Encyclopedia of Global Resources part 140 pptx

Vermiculite Category: Mineral and other nonliving resources Where Found Vermiculite is found in various parts of the world.. Technical Definition Vermiculite is the geological name given

mina is smelted by subjecting it to electrolytic

reduc-tion in a molten bath of natural or synthetic cryolite to

produce aluminum metal In 2008, six companies

operated fourteen primary aluminum smelters The

smelting required a huge amount of electricity, which

hydroelectric dams produced relatively cheaply The

operation of those smelters placed Venezuela fifteenth

in production of aluminum, behind Mozambique but

ahead of Tajikistan In the same year, the country

ac-counted for 2.9 percent of the world’s bauxite and 1.4

percent of aluminum output Venezuela consistently

ranks in the top twenty-five exporters of aluminum

The main importers of Venezuelan aluminum are

the United States, Mexico, Japan, the Netherlands,

and Colombia, in descending order

Other Resources

Venezuela also is a producer of other mineral

com-modities, although none holds more than minor

rank-ing in global exports These minerals are sulfur (6

percent), feldspar (2 percent), and silica sand (1

per-cent) Miscellaneous commodities ranking less than 1

percent include coal, lead, zinc, copper, nickel, gold,

titanium, diamonds, and uranium Most of these

com-modities come from mining activities in the Andes

Mountains or Guiana Highlands

Richard A Crooker

Further Reading

Arnold, Guy The Resources of the Third World Chicago:

Taylor and Francis, 1997

Crooker, Richard A Venezuela Philadelphia: Chelsea

House, 2006

Kogel, Jessica Elzea, et al., eds Industrial Minerals and

Rocks: Commodities, Markets, and Uses 7th ed

Little-ton, Colo.: U.S Society for Mining, Metallurgy, and

Exploration, 2006

Kozloff, Nikolas Hugo Chávez: Oil, Politics, and the

Challenge to the United States New York: Palgrave

Macmillan, 2006

Salazar-Carrillo, Jorge, and Bernadette West Oil and

Development in Venezuela During the Twentieth Century.

Westport, Conn.: Praeger, 2004

Web Sites

Central Intelligence Agency

The World Fact Book

https://www.cia.gov/library/publications/the-world-factbook/index.html

Energy Information Administration: Official Energy Statistics from the U.S Government Venezuela Natural Gas

http://www.eia.doe.gov/cabs/Venezuela/

NaturalGas.html International Trade Centre Countries

http://www.intracen.org/menus/countries.htm U.S Geological Survey

2006 Minerals Yearbook, Venezuela http://minerals.usgs.gov/minerals/pubs/country/ 2006/myb3-2006-ve.pdf

U.S Geological Survey Aluminum

http://minerals.usgs.gov/minerals/pubs/

commodity/aluminum/mcs-2009-alumi.pdf U.S Geological Survey

Bauxite and Alumina http://minerals.usgs.gov/minerals/pubs/

commodity/bauxite/myb1-2007-bauxi.pdf U.S Geological Survey

Iron http://minerals.usgs.gov/minerals/pubs/

commodity/iron_ore/mcs-2009-feore.pdf See also: Aluminum; Developing countries; Energy politics; Hydroenergy; Iron; Oil and natural gas distri-bution; Organization of Petroleum Exporting Coun-tries; Resources as a source of international conflict

Vermiculite

Category: Mineral and other nonliving resources Where Found

Vermiculite is found in various parts of the world Commercial mines for vermiculite are located in Aus-tralia, Brazil, China, Russia, Kenya, Zimbabwe, South Africa, and the United States

Primary Uses Vermiculite has a number of applications in a variety

of industries Some uses for vermiculite include con-struction, agricultural, and horticultural applications

