Open refuse dumps are not only a blight on the landscape, but they can also pose a health and safety risk to humans. Waste that is dumped onto landfill sites consists of both organic and inorganic waste. Rotting organic waste presents a health hazard when it decomposes aerobically on the surface of the landfill by attracting flies that can carry disease. However, the problem does not end there – as the organic material is covered up and compacted, it decomposes in an oxygen-free environment by anaerobic bacteria, producing methane, as a byproduct of the decomposition process. In addition, any water that enters the landfill site can leach toxins from the waste, which can filter through the soil and contaminate groundwater if not managed properly.
Environmental Problems Caused by Landfill Gas
According to the Agency for Toxic Substances and Disease Registry, landfill gas is composed of approximately 45-60% methane, 40-60% carbon dioxide, 2-5% nitrogen, and trace amounts of oxygen, ammonia, NMOCs (non-methane organic compounds), sulphides, hydrogen and carbon monoxide. The U.S. Environmental Protection Agency lists landfill gas as the third largest contributor of methane emissions from human-related sources in the US, topped only by methane produced by fermentation in livestock, and methane emissions due to natural gas extraction. Landfill gas forms under the layers of accumulated waste, and when the concentrations become high, the gas will move from a high concentration to a low concentration, migrating upwards or sideways, depending on the structure and porosity of the surrounding soil. If the landfill gas permeates upwards, it makes its way to the surface through air pockets; if it migrates sideways, it can enter underground manholes, pipes, or basements, through cracks, or it may eventually surface some distance from the landfill site. High concentrations of landfill gas on a landfill site can be extremely hazardous to workers managing the site, and to neighboring communities. Due to its colorless, odorless and highly flammable properties, it is not easily detected, and can easily explode or cause asphyxiation in confined spaces. Several incidences of landfill gas explosions with casualties and/or fatalities have been recorded off-site following landfill gas migrating to neighboring structures.
Environmental Problems Caused by Leachate
Water is a powerful corrosive agent, and in a landfill site heavy metals, chemicals, and hazardous toxins can easily enter the groundwater through runoff or leaching if safety measures are not taken. If toxins enter the groundwater they can impact the natural environment and can also pose a risk to human health if this water is used as a source of drinking water, or for recreational activities. Many environmental pollutants bioaccumulate, becoming more concentrated in animals further up the food chain, and can effect the biology and health of higher organisms – including humans – that feed on contaminated prey.
Design and Construction of a Municipal Solid Waste Landfill Site
To prevent hazardous landfill gas and leachate from escaping from municipal solid waste landfill sites and contaminating the surrounding atmosphere and groundwater, some form of landfill management is required to deal with the gas and leachate that accumulates within the waste. Since 1979 various federal regulations have been implemented in the US to prevent migration of methane in landfill gas, and to control landfill gas emissions in larger landfill sites. Modern municipal solid waste landfill sites are constructed using special linings and drainage mechanisms to collect both gas and leachate, to prevent contamination of the surrounding environment, and to minimize the health risk to local communities.
Leachate Collection System
A well designed municipal solid waste landfill site usually consists of a base layer of compacted clay, onto which a durable impermeable bottom plastic liner is placed to prevent leachate from escaping and contaminating the groundwater. Perforated or slotted pipes are placed onto this bottom plastic layer to collect the leachate that drains from the waste. The leachate that is collected flows into a leachate collection pipe, and is then pumped to a separate leachate collection pond, where it may be treated on-site to remove chemical and biological contaminants, or it may be released to a waste water treatment plant for treatment. The bottom plastic liner may sometimes be sandwiched between a geotextile mat to protect it from being punctured by surrounding rocks and gravel. This is then covered with a drainage layer consisting of several layers: gravel, course sand, and soil, to allow leachate to drain from the waste above. Waste is then added to the landfill, where it is compacted into cells and covered with soil. More waste is added on top of these older cells and these are again covered with soil. An older landfill site will consist of several layers of compacted cells.
Landfill Gas Collection System
In addition to the leachate collection system, a well run municipal solid waste landfill site will also have a methane gas collection system to collect and manage the landfill gas that accumulates underground. In larger landfill sites this is essential to prevent the gas from freely venting to the atmosphere, migrating to neighboring communities, and to prevent fires or explosions occurring both on- and off-site. Impermeable liners (plastic, clay or geotextile matting) on the bottom, sides, and top of the landfill can be used to contain landfill gas, and to channel it to preferred migration pathways for collection. The method used to collect the gas may consist of either a passive collection system, or an active collection system. Both methods of collection typically consist of a series of vertical wells that are drilled throughout the site, and possibly also horizontal wells positioned below the surface layers to facilitate free movement of gas to the recovery point. Drilling the wells is quite an intensive operation, as there are many layers of waste, made up of all sorts of materials, which have to be drilled through. Once the wells have been drilled, a heavy-duty perforated or slotted polyethylene pipe is inserted into the wells. The pipe is surrounded by stone to facilitate the free movement of gas into the pipe. The top six meters of the pipe consists of a solid, unperforated section of pipe, surrounded by a less porous clay material to seal the well off from the atmosphere. This is to prevent the gas from escaping near the surface, and in an active collection system, it also prevents air from getting sucked into the pipes.
