With the acceleration of urbanization and the improvement of residents’ living standards, the number of municipal waste solution has increased. In recent years, municipal solid waste has grown at an annual rate of 8% to 10%, and the growth of urban domestic waste has become a potential factor restricting the sustainable development of various countries. How to properly manage urban domestic waste is of great significance to China and the world. Urban living garbage is complex and large in quantity, and it is one of the main sources of pollution in the environment. The landfill gas generated by landfill degradation is a greenhouse gas. About 3% to 4% of the annual anthropogenic greenhouse gas emissions are from methane in landfill gas.
At present, the most common methods of domestic garbage disposal are landfill and incineration. About 80% of the existing waste disposal implementation is landfill. Due to the characteristics of domestic garbage and economic development, landfill will remain the main disposal method for municipal domestic garbage in the future for a long time to come. In recent years, waste incineration technology has caused great concern to all sectors of society because of the harmlessness, reduction and resource level of waste disposal. Incineration power generation technology has become an emerging industry with great development potential. The National Development and Reform Commission has identified municipal solid waste incineration power generation as a key support industry.
1. Landfill treatment
Economic and technical analysis: Landfill treatment is a commonly used treatment method. It can handle all kinds of garbage, and the method is simple and saves investment. Therefore, this method is widely used in countries all over the world. From uncontrolled landfill to sanitary landfill, including drainage and landfill, compressed landfill, and landfill.
The landfill treatment method firstly prevents the liquid leaching and rainwater runoff from the waste from polluting the groundwater. The general specification requires that the elevation at the lowest point of the backfill should be more than 3.3 m above the water table, and the lower part of the backfill should have an impervious rock or clay layer. Otherwise, an impervious layer such as clay, asphalt or plastic film should be set up. Secondly, the landfill should be provided with an exhaust port to allow methane and other gases released during the decomposition of anaerobic microorganisms to escape in time to avoid explosion.
Impact on the environment: After the backfilling, it is generally not suitable to build houses on it within 20 years to avoid structural damage caused by uneven subsidence of the backfill, but it can be used for green space, farmland and pasture. For landfill disposal, try to use natural or artificially excavated depressions, such as waste clay pits, waste quarries, and waste pits after development resources. Landfilling in the pit is conducive to restoring the landform and maintaining the ecological balance. However, if backfilling in large areas of depressions, harbors, valleys, etc., it is necessary to consider whether it will destroy the ecological environment. The biggest advantage of landfill is that its processing cost is relatively low. However, it has a large land resource. At the same time, landfill disposal will also cause serious environmental problems. The most important thing is malodor and methane emission. And landfill leachate, landfill leachate is a highly toxic substance, so its pollution is very serious. If the anti-seepage measures are not good, or the landfill leachate is out, the treatment will be bad, which will cause serious pollution to groundwater, surface water and soil, and bring serious environmental problems.
2. Composting treatment
Economic and technical analysis: Waste composting is a method of treating and utilizing garbage. It uses microorganisms such as bacteria, yeasts, fungi and actinomycetes present in garbage or soil to cause biochemical reactions in the garbage to be degraded (digested). ), forming a substance similar to corrosive soil, used as a fertilizer and used to improve the soil. According to the principle of bacterial decomposition, the garbage composting technology is divided into high temperature (good) oxygen method and low temperature anaerobic method composting.
According to the composting method, it is divided into open-air composting method and mechanical composting. The composting operation is generally divided into 4 steps:
(1) Pretreatment, removing large and inorganic impurities, and dividing the waste crushing sieve into a homogeneous shape. The optimum moisture content of the homogeneous garbage is 45-60%, and the carbon to nitrogen ratio is about (20-30): 1. Can be mixed with sludge or manure when it is not needed;
(2) Bacterial decomposition (or fermentation), under the appropriate conditions of temperature, moisture and oxygen, aerobic or anaerobic microorganisms rapidly multiply, and the waste begins to decompose, converting various organic matter into harmless fertilizer;
(3) It is decomposed, stable and fat, and can be applied after it is completely decomposed;
(4) Storage or disposal, storage of fertilizers, and disposal of fertilizers for landfill disposal.
The effect on the environment:
The composting treatment has a long cycle, a large area and poor sanitary conditions; the products produced by composting are farmyard manure, the fertilizer efficiency is poor, and the sales are relatively difficult.