It is also used as fire protection, as insulation, and

in various industrial markets Vermiculite is used as

packaging material for safe shipment of hazardous

compounds Recent uses include nanocomposites for

films and coatings

Technical Definition

Vermiculite is the geological name given to a large

group of hydrated laminar, or layered, minerals that

are aluminum-iron magnesium silicates that resemble

mica in appearance Vermiculite is a member of the

phyllosilicate group of minerals, a group with the

characteristic property of expanding into long,

worm-like strands with heating This expansion process is

called exfoliation and forms the basis for commercial

use of the mineral

Commercial vermiculite typically contains 38 to 46

percent silicon dioxide (SiO2), 16 to 35 percent

mag-nesium oxide (MgO), 10 to 16 percent aluminum

oxide (A12O3), 8 to 16 percent water, and smaller

amounts of several other chemicals When

vermicu-lite is heated and expanded, a color change occurs

that depends on the chemicals present and the

tem-perature of the furnace Generally, however,

vermicu-lite is gold-brown in color When vermicuvermicu-lite is heated,

it increases ten to thirty times in

vol-ume The bulk density of crude

ver-miculite is approximately 640 to 1,120

kilograms per cubic meter

Depend-ing on the size of the granules, the

bulk density of expanded vermiculite

is about 64 to 160 kilograms per cubic

meter

Description, Distribution, and

Forms

The name vermiculite is derived from

a combination of the Latin word

ver-miculare, meaning “to breed worms,”

and the English suffix “-ite,” which

means mineral or rock The term

“ver-miculite” applies to a group of

miner-als that have the property of

expand-ing into long, wormlike particles when

heated When vermiculite ores

exfoli-ate, they expand to many times their

original volume

There are two key components of

vermiculite’s unique properties First,

vermiculite has a laminar, or layered,

crystalline structure with connected

layers that expand or unfold linearly,

like an accordion The second key component is trapped water held within vermiculite When vermic-ulite is heated, this water is rapidly converted into steam, which forces the layers to separate and open,

or exfoliate After exfoliation, the lightweight mate-rial that results is chemically inert, fire-resistant, and odorless In its expanded form, vermiculite has very low density and thermal conductivity, which makes it useful in many applications The surface area of exfo-liated vermiculite is large and chemically active, a fea-ture that makes it useful in some chemical processes

as an absorbent

Several naturally occurring vermiculite minerals and soils exist, and the identification of specific ones requires scientific analysis One of the common forms

of vermiculite, however, is known as commercial ver-miculite This is the form that is mined and processed for various industrial and residential uses Vermiculite ores from mines are derived from rocks that contain large crystals of the minerals biotite and iron-bearing phlogopite Chemically, vermiculite is a hydrated mag-nesium aluminum silicate

All vermiculite ores contain a range of other

Vermiculite is used in construction, agriculture, and horticulture (USGS)

erals that were formed along with the vermiculite

in the rock Although vermiculite ores from some

sources have been found to contain asbestos, asbestos

is not intrinsic to vermiculite Only a few vermiculite

ores have been found to contain asbestos and

gener-ally not more than trace amounts One vermiculite

mine, in Libby, Montana, was found to be

contami-nated with substantial amounts of asbestos and was

subsequently closed down Overall, vermiculite is

clas-sified a “generally recognized as safe” (GRAS)

min-eral, a designation bestowed by the U.S Food and

Drug Administration

History

Most accounts indicate that vermiculite and its unique

properties were known as early as 1824, when Thomas

Webb experimented with the mineral in Worcester,

Massachusetts During his experimentation, he

ob-served that heating the mineral resulted in the

forma-tion of long, wormlike particles Because of this

prop-erty, he named the mineral vermiculite, or worm

breeder, because the heated mineral looked like a

mass of worms Other accounts suggest that

vermicu-lite was discovered in 1881 in Libby, Montana, by gold miners, and that in 1919, Edward Alley discovered its unique properties

Vermiculite was thought to be mostly a scientific curiosity until the early 1900’s, when more practical uses for the mineral were discovered In 1915, the first commercial mining effort of vermiculite was initiated

in Colorado, where the mineral was sold as tung ash There were not enough buyers, however, and the min-ing effort failed The Zonolite Company started the first successful vermiculite mine in 1923 in Libby, Montana In 1963, W R Grace bought the Zonolite mine, which continued to operate until 1990 While

in operation, this mine produced about 80 percent of the world’s vermiculite supply Vermiculite from the Libby mine was found to be contaminated with a toxic form of naturally occurring asbestos

Obtaining Vermiculite Obtaining vermiculite requires mining There are many commercial mining operations throughout the world Locations of some of the predominant com-mercial mines are in Australia, Brazil, China, Kenya,

Data from the U.S Geological Survey, U.S Government Printing Office, 2009.

Source: Mineral Commodity Summaries, 2009

110,000

25,000

200,000

100,000

25,000

Metric Tons

210,000 180,000

150,000 120,000

90,000 60,000

30,000 Zimbabwe

Russia

China

Brazil

Australia

South Africa

United States

Other countries

15,000 20,000

15,000

Vermiculite: World Mine Production, 2008

South Africa, the United States, and Zimbabwe.

