Food waste accounts for a large proportion of municipal solid waste, accounting for about 51%. Because kitchen waste is a "high-quality pest culture medium by-product of human beings", it is easy to spontaneously breed mosquitoes and flies, produce toxic, malodorous and combustible gases, and bring fire hazards and environmental protection and public health problems. Waste cooking oil and swill pigs that are not properly used are harmful. Because of their rich contents of oil and other organic substances and high calorific value after dehydration, kitchen waste is an available resource, so it is necessary to sort out the treatment forms of kitchen waste.
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According to the treatment media, it can be divided into abiotic treatment and biological treatment, in which abiotic treatment includes mechanical crushing, sanitary landfill, incineration for power generation, gasification, etc. Biological treatment includes anaerobic fermentation, aerobic fermentation and preparation of ecological feed.
(1) mechanical crushing. Make full use of the urban sewage treatment system, and break the domestic garbage through the garbage crusher and then enter the municipal sewage system for treatment. In the process of mechanical crushing, there is no chemical change in the kitchen waste, only the particle size is reduced. This method is essentially to sludge the kitchen waste sewage, not to digest it, but to transfer the waste. It is only suitable for cities or regions with new plans that can accept this part of the waste treatment, otherwise it will inevitably pose challenges to the original municipal sewage treatment system.
(2) sanitary landfill. Burying kitchen waste underground, including the process of continuous microbial degradation of organic matter, is the sanitary landfill of kitchen waste. The biochemical treatment process of microorganisms is a hot research topic in recent years. The general conclusion is that microorganisms preferentially decompose organic matter and convert it into nutrients needed for their own growth and reproduction, which can further degrade organic matter into inorganic matter to increase the fertility of soil when organic matter is surplus.
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Sanitary landfill has the advantages of simple operation and low cost. At the same time, it occupies a large area and takes a long time. There is secondary pollution of leachate, which releases odor, and the by-product biogas is easy to cause major safety hazards of explosion. As people pay more attention to environmental problems and land resources become increasingly tense, the sanitary landfill rate of kitchen waste is decreasing year by year.
(3) incineration for power generation. Kitchen waste is not suitable for direct combustion because of its high moisture content, so it must be dehydrated first, and then thoroughly oxidized and decomposed in an incinerator. Generally, the solid can be reduced by 65% (1.5%) by peroxide combustion at a temperature not lower than 1,111℃. The high-grade heat of combustion is generally used for power generation, and the low-grade heat can be used for heating. Ash can be used as raw materials for building materials. In order to ensure the continuous and stable incineration process, generally 21% coal can be blended.
At present, garbage incineration technology is mainly divided into three categories: layered combustion, fluidized bed combustion and rotary combustion technology (i.e. rotary kiln type).
This method has a high treatment rate, and the power steam generated by the conversion is used for power generation, and the low-grade heat is used for heating, which has a great degree of reduction and realizes the resource and reduction of kitchen waste; However, conventional garbage incineration inevitably produces air pollutants such as furan, dioxin, fly ash, etc. In addition, the content of heavy metals in incinerator ash (generally 5% ~ 21% before incineration) is high, which pollutes the environment seriously.
(4) anaerobic fermentation. Anaerobic fermentation means that some organic matter is transformed into microbial body by microorganisms under anaerobic conditions with insufficient or even no oxygen, and the other part is decomposed into CH4 (some processes can be directly decomposed into H2) and CO2. Organic acids or alcohols can also be prepared by selecting different microorganisms. Temperature, pH, C/N ratio, trace elements and oxygen content significantly restrict the rate and conversion rate of anaerobic fermentation [3]. At present, anaerobic fermentation technology has been greatly developed in Europe, and the representative process in China is Biomax.
centralized anaerobic fermentation technology is relatively mature, with the advantages of high automation, production of fuel gas, organic acids and alcohols, good recycling degree and high economy, and the residue can be made into organic fertilizer, which is widely used in the treatment of municipal solid waste and is an effective way to clean and recycle kitchen waste.
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(5) Aerobic fermentation. Aerobic composting of kitchen waste [1] refers to the biodegradation process of kitchen waste by artificially cultivated special-effect bacteria or natural aerobic microorganisms under the condition of abundant oxygen. Similar to anaerobic fermentation, part of organic matter is transformed into microbial body, but different from anaerobic fermentation, on the other hand, it is transformed into organic fertilizer without producing CH4 or H2.
