Scrubbers are air-pollution-control devices that remove harmful gases and particulates from the smokestacks of incinerators, chemical manufacturing facilities, and electric power plants before they enter the atmosphere. There are different types of scrubbers, including wet and dry, regenerative and nonregenerative. Regenerative scrubbers recycle the material that extracts the pollutants.

The nonregenerative wet scrubber is most commonly used to capture sulfur dioxide emitted from coal and oil burning power plants. It works by spraying limestone and water slurry into the flue gases. Sulfur dioxide reacts with limestone to form gypsum or calcium sulfate. The gypsum sludge is disposed of in landfills or recycled in saleable byproducts such as wallboard, concrete, and fertilizer.

Regenerative scrubbers can also be used; one reacts sodium sulfite with sulfur dioxide to form sodium bisulfite, from which sodium sulfite is recovered by adding alkali. The released sulfur is trapped in water to produce sulfuric acid, which is sold to offset the cost of installing the scrubber.

Particulates can be removed using venturi and centrifugal or condensation scrubbers. Flue gas enters through the top of the cone-shaped venturi scrubber and water, injected horizontally, forms droplets that absorb dust and other particles. The resulting slurry discharges from the bottom of the unit or can be separated from the clean gas by centrifugation or spinning at high speed. Copper oxide regenerable scrubbers that absorb sulfur and simultaneously convert nitrogen oxides to nitrogen are being researched.

In 1971 the EPA set a maximum limit on sulfur dioxide in air. To help meet this limit, revisions to the Clean Air Act in 1977 required all new power plants to install scrubbers to remove sulfur dioxide. Most spray tower scrubbers remove at least 90 percent of sulfur dioxide, according to the EPA.

Wet scrubber diagram
Wet scrubber diagram

In 1990 further revisions to the Clean Air Act under the Acid Rain Program allotted allowable amounts of sulfur dioxide emissions to electric utilities, which could trade allowances to meet their quotas. Sulfur dioxide emissions from power plants in 2001 were 33 percent lower than in 1990 and 5 percent lower than in 2000 according to the EPA.



The reuse of products, materials, and parts can have significant environmental and economic benefits. Waste is not just created when consumers throw items away. Waste is generated throughout the life cycle of a product, from extraction of raw materials, to transportation to processing and manufacturing facilities, to manufacture and use.

Reusing items or making them with less material decrease waste dramatically. Ultimately, less material will need to be recycled or sent to landfills or waste-combustion facilities.

Used goods are widely available to industries, businesses, institutions, and individuals. There are secondhand markets for entire industrial production facilities, such as breweries and chemical production plants, as well as for industrial, construction, and medical equipment.

Used goods for individuals include cars, clothes, books, furniture, household items, sports equipment, and musical instruments. Sources of used goods include on-line auctions and markets, secondhand merchandise stores, classified advertisements, estate sales, auctions, rummage sales, yard sales, salvage yards, materials exchanges, trash salvaging or “dumpster diving.”

Amount of Reuse

In the United States, several secondhand markets are $100 billion dollar industries, and several more fall in the $1 to $10 billion range. Each year 40 million used cars are sold in the United States, nearly three times the number of new cars purchased.

Overall, secondhand markets are almost as large as consumer recycling in terms of the amount of material processed (approximately fifty million tons of paper and ten million tons of glass are recycled annually in the United States), and the economic value of secondhand markets is far greater than those for recycling.

Used cars
Used cars

A considerable percentage of secondhand goods are exported from the United States, especially clothing; automobiles; and industrial, construction, and medical equipment. In a number of countries, including the Czech Republic, Nigeria, Uganda, and Zimbabwe, imports of used clothing compete strongly with the domestic production of new clothes.

Theory of Reuse

Reuse can reduce the pollution and resource use associated with manufacturing a new item, and can delay or eliminate disposal of the item. In order to experience the greatest environmental benefits, reuse of an item needs to replace, at least partially, the purchase and production of a new item.

