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A poorly managed landfill is easy to spot; it can be smelled long before it is actually seen. By Daniel P. Duffy Odors are only the most obvious of issues likely to occur as the result of landfill operations. In addition to obvious odors, there are odorless emissions of methane and carbon dioxide to be managed. Invisible gases are also accompanied by visible emissions of dust, particulates, and blown litter. A landfill operator will be required both by environmental regulations and his site’s own operating permit conditions to address these air-quality issues. The operator will have to install mechanical systems and initiate operating procedures to minimize the following problems: landfill gas, dust and particulates, odors, and minor emission sources. Landfill Gas But the primary source of LFG remains microbial activity, which turns the organic compounds found in waste into methane, carbon dioxide, and trace amounts of other gases. LFG is about 50%–55% methane, 45%–50% carbon dioxide, and 5% or less trace gases. Methane is odorless and not directly harmful to human health. However, in confined spaces the concentration of methane can accumulate until oxygen is displaced, leading to threat of suffocation. At high concentrations a single spark could explode methane. Carbon dioxide in itself does not represent a threat to the environment since it is already a major part of the Earth’s atmosphere. However, it has been designated as a “greenhouse gas,” the primary potential cause of global warming. Recent Supreme Court rulings that the EPA can and should regulate carbon-dioxide emissions could be applied to nonpoint sources like landfills. There are several factors affecting both the quantity and quality of LFG. Increased temperature accelerates microbiological activity up to an optimum temperature level. Moisture content can accelerate bacterial activity or smother it completely if the waste is completely saturated. Analytical studies have shown that throughout the LFG production process, water and moisture content have a significant effect on all kinds of bacterial activity. The least amount of LFG is produced at the extremes of moisture content, without water and leachate irrigation (dry wastes) and—at the other extreme—in completely saturated wastes. The greatest production of landfill gas has been observed in conditions where waste is irrigated by recirculating leachate at a rate equivalent to atmospheric precipitation. The waste composition itself (especially the proportion of organics present in the waste) will determine the amount of LFG produced over time. Waste with a high percentage of inorganics, such as construction-and-demolition (C&D) debris, metals, and plastics, will produce less LFG than waste containing large quantities of organic sludges. Since waste is very heterogeneous, the amount of organic material and subsequent LFG production can vary considerably in different parts of the same landfill. Not only organic quantities but also the characteristics of the various chemicals that may be present in the waste will affect LFG production, especially the amounts and kinds of trace gases. The production of LFG over the lifetime of the landfill goes through five stages, each with a different production mechanism and resulting gaseous emission. Aerobic Decomposition—This first stage is driven by aerobic bacteria and begins almost immediately after waste disposal. The waste’s organic fraction is subject to both hydrolysis (chemical reactions with moisture and water present in the waste mass that result in the breakdown of complex organic molecules, such as carbohydrates, into simpler ones, such as sugar) and aerobic degradation. This is an exothermic reaction (a chemical process that produces excess heat energy) that raises the temperature of the waste up to 70°C (158°F) while producing both carbon dioxide and water vapor as the aerobic microbes consume the available oxygen in the deposited waste. Once the available oxygen has been almost completely removed, an anaerobic state is achieved, triggering the next stage. Acidogenesis—Hydrolysis continues under an anaerobic regime by microbes that are actually poisoned by oxygen. This is actually a form of fermentation (the process of energy production in a cell in an anaerobic environment) that produces organic acids, hydrogen, carbon dioxide, water vapor, and ammonia nitrogen. The hydrogen and carbon dioxide are produced as byproducts of acidogenesis of the simpler organic monomers previously produced by aerobic hydrolysis (the process converts the simpler molecules into volatile fatty acids). During this stage, sulfur-reducing bacteria produce hydrogen sulfide. Unlike aerobic decomposition, anaerobic decomposition is an endothermic process (requiring heat energy) and the temperature of the waste usually falls. Acetogenesis—The conversion of the volatile fatty acids produced by the previous stage’s acidogenesis activities into acetic acid, carbon dioxide, and hydrogen occurs in this stage. By this stage, the waste’s temperature has fallen to between 20°C and 40°C (68°F and 104°F). Though gas production is less than optimal, acetogenesis sets the stage for the landfill’s long-term stage of stable LFG production. Methanogenesis—The fourth and most productive