For municipal solid waste, the United States is the country with the highest proportion of geological landfill, reaching 95%. Since 1930s, more than 65,438+0,400 cities in the United States have used landfill to treat garbage. By the early 1990s, there were as many as 75,000 landfills in the United States. At that time, there were 2,200 geological waste landfills in Canada. In addition, Britain, Australia, New Zealand, Norway, France, Denmark, Germany, Italy and other countries have established a large number of geological landfills. Governments, scientific research institutions and institutions of higher learning in these developed countries attach great importance to the study of methods and theories of geological landfill. Through the analysis of a large number of documents, it can be found that the research and development history of landfill disposal methods and theories can be divided into the following stages:
1. Early
In the 1940s and 1950s, people didn't realize the seriousness of garbage's harm to the environment and human beings, and the geological landfill method was rarely used. Garbage is mainly piled up in the open air, and the site selection is mainly based on convenient transportation conditions, and the suitability of topography, resource protection and geological environment conditions is rarely considered.
2. Primitive period
During this period, from the 1960s to the early 1970s, simple open-air garbage dumping caused serious environmental pollution and destruction. Especially after 1972 Stockholm held the "Human Environment Conference", environmental problems have attracted worldwide attention, and the impact of garbage stacking or simple landfill on the environment has aroused widespread vigilance and attention. At this stage, when selecting the disposal site, geological, hydrogeological, engineering geological, environmental geological and other conditions become the objects of investigation, analysis and evaluation. For example, Stewar et al. (1970) analyzed the hydrogeological factors affecting the Hillshorough landfill in Florida, Brown( 1973) evaluated the geological conditions of Whiteman landfill, the impacts of Matin( 1975) and Lee County landfill on surface water, and Ronsin (1. The main feature of this stage is the qualitative analysis and evaluation of the website.
3. Research stage of geological environment impact mechanism
This stage started from the middle and late 1970s to the mid-1980s. With the continuous development of the initial work, the interaction and influence between site waste and geological environment are gradually exposed and have entered this stage. The main characteristics of this stage are: focusing on the interaction mechanism between a single pollution component and mineral components in groundwater or strata. Methods and means include geophysical exploration, isotope method, tracer technique, solute transport analytical model, numerical model, zero flux surface calculation and so on. Methods From qualitative to quantitative. For example, Gureghian, A.B et al. successfully studied the migration of Cl- in groundwater of Long Island Geological Disposal Site in the United States with finite element method in 198 1. 1982, Sykes, J.F and others published the simulation research results of organic matter degradation in garbage disposal site for the first time. Based on the consideration of convection and diffusion and focusing on the process of microbial action, the prediction model of COD migration and transformation in leachate is established. 1984, 1986, AC. demetracop, etc. The sensitivity of unsaturated infiltration to landfill is simulated and analyzed, and the specific measures to collect leachate at the bottom of the site under different conditions are pointed out. In 1984, Gerhart, R.A. developed a set of methods for collecting vadose zone leachate, and conducted experiments in layered and non-layered soils. These results are of great significance to the study of geological disposal of garbage.
4. Comprehensive and systematic research stage
Since the late 1980s, the research on geological disposal sites has been more comprehensive, in-depth and systematic. Specific performance in:
(1) The design of the disposal site is more systematic and perfect. The most representative one is the paper "Geo-evaluation of geotechnical engineering barriers for garbage disposal engineering" published by M. Langer in Germany (1994+0994). This paper points out that a permanent and perfect geological disposal site must have a geotechnical barrier corresponding to the geological barrier. In addition to engineering geology, hydrogeology and environmental geology, there are also geotechnical engineering safety analysis (geotechnical bearing capacity, multi-layer pathogen-free evaluation, earthquake and structure analysis, hydrogeological simulation, slope stability analysis, etc.). ) and safety plan (measures to reduce or avoid danger, methods to prevent rock and soil collapse and site damage, etc.). ) It should be made. Based on a large number of indoor and outdoor tests and observations, the engineering safety is evaluated. At the same time, it is necessary to observe the operation and disposal site for a long time and implement a series of safety protection measures.