In some situations, reuse may not incur any real benefits. For example, if a car owner sells or gives a car to someone who would not otherwise possess a car, and then buys a new car to replace the old one, the result is that there are now two operating cars rather than one. In other situations, the reuse of an item may have zero effect on the production or purchase of new items.

For example, if someone buys a “white elephant” at a rummage sale (perhaps a necklace or a used compact disc), that purchase will not in any way prevent or replace the purchase of a new item. However, even if reuse has no tangible environmental benefits, it can have economic and social welfare benefits.

If the car example above is reconsidered, for instance, two people, not just one, now own a useful vehicle. In the compact disc example, the buyer acquires another disc for his or her pleasure, and the seller earns some perhaps much needed cash.

Reuse can replace the production and purchase of new items, especially when the first owner does not sell in order to be able to buy a new item. Examples of this sort include clothes and furniture, which are typically given away or sold at low prices by the first owners, and which second-hand buyers often buy instead of new items.

Role of Government and Industry

The U.S. government is one of the largest purveyors of used goods in the United States; it regularly sells surplus items through sealed bids, auctions, silent auctions, and fixed-price sales. On the other hand, government regulations largely prevent the purchase of used items by the U.S. government and require the labeling of products containing used parts in a way that may discourage the use of used parts by industry.

There are both incentives and disincentives for reuse by industry. Reuse, remanufacturing, repair, and refurbishment of products and parts can be economically beneficial for industry. For example, used copiers are often remanufactured and refurbished. A number of companies now sell modular, reusable carpet. On the other hand, firms in some cases have an incentive to discourage reuse of their products, in order to maintain and increase production of new goods.

Reuse by the Individual

Individuals can maximize the environmental and economic benefits of their own reuse efforts by carefully contemplating their reuse strategies, by developing the ability to make repairs, and by learning about local sources of used goods and replacement parts. The environmental and economic benefits of reuse typically increase as the size and cost of the item increase. For example, new furniture is both resource-intensive and expensive.

Repair, repainting, and reupholstering of used furniture can replace the purchase of new furniture. The regular repair of shoes can considerably extend their life. Used clothing, ranging from designer clothes at consignment stores to basic items at rummage sales, is widely available.

Used books, sports equipment, and musical instruments are also available at local stores and on-line. Used building materials (doors, windows, hardware, etc.) are increasingly available at salvage yards such as Urban Ore in Berkeley, California.

Reuse can have significant environmental and economic benefits by replacing the purchase of a new item. Secondhand items range from large industrial facilities and equipment to cars, sports equipment, clothes, and toys for individuals. Businesses can benefit from secondhand markets both by buying secondhand equipment and by selling surplus equipment for reuse.

Individuals can make a valuable contribution to the environment and their own finances by learning to make repairs, by wisely shopping for secondhand goods, and by selling or donating their unwanted goods so that others may use them.



Sediments in the aquatic ecosystem are analogous to soil in the terrestrial ecosystem as they are the source of substrate nutrients, and micro- and macroflora and -fauna that are the basis of support to living aquatic resources.

Sediments are the key catalysts of environmental food cycles and the dynamics of water quality. Aquatic sediments are derived from and composed of natural physical, chemical, and biological components generally related to their watersheds.

Sediments range in particle distribution from micron-sized clay particles through silt, sand, gravel, rock, and boulders. Sediments originate from bed load transport, beach and bank erosion, and land runoff. They are naturally sorted by size through prevalent hydrodynamic conditions.

In general, fast-moving water will contain coarse-grained sediments and quiescent water will contain fine-grained sediments. Mineralogical characteristics of sediments vary widely and reflect watershed characteristics. Organic material in sediments is derived from the decomposed tissues of plants and animals, from aquatic and terrestrial sources, and from various point and nonpoint wastewater discharges.

The content of organic matter increases in concentration as the size of sediment mineral particles decreases. Dissolved chemicals in the overlying and sediment pore waters are a product of inorganic and organic sedimentary materials, as well as runoff and ground water that range from fresh to marine in salinity.

