce or eliminate environmental pollution.
Physical treatment: Physical treatment is to change the structure of solid waste through concentration or phase change, making it a form convenient for transportation, storage, utilization and disposal. Physical treatment methods include compaction, crushing, sorting, concentration, adsorption and extraction. Physical treatment is often used as an important means to recover useful substances from solid waste.
L chemical treatment: chemical treatment is to use chemical methods to destroy harmful components in solid waste, make it harmless, or convert it into a form suitable for further treatment and disposal. Due to the complex chemical reaction conditions and many influencing factors, chemical treatment methods are usually only used for waste treatment with a single component or several similar chemical components. For mixed waste, chemical treatment may not achieve the expected purpose. Chemical treatment methods include oxidation, reduction, neutralization, chemical precipitation and chemical dissolution. Some hazardous solid wastes may produce residues rich in toxic components after chemical treatment, and these residues must be detoxified or safely disposed of.
L biological treatment: biological treatment is to decompose degradable organic matter in solid waste by microorganisms, so as to realize harmless and comprehensive utilization. After biological treatment, the volume, shape and composition of solid waste have changed greatly, which is convenient for transportation, storage, utilization and disposal. Biological treatment methods include aerobic treatment, anaerobic treatment and facultative anaerobic treatment. Compared with chemical treatment methods, biological treatment is generally cheaper and more widely used, but the treatment process takes a long time and the treatment efficiency is sometimes unstable.
(1) Composting: It is a process of artificially promoting the biotransformation of biodegradable organic matter into stable humus according to microorganisms such as bacteria, actinomycetes and fungi widely distributed in nature. The product of compost is called compost, which is a cheap and high-quality soil improvement fertilizer. It has many functions, such as improving soil structure, increasing soil water capacity, reducing inorganic nitrogen loss, promoting the transformation from insoluble phosphorus to soluble phosphorus, increasing soil buffering capacity and fertilizer efficiency.
According to the relationship between microbial oxygen demand in composting process, it can be divided into anaerobic composting and aerobic composting. Aerobic composting is widely used because of its advantages of high composting temperature, complete matrix decomposition, short composting cycle and low odor. According to different composting methods, aerobic composting can be divided into open-air composting and rapid composting.
Modern compost production usually consists of five processes: pretreatment, main fermentation (primary fermentation), post-fermentation (secondary fermentation), post-treatment and storage. Main fermentation is the key to the whole production process, and fermentation conditions such as ventilation, temperature, moisture, C/N ratio, C/P ratio and pH should be well controlled.
(2) Biogas: Biogas, also known as anaerobic fermentation, is a process in which carbohydrates, protein, fats and other organic substances in solid wastes generate combustible gas under the action of various microorganisms in an anaerobic environment with artificially controlled temperature, humidity and pH value. This technology has been widely used in the treatment of municipal sewage sludge, agricultural solid waste and feces. It not only plays a stable and harmless role in solid waste, but more importantly, it can produce an energy source that is convenient for storage and effective utilization. It is estimated that rural areas in China produce more than 500 million tons of crop straw every year. If half of them are used to produce biogas, 50-60 billion cubic meters of biogas can be produced every year. Therefore, biogas technology is an important way to control pollution and change rural energy structure.
(3) Saccharification technology of waste cellulose: Saccharification of waste cellulose is to convert it into monomer glucose through enzymatic hydrolysis technology, and then to chemical raw materials through chemical reaction or to single cell protein or microbial protein through biochemical reaction.
It is estimated that the global annual net output of cellulose is about 654.38+000 billion tons, and the recycling of waste cellulose is a very important worldwide topic. Japan and the United States have successfully developed the saccharification process of waste fiber. At present, it is technically feasible and its economic benefits need to be demonstrated. How to develop low-cost treatment methods, find better enzyme species, improve the unit biodegradability of enzymes and improve fermentation technology need further exploration.
(4) Feed waste cellulose-the technology of producing single-cell protein: this technology does not need saccharification process, but directly produces single-cell protein or microbial protein by using the microbial action of waste fiber. At present, it is feasible to use waste cellulose as feed to produce single cell protein, but there are still many problems to be solved to be economically competitive.
