城市生活垃圾填埋場畢業(yè)設(shè)計翻譯

1、Environmental Impacts of Solid Waste LandfillingAbstract:Inevitable consequences of the practice of solid waste disposal in landfills are gas and leachate generation due primarily to microbial decomposition, climatic conditions, refuse characteristics and landfilling operations. The migration of gas

2、 and leachate away from the landfill boundaries and their release into the surrounding environment present serious environmental concerns at both existing and new facilities. Besides potential health hazards, these concerns include, and are not limited to, fires and explosions, vegetation damage, un

3、pleasant odors, landfill settlement, ground water pollution, air pollution and global warming. This paper presents an overview of gas and leachate formation mechanisms in landfills and their adverse environmental impacts, and describes control methods to eliminate or minimize these impacts. Keywords

4、: landfill, solid waste disposal, biodegradation, gas and leachate generation, environmental impacts, control methods1.Introduction Solid waste disposal in landfills remains the most economic form of disposal in the vast majority of cases (Thompson and Zandi, 1975; Rushbrook, 1983; Carr and Cossu, 1

5、990). Therefore, landfills will continue to be the most attractive disposal route for solid waste. Indeed, depending on location, up to 95% of solid waste generated world- wide is currently disposed of in landfills (Bingemer and Crutzen, 1987; Cossu, 1989; Nozhevnikova et al., 1992; Gendebien et al.

6、, 1992). Alternatives to landfilling are considered as volume reduction processes because they produce waste fractions (e.g.ashes and slag from combustion processes that represent the second leading method of waste disposal) which ultimately must be landfilled (Emberton and Parker, 1987). Resorting

7、to landfills is not limited to the disposal of municipal solid waste, but it icludes most other industrial wastes. For instance, nearly 80% of hazardous wastes generated in the U.S. is dumped in landfills (Eichenberger et al., 1978). Solid waste composition varies substantially with socio-economic c

8、onditions, location, Most organic materials are biodegredable and can be broken down into simpler compounds by aerobic and anaerobic microorganisms, leading to the formation of gas and leachate. The following sections provide an overview of the mechanisms of gas and leachate formation in landlls, th

9、eir environmental impacts, and appropriate control methods to eliminate or minimize these impacts.2. Landll Gas Formation MechanismsAt the time of waste deposition in a landll, oxygen is present in the void space, giving rise to aerobic decomposition during which biodegradable organic materials reac

10、t quickly with oxygen to form carbon dioxide, water, and other by-products (e.g. bacterialcells). Carbon dioxide is produced in approximate molar equivalents to the oxygenconsumed. Oxygen depletion within the landll marks the onset of the anaerobic decompositon phase. Although a landll ecosystem und

11、ergoes an initial short aerobicdecomposition phase, the subsequent anaerobic phase is the dominant phase in its age and the more important one from the perspective of gas formation.Much of what is known or assumed concerning anaerobic processes in landlls has primarily come from work with anaerobic

12、digesters. Microbial populations in both environments appear to be similar however, the major dierence is that the substrates may vary in their relative content of fat, protein, and carbohydrates, and conversely to landlls, the environment in anaerobic digesters is well controlled and often under op

13、timal conditions.Investigators have recognized several major steps to describe the anaerobic de-composition phase during which organic materials are converted to methane and carbon dioxide (Alexander, 1971; Zehnder, 1978; Wolfe, 1979; McCarty 1981; Zehnder et al.,1982; Mosey, 1983; Archer and Robert

14、son, 1986; Balba, 1987). These steps are highly inter-dependent and include hydrolysis, acidogenesis, acetogenesis, and methanogenesis(Figure 1).Generally, the breakdown of organic matter in anaerobic ecosystems proceedssequentially from the complex to the simple starting with the hydrolysis of comp

15、lex particulate matter to simpler polymers like proteins, carbohydrates and lipids which are further hydrolyzed to yield biomonomers like amino acids, sugars, and high molecular fatty acids. Amino acids and sugars are converted into either intermediate by-products (e.g. propionic, butyric and other