This sediment/water environment varies significantly over space and time and its characteristics are driven by complex biogeochemical interaction between the inorganic, living, and nonliving organic components. The sediment biotic community includes micro-, meso-, and macrofauna and -flora that are interdependent of each other and their host sediment’s biogeochemical characteristics.

Sedimentation is the direct result of the loss (erosion) of sediments from other aquatic areas or land-based areas. Sedimentation can be detrimental or beneficial to aquatic environments. Moreover, sediment impoverishment (erosion or lack of replenishment) in an area can be as bad as too much sedimentation. Sedimentation in one area is linked to erosion or impoverishment in another area and is a natural process of all water bodies (i.e., lakes, rivers, estuaries, coastal zones, and even the deep ocean).

As an example, detrimental effects can be related to the burial of bottom-dwelling organisms and beneficial effects can be related to the building of new substrates for the development of marshes. These natural physical processes will continue whether or not they are influenced by the activities of humankind.

Human activities, however, have significantly enhanced sedimentation as well as sediment loss. Sedimentation activities can be land-based (i.e., agriculture, forestry, construction, urbanization, recreation) and water-based (i.e., dams, navigation, port activities, drag fishing, channelization, water diversions, wetlands loss, other large-scale hydrological modifications). Sediment impoverishment or loss is generally due to retention behind dams, bank or beach protection activities, water diversions, and many of the aquatic activities cited here.

Morphological changes (physical changes over a large area) to large aquatic systems can also result in major changes in natural sediment erosion and sedimentation patterns. As an example, the change in the size and shape of a water body will result in new water flow patterns leading to erosion or sediment removal from sensitive areas.

The environmental impacts of sedimentation include the following: loss of important or sensitive aquatic habitat, decrease in fishery resources, loss of recreation attributes, loss of coral reef communities, human health concerns, changes in fish migration, increases in erosion, loss of wetlands, nutrient balance changes, circulation changes, increases in turbidity, loss of submerged vegetation, and coastline alteration.

Abatement or control of sedimentation can be successful if implemented on a broad land area or watershed scale and is directly related to improvement in land-use practices. Agriculture and forestry (logging) improvements where soil loss is minimized are not only technically feasible: They can be carried out at a moderate cost and with net benefits.

The U.S. Department of Agriculture has a wide range of training and implementation programs for these types of activities. The United Nations Environmental Programme also has global programs, their Regional Seas activities, to guide countries in the management of land-based activities negatively impacting the coastal zone.

Improved land-use practices are the primary measures to control sediment sources: terracing, low tillage, modified cropping, reduced agricultural intensity (e.g., no-till buffer zones), and wetlands construction as sediment interceptors. Forestry practices such as clear-cutting to the water’s edge without replacement tree planting must be seriously curtailed because base soil in exposed areas will erode and import sediment to sensitive aqueous areas.

Wetlands that separate upland areas from aquatic areas serve as natural filters for the runoff from the adjacent land. Wetlands thus serve to trap soil particles and associated agricultural contaminants. The construction of natural buffer zones and wetlands replenishment adjacent to logging areas are effective techniques.

Watershed construction activities such as port expansion, water diversions, channel deepening, and new channel construction must undergo a complete environmental assessment, coupled with predictive sediment resuspension and transport modeling, so alternative courses of action and activities to minimize the negative impacts of sedimentation may be chosen.

Sediment impoverishment is equally important in coastal areas, such as coastal Louisiana where twenty-five to thirty square miles of wetlands are being lost each year. This loss primarily results from the Mississippi River levee system halting the annual natural replenishment of sediments that rebuilds the marsh system.

Engineered water diversion can replace sediment in the natural system to decrease losses due to dams, levees, jetties, and other structures built to control the flow of water and thus sediments. Proper placement of sediments from navigation dredging can also be a useful abatement technique.

