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Trace gas compound emissions from municipal landfill sanitary sites
Pergamon
Atmospheric Environment Vol. 28, No. 2, pp. 285 293, 1994 © 1994 Elsevier Science Lid Printed in Great Britain. All rights reserved 1352-2310/94 $6.00+0.00
TRACE GAS C O M P O U N D EMISSIONS FROM MUNICIPAL LANDFILL SANITARY SITES JOSI~E BROSSEAU a n d MICH~LE HEITZ D6partement de g6nie chimique, Facult6 des sciences appliqu6cs, Universit6 de Sherbrooke, Sherhrooke (Qu6boc), J1K 2R1, Canada
(First received 16 July 1992 and in final form 13 June 1993) Abstract--The literature on certain aspects of trace gas compounds emitted from Municipal Landfill Sanitary Sites* is reviewed. Aspects covered are the formation, nature and origin of such compounds, as well as the problems caused by them. Risks posed to human health and the environment by even low concentrations of these compounds arc examined and methods to reduce and control them discussed.
Key word index: Control, health, migration, monitoring, VOC. INTRODUCTION
The composition of the domestic waste generated by North American society is extremely varied. By domestic waste is meant household and commercial waste disposed of in Municipal Landfill Sites (M.L.S.). In previous years, disposal of industrial waste, of various degrees of contamination, was nevertheless permitted at these sites. This can no longer be said: current legislation is much stricter with respect to the types and quantities of industrial waste permitted at M.L.S. In Canada, the legislation does not yet take account of gas effluent emitted from M.L.S., and, still less, of trace gas compound emissions at large landfill sites, where risk of such emissions is increased. Recently, in the United States of America certain concerns related to toxic landfill gas emissions have been brought to light. Questions were raised in recent studies which pointed to the fact that trace compounds are dispersed near ground level, where risk of exposure is found to be sufficiently serious for legislation reassessing control standards to be enacted in respect of the contaminants in question. Furthermore, the presence of trace compounds leads to problems which become evident when the landfill gas is recovered for use as an energy source. This paper presents a review of the literature relating to the nature, problems and management of trace gas compounds. TRACE GAS COMPOUNDS
Type of landfill gases generated by Municipal Landfill Sites The relative composition of the decomposition gas emitted from a Municipal Landfill Site (M.L.S.) may * Municipal Landfill Sanitary Sites: sites usually used for disposal of domestic waste. :~Montr6ai, Qu6bec, Canada.
vary from one site to another depending on the type of waste buried, the stage reached in the decomposition process as well as operating conditions at the site (i.e. covering of the landfill, etc.). Table 1, taken from Crutcher et al. (1988), lists the principal constituents of landfill gas (major constituents: methane, carbon dioxide, hydrogen, nitrogen and oxygen; minor constituents: trace gases) and gives the expected limits for each in percentages by volume (ranges given for both aerobic and anaerobic decomposition). By way of example, Table 2 presents the results of landfill gas analysis carried out at Palos Verdes, CA, M.L.S. (anaerobic decomposition stage). Specifically, this is a large landfill site where biogas--methane and carbon dioxide--is recovered for commercial purposes. If these results are compared to those related to the principal constituents referred to in Table 1 (anaerobic decomposition stage), they correspond to the expected averages. Concentrations of methane (53.3 %), as well as those of carbon dioxide (45.6%), hydrogen (0.06%), oxygen (0.07%) and nitrogen (0.3%) are in the range of those presented in Table 1 in relation to the anaerobic decomposition stage. The amount of trace gases is equivalent to more than 0.7% of the landfill gases total. The results dating from 1979 may cover a smaller variety of trace gases than those given in Table 1 (1988) where the composition of the trace gases is not mentioned. A further example is supplied by D6sourdyBiothermica Inc. (1989) which presents results related to the main components of landfill gas actively extracted from the "Centre de Tri et d'Elimination des D6chets de Montr6al ~;(C.T.I~.D.)" (Montr6al's Sorting and Waste Disposal Centre). The percentage by volume of methane is 60% and that of carbon dioxide is 37 %: these values are in agreement with those given in Table 1. Trace gases and hydrogen (included in the category "Other compounds") are present in very small quantities (0.1% of total volume).
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Table 1. Principal constituents of M.L.S. gas* Gas
Range of constituents resulting from aerobic and anaerobic decomposition stages (% per volume)
Range of constituents resulting from anaerobic decomposition stager (% per volume)
0-70 0-90 0-90 < 2.0 < t.0 <5
40-70 30-50 Traces
Methane Carbon dioxide Hydrogen Nitrogen Oxygen Trace gases
<5
* Crutcher et al., 1988. t Methane production stage. ++Reproduced with kind permission from T. Crutcher, Conestoga-Rovers & Associates Ltd, Ontario, Canada. Table 2. Municipal landfill sites gas constituents (anaerobic phase) Elements
Palos Verdes, CA % Vol.*
Methane Carbon dioxide Hydrogen Oxygen Nitrogen Trace gases: Heptane Octane Nonane Hexane n-Pentane iso-Pentane Propane n-Butane iso-Butane Hydrogen sulphide Other compounds Total
53.283 45.588 0.056 0.070 0.272 0.290 0.206 0.064 0.128 0.014 0.010 0.007 0.006 0.004 0.002 100.00
C.T.E.D.,Montrral QC % VoLt 60 37 0.1 2.8
..... 0.1 100.00
* Tabasaran, 1979. t I~sourdy-Biothermica Inc., 1989.
These three examples indicate that the five principal constituents (CH,, CO2, H2, 02, N2) are found in every M.L.S., given the type and relative proportions of buried waste, which do not vary substantially for landfill sites associated with large urban agglomerations.
Type of trace compounds generated by M.L.S. The main classes of trace compounds found in M.L.S. gases are the following (Keller, 1988): • Saturated and unsaturated hydrocarbons: they have a higher molecular weight than that of methane and are insoluble in water. • Acidic hydrocarbons and organic alcohols--most of which are water soluble. • Aromatic hydrocarbons. • Halogenated hydrocarbons (chlorinated, for the most part). • Sulphur compounds, such as hydrogen sulphides and carbonyl, carbon disulphide and mercaptans.
• Inorganic compounds, inerts, and others that are not in any of the aforementioned categories; e.g. toxic metals (Greenberg, 1987). Some of these trace compounds have been sampled in many M.L.S. Table 3 shows the expected concentration ranges for these compounds. It is important to note that, for the same compound, results may sometimes differ enormously: the M.L.S. being different according to their content and age, and also the fact that measuring methods have evolved since 1981. It has been observed that gas generated by domestic wastes which have been buried for some time is composed mainly of hydrocarbons. A much wider variety of compounds are present in gases generated by landfill sites containing recently buried domestic or industrial wastes: thus, at such sites it was possible to identify esters (e.g. methyl acetate), terpenes (e.g. limonene) and organic sulphur compounds (e.g. ethyl mercaptan). Two gas compounds, vinyl chloride and/or benzene which is a known human carcinogen,
Trace gas compound emissions Table 3. Gaseous trace compounds identified in M.L.S. gas Compound Acetone Alpha terpinene Benzene Butyl alcohol Cblorobenzene Chloroform Dichlorobenzene 1,l-Dichloroethane 1,2-Dichloroethane 1,1-Dichlorcethylene Dichloromethane Dietbylene chloride Ethylbenzene Ethylene dichloride 2-Ethyl-l-hexanol Ethyl mercaptan Limonene 2-Methyl furan Methyl ethyl ketone Styrenes Terpene Tetrachloroethane Tetrachloroethylene Toluene 1,1,l-Trichloroethane Trichloroethylene Trichloromethane Vinyl acetate Vinyl chloride Xylene
Average concentrations (ppbV)* 32,500(1); 6838(2) 11,100(1) 5500(1); 2057(2); 960(3) 5200(1) 82(2) 245(2); 440(3) 4110(1) 2801(2) 36(2); 20(3) 130(2) 25,694(2);21,950(3) 2835(2) 21,400(1); 7334(2) 59(2) 6200(1) 21,100(1) 26,200(1) 6900(1) 5200(1);3092(2) 1517(2) 12,400(1) 246(2);550(3) 5244(2) 20,400(1); 34,907(2); 76,700(3) 615(2); 150(3) 2079(2);710(3) 440(3) 5663(2) 3508(2); 7470(3) 14,900(1); 2651(2); 10,900(3)
(1)Zimmerman and Goodkind, 1981. (2) Lang, 1989. (3) D6sourdy-Biothermica Inc., 1989. * ppbV: parts per billion per volume.
were found in 85 % of 23 Californian M.L.S. receiving, in principle, only domestic wastes (Wood and Porter, 1986a). Since, in the U.S.A., California is known for its strict environmental regulations, it is probable then that, in the United States, these two compounds, alone, present a nationwide problem (Wood and Porter, 1986b). Identifying the source of trace compounds should make it possible for their impact to be reduced and, perhaps, allow better control of them.