(2) The study of site geological environment impact assessment is comprehensive, systematic and quantitative. Since the mid-1980s, some scholars have begun to comprehensively study the geological and environmental effects of waste geological disposal sites. For example, 1984, the "hydraulic evaluation model of geological landfill" developed by American scholar Schroeder and others, that is, the HELP model, grasps the key factors of the influence of landfill leachate on geological environment, gives the estimation method of leachate leakage under different structural combinations, and comprehensively calculates the transformation of site water. 1988 Peyton, R.L. et al. verified the reliability and application scope of HELP model through the long-term simulation experiment of 17 geological landfill, and Hollings-Head, S.C. et al. (1989) used this model to analyze the factors such as clay aquiclude thickness, permeability coefficient and topsoil. Freeze, R.A. and Massman, J. et al. (1990, 199 1, 1992) introduced the decision analysis method into this field. This method is based on the risk theory in engineering design and aims at system engineering optimization. The coupling problem based on risk-cost-benefit target decision-making model, groundwater flow and solute transport model and uncertainty model is studied. The research results are of great significance for understanding the impact of garbage on geological environment and guiding the optimal design, rational operation and management of garbage landfill. Some scholars have also comprehensively studied organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter, organic matter.
(3) The research on the properties of barrier materials in geological disposal sites is increasingly active. Scholars have studied the interaction and transformation mechanism of garbage pollutants with geological minerals and groundwater, and also studied the properties of pollutant barrier materials. For example, Canadian scholar D.A.Dixon( 1992) studied the pollutant barrier ability of bentonite from the aspects of permeability coefficient, adsorption performance, desorption ability and pore water migration law. Roland Pushch (1989), another scholar, studied the appropriate amount of additives suitable for the hydraulic and chemical properties of landfill liner clay, so as to enhance the adsorption performance and ion exchange capacity, change the critical hydraulic gradient or initial hydraulic gradient of pore water in soil, and improve its anti-seepage ability. Beeman( 1987)' s article "Study on Microbes Contaminating Shallow Aquifers by Landfill Leachate" comprehensively considers the degradation characteristics of organic matter under the joint action of microorganisms.
(4) The application of computer artificial intelligence system in this field. 199 1 year, Modymont, C.L and others applied the knowledge system of artificial intelligence to the evaluation of the impact of landfill on groundwater environment; C.Irigar et al. (1994) introduced in detail the application of GIS in geotechnical engineering design and environmental assessment in the site selection of municipal waste treatment plant in Granada, Spain.
It should be pointed out that the above stages are only divided according to their main characteristics, and they are not completely separated. The mutual penetration of the research in each stage reflects the trend and process of the development of the research on geological disposal of garbage from primary to advanced.
Second, the research status of geological disposal in China
Compared with developed countries, there is still a big gap in the research on geological disposal of garbage in China.
During the "Seventh Five-Year Plan" period, China carried out pilot research on the harmless treatment technology of urban garbage nationwide. Based on the advanced experience of developed countries, various pilot projects such as geological landfill, composting, incineration and resource utilization are comprehensively analyzed and evaluated in economy and technology, and it is confirmed that geological landfill disposal technology is one of the most practical disposal technologies suitable for China's national conditions. 199 1 in September, the Ministry of Construction and the Science and Technology Commission formally determined that the geological landfill disposal technology was the first choice for China's recent promotion. Hangzhou, Shanghai, Tianjin, Guangzhou and other cities have carried out geological landfill of municipal solid waste, and whether to establish geological landfill of municipal solid waste is regarded as an important symbol of national sanitary city. This indicates that the geological disposal of urban garbage in China has entered a new stage. Its research status and development trend can be summarized as follows.
Generally speaking, China's current research level in this field is equivalent to the second or third stage of foreign research, but some research results have reached the fourth stage of foreign research.
During the period of 1988, Shanghai Environmental Protection Research Institute compiled the environmental impact reports of Jiangzhen and Laogang garbage dumps in Shanghai, covering geological, hydrogeological conditions, geological structure characteristics, garbage composition and numerical simulation calculation of leachate. 199 1 years ago, the published papers were mainly literature review and summary. Ru Lin (1984), Huang Mingmin (1988), Zhang He Zheng Xixin (1988), Meng Yuee (1990), Liang (199/kloc-). Liu Changli (199 1) introduced the site selection principle, environmental geological investigation, sampling analysis, monitoring design, geochemical process and simulation, main environmental geological problems caused by landfill and groundwater pollution control in foreign countries. Since 1990s, some concrete preliminary research results have begun to appear. For example, Zuo (1990) and (199 1) respectively introduced the simulated experimental results of the composition characteristics and variation laws of leachate after dynamic compaction in Guocikou Landfill of Tianjin Ma Steel Plant and Wuhan Environmental Sanitation Institute. In addition, some graduate students began to take this research as their dissertation.
