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| Pollution Shifting |
Pollution shifting is defined as the transfer of pollution from one medium (air, water, or soil) to another. Early legal efforts to control pollution focused on single media. For example, in the United States, the Clean Air Act covers air and the Clean Water Act covers water. However, pollution is not constrained by statute; it can shift between media by both natural and human action. Pollution management is improved when all media are considered.
Intentional pollution shifting may occur to destroy a pollutant, convert it to a safer form, or reduce its quantity or concentration. Examples of intentional pollution shifting include combustion, air stripping, air scrubbers, and adsorption. Intentional pollution shifting is accomplished by chemical reaction and/or mass transfer.
Chemical reactions can convert reactants in one media into products in a different media. In mass transfer shifting, differences in concentration are used to transfer pollutants from one media to another. For example, volatile compounds will transfer from relatively contaminated water to relatively clean air.
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Combustion, Air Stripping, and Adsorption
Combustion is the process of burning, a chemical reaction. It involves combining combustible material with oxygen under conditions that produce light and heat in addition to by-products. The combustion of wastes, such as municipal solid waste, sludge, or hazardous waste, results in gaseous emissions and a solid ash residue. It significantly reduces the volume and mass of waste requiring disposal, by shifting some wastes to gaseous form.
Although carbon dioxide has been implicated in global warming, many of the gaseous emissions have no negative health impact, such as nitrogen gas, carbon dioxide, and water vapor. However, pollutants can also be present, including nitrogen oxides, sulfur dioxide, carbon monoxide, particulate matter, metals, acid gases, dioxins, and furans.
Contaminants in exhaust gases are minimized by optimization of the combustion process, for example, maintaining proper temperature and oxygen levels. They can also be captured with pollution-control equipment, such as air scrubbers and filters. In addition, the ash may contain hazardous compounds, such as heavy metals.
In air stripping, contaminates dissolved in water are transferred to gaseous form by contact with relatively clean air, an example of mass transfer. Air stripping works best with volatile organic compounds (VOCs) and dissolved gases. VOCs are compounds with high vapor pressures, that is, compounds that tend to evaporate quickly. A common application of air stripping is the cleanup of groundwater contaminated by leaking fuel storage tanks.
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| Air stripping |
Air stripping is optimized by maximizing the surface area between the contaminated water and clean air, accomplished by creating fine water droplets in air or small air bubbles in water. Systems can be located away from the contamination (e.g., a system cleaning groundwater that is located on the earth’s surface), or located within the contaminated zone (e.g., a system located in wells installed in contaminated groundwater).
In some cases, the contaminated air from air stripping is released to the atmosphere, where the pollutants are destroyed by sunlight or reaction with other chemicals, adsorbed into soil or water, or diluted. Preferably, the organics in the exhaust from air stripping are destroyed by incineration or oxidation, or captured by adsorption.
The air stripping process may also be reversed. In air scrubbing, pollutants are transferred from contaminated air to clean water. However, a chemical reaction is often incorporated into air scrubbing, converting pollutants to a safer form.
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For example, sulfur dioxide produced during coal combustion can be removed from exhaust gas by mass transfer to water containing sodium hydroxide or carbonate, which converts the sulfur dioxide to calcium carbonate. Natural air stripping and air scrubbing also occur. Surface waters, such as lakes and oceans, serve as sinks for pollutants released to the atmosphere. Contaminated water left exposed to the atmosphere will release VOCs.
The final pollution shift considered here is adsorption, in which a contaminant in water or air is adsorbed onto a solid material. Adsorption is use for off-gas control, groundwater remediation, landfill leachate treatment, industrial wastewater treatment, and water treatment for drinking or industrial purposes.
The most commonly used adsorbent is granular activated carbon (GAC). GAC has a tremendous amount of surface area per mass, on the order of one thousand square meters per gram. Its surface attracts many organic compounds; thus, a small amount of GAC can adsorb a significan amount of organic material. GAC may be regenerated, during which con taminants are destroyed.
Multimedia Approach
The multimedia approach to environmental management considers all media. It can be applied to single facilities, entire companies, and regions. According to the U.S. Environmental Protection Agency Multimedia Enforcement Division, it can result in:
- Improved detection and resolution of environmental compliance problems
- Achievement of optimal enforcement results
- More effective enforcement
- More efficient use of resources
- Fundamental changes in the regulated community’s perceptions and behavior regarding environmental compliance
