Secondary pollutants produced by microbiological and chemical reactions of the waste inside the landfill mainly include landfill gas and leachate.
1.1 Landfill gas
Landfill gas refers to the gas generated by the anaerobic digestion and degradation of microorganisms during the landfill process of a large amount of organic matter contained in municipal garbage that has been piled up or landfilled. Landfill gas is a mixture of gases, the generation and release of which are determined by the components of the garbage, the moisture status of the landfill, the temperature of the garbage, the pH value, the meteorological conditions, the age of the garbage, the construction of the landfill and the environmental geology and other conditions.
Landfill gas has various release pathways: on the one hand, it can diffuse to the atmosphere in an unorganized manner through the surface of the landfill; on the other hand, the landfill gas can migrate and diffuse horizontally and horizontally through the geologic formations in the subsurface rock and soil, such as fissures, etc. to the surrounding areas of the landfill, so that the landfill gas can migrate to the place farther away from the landfill before it is released into the atmosphere.
The composition of landfill gas is mainly divided into three categories:
(1) methane and carbon dioxide, the total volume of landfill gas accounted for 95% to 99% of the total volume of landfill gas, of which methane accounted for 50% to 70%, carbon dioxide accounted for 30% to 50%. According to statistics, each ton of garbage can produce about 100 to 200m3 of landfill gas during the life of the landfill. Landfills in the United Kingdom emit 2.2 million tons of methane to the outside annually, and landfills in the United States emit 11.6 million tons of methane to the outside annually №1. Methane and carbon dioxide are the main greenhouse gases, and the greenhouse effect they produce will have a great impact on the global environment, and the potential greenhouse effect of the equivalent volume of methane in them is 21 times that of carbon dioxide. With the growth of waste generation, the share of methane emissions from landfills in China's total greenhouse gas emissions will increase from 3.38% in 2000 to 7.19% in 2020, and the control and utilization of landfill gas has become an important issue in the field of municipal waste management in China. Carbon dioxide, which is 1.5 times denser than air and 2.8 times denser than methane, has a tendency to move toward the bottom of the landfill and may eventually accumulate at the bottom of the landfill.
(2) sulfide, ammonia and nitrogen, etc., the content of these gases accounted for about 5% of the total volume of landfill gas; these gases are the main cause of malodor in the atmosphere around the landfill, the air as long as it contains a volume concentration of 1 × 10-5 hydrogen sulfide, 2 × 10-5 sulfur dioxide or 2 × 10-1 mercaptan, people will be able to smell the malodor. The scope of malodor pollution caused by landfills is generally within the 2km zone, and the scope of malodor can reach more than 6km under unfavorable inverse temperature conditions. The survey results of 329 urban waste disposal sites in China show that the landfill site's unorganized emissions of hydrogen sulfide in the exhaust gas exceeded the standard rate of 7.6%, exceeding the multiplier of 0.5 to 24.
(3) trace gases, these trace gases are diverse and complex, mainly including some trace volatile organic compounds (volatile organic compounds, VOCs), which can be detected in the landfill. compounds, VOCs), such as aromatic, aliphatic hydrocarbons and other trace gases, totaling more than 100 species. Young et al. measured trace VOCs in the air of three landfills in the United Kingdom, *** detected 154 kinds of compounds, of which 116 can be detected in the landfills. The California Atmospheric Resources Commission (CARC) in the United States investigated 66 landfills, all of which detected toxic and hazardous organic pollutants such as benzene, toluene, trichloroethylene, chloroform and vinyl chloride. Zou Shichun et al. analyzed qualitatively and quantitatively the VOCs in the atmosphere of a landfill site in Guangzhou, and the results showed that of the 60 trace gases detected, 17 were priority control pollutants of the U.S. Environmental Protection Agency (USEPA), and benzene and alkylbenzene compounds had the highest concentrations, ranging from 2.54 to 1508.48 g/m3. Although the volume percentage of trace gases in landfill gas is less than 1%, these gases are more volatile and toxic, and their potential toxicity to the environment is more serious, and their potential toxicity has aroused widespread concern. #p#Page title #e#
The distribution pattern of the concentration of hazardous gases in the air of the landfill site is roughly as follows: the new landfill area is higher than the landfill area, the site is higher than the off-site, the low terrain is higher than the high terrain, and the downwind place is higher than the upwind place.
The release of landfill gas adversely affects the environment around the landfill, and its potential ignition and explosion hazards are extremely harmful and threatening to the health and safety of landfill workers. In recent years, there have been several accidents of landfill gas explosion and combustion in landfill sites causing personal injury or death.
1.2 Landfill leachate
Landfill leachate is a kind of sewage containing high concentration of suspended solids and high concentration of organic or inorganic constituents filtered out from garbage in the process of stacking and landfilling by biological and chemical degradation, showering and flushing of rainwater, as well as soaking of surface water and groundwater, and the specificity of its formation leads to the complexity of its water quality.