At present, the core technology of aerobic fermentation of kitchen waste mainly focuses on the optimization and domestication of aerobic microorganisms and the rationalization and improvement of reactors. Aerobic fermentation of kitchen waste mainly includes squeezing dehydration, oil-water separation, biochemical degradation, tail gas treatment, wastewater purification, organic fertilizer treatment, etc. In the biochemical degradation process, kitchen waste with particle size of 1.5 ~ 8 cm is usually used as raw material, and strains and auxiliary materials are added for fermentation for about 4 weeks.
If the oil and salt content of kitchen waste are too high, the activity of microorganisms will be reduced. In addition, centralized aerobic fermentation will occupy a large area, produce polluting odor and have low economic benefits. The number of harmless composting plants in China is decreasing year by year, which is related to the mixed stacking of municipal waste, high separation cost and low economic benefits of composting enterprises.
It is recommended that the high-temperature aerobic rapid degradation technology is used to convert kitchen waste into organic fertilizer by aerobic fermentation for 1 days. This method has the advantages of high speed, good salinity adaptability and good high-temperature activity (even reaching 81℃). Compared with aerobic composting, the high-temperature aerobic reaction process does not produce any harmful substances, and the produced gas is relatively small, time-consuming, controllable, easy to operate, fast in degradation, good in recycling effect, capable of treating mixed environmental protection garbage and low in operating cost. Moreover, 91% of the organic garbage is discharged up to the standard and the oil is recycled during the pretreatment of the previous garbage. 11% of solid emissions are used as organic fertilizer raw materials for ecological planting and breeding. The whole treatment process is pollution-free and there is no secondary pollution. In line with the national principle of "reduction, harmlessness and resource" in the treatment of kitchen waste. At present, integrated kitchen waste equipment has been put into the market and has been popularized and applied.
(6) preparation of ecological feed. Ecological feed [1] refers to the feed prepared with kitchen waste as raw material. The treatment methods of kitchen waste for preparing ecological feed are mainly divided into three categories: fermentation to produce bacterial protein, silage method and dehydration method. Fermentation is widely used in China to prepare bacterial protein; The typical practice of silage method is lactic acid bacteria fermentation; Dehydration to prepare dry feed usually includes direct high temperature drying, boiling drying, vacuum frying and other methods.
(7) gasification. Although there are many kinds of mainstream gasification technologies in fields such as coal chemical industry, there are only a few gasification technologies that can be applied to the treatment of kitchen waste in a stable, economical and reasonable way due to the high water content of kitchen waste. Here, only the high-temperature plasma torch gasification technology is recommended [8]. This technology has the characteristics of fast reaction rate, high added value and environmental protection. The core technology is mastered abroad and mainly applied in the United States, Canada and Japan. With this technology, the solid after simple dehydration can be gasified together with oil. Because of the high temperature plasma gasification reaction and reasonable gas flow residence time, organic pollutants such as dioxins and furans are basically not produced, and the effective components of synthesis gas are CO and H2. Solid slag can be used as raw material for building insulation, but it is not widely used in China at present, so it is necessary to introduce core technology.
2 Conclusion
At present, the mainstream treatment method of non-living kitchen waste in China is incineration for power generation, and high-temperature plasma torch gasification power generation technology is the most promising technology to replace incineration. Large-scale garbage gasification technology is still in the stage of popularization and application, and the market potential is huge.
Anaerobic fermentation and traditional aerobic composting can realize the purpose of recycling and reducing kitchen waste. Generally speaking, the advantages of microbial treatment of kitchen waste are simple process, low initial investment, high degree of automation and little secondary pollution; Although it has the disadvantages of high energy consumption, small reduction degree, poor salinity adaptability, and often needs to add auxiliary materials, it can be recycled and has a certain output. At present, as a new garbage solution, miniaturized integrated kitchen waste equipment is widely used in government canteens, school canteens, hotels, hotels, restaurants, residential quarters, food processing plants, airports, street communities, large factories, agricultural wholesale markets, fruit and vegetable production bases, food processing plants and so on.
source: www.lbome.cn.