3. Incineration treatment
Economic and technical analysis: Waste incineration treatment refers to the process in which combustibles in waste are burned with oxygen in an incinerator. The essence is the chemical reaction of carbon, hydrogen, sulfur and other elements with oxygen. After the waste is incinerated, heat is released, and at the same time, smoke and solid residue are generated. The heat is recovered, the flue gas is purified, and the residue is digested. This is an indispensable process for incineration. The incineration treatment technology is characterized by large processing capacity, good volume reduction, and complete harmlessness. The heat generated by the incineration process can be used to generate electricity to realize the energyization of waste. Therefore, it is a waste treatment technology commonly used in developed countries. Garbage can be oxidized and decomposed by incineration to remove toxicity, recover energy and obtain by-products. Almost all organic waste can be treated by incineration. For inorganic, organic mixed solid waste, if the organic matter is a toxic and hazardous substance, it is generally best to use incineration. The incineration method is suitable for treating garbage with more combustibles. By incineration, care must be taken not to cause secondary pollution of the air. Incineration is a waste disposal technology that destroys the heat of waste utilization. However, it is only possible to recycle waste with thermal energy, and waste incineration is scientific and reasonable.
The effect on the environment:
(1) The destruction of available resources in garbage during incineration is a waste of resources;
(2) Incineration treatment has certain requirements for the low calorific value of garbage, and no garbage can be incinerated;
(3) The flue gas generated by incineration must be purified, and the purification technology is difficult and the operation cost is high;
(4) One-time investment in incineration equipment is high and the operating cost is high.
Organic solid waste → gas (H2, CH4, CO, CO2) + liquid (organic acid, aromatic hydrocarbon, tar, etc.) + solid (carbon black, slag, etc.)
Economic and technical analysis: Pyrolysis is a combustion process under anoxic or anaerobic conditions. It is an endothermic decomposition reaction under low electrode potential reduction conditions, also known as dry distillation or carbonization process (gas engineering, and coking is heat) Solution process).
The difference between pyrolysis and incineration can be summarized in the table below.
|Thermal effect||Exothermic, oxidizing||Endothermic, reduction|
|Reaction product||Carbon dioxide, water||Combustible low molecular compound|
|Release method and application||The heat generated can only be used nearby (power generation, heating water or steam generation)||Produces fuel oil and gas, can be stored and transported over long distances|
Environmental impact: It converts organic matter from solid waste into storage energy such as fuel gas, fuel oil, and carbon black. Pyrolysis is an anoxic decomposition that produces less exhaust gas and, accordingly, less exhaust gas, which helps to reduce environmental air pollution. Most of the harmful components such as sulfur and heavy metals in the waste are fixed in the carbon black. The trivalent chromium is not converted to hexavalent chromium because it is maintained under reducing conditions. The amount of nitrogen oxides produced is small.
In order to make urban garbage disposal resources, the following points should be achieved:
(1) Strengthen the classification and recycling of municipal waste. The classification and recycling of municipal waste is an important prerequisite for realizing urban waste recycling. The mixed recycling of urban garbage has increased the difficulty of harmlessness, reduction and resource treatment of garbage, resulting in a waste of a large amount of resources. In order to promote the classification and recycling of garbage, garbage containers with different signs should be placed in residential areas, commercial, cultural and entertainment centers, and corresponding systems and reward and punishment measures should be formulated to ensure that residents consciously classify and place garbage to realize classified collection of garbage.
(2) Strengthen the recycling of waste materials and promote the development of the old goods market.
(3) Strengthen the management of urban garbage resources. To recycle urban garbage, we must first change the garbage management system of government and enterprise integration, and establish a municipal solid waste management system that is compatible with the market economy. Secondly, we must establish an information management system for urban waste recycling and improve management capabilities. Technicalization of urban waste treatment to improve the technical level of harmless, reduced and resourced urban waste treatment
The traditional landfill method takes up land and forms secondary pollution, which does not meet the requirements of harmlessness. Incineration costs are high and resource utilization is low. Urban waste has complex composition, high water content and less combustibles. Therefore, the garbage disposal is suitable for the comprehensive treatment technical route of garbage sorting, recycling, organic fermentation, fertilizer, and combustibles for heat generation. The comprehensive treatment technology of waste recycling involves many fields such as machinery, chemical industry, biology, electronics, etc. Therefore, it is more important to improve the technology integration capability.
(4) Vigorously develop urban waste recycling technology. Faced with the challenge of garbage, many scientists have suggested that high-tech should be applied in full, turning waste into treasure and benefiting from garbage. Science and technology are the primary productive forces, and the recycling of urban waste is largely related to resource-based technologies.
Advances in science and technology can increase the types of wastes that can be utilized and their utilization depth, and improve the comprehensive utilization rate, thereby promoting the rapid development of the waste resource industry. Therefore, it is very necessary to strengthen the research of garbage resource technology and provide hardware support for urban garbage resource.