16.4: Waste Disposal

There are three primary methods for waste disposal: open dumps, sanitary landfills, and incineration. Sanitary landfills and incineration prevent reuse, recycling, and proper decomposition. While open dumps promote decomposition better than other methods of waste disposal and allow discarded materials to be salvaged or recycled, they promote disease spread and cause water pollution. They are thus illegal in many countries.

Open Dumps

Open dumps involve simply piling up trash in a designated area and is thus the easiest method of waste disposal (figure \(\PageIndex{a}\)). Open dumps can support populations of organisms that house and transmit disease (reservoirs and vectors, respectively). Additionally, contaminants from the trash mix with rain water forming leachate, which infiltrates into the ground or runs off. This liquid leachate may contain toxic chemicals such as dioxin (a persistent organic pollutant), mercury, and pesticides.

Figure \(\PageIndex{a}\): An open dump in Vietnam. Image by Julien Belli (CC-BY).

Sanitary Landfills

Despite advances in reuse and recycling, landfill disposal in sanitary landfills remains the primary waste disposal method in the United States. The rate of municipal solid waste (MSW) generation continues to increase, but overall landfill capacity is decreasing. Older existing landfills are increasingly reaching their authorized capacity. New regulations concerning proper waste disposal and the use of innovative liner systems to minimize the potential of groundwater contamination from leachate infiltration and migration have resulted in a substantial increase in the costs of landfill disposal. Finally, public opposition delays or, in many cases, prevents the approval of new landfills or expansion of existing facilities. Public opposition is partially inspired by memories of historic uncontrolled dumping practices the resulting undesirable side effects of uncontrolled vectors, contaminated groundwater, unmitigated odors, and subsequent diminished property values.

In sanitary landfills, trash is sealed from the top and the bottom to reduce contamination of surroundings (figure \(\PageIndex{c}\)). Rainwater that percolates through a sanitary landfill is collected in the bottom liner, and this bottom layer thus prevents contamination of groundwater. The groundwater near the landfill is closely monitored for signs of contamination from the leachate. Layers of soil on top prevent disease spread. Each day after garbage is dumped in the landfill, it is covered with clay or plastic to prevent redistribution by animals or the wind.

Figure \(\PageIndex{b}\): A sanitary landfill in 1972. Image by Bill ShroutEPA (public domain).

Figure \(\PageIndex{c}\): A sanitary landfill. Trash is compacted and stored underground. A landfill liner prevents leachate from draining into the aquifer and contaminating groundwater, which is monitored using a well. Additionally, leachate is collected and treated (leachate treatment system). A clay cap prevents animals or wind from accessing the trash. A methane gas recovery system captures the potent greenhouse gas, which can be used as a biofuel. Image by EPA/National Energy Education Development Project (public domain).

Several practices can reduce the environmental impact of sanitary landfills. Compacting in landfills reduces water and oxygen levels, slowing decomposition and promoting methane release. In the U.S., the Clear Air Act requires that landfills of a certain size collect landfill gas (biogas), which can be used as a biofuel for heating or electricity generation. Other gases such as ammonia and hydrogen sulfide may also be released by the landfill, contributing to air pollution. These gases are also monitored and, if necessary, collected for disposal. To address the often dry condition of wastes within landfills, the concept of bioreactor landfills has emerged. These recirculate leachate and/or inject other liquids to increase moisture and promote decomposition (and therefore increasing the rate of biogas production). Upon closure, many landfills undergo "land recycling" and can be redeveloped as golf courses, recreational parks, and other beneficial uses.

Incineration

Incineration is simply burning trash. This has several advantages: it reduces volume and can be used to generate electricity (waste-to-energy). In fact, the sheer volume of the waste is reduced by about 85%. Incineration is costly, however, and it pollutes air and water. Air pollutants released by incineration include particulates, sulfur dioxide, nitrogen oxides, methane, heavy metals (such as lead and mercury), and dioxins. The byproduct of incineration, ash, is often toxic. Depending on its composition, ash might require special disposal; other types of ash can be repurposed. Incineration is widely used in developed countries due to landfill space limitations. It is estimated that about 130 million tons of waste are annually combusted in more than 600 plants in 35 countries. Further, incineration is often used to effectively mitigate hazardous wastes such as chlorinated hydrocarbons, oils, solvents, medical wastes, and pesticides.

An incinerator processes trash and burns it in a combustion chamber (figure \(\PageIndex{d-e}\)). The heat boils water, and the resultant steam is used to generate electricity. The smoke (called flue gases) goes through a pollution-removal before it is released, but it still contains some pollutants. The U.S. incinerated 11.8% of MSW in 2018.

Figure \(\PageIndex{d}\): Wheelabrator Technologies' Waste-to-Energy plant in Saugus, Massachusetts has been in service since 1975. Image and caption (modified) from Fletcher6 (CC-BY).

Figure \(\PageIndex{e}\): A schematic diagram of a waste incineration facility (waste-to energy system). A grabber collects the waste and transfers it into the incinerator. Heat from combusting the trash is used to produce steam and generate electricity. Emissions (flue gases) pass through a scrubber and particulate removal system to limit pollution before the gases are released through a chimney. Ash remains after incineration. Image by Kaza, Silpa; Bhada-Tata, Perinaz. 2018. Decision Maker’s Guides for Solid Waste Management Technologies. Urban Development Series Knowledge Papers;. World Bank, Washington, DC. © World Bank. (CC-BY)

There are two kinds of waste-to-energy systems: mass burn incinerators and refuse-derived incinerators. In mass burn incinerators all of the solid waste is incinerated. The heat from the incineration process is used to produce steam. This steam is used to drive electric power generators. Acid gases from the burning are removed by chemical scrubbers. Any particulates (small particles that remain suspended in the air) in the combustion gases are removed by electrostatic precipitators, which charge particulates and remove them with electrodes. The cleaned gases are then released into the atmosphere through a tall stack. The ashes from the combustion are sent to a landfill for disposal.

It is best if only combustible items (paper, wood products, and plastics) are burned. In a refuse-derived incinerator, non-combustible materials are separated from the waste. Items such as glass and metals may be recycled. The combustible wastes are then formed into fuel pellets which can be burned in standard steam boilers. This system has the advantage of removing potentially harmful materials from waste before it is burned. It also provides for some recycling of materials.

Attribution

Modified by Kyle Whittinghill and Melissa Ha from the following sources:

• Systems of Waste Management from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (CC-BY). Download for free at CNX.
• Solid Waste from AP Environmental Science by University of California College Prep (CC-BY). Download for free at CNX.
• Biomass explained: Landfill gas and biogas. 2020. EIA. Accessed 01-18-2021 (public domain).
• 15.2 Waste Management Strategies from Essentials of Environmental Science by Kamala Dorsner and CK-12 (CC-BY)