As of 2009, the largest vermiculite mining

opera-tion in the world was located in the Phalaborwa,

or Palabora, region of northeastern Transvaal in

South Africa Other large mining operations are

located in the northwestern corners of China and

in the United States, along the eastern

Appala-chian range (in Virginia and South Carolina)

Some other countries producing significant

amounts of vermiculite include Russia, Brazil,

and Japan

Vermiculite mines are surface operations in

which ore is separated from other minerals Rocks

containing vermiculite are detonated and the

loose rocks are fed through crushers and screens

to separate the vermiculite from surrounding

rocks Vermiculite flakes are shipped to

exfolia-tion plants, where they are heated in a furnace to

approximately 540° to 810° Celsius, which causes

trapped water to convert rapidly to steam and

ver-miculite flakes to expand into wormlike particles

Vermiculite ores may also contain other

materi-als, such as mica, quartz, feldspar, and possibly

as-bestos None of the mines in operation poses an

asbestos health risk

Uses of Vermiculite

Vermiculite has thousands of applications in a variety

of industries and has been in use for more than eighty

years Vermiculite is used in construction,

agricul-tural, horticulagricul-tural, and industrial markets It has

ap-plications ranging from use as building insulation to

improving potting soil It is used by pool contractors,

by greenhouse growers, in fireproofing, and in many

other commercial businesses

Vermiculite has been used extensively as a soil

con-ditioner and as an amendment in potting soils It is

used in soil mixes for root cuttings, seed germination,

turf grass, plantings, and gardens Recently,

vermicu-lite has been used increasingly in hydroponic

garden-ing and for water conservation Vermiculite improves

soil aeration and drainage, while retaining moisture

and nutrients necessary for plant growth Vermiculite

is readily mixed with soil, peat, composted bark, and

organic compost and creates air channels to allow the

soil mix to breathe, while at the same time holding

water and nutrients needed by the plant When used

as a carrier for fertilizers, pesticides, or herbicides, or

as a bulking agent, vermiculite ensures better

distri-bution Vermiculite has cation exchange properties,

which help the growing plant access necessary nutri-ents such as ammonium, potassium, calcium, and magnesium In the agricultural industry, vermiculite

is used in animal feed as a carrier for supplements and nutrients

In construction, vermiculite is used in acoustic fin-ishes, in lightweight insulating concrete, in gypsum plaster, and as loft insulation and fire protection Ver-miculite can be used in combination with many typi-cal binders, such as portland cement, clay, gypsum, and resins In pools, vermiculite has been used in place of packed sand Vermiculite has been used as loose-fill insulation in insulated masonry wall systems and as a lightweight aggregate for plaster by mixing with either gypsum or portland cement Vermiculite is

a major ingredient in most fireproof door cores and safes Vermiculite is ideal for filling gaps or spaces in existing insulation and was one of the first home in-sulation products used in the United States When ground into a powder, vermiculite is useful as filler in paints, plastics, and other materials

The absorption properties of vermiculite make it useful as an absorbent packaging material for safe shipment of hazardous liquids It can hold liquids such as oils, nutrients, chemical mixtures, and special

Light aggregates 35%

Horticulture 30%

Insulation 5%

Other 30%

Summaries, 2009

Data from the U.S Geological Survey,

U.S Government Printing Office, 2009.

U.S End Uses of Vermiculite

coatings It is also used to transport liquids such as

fer-tilizers, herbicides, and insecticides, as free-flow

sol-ids Vermiculite is also used to insulate cryogenic

tanks Vermiculite is used in fixation of hazardous

material and for nuclear-waste disposal In the

auto-mobile industry, vermiculite is used in brake pads and

shoes

C J Stewart

Further Reading

Kogel, Jessica Elzea, et al., eds Industrial Minerals and

Rocks: Commodities, Markets, and Uses 7th ed

Little-ton, Colo.: U.S Society for Mining, Metallurgy, and

Exploration, 2006

Middleton, Gerald V., et al Encyclopedia of Sediments

and Sedimentary Rocks New York: Springer, 2003.