Sediments are absolutely necessary for aquatic plant and animal life. Managed properly, sediments are a resource; improper sediment management results in the destruction of aquatic habitat that would have otherwise depended on their presence.

The United Nations Group of Experts on the Scientific Aspects of Marine Environmental Protection recently recognized that on a global basis, changes in sediment flows are one of the five most serious problems affecting the quality and uses of the marine and coastal environment.



Mined ores are processed to concentrate the minerals of interest. In the case of metal ores, these mineral concentrates usually need to be further processed to separate the metal from other elements in the ore minerals.

Smelting is the process of separating the metal from impurities by heating the concentrate to a high temperature to cause the metal to melt. Smelting the concentrate produces a metal or a high-grade metallic mixture along with a solid waste product called slag.

The principal sources of pollution caused by smelting are contaminantladen air emissions and process wastes such as wastewater and slag.

One type of pollution attributed to air emissions is acid rain. The smelting of sulfide ores results in the emission of sulfur dioxide gas, which reacts chemically in the atmosphere to form a sulfuric acid mist. As this acid rain falls to the earth, it increases the acidity of soils, streams, and lakes, harming the health of vegetation and fish and wildlife populations.

In older smelters, air emissions contained elevated levels of various metals. Copper and selenium, for example, which can be released from copper smelters, are essential to organisms as trace elements, but they are toxic if they are overabundant.

Furnace diagram
Furnace diagram

These metals can contaminate the soil in the vicinity of smelters, destroying much of the vegetation. In addition, particulate matter emitted from smelters may include oxides of such toxic metals as arsenic (cumulative poison), cadmium (heart disease), and mercury (nerve damage).

When compared to pollution caused by air emissions, process wastes and slag are of less concern. In modern smelters, much of the wastewater generated is returned to the process. If the economic value of the metal concentrate in slag is high enough, the slag may be returned to the process, thereby reducing the amount requiring permanent disposal.

New technologies are playing an important role in reducing or even preventing smelter pollution. Older smelters emitted most of the sulfur dioxide generated, and now almost all of it is captured prior to emission using new technologies, such as electrostatic precipitators, which capture dust particles and return them to the process. Raw material substitution or elimination, such as recycling lead batteries and aluminum cans, decreases the need to process ore, which reduces pollution.

Some of the major federal statutes and regulations that apply to smelting are the same as those that have applied to mining since the Clean Air Act (CAA) of 1970 became law. The CAA established nationally uniform standards that control particular hazardous air pollutants.

Sudbury, in Ontario, Canada, is one of the world’s largest smelting complexes, with an international reputation as a highly polluted area that has been mined for more than one hundred years. The environmental impact was completely or partially denuded vegetation on over 46,000 hectares and 7,000 acid-damaged lakes.

Smelting caused much of the ecological damage via acid rain and elevated levels of copper and nickel in the vicinity of the smelters. Efforts by government and industry since the 1970s have eliminated most of the sulfur dioxide emissions in the area, and there has been significant progress toward achieving sustainable ecosystems.



Originally, the term smog was coined to describe the mixture of smoke and fog that lowered visibility and led to respiratory problems in industrial cities. More recently, the term has come to mean any decrease in air quality whether associated with reduced visibility or a noticeable impact on human health.

Smog occurs when emissions of gases and particles from industrial or transportation sources are trapped by the local meteorology so the concentrations rise and chemical reactions occur. It is common to distinguish between two types of smog: London smog and Los Angeles smog.

London, or sulphurous, smog was noted following the introduction of coal into cities. It is most prevalent in the fall or winter when cool conditions naturally produce a thick surface fog. This fog mixes with the smoke and gases from burning coal to produce a dark, thick, acrid sulphurous atmosphere.

Normally, the unpolluted fog would disperse during the day and be reformed at night. However, the presence of smoke particles makes the fog so thick that sunlight cannot penetrate it and so only a major change in meteorology can disperse it. The smog has been shown to contribute to an increased death rate, primarily due to respiratory problems.