stage is methanogenesis (available acetate is converted to methane and carbon dioxide while using up hydrogen). The process can also involve carbon-dioxide reduction by free hydrogen molecules. This phase is the longest in duration, often lasting as long as or longer than all the other phases combined. The durations of each of the other stages are measured in terms of years; this fourth stage can last for decades. Settlement of the waste through the process of decomposition also achieves maximum volume reduction at this time. Production of nitrogen effectively disappears during this stage. There is a direct relationship between waste composition and methanogenesis. The act of creating methane removes organics from the wastestream. This in turn eventually reduces the quantities of methane being generated. The higher the initial organic waste content, the higher the peak LFG production rate. Aerobic Decomposition—Once all of the available acetate is converted into methane, the landfill is ready to return to its initial aerobic stage. As there is no more feedstock for anaerobic microbes, they are displaced with aerobic microbes. Methane production falls off to zero, while the landfill begins to once again emit nitrogen and oxygen. This stage, by definition, lasts the longest though most landfills have not existed long enough for this stage to begin much less be fully played out. In addition to moisture content, waste composition, and oxygen availability, the temperature of the waste will affect the anaerobic decomposition process. Though anaerobic bacteria can survive temperatures ranging from freezing to above 135°F, they are most active at either 98°F (mesophilic) or 130°F (thermophilic). Between these two temperatures and below the mesophilic range, LFG production falls off. Of the two peak ranges, greater gas production occurs at the thermophilic range. Of secondary importance to gas production is its relationship to the in-place compaction density of the waste. By definition, a higher-density waste (all other factors such as organic and water content being equal) will produce more gas per unit volume than low-density waste of the same composition. But this is rather simplistic since waste compaction indirectly affects these other factors. For example, high-density waste has less void volumes and thus less potential for higher water content or even saturation. Higher-density waste will also have less starting oxygen for the same reason. Compacted waste will also generate more heat per unit volume than loose waste. However, since there are so many factors and municipal solid waste is so heterogeneous, it is difficult to impossible to establish a direct, straight-line relationship between waste compaction and gas production. Though it may seem odd to discuss air-quality issues in the context of underground gas migration, it is below the ground surface where landfill gas can do the most physical damage. Most landfills have impermeable geomembrane covers, and these can blow up like a bullfrog’s cheeks if LFG is allowed to accumulate underneath. If allowed to go unchecked, this can result in dangerous structural and slope stability problems since waste can shift and cover soils can slough off. High concentrations of LFG can also case vegetative distress by poisoning the root systems of grasses growing on the landfill’s cover or adjacent to the landfill. LFG migrating offsite tends to follow natural seams in the local stratigraphy or man-made paths, such as sewers or other buried utilities. Once it reaches a confined terminus, migrating LFG tends to accumulate over time to dangerously high levels. The worst case occurs as landfill gas concentrations reach a level between the lower explosive limit (LEL, or 5% by volume) of the gas and its upper explosive limit (UEL, or 15% by volume). Such accumulations can result in serious explosion dangers to nearby homes, shops, and businesses. Lower than the LEL, there is not enough methane to explode, while above the UEL there is not enough oxygen to create a violent reaction. Explosive or not, anyone entering a confined space with such a high concentration of LFG faces the risk of asphyxiation. So what can be done about accumulated LFG, especially during the critical fourth stage of the anaerobic production of methane? Extraction systems can range from the simple passive venting of LFG to the mechanical, active extraction via a system of wells connected by header pipelines to a blower apparatus. Passive systems usually contain a series of relatively shallow wells consisting of slotted pipes set vertically through the landfill cover to a depth sufficient to tap into the accumulated gas. These wells extend through the cap via a solid pipe segment that reaches a sufficient height for safe LFG venting. Set on top of this solid pipe segment is either a U-shaped piece of tubing that gives the wellhead a “candy cane” shape or a whirligig that spins with blowing wind and creates some negative pressure to increase the extraction rate. Passive wells tend to be mostly used around the perimeter of landfills or in landfills that do not have a geomembrane final cover. Since they apply less negative pressure than an active system, each passive well’s zone of influence (the outward distance from the well point to the point where gas