Formation and (or) orioin of trace compounds Trace compounds usually result from the anaerobic decomposition of products for domestic use which are manufactured industrially, or they derive from the volatilization of the most volatile compounds found in this type of waste. None the less, household wastes do not constitute the sole source of trace compound emissions from M.L.S.: some of the industrial wastes, buried in the past or dumped illicitly in landfills that are now restricted to accepting municipal and commercial wastes, are also responsible for a portion of these emissions. As an example, benzene and vinyl chloride are more than likely to be present in biogas if organic wastes of industrial origin are contained in a
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M.L.S. Even if those increases are generally low, both of these organic compounds are present at concentrations which already often exceed air emission standards (Young and Heasman, 1985). Trace gases are, therefore, the result of the volatilization of an organic liquified waste, or of chemical and/or biological conversion of a given substance (Lang, 1989). Trace compounds may thus be classified into four categories according to their origin or formation: (a) (b) (c) (d)
volatilization of substances buried; decomposition of buried wastes; volatilization and decomposition, separately; other mechanisms.
(a) Basic concentrations of volatile organic compounds (VOCs) derived from household wastes are not very high according to Vogt and Walsh (1985); it is the disposing of municipal, industrial and hazardous wastes together in the one landfill which is responsible for the presence of additional VOCs in the basic concentration levels. (b) The chemical composition of a landfill site is not static. As the organic material in the landfill is digested by microorganisms, a wide variety of organic compounds is produced. Allowed to run its course, this process would appear to offer a method for the detoxification of a landfill site which would seem to be attractive for the suppression of halogenated organic compounds; the fact is, however, that during this process toxic substances such as vinyl chloride, in this particular case, are produced (Wood and Porter, 1986a; b). (c) Benzene, toluene and vinyl chloride derive from both these types of origins. Background concentrations were found in respect to these compounds in particular at all sites sampled (Young and Parker, 1983), both residential and mixed (residential and industrial). (d) With respect to metals, Young and Heasman (1985) think that the addition of wastes containing metal to municipal wastes does not lead to significant emissions by way of the biogas. It is evaporation of water from biogas that allows air dispersion of the metals. PROBLEMSCAUSEDBY GASEOUSTRACECOMPOUNDS Six of the problems giving most cause for concern are described below.
Harmful effects on human health It is known that trace gas emissions from M.L.S. affect the ambient air quality of the surrounding area and contribute to increased concentration levels of specific contaminants (vinyl chloride, benzene, trichloromethane) (Cenei and Emerson, 1989). Exposure to one or more of these contaminants may have acute
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or chronic effects on human health, or increase the risk of cancer. For instance, the increased presence of volatile organic compounds (VOCs) caused by M.L.S. could (when combined with nitrogen oxides from other sources, e.g. automotive vehicles) lead to conditions favorable to local production of ozone--a lung irritant gas. The toxicity of M.L.S. gas, as such, depends on the cumulative effect of more than a hundred groups of compounds (Young and Heasman, 1985). Carcinogenicity/mutagenicity of trace compounds. The risk of cancer due to a particular air contaminant is typically assessed on the basis of a "hypothetical" person living close to a site emitting that contaminant, during a period of 70 years, and is expressed as the increased probability that that person will develop a cancer (Niemi and Rose, 1990). Only a small portion of inhaled gases is filtered through the blood. If these gases are qualified as irritant (e.g. H2S) and are relatively soluble, they are absorbed by the moist secretions of the mucous membranes of the upper respiratory system. They may result in an irritation of the undulating hair-like cilia and, if this occurs repeatedly, cell modifications may result (proliferation or mutation of cells). Upon reaching the lungs, non-irritant and insoluble gases may be absorbed into the blood through the pulmonary alveoli and, eventually, transported to vital organs where they may accumulate. This is the case, for example, with vinyl chloride which is associated with a rare form of liver cancer (Bisson, 1986). Certain VOCs are known or suspected human carcinogens. A study carried out by Bozzelli and Kebbekus (1982) shows that some of those VOCs are habitually found in the ambient air of large cities of North America as a result of industrial activities. The presence of these carcinogenic species in the ambient air of large landfills which, in addition, are located in or close to these urban centres, gives rise to concern about the long term health of people working at the recovery of landfill gas. A report by Kinman et al. (1986) implicated certain VOCs as carcinogens or mutagens which could present a risk to the health of such workers.
Atmospheric pollution resulting from combustion of trace components found in M.L.S. gas Emissions of air pollutants vary according to the mode of combustion employed to burn the landfill gases. For example, atmospheric emissions from a gas turbine are less significant than those derived from an internal combustion engine because of its great excess of air and high combustion temperature. It is generally admitted that destruction efficiency with respect to trace constituents is in the 95-99% range (Greenberg, 1987). No reliable data are reported in scientific papers related to the destruction efficiency of internal combustion engines, which together with flares are the destruction methods most frequently employed in
North America (Greenberg, 1987). Yet, where there is an available source of chlorine, polychlorinated dibenzodioxins and dibenzofurans are emitted by internal combustion engines. The precise identity and concentrations emitted remain to be characterized. Thus, because of the significant quantities of chlorinated hydrocarbons in landfill gas (idem), employing an internal combustion engine to burn such gases will not only allow unburned contaminants to be emitted but will also release into the ambient air products that are the result of incomplete combustion (Moss and Manley, 1986). According to Grecnberg (1987), most of the systems used to burn landfill gas emit toxic compounds at levels significantly higher than those permitted for incinerators of hazardous wastes regulated under the "Resource Conservation & Recovery Act" of 1976; such systems could present a risk to workers at the site and also to nearby residents. Therefore, it would appear to be essential to treat M.L.S. gas prior to combustion; a statement on this subject by Keller (1988) reads as follows: "Trace constituents removal from the methane product stream is normally required since sufficient quantities may result in a human health hazard after combustion." It is still not fully understood what becomes of the substances produced by the combustion of landfill gas; the same is true for exposure mechanisms with respect to individuals. Nevertheless, since the problems arising from the burning of trace compounds may increase or create risks, that are difficult to measure accurately, it seems reasonable to suggest that preventive action be taken in the form of treatment of trace gases prior to or after combustion.