(5) Bacterial leaching: chemoautotrophic bacteria oxidize bivalent iron into ferric iron (trivalent iron), sulfur and reducing sulfide into sulfuric acid to obtain energy, and absorb carbon dioxide, oxygen and other trace elements (such as N and P) from the air to synthesize cytoplasm. These bacteria grow in a simple inorganic medium and can tolerate higher concentrations of metal ions and hydrogen ions. Using the unique physiological characteristics of chemoautotrophic bacteria, the process of dissolving some metals from mineral materials and then extracting metals from leachate is generally called bacterial leaching. This method is mainly used to treat copper ore and uranium ore wastes, such as copper sulfide and general oxides (Cu2O, CuO), and recover copper and uranium. Manganese, arsenic, nickel, zinc, molybdenum and some rare elements also have application prospects. At present, bacterial leaching has been widely used in industry at home and abroad.
L heat treatment: heat treatment is to destroy and change the composition and structure of solids through high temperature, and at the same time achieve the purpose of volume reduction, harmlessness or comprehensive utilization. The heat treatment methods include incineration, pyrolysis, wet oxidation, roasting and sintering.
(1) Incineration treatment: Incineration treatment is to convert combustible components in solids into inert residues at high temperature (800 ~ 1000℃) and recover heat energy at the same time, which undoubtedly plays an important role in the world of energy crisis, and is also the reason why this technology has been widely used in developed countries in recent years. Through combustion, the volume of solid waste can be further reduced. After incineration, the volume and weight of municipal waste can be reduced by 80-90% and 75-80% respectively. At the same time, it can completely eliminate all kinds of pathogens and eliminate the source of corruption. In contrast, the combustion process has:
Combustion generally goes through dehydration, degassing, light-off, combustion, extinction and other processes. There are three main factors that control this process, namely, time, temperature and turbulent mixing degree of fuel and air (commonly known as three T). It is generally believed that the combustion time is approximately proportional to the square of the particle size of solid waste. The finer the particles, the larger the contact area with air, the faster the combustion and the shorter the residence time of waste. In addition, the higher the oxygen concentration in combustion, the higher the combustion speed and quality. Therefore, it is necessary to make enough air flow in the fuel. The higher the turbulent mixing degree between fuel and air, the better the combustion.
Generally speaking, the combustion process includes solid waste storage, pretreatment, feeding system, combustion chamber, waste gas emission and pollution control, slag discharge, monitoring and testing, energy recovery, etc. 12 system.
(2) Pyrolysis: under anaerobic or anoxic conditions, organic matter is heated at high temperature (500 ~ 1000℃) to decompose into gas, liquid and solid products, including combustible gases such as hydrogen, methane, hydrocarbon mixture and carbon monoxide; Liquid fuel oil containing methanol, acetone, acetic acid, acetaldehyde and other components; Solids are mainly solid carbon. The main advantage of this method is that it can convert the organic matter in the waste into useful fuel which is convenient for storage and transportation, with less tail gas emission and residue, and it is a low-pollution treatment and resource utilization technology.
(3) Wet oxidation: Wet oxidation is also called wet combustion method. It refers to the rapid oxidation process of organic substances in the presence of water medium at appropriate temperature and pressure. Organic materials should be in a flowing state and can be added to the wet oxidation system through a pump. Because the oxidation process of organic matter is exothermic, once the reaction starts, it will automatically proceed under the action of the heat released by the oxidation of organic matter, without adding auxiliary fuel. The discharged tail gas mainly contains gases such as carbon dioxide, nitrogen and excess oxygen, and the liquid phase contains residual metal salts and unreacted organic matter. The degree of oxidation of organic matter depends on reaction temperature, pressure and residence time of waste in the reactor. Increasing temperature and pressure can speed up the reaction and improve the conversion rate of COD, but the maximum temperature cannot exceed the critical temperature of water.
Microwave treatment: The latest research results show that microwave technology can be successfully applied to radioactive waste treatment, soil purification, industrial crude oil and sludge treatment. Although it is still in the laboratory research stage, experts point out that microwave technology will definitely play its potential in waste treatment in the future.
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