movements are affected by the well) is considerably smaller than those of active extraction wells. Each well has a smaller area of coverage as a result, and a passive system would need to employ many times more wells than an active system extracting the same amount of LFG. Fortunately, passive systems are usually used only in much older landfills that are well past their peak rates of gas production. Active systems apply negative air pressure to the waste mass to expedite the removal of LFG. This extraction is performed by a series of evenly spaced wells with overlapping zones of influence to ensure complete coverage. The wells are set to a depth necessary to achieve compete extraction. Each well is connected to a header pipeline encircling the landfill via lateral-branch pipelines. The header pipeline is connected to an industrial blower that applies the negative pressure needed to extract the LFG and overcome resistance head losses from the pipelines and their appurtenances. The gas is blown out to the air if the LFG concentration is relatively low, or it is ignited by a flare stack if methane is collected in significant quantities. Even if the gas concentration is too low to allow for continuous flaring, an inefficient flare operation is often preferable to the odor problems and air pollution issues resulting from direct emission from the blower. Direct operation of the LFG system is only half the story; system maintenance and monitoring is equally important. LFG extraction systems have to be balanced on a regular basis since the amount of gas generated in a particular region of the landfill can vary with time, the amount of and type of waste, and the presence of final cover. As part of the LFG system maintenance, the integrity of the final cover system needs to be maintained. Any breaches in the final cover could allow for air inflow into the waste mass. Such excess oxygen entering the landfill can alter the anaerobic nature (and resultant gas production) of the region near the breach, further unbalancing the system and even presenting a potential fire hazard. Monitoring both the pressures in the pipeline at the wellheads (in addition to standard monitoring of offsite gas migration) provides the information needed to adjust and operate the system. Particulates and Dust Dust suppressants are regularly applied to the dust-producing areas. These can include chemical sprays, water, and sometimes oil applied to dust-producing areas outside of the landfill (contaminated liquids, such as raw leachate, should not be used under any circumstances). Dust-suppressant liquids are applied either with spray hoses or (more commonly) water trucks. While spray hoses are limited to a particular area, the more mobile spray truck can maneuver over dust-generating access roads and can apply the liquid in a more complete and uniform manner. Though water is the cheapest and most common dust-suppressant liquid, other applicants are used where allowed. One example would be water-insoluble elastomeric polymers combined with oil. Though more expensive, such nonwater suppressants may actually need fewer applications. While liquids can be applied outside of the disposal area, the working face is a different matter, since landfill regulations will typically not allow the application of significant amounts of water or other liquids directly into a landfill. This is to prevent unnecessary leachate production, especially during the early stages of a cell’s operational lifetime when there is little waste in place. Certain waste deposits, such as ash from coal combustion, will be very prone to dust generation. To minimize blown dust from these types of waste, it is best to cover it with other waste or daily cover shortly after delivery. The working face is another source of particulates, blown litter, and debris. This is a significant problem on windy and dry days. Like dust, blown litter can be controlled by proper application of daily cover and by keeping the size of the working face as small as possible. As a further precaution, portable litter control fences should be erected immediately downwind of the current working face. Since litter can be blown high into the air, these fences are typically netting strung between upright flagpoles set in a heavy base (such as a used truck tire backfilled with concrete). Man-hours will have to be allocated for policing the landfill in general and picking accumulated litter from the fence in particular. Odor Control As mentioned, the vast bulk of the gas produced by a landfill is made up of carbon dioxide and methane. Ironically, both of these are odorless gasses. In fact, the odors come almost exclusively from trace gases such as hydrogen sulfide. Some odor is going to be unavoidable due to the very nature of decomposing waste. For landfills handling C&D debris, the source of the odor isn’t primarily biological (C&D debris has little if any organic material), but is caused by the hydrogen-sulfide emissions that can occur when large quantities of drywall disposed in the landfill get wet. In municipal sold waste landfills (or other landfills that receive large quantities of organic materials) the odor is caused by a mixture of hydrogen sulfide and organic compounds. In this case, the hydrogen sulfide is caused by the anaerobic bacteria that produce LFG.
