A landfill is a carefully designed structure built into or on top of the ground in which trash is isolated from the surrounding environment (groundwater, air, rain); it is the oldest form of waste treatment. Before the introduction of landfill sites where solid wastes are being buried, refuse was just left in piles or thrown into pits. Unfortunately, most developing nations still practice the old method of landfill which is to dump refuse at any designated place with no barrier or liner. This allows leachates to percolate through the waste; pick-up harmful contaminates, and then enters into the soil and the underground aquifers. New improved and modern landfill design requires that protective liners, made of clay or plastic, provide a barrier between the disposed waste and the ground below. Now any liquid that develops among the waste is collected and treated to prevent contamination.
Environmental and Socio-Economic Impacts of Landfill Emissions
Inevitable consequences of the practice of solid waste disposal in landfills are gas and leachate generation due primarily to microbial decomposition, climatic conditions, refuse characteristics and landfilling operations. The migration of gas and leachate away from the landfill boundaries and their release into the surrounding environment present serious environmental concerns at both existing and new facilities. Besides potential health hazards, these concerns include, and are not limited to, fires and explosions, vegetation damage, unpleasant odours, landfill settlement, ground water pollution, air pollution and global warming.
Hazardous gas emissions: Overtime the degradation of wastes in the landfill results in production of leachate and emission of tons of hazardous greenhouse gases such as methane and carbon dioxide which makes up 90 to 98% of landfill gas. The remaining 2 to 10% includes nitrogen, oxygen, ammonia, sulfides, hydrogen and various other gases.
In addition to its effect in the ozone layer, methane can be highly combustible and may be responsible for various explosion hazards in and around landfills. Methane reacts with hydroxyl radicals and oxygen in the atmosphere to generate carbon dioxide within a period of days to a few years, thereby losing some of their greenhouse gas potential. Small amounts of methane are also consumed after absorption by soil. These emissions are potential threats to human health and to the quality of the environment, and contribute 20% of the global anthropogenic methane emissions. Nevertheless, control of these emissions at the source is necessary from an environmental protection viewpoint and to address the obligations under the Kyoto protocol.
Gaseous pollutants have significant effects on plants, animals and entire ecosystems. The lateral migration of gas through soil beyond landfill boundaries causes the displacement of oxygen from soil. This results in a decline in soil faunal populations and burrowing animals and causes vegetation dieback. Mainly, the vegetation around the landfill and the newly planted vegetation on a closed landfill can be damaged due to the suppression of air around the roots by migrated landfill gas.
The acidic gaseous constituents contribute to the phenomenon of acid rains and its secondary effects on the acidification of soils and ecosystems. Ammonia is a major acidic constituent which can be found in the landfill gas. It is a secondary acidifying agent following its atmospheric oxidation to nitric acid. It has effects on plants, causing a loss of stomatal control, a reduction in photosynthesis, enzyme inhibition, changes in synthetic pathways and depressed growth and yield. Hydrogen sulfide also has a considerable impact on ecosystem. It is an extremely biotoxic gas, effective at a few parts per billion (ppb) in mammals. Plants are far less sensitive to direct toxicity effects but the most severe impact on plants is inhibition and destruction of root growth and vegetation cover due to the anaerobic soil conditions created by high concentration of sulfides which laterally seep from landfill sites. Volatile Organic Compounds (VOCs) play a significant role in formation of ground level ozone. High concentrations of ground level ozone tend to inhibit the photosynthesis, reduce growth and depress the agricultural yields.
Water Quality Contamination: Leachate can migrate to groundwater or even to surface water through the flaws in the barriers or liners and this poses a serious problem as aquifers require extensive time for rehabilitation. The seepage of leachate into the underground aquifers is a clear environmental liability that degrades the quality of the water gotten from these aquifers. The main constituents of landfill leachate are dissolved methane, fatty acids, sulfate, nitrate, nitrite, phosphates, calcium, sodium, chloride, magnesium, potassium and trace metals like chromium, manganese, iron, nickel, copper, zinc, cadmium, mercury and lead. Moreover, soil can retain the constituents of the leachate like metals and nutrients and can cause adverse impacts on the eco system.