Corrosion of landfill gas collection and combustion systems Gases containing sulphur (e.g. H2S ) or chlorine (e.g. methyl chloride) react with oxygen during the combustion process to form powerful acids corrosive to combustion systems (Zimmerman et al., 1985; Schleifer, 1988). According to research carried out by Dent et al. (1986) in the United Kingdom, on three M.L.S., representative of the average site of this type, concentrations higher than 100 mg of chlorine per m 3 may be recorded up to two and a half years after the burying of strictly domestic wastes; concentrations exceeding that level have been shown to be corrosive. It was also noted in the course of their research that, in general, chlorine concentrations are at their maximum in the immediate period following the burying of waste or at the time the final covering is put in place (Dent et al., 1986). It is known that removal of certain trace constituents (e.g. carbon disulphide) from gas intended for the commercial market is normally required (Keller, 1988), since in sufficient concentration their presence can cause a breakdown in the end-users combustion equipment, due to corrosion. Costs may also be
Trace gas compound emissions incurred because of the deteriorating effect of these constituents on equipment employed for landfill gas collection or combustion, costs that can be measured in terms of public health and safety and environmental quality. Sub-surface migration of certain trace compounds to residences close to landfill sites According to Wood and Porter (1986), landfill gas trace constituents may have an impact on residences in the area surrounding a M.L.S. due to gas migration via sub-surface cracks. These cracks may occur naturally, be formed as a result of gas pressure, or a layer of porous soil can allow gas to migrate. Gas may then escape to the surface via the cracks or make its way out around pipes at points where they enter the ground. Furthermore, the Battelle report to the "California Air Resources Board" warns that the most popular method employed for the control of emissions generated by M.L.S. (water-tight covering) may cause an increase in the risks to public health, rather than a reduction: the build-up of gas pressure within the landfill site can force gas to migrate into basements of neighbouring dwellings (Molton et al., 1987). It is therefore possible, according to Greenberg (1987), that methods employed for trace gas air dispersion modelling may significantly underestimate sub-surface migration of these gases and so seriously underestimate receptor exposure and also the public health risks presented by toxic substances in the trace gases. An example of this is given in the case of vinyl chloride which can precede odourous compounds in the course of M.L.S. sub-surface gas migration; this was evidenced by two studies done by Wood and Porter (1986). In one of those studies, vinyl chloride concentrations present in the dwellings were found to exceed by up to 3 or 5 times (24 h average) the standard set by California, without any odour being detected. At the present time, after making a careful assessment of the distribution area of toxic trace compounds and methane emissions from a M.L.S., it would seem to be appropriate to use methane as an indicator of sub-surface migration. Since methane will either precede or accompany the toxic trace constituents, detecting it could prove to be very useful according to Wood and Porter (1986). After methane has been detected, then further analysis of the toxic substances would be required. Indoor air of dwellings close to an M.L.S. should be included in any assessment of trace compounds exposure since it is recognized that the majority of people spend up to 80-90% of their time indoors and that concentrations of many trace compounds are often higher inside buildings. Furthermore, according to Hodgson et at. (1988, 1992), relatively significant concentrations of VOCs were found in the indoor air of dwellings located near to M.L.S. licensed to accept only "non hazardous"
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wastes. Therefore, in recent years, there has been a general modification in the way that exposure of people in those areas to substances such as VOCs, for example, has been assessed: from an almost exclusive reliance on an analysis of the landfill's ambient air, there is now a shift to include an analysis oftbe indoor air of dwellings which can also influence receptor exposure (Hodgson et al., 1988). Odours The odours which are perceived do not derive from the major components of M.L.S. gas: CH 4, CO 2, N 2, to list only the principal ones (these gases being odourless). Odours result from certain trace gas compounds releasing themselves into the air or escaping into the atmosphere via the M.L.S. gas. Trace compounds such as hydrogen sulphide and mercaptans can be perceived at concentrations as low as 1 ppb. Some M.L.S. gases require air dispersion equivalent to a dilution factor of 106 in order to bring them below their detectable odour threshold (Young and Parker, 1983; Young and Heasman, 1985). According to Young and Heasman (1985), this indicates that M.L.S. could be the source of widespread odour plumes if the landfill gas could not be dispersed easily due to unfavorable weather conditions. In addition, they measured the strength of the odours emitted by some of the trace compounds. According to their studies, only 10% of the total number of those compounds were likely to present an odour problem. Compounds emitted at various stages in the decomposition process and ranked in the first five according to odour strength, determined by these researchers, are: (1) organosulphurs--methyl mercaptan; (2) esters--methyl butyrate, ethyl butyrate, propionate; (3) acids--butyric acid; (4) hydrocarbons--limonene; (5) alcohols--butan-2-ol.
MANAGEMENT AND CONTROL OF TRACE COMPOUNDS Odour combating methods Lavalin Inc. (1988) suggested the use of flare gas burning as a simple means of eliminating the odours from collected gas. However, many M.L.S. have in the end opted for other solutions (covering material, biofilters, mineral or organic granules, collection) for fear of producing air contaminants by burning trace compounds, as was the case in Wisconsin according to Maier et al. (1988). There has been an evolution in odour problem management methods by use of means that take into account possible impacts from trace compounds. Recently, it was shown by Stiegler et al. (1989) that sorption (ab- and adsorption) of trace compounds, derived from the gas phase, is enhanced by increasing the carbon content of the absorption medium. Bohn
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and Bohn (1988) experimented with a biological filter made of granulated peat (this offers a greater contact surface area) of sufficient thickness to give a retention time long enough for the degradation process to be completed. Therefore, we would recommend installing a surface gas collection system over the biological filter which would trap any non-degraded compounds and conduct them to chimneys equipped with carbon filters; carbon being an adsorbent which destroys odours and organic vapours very efficiently (Bisson, 1986; O'Leary and Tansel, 1986). Measures such as these would likely bring about a reduction in odour problems. Monitoring trace compounds in M.L.S. emissions In California, the initial approach adopted under the current trace gas emissions control policy is the establishing of a database of information from inactive M.L.S. Then, selected sites will be subjected to more or less the same mandatory tests as active sites which according to Cenci and Emerson (1989) are: • monitoring of 10 trace compounds specified in the California legislation in an air corridor adjacent to the M.L.S.; • integrated sampling of surface level air; • monitoring of landfill gas; sampling is done from within the landfill wells; • analyzing gas composition in the layer just below the surface, around the edge of the site. Results from these tests should help identify those sites needing further study and the remedial action to be taken. The state of vegetation in the area may also be an indication of the overall quality of the ambient air at the site. The use of aerial photography, either colour or infrared photographs, can easily identify areas of stress. Watson (1988) reported that this could be done on an annual basis at inactive sites and would provide a method for determining whether trace compounds were the source of stress. Collection of landfill gas According to the Environmental Protection Agency (EPA), emissions from M.L.S., in particular trace compound emissions, may be controlled by collecting the gas and then monitoring (or recovering) the gas extracted. Several states already practice monitoring or recovery of such gases, they are: California, Maryland, New Jersey, New York, Ohio, Oregon, Texas, Wisconsin and Washington.* Landfill gas collection systems linked to some form of control of air pollution derived from trace compounds (e.g. extraction, incineration), are considered
* lnformations transmitted to D6sourdy-Biothermica Inc. by SCS Engineers (KY) (1989), obtained through M. Vezeau, City of Montr6al, April 1990.
to be the basic techniques to employ (Cenci and Emerson, 1989). The EPA has issued regulations based on the collection of landfill gases (Resource Conservation and Recovery Act, subtitle D regulations, 1991) and the control of their emissions at municipal landfill sites (Clean Air Act, section 112, 1981). These regulations should allow deficiencies to be corrected in systems that are employed for either the collection or the control of landfill gases. Although certain individual VOCs were proposed to be added to the list of hazardous air contaminants in 1985, standards regarding these have not yet been announced. Extraction of trace constituents In the past, the removal of trace constituents from landfill gas was achieved by using a condensate trap, a molecular sieve or activated carbon filter, which were added to the extraction systems. Such methods were effective in significantly reducing levels of inorganic and organic compounds in gas intended for residential or commercial use (Flynn et al., 1982). Trace compounds harmful to end-users' health or to combustion systems were targeted according to the end use envisaged. Table 4, taken from Pilarczyk et al. (1987), lists some of those compounds together with the type of treatment required in accordance with the end use. From this table, it may be seen that removal is not carried out in every case. Regulations in respect to some of the uses are less strict, depending on the risk to human health. However, this attitude is in the process of changing: the preservation of the integrity of a healthy environment is now the aim of a growing number of legislators, independently of the level of human contact with trace constituents. Likewise, in the past few years, methods for recovering various trace constituents from landfill gas have been improved or created. A report written by Keller (1988) lists an inventory and establishes the assessment of the most recent methods tried out in the United States of America. Destruction of trace constituents in landfill 9as by incineration Three types of incineration are employed: (1) direct: flare gas burning; (2) thermal; (3) catalytic. Destruction efficiency for trace constituents using the flare method varies between 95 and 99% (Greenberg, 1987). In order to be efficacious, such systems have to operate at temperatures of > 1100°C (Bisson, 1986). The following two processes, thermal and catalytic, require temperatures of about 760 to 870°C or higher in order to destroy trace constituents. The constituents are completely oxidized into carbon dioxide and water (Keller, 1988). However, chimney gas may require further treatment to remove sulphuric
Trace gas compound emissions
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Table 4. Landfill gas treatment according to end-use* Use of landfill gas Trace component to Fuel be extracted (industrial utility)
Electricity production
Fuel engine
Injection in a natural gas network
H2S Ammoniac Halogenated hydrocarbons Hg
~ --
~ --
~ 0
do
0 0
do 0
do 0
dO dO
CO
--
--
dO
dO
2
*: Pilarczyk et al., 1987. do: required extraction. 0: desirable extraction. - - : non applicable. Reproduced with kind permission from Elsevier Science Publishers B.V., Amsterdam.
and hydrochloric acids. A horizontal combustion unit (thermal type), employed at Long Island (NY) since 1984, permits the destruction of 94-100% of the VOCs. It is most often recommended that thermal or catalytic incineration modes be used rather than flare gas burning, as the former methods are capable of destroying trace constituents even if their concentrations are lower than the flammability limit (Bisson, 1986).