The most notable example of this kind of smog occurred in London, from December 4 to 10, 1954, when some four thousand deaths in excess of normal averages resulted. A similar episode in Donora, Pennsylvania, in 1948 involved approximately twenty excess deaths. Most jurisdictions have instituted control measures to prevent this level of disaster from happening again.

They have moved industries out of cities, demanded lower industrial emissions, and increased the heights of smokestacks so emissions are not trapped by local meteorology. These approaches have been largely successful, at least in controlling the most extreme events.

Thick smog
Thick smog

Los Angeles, or photochemical, smog first became apparent in the late 1940s in warm sunny cities that did not have significant coal-burning industries. It is a daytime phenomenon characterized by a white haze and contains oxidants, such as ozone, that cause eyes to water, breathing to become labored, and plants to be damaged.

It results from the action of sunlight on the combination of hydrocarbons and nitrogen oxides (NOx), known as precursor gases. These are emitted from combustion sources to produce a range of oxidized products and oxidants.

These compounds have been shown to produce respiratory and cardiac problems in individuals sensitive to pollution, and the damage inflicted on crops can cause significant decreases in yield. In most cities, the automobile is the primary contributor of smog’s precursor gases.

As the name would suggest, the most notable example of this type of smog occurs in Los Angeles, California, but it has also been experienced in a large number of cities where the weather is dry, sunlight is plentiful, and there are many automobiles or petroleum industries (e.g., Houston, Athens, and Mexico City.)

The control of photochemical smog is more difficult than for sulphurous smog because the compounds responsible for human and crop impacts are not directly emitted, but produced by chemistry in the atmosphere. Thus, greater knowledge on the emissions of gases, their reactions in the atmosphere, and their lifetime is needed. Most jurisdictions continue to focus their control strategies on reducing ozone concentrations, although particle concentrations are receiving increasing attention.

Another thick smog
Another thick smog

Because smog results from the sunlight-initiated chemistry of hydrocarbons and nitrous oxides, the most common approach to smog control is to decrease the emission of these compounds at their source. Lower volatility gasolines and systems to capture gasoline vapors are used to reduce hydrocarbon emissions while tailpipe controls (catalytic converters) reduce emissions of both hydrocarbons and nitrogen oxides.

The emission control systems of the twenty-first century mean that a car typically emits 70 percent less nitrogen oxides and 80 to 90 percent less hydrocarbons than the uncontrolled cars of the 1960s. The expected improvement in air quality, as a result of increasing controls, is estimated by using computer models of the atmosphere and its chemistry.

Soil Pollution

Polluted soil
Polluted soil

Soil pollution comprises the pollution of soils with materials, mostly chemicals, that are out of place or are present at concentrations higher than normal which may have adverse effects on humans or other organisms.

It is difficult to define soil pollution exactly because different opinions exist on how to characterize a pollutant; while some consider the use of pesticides acceptable if their effect does not exceed the intended result, others do not consider any use of pesticides or even chemical fertilizers acceptable.

However, soil pollution is also caused by means other than the direct addition of xenobiotic (man-made) chemicals such as agricultural runoff waters, industrial waste materials, acidic precipitates, and radioactive fallout.

Both organic (those that contain carbon) and inorganic (those that don’t) contaminants are important in soil. The most prominent chemical groups of organic contaminants are fuel hydrocarbons, polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), chlorinated aromatic compounds, detergents, and pesticides.

Inorganic species include nitrates, phosphates, and heavy metals such as cadmium, chromium and lead; inorganic acids; and radionuclides (radioactive substances). Among the sources of these contaminants are agricultural runoffs, acidic precipitates, industrial waste materials, and radioactive fallout.

Soil pollution can lead to water pollution if toxic chemicals leach into groundwater, or if contaminated runoff reaches streams, lakes, or oceans. Soil also naturally contributes to air pollution by releasing volatile compounds into the atmosphere. Nitrogen escapes through ammonia volatilization and denitrification. The decomposition of organic materials in soil can release sulfur dioxide and other sulfur compounds, causing acid rain.