16、volatile acids) or directly fermented to acetic acid. High molecular fatty acids are oxidized to intermediate by-products and hydrogen. Methane and carbon dioxide generation occurs primarily through acetate cleavage. Methane is also produced through carbon dioxide reduction with hydrogen.In a landll

17、 environment, methane generation from the latter route is often limited bythe lack of hydrogen which is consumed by sulfate reducers (Kasali, 1986).Qualitatively, landll gas is highly dependent on the decomposition stage within the landll (Rovers and Farquhar, 1973; Rees, 1980; Pohland et al., 1983;

18、 Barlaz et al.,1989c). Under a stabilized methanogenic condition which is the stage of interest from a benecial recovery perspective, methane and carbon dioxide are by far the two principal components of landll gas and form more than 90% of the total gas generated.Nitrogen and oxygen are normally pr

19、esent in small quantities primarily as a result of air entrapment during waste deposition, atmospheric air diusion through the landll cover especially in the near surface layers, or air intrusion from negative landll pressure when landll gas is extracted. 3. Leachate Formation MechanismsLeachate for

20、mation is the result of the removal of soluble compounds by the non-uniform and intermittent percolation of water through the refuse mass. Soluble compounds aregenerally encountered in the refuse at emplacement or are formed in chemical and biological processes. The sources of percolating water are

21、primarily the precipitation,irrigation, and run o which cause inltration through the landll cover; ground water intrusion, and to a lesser extent, the initial refusemoisture content.Refuse decomposition due to microbial activity may also contribute to leachate formation but in smaller amounts. The q

22、uantity of leachate generated is site-specic and a function of water availability and weather conditions as well as the characteristics of the refuse, the landll surface, and underlying soil.The quality of landll leachate is highly dependent upon the stage of fermentation in the landll, waste compos

23、ition, operational procedures, and co-disposal of industrial wastes (Hoeks and Harmsen, 1980; Parker and Williams, 1981; Harmen, 1983; Pohland et al., 1983). Many chemicals (e.g. metals, aliphatics, acyclics, terpenes, and aromatics) have been detected in landll leachate from domestic, commercial, i

24、ndustrial, and co-disposal sites. 4. Environmental ImpactsHistorically, landlls were initiated largely as a result of a need to protect the environment and society from adverse impacts of alternative methods of refuse disposal such as open-air burning, open-pit dumping, and ocean dumping (Senior, 19

25、90). Although landlls eliminated some impacts of old practices, new ones arose, primarily due to gas and leachate formation. Besides potential health hazards, these concerns include res and explosions, vegetation damage, unpleasant odors, landll settlement, ground water pollution, air pollution and

26、global warming.4.1 Fire and Explosion Hazards Although landll gas rich in methane provides an energy recovery opportunity, it has often been considered to be a liability because of its ammability, its ability to form explosive mixtures with air, and its tendency to migrate away from the landll bound

27、aries by diusion and advection. Diusion is the physical process that causes a gas to seek a uniform concentration throughout the landll volume, hence the gas moves from areas of higher to areas of lower concentration. Advection results from pressure gradients where gas moves from zones of higher to

28、zones of lower pressure. Diusion and advection rates depend primarily on the physical properties and generation rates of the landll gas, refuse permeability, internal landll temperature, moisture content surrounding soil formation and changes in barometric pressure.4.2.Vegetation Damage At closure,

29、many landll sites are converted to parks, golf courses, agricultural elds, and in some cases, commercial developments. Vegetation damage at or nearby to such sites is well documented in the literature (Flower et al., 1977, 1981; Leone et al., 1977; Leone and Flower, 1982; Gilman, 1980; Gilman et al.