Hydrogen sulfide is a flammable, poisonous gas that has an odor similar to rotten eggs. Hydrogen sulfide generated by decaying organic matter is generally the product of anaerobic digestion (the breakdown of organic material by microorganisms in the absence of oxygen). It is produced not only in solid waste landfills, but also in papermill waste, cattle feed lots, poultry farms, and sewage treatment facilities, to name a few. Any landfill receiving sludges from any of these operations is bound to have elevated production of hydrogen sulfide and other gases. Hydrogen sulfide has also been designated as a potentially lethal gas by OSHA in concentrations of 100 parts per million (0.01%) In addition to hydrogen sulfide, another odor-causing gas produced by landfills is nonmethane organic compounds. These have more of a sour smell or a cleaning solvent–type odor. Nonmethane organic compounds are complex organic compounds with more than one carbon atom. Various types combine carbon atoms with oxygen, sulfur, chlorine, and/or nitrogen. These are volatile organic compounds categorized as hazardous air pollutants. They include vinyl chloride, ethyl benzene, toluene, and benzene. While unavoidable and practically impossible to eliminate, landfill odors can be managed. First of all, most landfills have already been situated in isolated locations, usually with the aim of minimizing odor impacts on adjacent areas. People who come into contact with the landfill odor are usually transients (such as motorists traveling up a highway running adjacent to a landfill) instead of permanent residents who have to constantly put up with the odor. Isolation largely eliminates odor complaints. Odors can also be masked by other odors. Landfill odor-control sprays are, in effect, large perfume applicators with large, economy-size containers (55-gallon drums typically) holding the concentrated perfume compounds. They usually send a diluted aerosol spray of the perfume high into the air above the landfill, to be carried by the wind along with the landfill smells. These perfumes are typically water-based, nontoxic, and biodegradable compounds. Since the open face receiving waste and emitting odors is always moving, these perfume sprayers usually are mounted on mobile units that can be wheeled to various locations on the site. Another strategy for dealing with landfill odors utilizes foams that can be sprayed directly on the waste. Particles, likewise, can be spread over the landfill. These topical applications, applied at a rate between 400 and 600 pounds per acre, do not mask the landfill odor with another odor. Instead, they act like giant “odor eaters” that absorb and trap the odor-causing gases before they can escape downwind. As they absorb hydrogen sulfide or other sources of sulfur, they form a permanent chemical bond that is not broken down by moisture. Each application is subsequently buried by newer layers of deposited waste and becomes part of the landfill’s waste mass. As with most problems, the simple application of a proven remedy is often the best approach. In the case of landfill odors, this simple remedy is the use of daily and intermediate cover (for waste areas that will be exposed for extended periods) on a regular basis. Daily cover is applied to the current open working face at the end of each workday and usually consists of 6 inches or more of clean dirt spread out over the waste. However, alternate daily covers, such as tarps or spray-on foams, can be used, too. While often easier to apply and not taking up any valuable disposal airspace, tarps and foams usually cannot be used in high winds or cold-climate situations. Either way, by physically containing the waste the daily cover effectively contains the odor the waste produces. Other Issues While potentially these are all point sources of emissions, they remain of secondary importance to the landfill itself. On the landfill the best defense against poor air quality is a good final cap system and a more-than-good application of daily and intermediate cover. Daniel P. Duffy, P.E., is an environmental engineer with URS Corp.
MSW - September/October 2007
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