RECOMMENDATIONS
The main reason for controlling landfill gas emissions at M.L.S. has always been the problems caused by odours and methane. Where economically possible, utilization of this gas was the goal envisaged. We have seen that another reason may lead to the recuperation, or at least the monitoring, of landfill gas emissions: trace compounds. Since we inhale almost 14 kg of air per day in order to stay alive, while we absorb approximately 1.4 kg of food and about 2.4 kg of water, the quality of the air we breathe is most important to us (Bisson, 1986). Thus, trace compound emissions from M.L.S. represent an important factor for consideration when evaluating the impact generated on the environment by such sites since the toxic and odorous substances which may he present in those emissions could be a cause for concern for areas surrounding a landfill site. In the light of this review of the literature and after due reflection on our part, we recommend implementation of the following: • Systems should be put in place for collecting the recycling hazardous domestic waste as well as for the re-using and recycling of industrial-type waste so as to reduce potential production of toxic trace compounds in M.L.S. • Illegal dumping of hazardous industrial waste in M.L.S. should be stopped. • Technology for monitoring landfill gas emissions
at M.L.S. should be upgraded, especially in reference to trace compounds. • Further research should be done in order to expand our knowledge relating to air dispersion and migration of trace compounds in the lower levels of the troposphere. • Current methods for destruction of non-utilized landfill gas generated at M.L.S. should be improved. • Stricter controls should be introduced for the treatment of landfill gas utilized for energy purposes in order to reduce risks to end-users (health risks and damage to pipelines and combustion systems).
CONCLUSION In this review of the literature we have focussed on a number of aspects related to the trace compounds found in landfill gas emissions at Municipal Landfill Sites (M.L.S.). Our primary emphasis has been on the nature of the compounds encountered and the ways in which they are formed. Trace compounds are not solely derived merely from waste degradation. We have tried to pinpoint their source in accordance with the data in the literature and also to indicate current control methods practised at M.L.S. in respect of such substances. In North America, control of trace gas compounds is being enforced due to the potential impact these substances may have on human health and on the environment. Some of the papers reviewed discuss suspected impacts, others deal with those already known. All of the problems reveiwed have been raised in recent studies which presented evidence showing trace gas compounds are dispersed near ground level; in other words, exactly where risks of exposure are greatest. Researchers tackling the problems are restricted by the fact that there is only a limited number of studies to which they can have access, given that interest in these contaminants is of relatively recent
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origin. In addition, appreciable difficulties are encountered when carrying out representative sampling according to actual emission conditions of trace compounds in ambient air. We have mentioned the ways in which trace compounds are dispersed as well as various methods of monitoring needed in order to obtain an accurate reflection of the situation. Substances present in concentrations that are quite low but, none the less, capable of causing problems considered by many to be serious, obviously create difficulties in regard to their control; we have indicated some of those which we consider relevant. In the context of any lasting development, nonutilization of the gas generated by a large M.L.S. cannot be envisaged; unless such use would be economically unsound. In the event of the latter, this would not rule out the need for control of landfill gas, in particular, trace compound emissions. Proposals for management and control are discussed keeping in mind the best options so that impacts on human health and on the environment are minimized. Finally, we have restated recommendations put forward by some of the researchers in reference to certain aspects of these problems which require to be developed further. Acknowledgementv-Thanks are due to the Natural Science and Engineering Research Council (NSERC) of Canada and to the Sciences and Applied Sciences Faculties of the University of Sherbrooke (Qurbec). The authors wish to express their gratitude to Marie Lurline Brown.
REFERENCES Bajeat P. (1988) I~tude sur la valorisation du biogaz de lieu d'enfouissement sanitaire. Essay for M.Env. grade, University of Sherbrooke, 96 pp. Bisson M. (1986) Introduction fi la pollution atmosphrrique. MEnviQ, Direction de I'assainissement de Fair, Les Publications du Qurbec ed., 135 pp. Bohn H. and Bohn R. (1988) Soil beds weed out air pollutants. Chem. Engng 95, 73-76. Bozzelli J. W. and Kebbekus B. B. (1982) A study of some aromatics and halocarbons vapours in the ambient atmosphere of New Jersey. J. envir. Sci. Health A17, 693-711. Cenei C. and Emerson C. (1989) The Landfill Gas Testing Program: a Second Report to the California Legislature. State of California, Air Resources Board: Stationary Source Division (June), 52 p. Crutcher A. J., Van Norman A. W. and Turchan G. T. (1988) Landfill gas collection and utilization. The Canadian Geotechnical Society--Southern Ontario Section Symposium: "Solid Waste Management--Landfill design: from concept to completion" (March), 30 pp. Dent C. G., Scott P. and Baldwin G. (1986) Study of landfill gas composition at three U.K. domestic waste disposal sites, pp. 130-149. Proceedings of a conference jointly sponsored by the U.K. and the U.S. departments of energy (Oct.), Solihull, West Midlands, U.K., 505 pp. Drsourdy-Biothermica Inc. (1989) Analyse,qualitative et quantitative du biogaz produit par le C.T.E.D. (Centre de traitement et d,rlimination des drehets). Final report, Ville de Montrral, 75 pp. Flynn N. W,, Guttman M., Hahn J. and Payne J. R. (1982) Trace chemical Characterization of Pollutants Occurring
in the Production of Landfill Gas from Shoreline Regional Park Sanitary Landfill. Mountain View, California, Dept. of Energy, Washington, DC. Report No. DOE/CS/202912 (Oct.), 113 pp. Greenberg A. J. (1987) Toxic substances emitted by M.S.W. landfills. Report prepared for Friends of Recycling and the Environment and North Country Resource Recovery Associates (August), 25 pp. Hodgson A. T., Garbesi K., Sextro R. G. and Daisey J. M. (1988) Evaluation of soil-gas transport of organic chemicals into residential buildings: final report. Lawrence Berkeley Lab., CA., report No. LBL-25465 (June), 92 pp. Hodgson A. T., Garbesi K., Sextro R. G. and Daisey J. M. (1992) Soil-gas contamination and entry of volatile organic compounds into a house near a landfill. J. Air Waste Man. Ass. 42(3), 277-283. Informations transmitted to Drsourdy-Biothermica Inc. by SCS Engineers (KY) (1989) obtained via M. Vezeau, City of Montrral, Division "Grnie de l'environement", April 1990. Keller A. P. (1988) Trace constituents in landfill gas. Task report on inventory and assessment of cleaning technologies. Gas Research Institute, Chicago, ILL., report No. GRI-87/001 8.2. (April), 140 pp. Kinman R. N., Rickabaugh J., Nutini D. and Lambert M. (1986) Gas characterization, microbiological analysis, and disposal of refuse in Gas Research Institute Landfill Simulators. EPA report No.: EPA/600/2-86/041, 94 p. Lang R. J, (1989) Modeling the movement of trace gases in municipal solid waste landfills; thesis. University of California, Davis, 215 pp. Lavalin Inc. (1988) Centre de tri et d'6limination des d6chets: am61iorations de la situation, voL 1 Report. City of Montreal, Public Works Service. Maier G., Keith S. and Benzschawel D. (1988) Landfill emissions--a municipal decision. Proc. l l th Annu. Madison Waste Conf., Madison, WIS., 13-14 Sept., pp. 142-150. Molton R. M. et al. (1987) Study of vinyl chloride formation at landfill sites in California. Prepared for the California Air Resources Board, Battelle Pacific Northwest Lab., Richland, Washington (cited by Greenberg A. J. (1987) and Lang R. J. (1989)). Moss H. D. and Manley B. J. W. (1986) Landfill gas abstraction and use operational schemes at Shanks and McEwan (southern) states. In Energy from Landfill Gas Conference Proc. (edited by Emberton J. R. and Emberton R. F.), pp. 285-293. Solihull, West Midlands, U.K., 28-31, Oct. Niemi C. and Rose R. S. (1990) Management of air borne toxics in the U.S.A. Proc. 4th Conf. on Toxic substances, Montreal, Quebec, Canada (April), pp. 81-86. O'Leary P. and Tansel B. (1986) Landfill gas movement, control and uses. Waste Age 17, 104-116. Pilarczyk E., Henning K. D. and Knoblauch K. (1987) Natural gas from landfill gases. Resources Conserv. 14, 283-294. Schleifer R. (1988) Easy landfill gas profits. Waste Age 19, 105-106. Stiegler L., Stallard M., Lang R. and Tchobanoglous G. (1989) A methodology for assessing the sorption phenomena of trace organic compounds found in landfill gas. Proc. 43rd Purdue Ind. Waste Conf., West Lafayette, IND. (May), pp. 213-219. Tabasaran O. (1979) Mean research topics in the department of waste management at the University of Stuggart and a report on current investigations of practical interest concerning landfill gas. Elmia, Jonkoping (Sweden). Vogt W. G. and Walsh J. J. (1985) Volatile organic compounds in gases from landfill simulators. Proc. 78th A.P.C.A. (Air Pollution Control Association) Ann. Meeting, Detroit, MI (June), 17 pp.