Prothero, Donald R., and Frederic L Schwab

Sedi-mentary Geology: An Introduction to SediSedi-mentary Rocks

and Stratigraphy 2d ed New York: Freeman and

Company, 2004

Velde, Bruce Origin and Mineralogy of Clays New York:

Springer, 1995

Web Site

The Vermiculite Association

http://www.vermiculite.org

See also: Agricultural products; Agriculture

indus-try; Agronomy; Clays; Fertilizers; Minerals, structure

and physical properties of; Sand and gravel; Silicates;

Soil

Volcanoes

Category: Geological processes and formations

Volcanoes or volcanic activity can be a valuable source

of natural resources Some of the economically

impor-tant resources derived from volcanic activity are

dia-monds, precious metallic minerals, native sulfur, and

a nutrient-rich soil produced by the weathering of

vol-canic rock.

Background

All volcanoes are related to the process of plate

tec-tonics Plate tectonics describes the continual

move-ment of immense sections (plates) of the Earth’s crust

relative to one another Although this process is

in-credibly slow, geologic time is equally long Both earthquakes and volcanoes most often occur along the boundaries of these plates They result from the buildup of intense pressure as one plate collides with,

or slides past, another Here old crustal rock is melted

as it plunges down into the upper mantle, or new rock forms as magma squeezes out from great fissures in the crust In the process, old crustal rock is recycled to form new rock that is rich in mineral resources Major metallic mineral deposits from around the world are associated with plate boundaries past and present The island of Cyprus is rich in copper that once formed on the seafloor of an ancient oceanic spreading center The same process has been happen-ing in the Red Sea, where copper-rich minerals are be-ing extruded through volcanic activity

The best evidence for submarine deposition of sul-fide minerals by volcanic activity comes from struc-tures called hydrothermal vents, also known as “black smokers.” In appearance, they resemble underwater geysers with cone-type vents emitting black smoke They result from the seepage of seawater into the hot oceanic basalt crust This heated seawater then inter-acts with the basalt by extracting iron, copper, sulfur, and other metals from it Once this mixture erupts onto the seafloor, it mixes with the cold seawater and precipitates sulfide minerals into massive deposits These become the resources for the future

Volcanoes come in three basic types, based on their particular chemistry They are named for the volcanic rock produced by each: basalt, andesite, and rhyolite The most common type of volcano is the basaltic vari-ety Varieties of basaltic volcanoes can be found along plate boundaries as well as plate centers (such as the one where the Hawaiian Islands formed) The princi-pal rock that underlies the world’s oceans is also basalt Basaltic Volcanoes

Basaltic volcanoes are usually low in silica (approxi-mately 50 percent) and gas content This type of vol-cano commonly produces fast-moving lava flows and

is generally not explosive The only mineral that is consistently associated with basaltic volcanoes is sul-fur It forms from sulfur-rich gases that escape from fissures in the cooling lava rock As the hot gases es-cape, sulfur quickly crystallizes, with its distinctive yel-low color present on the rock Sulfur is mined at vari-ous volcanic locations One is Mount Etna on the island of Sicily, where it is an important economic re-source

Andesitic Volcanoes

The second type of volcano results from andesitic

magma It is richer in silica (approximately 60

per-cent) and gas than basaltic volcanoes are This results

in a volcano that can be explosive and can produce a

large quantity of lava, depending upon slight

varia-tions in its chemical composition Volcanoes such as

this can be extremely dangerous since no one is ever

certain what will happen each time they erupt

Mount St Helens in the state of Washington and

Mount Fuji in Japan are two examples of andesitic

vol-canoes, which can remain dormant for hundreds of

years and then suddenly erupt The 1980 eruption of

Mount St Helens devastated the area around it In

the aftermath, a rich volcanic ash covered the region

Despite the fact that considerable vegetation was de-stroyed by the eruption and associated flooding, vig-orous plant life returned within a couple of years This was possible because of the nutrient-rich ash that cre-ated a new soil

Rhyolitic Volcanoes

A magma of rhyolitic composition produces the third volcanic type Compared to the other two, rhyolitic magma is the richest in both its silica (approximately

70 percent) and its gas content Both gases and fluids present are rich in dissolved metallic minerals The magma, as it nears the Earth’s surface, first cracks crustal rock and then may erupt with a violent explo-sion

Beginning of eruption at summit

Lava flow and deposition; eruption

at lower elevations

Subsidence or collapse of summit

Cooling; cessation of activity

Volcanic Eruption and Caldera Formation

Depending on their type and size, volcanoes produce craters or larger calderas Ancient calderas are the sites of many ore deposits; some more re-cent calderas are regions of geothermal energy.