The organic matter in the waste leachate can be divided into low molecular weight fatty acids, medium molecular weight fulvic acid-like substances and humus-like high molecular weight carbohydrates. At present, many scholars at home and abroad have analyzed the composition of organic matter in leachate, the results show that the percentage of organic pollutants in leachate is complex, including alkanes, aromatic hydrocarbons are more, in addition, there may also be a number of acids, alcohols, phenols, ketones, aldehydes, amides, etc., many of the components of the past nature has never been the emergence of synthetic organic compounds, a considerable number of compounds for carcinogenicity, carcinogenicity, co A considerable number of these compounds are carcinogenic, pro-carcinogenic and co-carcinogenic toxic organic pollutants. Especially when domestic waste is mixed with some industrial waste, the composition is more complicated. Zheng Manying et al. sampled and analyzed the Datanshan landfill in Guangzhou, and 77 main organic pollutants were detected in the leachate, including 29 aromatic hydrocarbons, 18 alkanes and olefins, 8 acids, 5 esters, 6 alcohols and phenols, 4 ketones and aldehydes, 2 amides, and 5 others. These 77 organic pollutants contained one identified carcinogen, which was the highest relative amount of the 77 organic pollutants, and five carcinogens or co-carcinogens.Pirbazari et al. conducted a study on the West Covina landfill leachate, which also detected 18 specific organic compounds.
Heavy metals are mostly present in leachate in the form of compounds or compatibilizers, which are closely related to the pH of the effluent and the concentration of organic matter in the water. Generally speaking, the concentration of metal ions in the leachate of landfills of domestic waste is usually low, but for landfills with mixed landfills of industrial and domestic waste, the leaching of heavy metal ions increases significantly.Jensen et al. investigated and analyzed the content of heavy metals in the leachate of four landfills in Denmark, and the results were as follows: Cadmium (0.2-3.6 μg/L), Nickel (28-84 μg/L) , zinc (85-5310 μg/L), copper (2-34 μg/L), chromium (0-188 μg/L), lead (0-16 μg/L). Investigations showed that most of the heavy metals were bound to colloids in the leachate, and different heavy metals were bound in different proportions. Usually the most biotoxic are free state ions (organic mercury, etc. are exceptions). China's garbage leachate treatment research started late, the special research on heavy metal ions in leachate is even less, a small amount of literature data varies greatly, which is related to China's vast geographical area, while the collection of garbage landfill is not standardized, the garbage contains different industrial waste and other factors.
The garbage contains a large number of microorganisms, and the types of microorganisms in the leachate are basically the same as those contained in the landfill garbage, which mainly contain seven types of bacteria, such as nitrosating and nitrifying bacteria, denitrifying bacteria, desulfurizing bacilli, dezincobacteria, ferro-bacteria, sulfate-reducing bacilli, and methanotrophic bacilli. In addition, leachate contains a large number of pathogenic bacteria and disease-causing microorganisms such as Salmonella spp. Viruses that may cause infectious diseases, such as Coxsackie virus, can also be detected in hospital waste illegally landfilled.
Solid waste, even toxic hazardous materials produced in the landfill process leachate contains a large number of toxic and hazardous substances, leachate to the ground for a long time seepage shabu training, such as not properly handled, will become serious pollution of the surrounding environment. Heavy metals in an old landfill in Wuhan, China, have a serious adverse effect on the environment, and the depth of heavy metal influence in the subsoil layer has reached 20 to 25 meters below the ground, and the concentrations of lead, mercury, cadmium, chromium, copper, and nickel have all increased considerably at this depth.
Hallbourg et al.'s study of groundwater and surface water at landfills in north-central Florida, USA, showed that there were large amounts of aromatic organic components and undecomposed prioritized control organic pollutants from solid waste in the water column. There are approximately 18,500 landfills in the U.S., and almost half are polluting water bodies. The Rhine River area has shut down many waterworks due to groundwater contamination from landfill leachate. Nearly 90% of our landfills currently have no water barrier. For a long time, most of the sewage infiltrated from landfills seeps into the groundwater. The results of two rounds of national groundwater resources evaluation by the Ministry of Land and Resources from 1981 to 1984 and from 2000 to 2002 show that the scope of groundwater pollution in China is expanding, with the quality of groundwater generally declining in two-thirds of the country's cities, and water supply being strained in more than 300 cities as a result of groundwater pollution. Not only is the composition of groundwater pollutants becoming more and more complex, but the degree and depth of pollution is also increasing. In some areas, pollutants have been detected in deep groundwater. Some of the wells in the water source areas of Yantan and Mataan in Lanzhou, China, have been abandoned due to leachate contamination.
Waste seepage into the soil on the ecological environment will also cause long-term impact can not be underestimated. Residual toxic substances are not only difficult to volatilize and dissipate in the soil, but also can kill microorganisms in the soil, change the nature and structure of the soil, impede the growth and development of plant roots, and accumulate in the plant body.