The metals retained by the soil after uptake by plants provide a key route for entry of metals into the food chain. Deposition of trace metals in the plants can affect crop growth and productivity and also pose a greater threat to animal health. Those metals such as lead, zinc and cadmium show differential mobility through the vegetation and invertebrate trophic levels. Uptake by plants is affected by soil pH and salinity and also cadmium and lead uptake is enhanced by the chloride complexation of the metals present in the leachate.
These metals can be present in the leachate either in large or small concentrations depending on the waste categories deposed in the landfills. Mercury is one of the most studied contaminants. It is one of the most toxic metals within the food chain, being readily absorbed by animals, fish and shellfish. Landfills are potential mercury emitters to the ecosystem due to the disposal of batteries and paint residues in the landfills.
Natural Habitat Degradation: The construction and management of landfills have ecological effects that may lead to landscape changes, loss of habitats and displacement of fauna. Often, this fertility cannot be completely reclaimed, even after the landfill is capped. Eutrophication is the most extensive threat when the leachate is mixed with the surface water with higher concentrations of nitrate and phosphates. Eutrophic conditions invariably cause excessive production of planktonic algae and cyanobacteria in the open sectors of the lakes. This excessive production of algae results in adverse impacts on fish species in the lake by limiting the light penetration into the lake.
Socio-Economic: Apart from the environmental impacts, landfills are sources for several socio-economic impacts like public health issues due to the exposure to landfill gas, ground and surface water contamination by landfill leachate, diffusion of litter into the wider environment and inadequate on-site recycling activities. Nuisances such as flies, foul odours, smoke and noise are frequently cited among the reasons why people do not want to reside close to landfills. Although modern landfill sites are well designed to reduce emissions, the emissions from landfills continue to give rise to concerns about the health effects on people living and working near these sites, both new and old. Several studies revealed that there is a higher risk of developing cancer among the people near landfill sites and the elevated risks were observed for cancers of the stomach, liver and intrahepatic bile ducts and trachea, bronchus, lung, cervix and prostate. The exposure to contaminants and emissions can be through direct contact, inhalation or ingestion of contaminated food and water.
Siting Resistance and Regulation: Site selection of waste management facilities can be a major issue as all infrastructural projects have the capacity to damage the ecology of the site on which they are developed, causing landscape changes, loss of habitats and displacement of fauna.
No one wants to live near a landfill, and as residential areas develop, it becomes more difficult to find land that is suitable for dumping and amenable to the surrounding population. Couple this with increasing regulation, and it becomes more difficult to efficiently and diplomatically site a landfill.
In addition to the health issues, landfills create considerable impacts on land value, land degradation and land availability. It has been observed that residential areas sited close to landfills are likely to have adverse impacts on housing values depending upon the actual distance from the landfill.
The Way Forward
Emissions from landfills can contribute directly to climate change when organic waste is left to biodegrade in a landfill. The solution is to either prevent organic waste being sent to landfill by separating at source or pre-processing the waste or, as a secondary measure, to capture the methane being emitted from the landfill or turn it into energy.
In light of the deleterious effects of landfills, it is necessary to utilize the methane gas that is being generated from these landfills as an energy source can be utilized to power our homes, factories and machineries. Instead of landfills being a nuisance and risk to the environment, the LFG that is extracted through wells in landfills can be used for electricity generation, direct use, cogeneration in combined heat and power projects (CHP), or used as alternate fuels, mostly in industrial units.