Trace gas compound emissions Walsh J. J., Conrad E. T., Stubing H. D. and Vogt W. G. (1988) Control of volatile organic compounds emissions at a landfill site in New York: a community perspective. Waste Man. Res. 6, 23-34. Watson K. P. (1988) Cost of landfill capping: Trail Road Landfill Site. Report to the Regional municipality of Ottawa-Carleton (Ontario); Environmental services department, 12 pp. Wood J. A. and Porter M. L. (1986a) Hazardous Pollutants in Class l l Landfills. E.P.A., 99 pp. Young P. J. and Heasman L. A. (1985) An assessment of the odor and toxicity of the trace components of landfill gas.
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Proc. G.R.C.D.A. 8th Int. Landfill Gas Syrup. (April), San Antonio, TX. 23 pp. Young P. J. and Parker A. (1983) The identification and possible environmental impact of trace gases and vapours in landfill gas. Waste Man. Res. 1, 213-226. Zimmerman R. E. and Goodkind M. E. (1981) Landfill Recovery. Part 1: Environmental Impacts. Gas Research Inst., Chicago, IL, report No. GRI-80/0084 (Oct), 100 pp. Zimmerman R. E., Stetter J. and Flynn N. W. (1985) Landfill Methane Recovery. Part 4: Instrumentation (Final report). Gas Research Inst., Chicago, IL, Report No. GRI85/00158 (February), 122 pp.
Atmospheric Environment Vol. 28, No. 2, pp. 285 293, 1994 © 1994 Elsevier Science Lid Printed in Great Britain. All rights reserved 1352-2310/94 $6.00+0.00
TRACE GAS C O M P O U N D EMISSIONS FROM MUNICIPAL LANDFILL SANITARY SITES JOSI~E BROSSEAU a n d MICH~LE HEITZ D6partement de g6nie chimique, Facult6 des sciences appliqu6cs, Universit6 de Sherbrooke, Sherhrooke (Qu6boc), J1K 2R1, Canada
(First received 16 July 1992 and in final form 13 June 1993) Abstract--The literature on certain aspects of trace gas compounds emitted from Municipal Landfill Sanitary Sites* is reviewed. Aspects covered are the formation, nature and origin of such compounds, as well as the problems caused by them. Risks posed to human health and the environment by even low concentrations of these compounds arc examined and methods to reduce and control them discussed.
Key word index: Control, health, migration, monitoring, VOC. INTRODUCTION
The composition of the domestic waste generated by North American society is extremely varied. By domestic waste is meant household and commercial waste disposed of in Municipal Landfill Sites (M.L.S.). In previous years, disposal of industrial waste, of various degrees of contamination, was nevertheless permitted at these sites. This can no longer be said: current legislation is much stricter with respect to the types and quantities of industrial waste permitted at M.L.S. In Canada, the legislation does not yet take account of gas effluent emitted from M.L.S., and, still less, of trace gas compound emissions at large landfill sites, where risk of such emissions is increased. Recently, in the United States of America certain concerns related to toxic landfill gas emissions have been brought to light. Questions were raised in recent studies which pointed to the fact that trace compounds are dispersed near ground level, where risk of exposure is found to be sufficiently serious for legislation reassessing control standards to be enacted in respect of the contaminants in question. Furthermore, the presence of trace compounds leads to problems which become evident when the landfill gas is recovered for use as an energy source. This paper presents a review of the literature relating to the nature, problems and management of trace gas compounds. TRACE GAS COMPOUNDS
Type of landfill gases generated by Municipal Landfill Sites The relative composition of the decomposition gas emitted from a Municipal Landfill Site (M.L.S.) may * Municipal Landfill Sanitary Sites: sites usually used for disposal of domestic waste. :~Montr6ai, Qu6bec, Canada.
vary from one site to another depending on the type of waste buried, the stage reached in the decomposition process as well as operating conditions at the site (i.e. covering of the landfill, etc.). Table 1, taken from Crutcher et al. (1988), lists the principal constituents of landfill gas (major constituents: methane, carbon dioxide, hydrogen, nitrogen and oxygen; minor constituents: trace gases) and gives the expected limits for each in percentages by volume (ranges given for both aerobic and anaerobic decomposition). By way of example, Table 2 presents the results of landfill gas analysis carried out at Palos Verdes, CA, M.L.S. (anaerobic decomposition stage). Specifically, this is a large landfill site where biogas--methane and carbon dioxide--is recovered for commercial purposes. If these results are compared to those related to the principal constituents referred to in Table 1 (anaerobic decomposition stage), they correspond to the expected averages. Concentrations of methane (53.3 %), as well as those of carbon dioxide (45.6%), hydrogen (0.06%), oxygen (0.07%) and nitrogen (0.3%) are in the range of those presented in Table 1 in relation to the anaerobic decomposition stage. The amount of trace gases is equivalent to more than 0.7% of the landfill gases total. The results dating from 1979 may cover a smaller variety of trace gases than those given in Table 1 (1988) where the composition of the trace gases is not mentioned. A further example is supplied by D6sourdyBiothermica Inc. (1989) which presents results related to the main components of landfill gas actively extracted from the "Centre de Tri et d'Elimination des D6chets de Montr6al ~;(C.T.I~.D.)" (Montr6al's Sorting and Waste Disposal Centre). The percentage by volume of methane is 60% and that of carbon dioxide is 37 %: these values are in agreement with those given in Table 1. Trace gases and hydrogen (included in the category "Other compounds") are present in very small quantities (0.1% of total volume).