Often, large hydrothermal mineral deposits are

as-sociated with rhyolitic volcanoes These are deposits of

various minerals such as malachite, chalcopyrite, and

pyrite, where a metallic element like copper or iron is

bonded with sulfur or bonded to a carbonate

mole-cule Such minerals tend to occur in veins where the

mineral-rich fluids penetrate fissures in existing rock

and then crystallize during cooling Often gold and/or

silver are deposited in this manner Although such

de-posits are common, they do not usually occur in large

quantities Most often, huge amounts of rock must be

mined in order to extract relatively small amounts of

the valuable metals The great Bingham copper mine

in Utah is an excellent example of such a deposit

Diamond Pipes

One important occurrence of a valuable mineral

as-sociated with volcanic activity is the diamond pipe

Diamond formation is typically associated with a

high-pressure, high-temperature environment Such

con-ditions are present in the Earth’s upper mantle at depths of approximately 200 kilometers Here dia-monds slowly crystallize within magma As a result of rapid upward movement, the diamonds are carried along with the magma column Eventually, upon cool-ing, the magma will form a pipe structure In shape it somewhat resembles a champagne glass

Most volcanic pipes do not reach the surface and produce a volcano The more probable situation is that they remain underground as a magma source for an erupting volcano In those pipes which contain diamonds, the diamonds are disseminated through-out a rock called kimberlite Erosion may eventually destroy evidence of the volcano, exposing the dia-mond pipe Erosion also acts as a natural means of ex-tracting the diamonds and then depositing them as sediment in rivers or on beaches The most important diamond pipes include those of South Africa, Siberia, and western Australia

Paul P Sipiera

A 1954 photograph detailing the eruption of Kilauea Volcano at Hawaii Volcanoes National Park (USGS)

Further Reading

Coleman, Robert G Geologic Evolution of the Red Sea.

New York: Oxford University Press, 1993

Decker, Robert, and Barbara Decker Volcanoes 4th

ed New York: W H Freeman, 2006

Francis, Peter, and Clive Oppenheimer Volcanoes 2d

ed New York: Oxford University Press, 2004

Martí, Joan, and Gerald Ernst, eds Volcanoes and the

Environment New York: Cambridge University

Press, 2005

Parfitt, Elisabeth A., and Lionel Wilson Fundamentals

of Physical Volcanology Malden, Mass.: Blackwell,

2008

Schmincke, Hans-Ulrich Volcanism New York:

Springer, 2004

Stanton, R L Ore Elements in Arc Lavas New York:

Ox-ford University Press, 1994

Tarbuck, Edward J., and Frederick K Lutgens Earth:

An Introduction to Physical Geology 9th ed Illustrated

by Dennis Tasa Upper Saddle River, N.J.: Pearson

Prentice Hall, 2008

Wood, Charles A., and Jürgen Kienle, eds Volcanoes of North America: United States and Canada New York:

Cambridge University Press, 1990

Zeilinga de Boer, Jelle, and Donald Theodore

San-ders Volcanoes in Human History: The Far-Reaching Effects of Major Eruptions Princeton, N.J.: Princeton

University Press, 2005

Web Sites U.S Geological Survey

Volcanoes, by I Robert Tilling: On-Line Edition

http://pubs.usgs.gov/gip/volc U.S Geological Survey, Volcano Hazards Program

About U.S Volcanoes http://volcanoes.usgs.gov/about See also: Diamond; Earth’s crust; Igneous processes, rocks, and mineral deposits; Magma crystallization; Placer deposits; Plate tectonics; Plutonic rocks and mineral deposits; Seafloor spreading

Waste management and sewage

disposal

Category: Pollution and waste disposal

Wastewater consists of domestic and industrial

efflu-ent that is collected by a sewage system and conveyed to

a central plant, where it is treated prior to release into

the ground or, more usually, into a surface

water-course For public health considerations, the proper

dis-posal of wastewater is a critical parameter in

environ-mental planning.