30、, 1981, 1982, 1985; Arthur et al.,1985). The damage occurs primarily due to oxygen deciency in the root zone resulting from a direct displacement of oxygen by landll gas. In the absence of a gas control measure, landll gas can migrate upward due to concentration and pressure gradients, and escape in

31、to the atmosphere by venting through the landll cover. During this process, oxygen is displaced and plant roots are exposed to high concentrations of methane and carbon dioxide, the two major constituents of landll gas. The lack of oxygen causes the death of plants of asphyxia.4.3. Unpleasants Odors

32、 Odors are mainly the result of the presence of small concentrations of odorous constituents (esters, hydrogen sulde, organosulphurs, alkylbenzenes, limonene and other hydrocarbons) in landll gas emitted into the atmosphere (Young and Parker,1983, 1984). The odorous nature of landll gas may vary wid

33、ely from relatively sweet to bitter and acrid depending on the concentration of the odorous constituents within the gas. These concentrations will vary with waste composition and age, decomposition stage and the rate of gas generation, and the nature of microbial populations within the waste, amongs

34、t other factors. Although many odorous trace compounds may be toxic, they have historically been perceived more as an environmental nuisance than as a direct health hazard (Young and Parker, 1984; Young and Heasman, 1985). The extent to which odors spread away from the landll boundaries depends prim

35、arily on weather conditions (wind, temperature, pressure, humidity).4.4 Ground Water Pollution Leachate occurrence is by far the most signicant threat to ground water. Once it reaches the bottom of the landll or an impermeable layer within the landll, leachate either travels laterally to a point whe

36、re it discharges to the grounds surface as a seep, or it will move through the base of the landll and into the subsurface formations. Depending upon the nature of these formations and in the absence of a leachate collection system, leachate has reportedly been associated with the contamination of aq

37、uifers underlying landlls which resulted in extensive investigations for the past four decades (Zanoni, 1972; Walls, 1975; Dunlap et al., 1976; Kelly, 1976; MacFarlane et al., 1983; Cheremissino et al., 1984; Reinhard et al., 1984; Ostendorf et al., 1984; Mackay et al., 1985; Albaiges et al., 1986;

38、Mann and Schmadeke, 1986). In fact, it is speculated that in the U.S.,contamination by municipal landlls, to which every household contributes more than a gallon of hazardous wastes per year (Lee et al.,1986), could become a bigger problem than contamination associated with the sole disposal of haza

39、rdous wastes in landlls (Senior, 1990). Currently, it is estimated that over 25% of the Superfund sites listed on the National Priority List are solid waste landlls (Arigala et al., 1995). 4.5. Air Pollution Although methane and carbon dioxide are the two major components of the gas emitted from lan

40、dlls, there is evidence that this gas contains numerous other constituents in trace amounts signicant enough to cause environmental and health concerns(Lytwynyshyn et al., 1982; Young and Parker, 1983; Karimi, 1983; Gianti et al., 1984; Harkov et al., 1985; Todd and Propper, 1985; Young and Heasman,

41、 1985; Wood and Porter, 1986; Rettenberg, 1984, 1987). Potential emissions of Volatile Organic Compounds (VOCs) from landlls can range from 4×104 to 1×103 kg/m2/day (US EPA, 1989).4.6 Global Warming. Atmospheric gas emission rates through a landll cover have been measured by several invest

42、igators. During dry soil conditions at a semi-arid landll site, Bogner et al. (1989) indicated that methane and carbon dioxide uxes may be as high as 630 and 950 kg/ m2/yr, respectively. Using ux box measurements, Lytwynyshyn et al. (1982) and Kunz and Lu (1979, 1980), estimated that methane diusion

43、 ux through landll covers ranged between 390 and 1200 kg/m2/yr. These measurements are likely to underestimate actual emission rates because of aerobic oxidation of methane near the surface by methanotrophs. Although emission rates from controlled experiments may not be representative of actual emis

44、sions from landlls, they clearly demonstrate the propensity of gas release into the atmosphere.5. Landll gas and leachate controlLandll gas control measures are essential in order to eliminate or minimize its associated adverse environmental impacts. In most cases the installation of a gas recovery,