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Table 1. Principal constituents of M.L.S. gas* Gas
Range of constituents resulting from aerobic and anaerobic decomposition stages (% per volume)
Range of constituents resulting from anaerobic decomposition stager (% per volume)
0-70 0-90 0-90 < 2.0 < t.0 <5
40-70 30-50 Traces
Methane Carbon dioxide Hydrogen Nitrogen Oxygen Trace gases
<5
* Crutcher et al., 1988. t Methane production stage. ++Reproduced with kind permission from T. Crutcher, Conestoga-Rovers & Associates Ltd, Ontario, Canada. Table 2. Municipal landfill sites gas constituents (anaerobic phase) Elements
Palos Verdes, CA % Vol.*
Methane Carbon dioxide Hydrogen Oxygen Nitrogen Trace gases: Heptane Octane Nonane Hexane n-Pentane iso-Pentane Propane n-Butane iso-Butane Hydrogen sulphide Other compounds Total
53.283 45.588 0.056 0.070 0.272 0.290 0.206 0.064 0.128 0.014 0.010 0.007 0.006 0.004 0.002 100.00
C.T.E.D.,Montrral QC % VoLt 60 37 0.1 2.8
..... 0.1 100.00
* Tabasaran, 1979. t I~sourdy-Biothermica Inc., 1989.
These three examples indicate that the five principal constituents (CH,, CO2, H2, 02, N2) are found in every M.L.S., given the type and relative proportions of buried waste, which do not vary substantially for landfill sites associated with large urban agglomerations.
Type of trace compounds generated by M.L.S. The main classes of trace compounds found in M.L.S. gases are the following (Keller, 1988): • Saturated and unsaturated hydrocarbons: they have a higher molecular weight than that of methane and are insoluble in water. • Acidic hydrocarbons and organic alcohols--most of which are water soluble. • Aromatic hydrocarbons. • Halogenated hydrocarbons (chlorinated, for the most part). • Sulphur compounds, such as hydrogen sulphides and carbonyl, carbon disulphide and mercaptans.
• Inorganic compounds, inerts, and others that are not in any of the aforementioned categories; e.g. toxic metals (Greenberg, 1987). Some of these trace compounds have been sampled in many M.L.S. Table 3 shows the expected concentration ranges for these compounds. It is important to note that, for the same compound, results may sometimes differ enormously: the M.L.S. being different according to their content and age, and also the fact that measuring methods have evolved since 1981. It has been observed that gas generated by domestic wastes which have been buried for some time is composed mainly of hydrocarbons. A much wider variety of compounds are present in gases generated by landfill sites containing recently buried domestic or industrial wastes: thus, at such sites it was possible to identify esters (e.g. methyl acetate), terpenes (e.g. limonene) and organic sulphur compounds (e.g. ethyl mercaptan). Two gas compounds, vinyl chloride and/or benzene which is a known human carcinogen,
Trace gas compound emissions Table 3. Gaseous trace compounds identified in M.L.S. gas Compound Acetone Alpha terpinene Benzene Butyl alcohol Cblorobenzene Chloroform Dichlorobenzene 1,l-Dichloroethane 1,2-Dichloroethane 1,1-Dichlorcethylene Dichloromethane Dietbylene chloride Ethylbenzene Ethylene dichloride 2-Ethyl-l-hexanol Ethyl mercaptan Limonene 2-Methyl furan Methyl ethyl ketone Styrenes Terpene Tetrachloroethane Tetrachloroethylene Toluene 1,1,l-Trichloroethane Trichloroethylene Trichloromethane Vinyl acetate Vinyl chloride Xylene
Average concentrations (ppbV)* 32,500(1); 6838(2) 11,100(1) 5500(1); 2057(2); 960(3) 5200(1) 82(2) 245(2); 440(3) 4110(1) 2801(2) 36(2); 20(3) 130(2) 25,694(2);21,950(3) 2835(2) 21,400(1); 7334(2) 59(2) 6200(1) 21,100(1) 26,200(1) 6900(1) 5200(1);3092(2) 1517(2) 12,400(1) 246(2);550(3) 5244(2) 20,400(1); 34,907(2); 76,700(3) 615(2); 150(3) 2079(2);710(3) 440(3) 5663(2) 3508(2); 7470(3) 14,900(1); 2651(2); 10,900(3)
(1)Zimmerman and Goodkind, 1981. (2) Lang, 1989. (3) D6sourdy-Biothermica Inc., 1989. * ppbV: parts per billion per volume.
were found in 85 % of 23 Californian M.L.S. receiving, in principle, only domestic wastes (Wood and Porter, 1986a). Since, in the U.S.A., California is known for its strict environmental regulations, it is probable then that, in the United States, these two compounds, alone, present a nationwide problem (Wood and Porter, 1986b). Identifying the source of trace compounds should make it possible for their impact to be reduced and, perhaps, allow better control of them.
Formation and (or) orioin of trace compounds Trace compounds usually result from the anaerobic decomposition of products for domestic use which are manufactured industrially, or they derive from the volatilization of the most volatile compounds found in this type of waste. None the less, household wastes do not constitute the sole source of trace compound emissions from M.L.S.: some of the industrial wastes, buried in the past or dumped illicitly in landfills that are now restricted to accepting municipal and commercial wastes, are also responsible for a portion of these emissions. As an example, benzene and vinyl chloride are more than likely to be present in biogas if organic wastes of industrial origin are contained in a
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M.L.S. Even if those increases are generally low, both of these organic compounds are present at concentrations which already often exceed air emission standards (Young and Heasman, 1985). Trace gases are, therefore, the result of the volatilization of an organic liquified waste, or of chemical and/or biological conversion of a given substance (Lang, 1989). Trace compounds may thus be classified into four categories according to their origin or formation: (a) (b) (c) (d)
volatilization of substances buried; decomposition of buried wastes; volatilization and decomposition, separately; other mechanisms.
(a) Basic concentrations of volatile organic compounds (VOCs) derived from household wastes are not very high according to Vogt and Walsh (1985); it is the disposing of municipal, industrial and hazardous wastes together in the one landfill which is responsible for the presence of additional VOCs in the basic concentration levels. (b) The chemical composition of a landfill site is not static. As the organic material in the landfill is digested by microorganisms, a wide variety of organic compounds is produced. Allowed to run its course, this process would appear to offer a method for the detoxification of a landfill site which would seem to be attractive for the suppression of halogenated organic compounds; the fact is, however, that during this process toxic substances such as vinyl chloride, in this particular case, are produced (Wood and Porter, 1986a; b). (c) Benzene, toluene and vinyl chloride derive from both these types of origins. Background concentrations were found in respect to these compounds in particular at all sites sampled (Young and Parker, 1983), both residential and mixed (residential and industrial). (d) With respect to metals, Young and Heasman (1985) think that the addition of wastes containing metal to municipal wastes does not lead to significant emissions by way of the biogas. It is evaporation of water from biogas that allows air dispersion of the metals. PROBLEMSCAUSEDBY GASEOUSTRACECOMPOUNDS Six of the problems giving most cause for concern are described below.
Harmful effects on human health It is known that trace gas emissions from M.L.S. affect the ambient air quality of the surrounding area and contribute to increased concentration levels of specific contaminants (vinyl chloride, benzene, trichloromethane) (Cenei and Emerson, 1989). Exposure to one or more of these contaminants may have acute
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or chronic effects on human health, or increase the risk of cancer. For instance, the increased presence of volatile organic compounds (VOCs) caused by M.L.S. could (when combined with nitrogen oxides from other sources, e.g. automotive vehicles) lead to conditions favorable to local production of ozone--a lung irritant gas. The toxicity of M.L.S. gas, as such, depends on the cumulative effect of more than a hundred groups of compounds (Young and Heasman, 1985). Carcinogenicity/mutagenicity of trace compounds. The risk of cancer due to a particular air contaminant is typically assessed on the basis of a "hypothetical" person living close to a site emitting that contaminant, during a period of 70 years, and is expressed as the increased probability that that person will develop a cancer (Niemi and Rose, 1990). Only a small portion of inhaled gases is filtered through the blood. If these gases are qualified as irritant (e.g. H2S) and are relatively soluble, they are absorbed by the moist secretions of the mucous membranes of the upper respiratory system. They may result in an irritation of the undulating hair-like cilia and, if this occurs repeatedly, cell modifications may result (proliferation or mutation of cells). Upon reaching the lungs, non-irritant and insoluble gases may be absorbed into the blood through the pulmonary alveoli and, eventually, transported to vital organs where they may accumulate. This is the case, for example, with vinyl chloride which is associated with a rare form of liver cancer (Bisson, 1986). Certain VOCs are known or suspected human carcinogens. A study carried out by Bozzelli and Kebbekus (1982) shows that some of those VOCs are habitually found in the ambient air of large cities of North America as a result of industrial activities. The presence of these carcinogenic species in the ambient air of large landfills which, in addition, are located in or close to these urban centres, gives rise to concern about the long term health of people working at the recovery of landfill gas. A report by Kinman et al. (1986) implicated certain VOCs as carcinogens or mutagens which could present a risk to the health of such workers.