Compared with the domestic scientific research achievements that can represent the research level in this field at present, the key scientific and technological projects of the Ministry of Geology and Minerals in the Eighth Five-Year Plan include the impact assessment of garbage dumps in Pudong New Area of Shanghai, the suitability assessment of garbage dumps in selected areas in Pudong New Area of Shanghai, the geochemical effects and mechanism of urban garbage dumps supported by the National Natural Science Foundation of China (1995 ~ 1997), and the research on the barrier capacity of cohesive soil cushion in garbage landfills supported by the National Natural Science Foundation of China. The main progress of these research projects is as follows:
(1) A quasi-two-dimensional joint mathematical model of organic nitrogen, phosphorus, phosphorus and chloride ion in landfill leachate was established by using physical model simulation experiment, and the environmental impact degree and speed of landfill site were simulated, calculated and predicted.
(2) The migration, transformation and interaction mechanism of microorganisms to nitrides and some metal pollution elements in the landfill-soil system, and the nitrification, denitrification and denitrification of nitrifying bacteria and denitrifying bacteria to COD were deeply and systematically studied. The conclusion is that all kinds of microorganisms in the landfill not only have the ability to remove organic matter and nitride, but also can purify some metal ions.
(3) In-depth, comprehensive and systematic research has been carried out on the environmental impact assessment methods of garbage disposal sites, and the environmental pollution of some garbage dumps has been analyzed and evaluated.
(4) Using the principles and methods of systems engineering, a set of comprehensive and systematic theories and methods for the optimization of waste geological disposal sites are explored.
(5) The migration law of pollutant solution in site soil in soil composed of different layers of pores was studied, and the calculation method of the distance between waste disposal site and surface water and groundwater, the thickness and density of clay liner at the bottom of site was obtained by using the critical hydraulic gradient of pore water.
(6) The barrier ability of various clays (clay, heavy clay, silty clay) and special soils (weathered coal, peat, bentonite) and their mixed materials in different proportions to various pollutants and the laying methods of various materials' liners are deeply and systematically studied, and the laying ideas, theories and methods of liner systems suitable for China's national conditions are put forward.
(7) Different landfill modes and management modes suitable for southern and northern landfills are obtained.
(8) The benefit engineering theory and method of geological landfill disposal are put forward. The core and goal of benefit engineering theory is to simultaneously consider economic benefits, environmental benefits and social benefits. From the selection of disposal site of municipal solid waste landfill, the treatment of unfavorable geological problems in the site, the design of landfill project, the laying of impervious liner system to the construction of the project, the treatment of leachate, the final development and utilization of landfill, etc. The analytic hierarchy process (AHP) and optimization theory of system engineering are adopted to realize the optimal allocation and perfect unity of economic benefits, environmental benefits and social benefits.
The overall level of these research results is in a leading position in China, and some of them have reached the international advanced level.
To sum up, the research hotspots and development directions in this field at home and abroad at this stage are as follows:
(1) Migration law and physical, chemical and biochemical mechanism of waste pollutants in site soil and groundwater.
(2) Regulation mechanism of microorganisms on accumulation, degradation and dissipation of pollutants in groundwater and vadose zone.
(3) The purification mechanism and barrier ability of different liner materials to waste pollutants.
(4) Theories and methods of biochemical treatment of garbage pollution.
(5) Because of China's large population and huge amount of garbage, the phenomenon of garbage besieged the city and endangering the environment is very common in large and medium-sized cities across the country. Therefore, with the support of the government, powerful scientific research institutions in China are evaluating the impact of garbage on the environment, and choosing the planning and location of landfill sites in some megacities (such as Beijing). With the continuous improvement of the country's economic strength, such work will be gradually carried out in other cities.