45、 collection and treatment system will assist in preventing gas migration away from the landll boundaries or gas emissions through the landll surface. Indeed many of the early gas recovery projects were developed as a consequence of, or as an adjunct to, existing gas migration control schemes. When l

46、andll gas is recovered appropriately its methane content represents an energy reservoir of great potential. It is estimated that annual gas generation potential in the US alone exceeds 6 billion m3 (Dawson, 1981;Marchant,1981). The energy represented by this gas could meet 1% of the total energy nee

47、ds or5% of the natural gas utilization in the U.S. (Lytwynyshyn et al., 1982). Reported estimates of worldwide annual gas generation potential vary widely, 30 to 430 billion m3 (Bingemer and Crutzen, 1987; Lagerkvist, 1987; Dessanti and Peter, 1984; Sheppard et al., 1982). The upper range is questio

48、nable, particularly when compared with methane yield data from actual landlls with a recovery system.The economic feasibility of landll gas recovery, processing, and utilization have indeed been demonstrated and reported by many investigators at sites under dierent climatic conditions (Boyle, 1976;

49、Lockman, 1979; Kaszynski et al., 1981; EMCON, 1983; Mouton, 1984; Wiqwist, 1986; Gendebien et al., 1992). New landlls can be designed to prevent landll gas accumulation even if no productive use of the gas is planned. Landll gas control systems have been well documented in engineering practice (Weis

50、s, 1974; Stone, 1978a,b; Goleuke, 1980; Shen, 1981; Stearns and Peyotan, 1981;EMCON, 1983; Pacey, 1984; Ghassemi et al., 1986; Walsh et al., 1988; Shen et al., 1990). In addition to gas recovery and active gas pumping, control measures include:(1) the installation of impermeable barriers before site

51、 operations to secure the perimeter of the landll (cement walls, clay trenches, impervious liner materials such as plastics, rubber, asphalt, polyvinyl chloride, high density polyethylene, etc.); (2) passive ventingconsisting of a trench installed beyond the landll boundary and backlled with coarse

52、material (e.g. gravel) to create a zone of high permeability which would be preferentially used by the gas; (3) a hybrid system consisting of any combination of impermeable barriers and an active or passive system (Alzaydi, 1980). Injection of lime slurry and y ash has also been reported to control

53、methane formation and stabilize landlls by inhibiting methanogenesis and stopping landll gas generation (Kinman et al., 1988).Leachate composition can be controlled to a limited extent by close monitoring and sorting of landll waste. However, decomposition byproducts dissolved in inltrating water wi

54、ll result in a leachate with elevated concentrations of numerous hazardous chemicals. Leachate treatment is often necessary to reduce these concentrations to levels that meet regulatory requirements. Most biological, physical and chemical processes used for the treatment of industrial wastewater hav

55、e been tested for treatment of landll leachates (Pohland and Harper, 1986). The selection of a particular treatment process will depend on the quality and strength of the leachate. 6. Summary and conclusionsGas and leachate generation are inevitable consequences of the practice of waste disposal in

56、landlls.Microbial decomposition, climatic conditions, refuse characteristics and landlling operations are amongst the many factors contributing the gas and leachate generation at landll sites. The migration of gas and leachate away from the landll boundaries and their release into the surrounding en

57、vironment present serious environmental concerns at both existing and new facilities including potential health hazards, res and explosions, damage to vegetation, unpleasant odors, landll settlement,ground water pollution, air pollution and global warming. An overview of gas and leachate formation m

58、echanisms in landlls and their associated adverse environmental impacts was presented and a description of control methods to eliminate or minimize these impacts was provided. In most cases the installation of a gas recovery, collection and treatment system will assist in preventing gas migration aw

59、ay from the landll boundaries or gas emissions through the landll surface. Hydraulic barriers (e.g. extraction and relief wells, gradient control wells and trenches) and collection systems are commonly used to control leachate problems.7ReferencesAbriola, L. and Pinder, G. F. (1985). A multiphase approach t