Atmospheric pollution resulting from combustion of trace components found in M.L.S. gas Emissions of air pollutants vary according to the mode of combustion employed to burn the landfill gases. For example, atmospheric emissions from a gas turbine are less significant than those derived from an internal combustion engine because of its great excess of air and high combustion temperature. It is generally admitted that destruction efficiency with respect to trace constituents is in the 95-99% range (Greenberg, 1987). No reliable data are reported in scientific papers related to the destruction efficiency of internal combustion engines, which together with flares are the destruction methods most frequently employed in
North America (Greenberg, 1987). Yet, where there is an available source of chlorine, polychlorinated dibenzodioxins and dibenzofurans are emitted by internal combustion engines. The precise identity and concentrations emitted remain to be characterized. Thus, because of the significant quantities of chlorinated hydrocarbons in landfill gas (idem), employing an internal combustion engine to burn such gases will not only allow unburned contaminants to be emitted but will also release into the ambient air products that are the result of incomplete combustion (Moss and Manley, 1986). According to Grecnberg (1987), most of the systems used to burn landfill gas emit toxic compounds at levels significantly higher than those permitted for incinerators of hazardous wastes regulated under the "Resource Conservation & Recovery Act" of 1976; such systems could present a risk to workers at the site and also to nearby residents. Therefore, it would appear to be essential to treat M.L.S. gas prior to combustion; a statement on this subject by Keller (1988) reads as follows: "Trace constituents removal from the methane product stream is normally required since sufficient quantities may result in a human health hazard after combustion." It is still not fully understood what becomes of the substances produced by the combustion of landfill gas; the same is true for exposure mechanisms with respect to individuals. Nevertheless, since the problems arising from the burning of trace compounds may increase or create risks, that are difficult to measure accurately, it seems reasonable to suggest that preventive action be taken in the form of treatment of trace gases prior to or after combustion.
Corrosion of landfill gas collection and combustion systems Gases containing sulphur (e.g. H2S ) or chlorine (e.g. methyl chloride) react with oxygen during the combustion process to form powerful acids corrosive to combustion systems (Zimmerman et al., 1985; Schleifer, 1988). According to research carried out by Dent et al. (1986) in the United Kingdom, on three M.L.S., representative of the average site of this type, concentrations higher than 100 mg of chlorine per m 3 may be recorded up to two and a half years after the burying of strictly domestic wastes; concentrations exceeding that level have been shown to be corrosive. It was also noted in the course of their research that, in general, chlorine concentrations are at their maximum in the immediate period following the burying of waste or at the time the final covering is put in place (Dent et al., 1986). It is known that removal of certain trace constituents (e.g. carbon disulphide) from gas intended for the commercial market is normally required (Keller, 1988), since in sufficient concentration their presence can cause a breakdown in the end-users combustion equipment, due to corrosion. Costs may also be
Trace gas compound emissions incurred because of the deteriorating effect of these constituents on equipment employed for landfill gas collection or combustion, costs that can be measured in terms of public health and safety and environmental quality. Sub-surface migration of certain trace compounds to residences close to landfill sites According to Wood and Porter (1986), landfill gas trace constituents may have an impact on residences in the area surrounding a M.L.S. due to gas migration via sub-surface cracks. These cracks may occur naturally, be formed as a result of gas pressure, or a layer of porous soil can allow gas to migrate. Gas may then escape to the surface via the cracks or make its way out around pipes at points where they enter the ground. Furthermore, the Battelle report to the "California Air Resources Board" warns that the most popular method employed for the control of emissions generated by M.L.S. (water-tight covering) may cause an increase in the risks to public health, rather than a reduction: the build-up of gas pressure within the landfill site can force gas to migrate into basements of neighbouring dwellings (Molton et al., 1987). It is therefore possible, according to Greenberg (1987), that methods employed for trace gas air dispersion modelling may significantly underestimate sub-surface migration of these gases and so seriously underestimate receptor exposure and also the public health risks presented by toxic substances in the trace gases. An example of this is given in the case of vinyl chloride which can precede odourous compounds in the course of M.L.S. sub-surface gas migration; this was evidenced by two studies done by Wood and Porter (1986). In one of those studies, vinyl chloride concentrations present in the dwellings were found to exceed by up to 3 or 5 times (24 h average) the standard set by California, without any odour being detected. At the present time, after making a careful assessment of the distribution area of toxic trace compounds and methane emissions from a M.L.S., it would seem to be appropriate to use methane as an indicator of sub-surface migration. Since methane will either precede or accompany the toxic trace constituents, detecting it could prove to be very useful according to Wood and Porter (1986). After methane has been detected, then further analysis of the toxic substances would be required. Indoor air of dwellings close to an M.L.S. should be included in any assessment of trace compounds exposure since it is recognized that the majority of people spend up to 80-90% of their time indoors and that concentrations of many trace compounds are often higher inside buildings. Furthermore, according to Hodgson et at. (1988, 1992), relatively significant concentrations of VOCs were found in the indoor air of dwellings located near to M.L.S. licensed to accept only "non hazardous"
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wastes. Therefore, in recent years, there has been a general modification in the way that exposure of people in those areas to substances such as VOCs, for example, has been assessed: from an almost exclusive reliance on an analysis of the landfill's ambient air, there is now a shift to include an analysis oftbe indoor air of dwellings which can also influence receptor exposure (Hodgson et al., 1988). Odours The odours which are perceived do not derive from the major components of M.L.S. gas: CH 4, CO 2, N 2, to list only the principal ones (these gases being odourless). Odours result from certain trace gas compounds releasing themselves into the air or escaping into the atmosphere via the M.L.S. gas. Trace compounds such as hydrogen sulphide and mercaptans can be perceived at concentrations as low as 1 ppb. Some M.L.S. gases require air dispersion equivalent to a dilution factor of 106 in order to bring them below their detectable odour threshold (Young and Parker, 1983; Young and Heasman, 1985). According to Young and Heasman (1985), this indicates that M.L.S. could be the source of widespread odour plumes if the landfill gas could not be dispersed easily due to unfavorable weather conditions. In addition, they measured the strength of the odours emitted by some of the trace compounds. According to their studies, only 10% of the total number of those compounds were likely to present an odour problem. Compounds emitted at various stages in the decomposition process and ranked in the first five according to odour strength, determined by these researchers, are: (1) organosulphurs--methyl mercaptan; (2) esters--methyl butyrate, ethyl butyrate, propionate; (3) acids--butyric acid; (4) hydrocarbons--limonene; (5) alcohols--butan-2-ol.