Heavy metals and other potentially toxic elements are the most serious soil pollutants in sewage. Sewage sludge contains heavy metals and, if applied repeatedly or in large amounts, the treated soil may accumulate heavy metals and consequently become unable to even support plant life.

In addition, chemicals that are not water soluble contaminate plants that grow on polluted soils, and they also tend to accumulate increasingly toward the top of the food chain.

Soil pollution by industrial sewage
Soil pollution by industrial sewage

The banning of the pesticide DDT in the United States resulted from its tendency to become more and more concentrated as it moved from soil to worms or fish, and then to birds and their eggs. This occurred as creatures higher on the food chain ingested animals that were already contaminated with the pesticide from eating plants and other lower animals.

Lake Michigan, as an example, has 2 parts per trillion (ppt) of DDT in the water, 14 parts per billion (ppb) in the bottom mud, 410 ppb in amphipods (tiny water fleas and similar creatures), 3 to 6 parts per million (ppm) in fish such as coho salmon and lake trout, and as much as 99 ppm in herring gulls at the top of the food chain.

The ever-increasing pollution of the environment has been one of the greatest concerns for science and the general public in the last fifty years. The rapid industrialization of agriculture, expansion of the chemical industry, and the need to generate cheap forms of energy has caused the continuous release of man-made organic chemicals into natural ecosystems. Consequently, the atmosphere, bodies of water, and many soil environments have become polluted by a large variety of toxic compounds.

Many of these compounds at high concentrations or following prolonged exposure have the potential to produce adverse effects in humans and other organisms: These include the danger of acute toxicity, mutagenesis (genetic changes), carcinogenesis, and teratogenesis (birth defects) for humans and other organisms. Some of these man-made toxic compounds are also resistant to physical, chemical, or biological degradation and thus represent an environmental burden of considerable magnitude.

Numerous attempts are being made to decontaminate polluted soils, including an array of both in situ (on-site, in the soil) and off-site (removal of contaminated soil for treatment) techniques. None of these is ideal for remediating contaminated soils, and often, more than one of the techniques may be necessary to optimize the cleanup effort.

The most common decontamination method for polluted soils is to remove the soil and deposit it in landfills or to incinerate it. These methods, however, often exchange one problem for another: landfilling merely confines the polluted soil while doing little to decontaminate it, and incineration removes toxic organic chemicals from the soil, but subsequently releases them into the air, in the process causing air pollution.

For the removal and recovery of heavy metals various soil washing techniques have been developed including physical methods, such as attrition scrubbing and wet-screening, and chemical methods consisting of treatments with organic and inorganic acids, bases, salts and chelating agents.

For example, chemicals used to extract radionuclides and toxic metals include hydrochloric, nitric, phosphoric and citric acids, sodium carbonate and sodium hydroxide and the chelating agents EDTA and DTPA. The problem with these methods, however, is again that they generate secondary waste products that may require additional hazardous waste treatments.

Spraying pesticide
Spraying pesticide

In contrast to the previously described methods, in situ methods are used directly at the contamination site. In this case, soil does not need to be excavated, and therefore the chance of causing further environmental harm is minimized. In situ biodegradation involves the enhancement of naturally occurring microorganisms by artificially stimulating their numbers and activity. The microorganisms then assist in degrading the soil contaminants.

A number of environmental, chemical, and management factors affect the biodegradation of soil pollutants, including moisture content, pH, temperature, the microbial community that is present, and the availability of nutrients. Biodegradation is facilitated by aerobic soil conditions and soil pH in the neutral range (between pH 5.5 to 8.0), with an optimum reading occurring at approximately pH 7, and a temperature in the range of 20 to 30°C.

These physical parameters can be influenced, thereby promoting the microorganisms’ ability to degrade chemical contaminants. Of all the econtamination methods bioremediation appears to be the least damaging and most environmentally acceptable technique.