Background

The Minoan civilization on the island of Crete near

Greece had one of the earliest known sewage

collec-tion systems in the world (c 1600 b.c.e.) Ancient

Greece had hot and cold water plumbing systems A

large sewer known as the Cloaca Maxima was built

during the sixth century b.c.e in ancient Rome to

drain the Forum The Romans also reused public

bathing water to flush public toilets London had a

drainage system by the thirteenth century, but

efflu-ent could not be discharged into it until 1815 Sewers

were constructed in Paris before the sixteenth

cen-tury but fewer than 5 percent of the homes were

con-nected to the system by 1893 In general, the

wide-spread introduction of sewage collection systems in

densely populated areas did not occur until the

mid-nineteenth century For example, the first sewer that

was carefully engineered was constructed in

Ham-burg, Germany, in 1848

Wastewater disposal systems usually consist of a

col-lection system of sewer pipes of varying diameters and

materials, a treatment plant of varying size and level

of treatment, and an outfall The outfall may be to the

ground or, more commonly, to a receiving

water-course such as a stream or (typically along a coast) the

ocean Older wastewater systems are generally

com-bined—domestic, industrial, and storm-water runoff

are conveyed in the same pipe to the treatment plant

Although cheaper to build initially, combined systems

are less desirable, as most of the effluent must bypass

the treatment plant during storms, when street runoff

increases rapidly Modern wastewater systems are

de-signed to be separate, with different pipes for waste-water and storm runoff

Wherever possible, sewage systems are designed to

be below the depth of frost and at a slope that allows gravity drainage In some low-lying locations and other areas with low relief, the effluent must be pumped, a process that adds expense

Wastewater Characteristics About 60 to 75 percent of the water supplied to a com-munity will wind up as effluent or spent water which must be treated and disposed of The remaining water

is used in industrial processes, lawn sprinkling, and other types of consumptive use Domestic sewage con-tains varying proportions of human excrement, pa-per, soap, dirt, food waste, and other substances Much of the waste substance is organic and can be used by organisms of decay (saprophytic microorgan-isms) Accordingly, domestic sewage is biodegrad-able (putrescible) and capbiodegrad-able of producing offensive odors The composition of industrial waste varies from relatively clean rinse water to effluent that can contain corrosive, toxic, flammable, or even explosive materials This is why communities usually insist on some form of pretreatment by industry before the ef-fluent enters the treatment plant

The organic material in sewage is decomposed by aerobic (oxygen-requiring) bacteria However, the oxygen that is dissolved in water (DO) can be used up

in the process of microbial decomposition If too much organic waste enters the water body, the bio-chemical oxygen demand (BOD) can exhaust the DO

in the water to the extent that the aquatic ecosystem is damaged Most species of fish die if the DO concen-tration falls below 4 milligrams per liter for periods of time Some species, such as trout, are even more sensi-tive to DO levels and do best when DO is 8 milligrams per liter or higher

The function of wastewater treatment plants is to produce a discharge that is free of odors, suspended solids, and objectionable bacteria Coliform bacteria, which are common in the lower intestines of mam-mals, may not be pathogenic themselves but are taken

as an indicator of contamination in the watercourse

Treatment processes are often categorized as

pri-mary, secondary, or tertiary The distinction among

the three processes is somewhat arbitrary, but the

main point is that higher levels of treatment result in a

more purified discharge that becomes increasingly

more expensive to attain Primary treatment is mostly

mechanical, as it involves the removal of floating and

suspended solids by screening and sedimentation in

settling basins As an optional step, chemicals that

flocculate or precipitate solids may be added as a

means of speeding the process This type of treatment

can remove 40 to 90 percent of the suspended solids

and 25 to 85 percent of the BOD The final effluent

may be chlorinated prior to release into a receiving

watercourse

Secondary treatment involves biological

process-ing after the wastewater has been through primary

treatment One of the two forms of biological

process-ing is by means of a tricklprocess-ing filter, in which wastewater

is sprayed over crushed stone and allowed to flow in

thin films over biologic growths that cover the stone

The organisms in the biologic growths, which include bacteria, fungi, and protozoa, decompose the dis-solved organic materials in the wastewater Some of the breakdown products in the wastewater, such as carbon dioxide, escape into the atmosphere; others, such as nitrate, which is a mobile ion, remain in solu-tion Still others are absorbed into the cells of the bio-logic growths These growths eventually slough off and are carried to settling tanks by the flow of the wastewater The other type of secondary treatment is known as the activated sludge process In this proce-dure, flocs of bacteria, fungi, and protozoa are stirred into the wastewater with results that are about the same as trickling filters Depending upon the effi-ciency of the plant and the nature of the incoming wastewater, both types of biological processes can re-move 50 to 95 percent of the suspended solids and BOD The efficiency of secondary treatment can be seriously lowered if the design capacity of the plant

is overloaded with excessive effluent coming from storm runoff in combined sewers This is one

A worker operates a garbage truck at the Norcal Waste facility in San Francisco (Getty Images)