Utilization and Management of emissions from landfills
The gas being generated from landfills is a complex mixture of different gases created by the action of microorganisms within a landfill. Landfill gas is approximately forty to sixty percent methane, with the remainder being mostly carbon dioxide. Trace amounts of other volatile organic compounds comprise the remainder (<1%). Methane is a potent greenhouse gas 28 to 36 times more effective than CO2 at trapping heat in the atmosphere over a 100-year period, per the latest Intergovernmental Panel on Climate Change (IPCC) assessment report (AR5).
Landfill gases undergo three processes which are:
- Evaporation of volatile organic compounds (e.g., solvents)
- Chemical reactions between waste components
- Microbial action, especially methanogenesis.
The first two depend strongly on the nature of the waste. The dominant process in most landfills is the third process whereby anaerobic bacteria decompose organic waste to produce biogas, which consists of methane and carbon dioxide together with traces of other compounds. Despite the heterogeneity of waste, the evolution of gases follows well defined kinetic pattern. Formation of methane and carbon dioxide commences about six months after depositing the landfill material. The evolution of gas reaches a maximum at about 20 years, then declines over the course of decades.
The gas generated from landfills must be treated to remove impurities, condensate, and particulates. The treatment system depends on the end use; minimal treatment is needed for the direct use of gas in boiler, furnaces, or kilns. Using the gas in electricity generation typically requires more in-depth treatment. Treatment systems are divided into primary and secondary treatment processing. Primary processing systems remove moisture and particulates. Gas cooling and compression are common in primary processing. Secondary treatment systems employ multiple clean-up processes, physical and chemical, depending on the specifications of the end use. Two constituents that may need to be removed are siloxanes and sulfur compounds, which are damaging to equipment and significantly increase maintenance cost. Adsorption and absorption are the most common technologies used in secondary treatment processing.
The gases that are produced within a landfill can be collected and used in various ways; it can be utilized directly on site by a boiler or any type of combustion system, providing heat. If the landfill gas extraction rate is large enough, a gas turbine or internal combustion engine could be used to produce electricity to sell commercially or use on site. The electricity is generated on site through the use of microturbines, steam turbines, or fuel cells. The landfill gas can also be sold off site and channelled into natural gas pipelines commercially. This approach requires the gas to be processed into pipeline quality by removing various contaminants and components. The efficiency of the gas being collected at landfills directly impacts the amount of energy that can be recovered – closed landfills (those no longer accepting waste) collect gas more efficiently than open landfills (those that are still accepting waste).
Safety and Monitoring: Landfill gas emissions can lead to environmental, hygiene and security problems in the landfill. Due to the risk presented by landfill gas, there is a clear need to monitor gas produced by landfills. In addition to the risk of fire and explosion, gas migration in the subsurface can result to contact of landfill gas with the groundwater if not properly managed. This, in turn, can result in contamination of groundwater by organic compounds present in nearly all landfill gas.
For the reason that gases produced by landfills are both valuable and sometimes hazardous, monitoring techniques have been developed;
- Flame ionization detectors can be used to measure methane levels as well as total VOC levels.
- Surface monitoring and sub-surface monitoring as well as monitoring of the ambient air are carried out. In the U.S., under the Clean Air Act of 1996, it is required that many large landfills install gas collection and control systems, which means that at the very least the facilities must collect and flare the gas.
Landfills are a potential threat to the quality of our environment due to their environmental, health and socio-economic impacts that are caused due to the emissions of greenhouse gases like methane and carbon dioxide which depletes the ozone layer and the leachates which seeps into the underground aquifers to degrade the water body. These impacts can be managed and channelled to be beneficial. The construction of modern landfills with well-engineered and managed disposal facilities can significantly lessen the impacts of landfill on soil, air, and water. Landfills that are well-designed and operated ensure compliance with environmental preservation requirements and it ultimately ensures that the environment is free from contaminants. The use of such designs also ensure that the landfills are not located in environmentally-sensitive areas and are incorporated with on-site environmental monitoring systems. With on-site environmental monitoring systems, signs of land fill gas and groundwater contamination can be easily detected and controlled.