MANAGEMENT AND CONTROL OF TRACE COMPOUNDS Odour combating methods Lavalin Inc. (1988) suggested the use of flare gas burning as a simple means of eliminating the odours from collected gas. However, many M.L.S. have in the end opted for other solutions (covering material, biofilters, mineral or organic granules, collection) for fear of producing air contaminants by burning trace compounds, as was the case in Wisconsin according to Maier et al. (1988). There has been an evolution in odour problem management methods by use of means that take into account possible impacts from trace compounds. Recently, it was shown by Stiegler et al. (1989) that sorption (ab- and adsorption) of trace compounds, derived from the gas phase, is enhanced by increasing the carbon content of the absorption medium. Bohn
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and Bohn (1988) experimented with a biological filter made of granulated peat (this offers a greater contact surface area) of sufficient thickness to give a retention time long enough for the degradation process to be completed. Therefore, we would recommend installing a surface gas collection system over the biological filter which would trap any non-degraded compounds and conduct them to chimneys equipped with carbon filters; carbon being an adsorbent which destroys odours and organic vapours very efficiently (Bisson, 1986; O'Leary and Tansel, 1986). Measures such as these would likely bring about a reduction in odour problems. Monitoring trace compounds in M.L.S. emissions In California, the initial approach adopted under the current trace gas emissions control policy is the establishing of a database of information from inactive M.L.S. Then, selected sites will be subjected to more or less the same mandatory tests as active sites which according to Cenci and Emerson (1989) are: • monitoring of 10 trace compounds specified in the California legislation in an air corridor adjacent to the M.L.S.; • integrated sampling of surface level air; • monitoring of landfill gas; sampling is done from within the landfill wells; • analyzing gas composition in the layer just below the surface, around the edge of the site. Results from these tests should help identify those sites needing further study and the remedial action to be taken. The state of vegetation in the area may also be an indication of the overall quality of the ambient air at the site. The use of aerial photography, either colour or infrared photographs, can easily identify areas of stress. Watson (1988) reported that this could be done on an annual basis at inactive sites and would provide a method for determining whether trace compounds were the source of stress. Collection of landfill gas According to the Environmental Protection Agency (EPA), emissions from M.L.S., in particular trace compound emissions, may be controlled by collecting the gas and then monitoring (or recovering) the gas extracted. Several states already practice monitoring or recovery of such gases, they are: California, Maryland, New Jersey, New York, Ohio, Oregon, Texas, Wisconsin and Washington.* Landfill gas collection systems linked to some form of control of air pollution derived from trace compounds (e.g. extraction, incineration), are considered
* lnformations transmitted to D6sourdy-Biothermica Inc. by SCS Engineers (KY) (1989), obtained through M. Vezeau, City of Montr6al, April 1990.
to be the basic techniques to employ (Cenci and Emerson, 1989). The EPA has issued regulations based on the collection of landfill gases (Resource Conservation and Recovery Act, subtitle D regulations, 1991) and the control of their emissions at municipal landfill sites (Clean Air Act, section 112, 1981). These regulations should allow deficiencies to be corrected in systems that are employed for either the collection or the control of landfill gases. Although certain individual VOCs were proposed to be added to the list of hazardous air contaminants in 1985, standards regarding these have not yet been announced. Extraction of trace constituents In the past, the removal of trace constituents from landfill gas was achieved by using a condensate trap, a molecular sieve or activated carbon filter, which were added to the extraction systems. Such methods were effective in significantly reducing levels of inorganic and organic compounds in gas intended for residential or commercial use (Flynn et al., 1982). Trace compounds harmful to end-users' health or to combustion systems were targeted according to the end use envisaged. Table 4, taken from Pilarczyk et al. (1987), lists some of those compounds together with the type of treatment required in accordance with the end use. From this table, it may be seen that removal is not carried out in every case. Regulations in respect to some of the uses are less strict, depending on the risk to human health. However, this attitude is in the process of changing: the preservation of the integrity of a healthy environment is now the aim of a growing number of legislators, independently of the level of human contact with trace constituents. Likewise, in the past few years, methods for recovering various trace constituents from landfill gas have been improved or created. A report written by Keller (1988) lists an inventory and establishes the assessment of the most recent methods tried out in the United States of America. Destruction of trace constituents in landfill 9as by incineration Three types of incineration are employed: (1) direct: flare gas burning; (2) thermal; (3) catalytic. Destruction efficiency for trace constituents using the flare method varies between 95 and 99% (Greenberg, 1987). In order to be efficacious, such systems have to operate at temperatures of > 1100°C (Bisson, 1986). The following two processes, thermal and catalytic, require temperatures of about 760 to 870°C or higher in order to destroy trace constituents. The constituents are completely oxidized into carbon dioxide and water (Keller, 1988). However, chimney gas may require further treatment to remove sulphuric
Trace gas compound emissions
291
Table 4. Landfill gas treatment according to end-use* Use of landfill gas Trace component to Fuel be extracted (industrial utility)
Electricity production
Fuel engine
Injection in a natural gas network
H2S Ammoniac Halogenated hydrocarbons Hg
~ --
~ --
~ 0
do
0 0
do 0
do 0
dO dO
CO
--
--
dO
dO
2
*: Pilarczyk et al., 1987. do: required extraction. 0: desirable extraction. - - : non applicable. Reproduced with kind permission from Elsevier Science Publishers B.V., Amsterdam.
and hydrochloric acids. A horizontal combustion unit (thermal type), employed at Long Island (NY) since 1984, permits the destruction of 94-100% of the VOCs. It is most often recommended that thermal or catalytic incineration modes be used rather than flare gas burning, as the former methods are capable of destroying trace constituents even if their concentrations are lower than the flammability limit (Bisson, 1986).
RECOMMENDATIONS
The main reason for controlling landfill gas emissions at M.L.S. has always been the problems caused by odours and methane. Where economically possible, utilization of this gas was the goal envisaged. We have seen that another reason may lead to the recuperation, or at least the monitoring, of landfill gas emissions: trace compounds. Since we inhale almost 14 kg of air per day in order to stay alive, while we absorb approximately 1.4 kg of food and about 2.4 kg of water, the quality of the air we breathe is most important to us (Bisson, 1986). Thus, trace compound emissions from M.L.S. represent an important factor for consideration when evaluating the impact generated on the environment by such sites since the toxic and odorous substances which may he present in those emissions could be a cause for concern for areas surrounding a landfill site. In the light of this review of the literature and after due reflection on our part, we recommend implementation of the following: • Systems should be put in place for collecting the recycling hazardous domestic waste as well as for the re-using and recycling of industrial-type waste so as to reduce potential production of toxic trace compounds in M.L.S. • Illegal dumping of hazardous industrial waste in M.L.S. should be stopped. • Technology for monitoring landfill gas emissions
at M.L.S. should be upgraded, especially in reference to trace compounds. • Further research should be done in order to expand our knowledge relating to air dispersion and migration of trace compounds in the lower levels of the troposphere. • Current methods for destruction of non-utilized landfill gas generated at M.L.S. should be improved. • Stricter controls should be introduced for the treatment of landfill gas utilized for energy purposes in order to reduce risks to end-users (health risks and damage to pipelines and combustion systems).
CONCLUSION In this review of the literature we have focussed on a number of aspects related to the trace compounds found in landfill gas emissions at Municipal Landfill Sites (M.L.S.). Our primary emphasis has been on the nature of the compounds encountered and the ways in which they are formed. Trace compounds are not solely derived merely from waste degradation. We have tried to pinpoint their source in accordance with the data in the literature and also to indicate current control methods practised at M.L.S. in respect of such substances. In North America, control of trace gas compounds is being enforced due to the potential impact these substances may have on human health and on the environment. Some of the papers reviewed discuss suspected impacts, others deal with those already known. All of the problems reveiwed have been raised in recent studies which presented evidence showing trace gas compounds are dispersed near ground level; in other words, exactly where risks of exposure are greatest. Researchers tackling the problems are restricted by the fact that there is only a limited number of studies to which they can have access, given that interest in these contaminants is of relatively recent
292
J. BROSSEAUand M. HEITZ
origin. In addition, appreciable difficulties are encountered when carrying out representative sampling according to actual emission conditions of trace compounds in ambient air. We have mentioned the ways in which trace compounds are dispersed as well as various methods of monitoring needed in order to obtain an accurate reflection of the situation. Substances present in concentrations that are quite low but, none the less, capable of causing problems considered by many to be serious, obviously create difficulties in regard to their control; we have indicated some of those which we consider relevant. In the context of any lasting development, nonutilization of the gas generated by a large M.L.S. cannot be envisaged; unless such use would be economically unsound. In the event of the latter, this would not rule out the need for control of landfill gas, in particular, trace compound emissions. Proposals for management and control are discussed keeping in mind the best options so that impacts on human health and on the environment are minimized. Finally, we have restated recommendations put forward by some of the researchers in reference to certain aspects of these problems which require to be developed further. Acknowledgementv-Thanks are due to the Natural Science and Engineering Research Council (NSERC) of Canada and to the Sciences and Applied Sciences Faculties of the University of Sherbrooke (Qurbec). The authors wish to express their gratitude to Marie Lurline Brown.
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