In a passive gas collection system, the gas moves through the collection system passively, due to differences in pressure gradients. An active gas collection system consists of more elaborate features, such as mainline piping fitted with control valves, pressure gauges, and sampling ports for adequate monitoring and control. It also includes the addition of a number of blowers or pumps that are attached to the pipes, creating a vacuum, which actively sucks the gas up from underground. The gas is then piped through the mainline to a central collection point, where it is controlled and treated to minimize environmental health and safety risks.
Landfill Gas Treatment
The gas collected may be treated in various ways. Gas collected using the passive collection method may simply be released to the atmosphere through vents. This method is not very environmentally friendly as methane, a potent greenhouse gas that adds to the problem of global warming, together with other hazardous pollutants, are released untreated.
Alternatively, the gas can be flared – burning off the methane – but carbon dioxide, a greenhouse gas less potent than methane, is released into the atmosphere. While both these methods reduce the concentrations of underground methane, and therefore the potential for explosions, neither of these two methods utilizes the methane component of landfill gas as an energy resource.
Another method of dealing with the collected landfill gas through combustion, is to use the gas to fuel boilers, gas turbines or combustion engines to generate energy or electricity. Many municipal solid waste landfill sites have realized the potential of landfill gas as an energy source, and have infrastructure in place to not only collect landfill gas, but to use it to produce energy. Methane is converted to carbon dioxide, which although still a greenhouse gas, is twenty times less potent than methane, and therefore poses a lesser impact when released to the atmosphere. However, there is a concern among certain groups that pollutants such as carbon monoxide, as well as nitrogen oxides and sulphur oxides – which are acidic, and can cause acid rain – and particulates, including highly toxic dioxins, and other hazardous pollutants such as mercury, are released into the atmosphere when landfill gas is combusted either by flaring, or to power engines that produce energy. Dioxins are known to cause cancer at extremely low concentrations, and can cause reproductive and developmental defects at even lower concentrations. In addition, they can damage the immune system, cause hormonal imbalances, and many other health problems. Mercury is another pollutant that is commonly found in landfill gas. Mercury can cause cancer, birth defects, and other serious health problems. Mercury is fat soluble, and consequently it is stored in the fatty tissue of animals that are exposed to it. As a result, it becomes more concentrated up the food chain, posing a risk to human health if contaminated animal products are consumed. Because of these risks, The Energy Justice Network recommends that the gas should be filtered through carbon filters to remove toxic contaminants before burning to produce energy. They also argue that landfill gas cannot be considered a clean, green, or renewable source of energy for a number of reasons. Firstly, due to the polluting nature of landfill gas emissions, much of which is not recovered, they consider it not to be clean; secondly, they advocate measures to reduce, reuse, and recycle the waste at source, and composting, as green alternatives to reduce landfill gas emissions; thirdly, the fact that landfill sites have a limited lifespan makes them a non-renewable resource in their opinion.
There are, however, cleaner methods of dealing with landfill gas, which do not involve combustion. These include both energy recovery, and gas-to-product conversion technologies. In both these methods the landfill gas undergoes a pretreatment to remove contaminants before being processed further. Energy recovery technologies generate energy directly from the recovered landfill gas by processing it through fuel cells. A series of chemical reactions take place within the system, releasing water, electricity, heat and waste gases. Gas-to-product conversion technologies convert landfill gas into commercially viable products such as methanol, compressed or liquefied natural gas, or purified carbon dioxide and methane. In both cases, waste gases are burned off in a flare, but these are environmentally safe, as the pollutants are removed in the pretreatment process.
Alternative Management Options
In order to get the most efficient return of energy from organic waste, another solution would be to have separate landfills designated specifically for organic waste disposal. These organic waste landfill sites would accumulate a relatively clean source of methane-rich gas, free from pollutants acquired from other waste products. Alternatively, the organic waste component could be composted and used as clean fill, or to enrich soils in garden landscaping. Compost derived from landfill waste is not recommended for food production as pathogens may be present.
Viability of Landfill Gas to Energy Projects
The infrastructure required to recover methane from a landfill site to generate electricity requires a substantial financial outlay; therefore a feasibility study to determine the economic viability of such a project, needs to be undertaken. Various factors need to be taken into account, including: size of the landfill site in terms of area, depth and waste generated; the potential lifespan of the landfill site; the climatic conditions – very cold or very dry climates can limit gas production; and the distance and accessibility of the landfill site to the end user of the energy source.
Landfill sites will be with us for a long time to come. As long as organic waste matter is being generated and added to landfills, they will continue to produce methane, and continue to do so years after closure. It makes sense to tap into this energy source to both reduce its impact on the environment, and to use it as an alternative source of energy, where viable. Whether it can be classified as clean, green, renewable energy in the same light as wind-, hydro-, or solar power is debatable. But just leaving it to vent off into the atmosphere seems counterproductive to improving energy efficiency, and reducing our carbon footprint. Alternative sources of energy often come with costs and benefits to the environment. Both wind power and hydro-electric power are produced with costs to the environment. Wind turbines cause high bird mortality due to collisions; hydro-electric dams disrupt the natural flow and functioning of river ecosystems, and methane is produced as vegetation decomposes in the anaerobic muddy sediments on the floor of the dam. The benefits are that they both offer a clean source of renewable energy – but are they truly green? Landfill gas may not be as clean, green, or renewable as we would like, but it is readily available, in constant supply, and by utilizing this resource we are minimizing the impact that it would otherwise be having on the environment and human health, and reducing our need for fossil fuels.
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