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Activated alumina
See Technology
Adsorption systems
See Process
Aeration systems
See Process
Air Stripping Systems
See Process
A measure of water’s ability to neutralize acids.
A geological layer or “formation,” typically sand, gravel, or limestone, that can store and transmit groundwater and allow it to be pumped in “useful” quantities.
A poisonous element that commonly occurs in the natural environment and can contaminate groundwater. Drinking arsenic-rich water over months and years causes arsenic to accumulate in the body. This can lead to arsenicosis.
The long-term poisoning that results from drinking water with high arsenic levels. The effects of arsenicosis include skin problems, cancers, and vascular disease.
Artificial recharge
The process of adding surface water to ground water reservoirs through human actions like induced infiltration or injection wells.
Atmospheric vapor
The water vapor suspended in Earth’s atmosphere as part of the natural water cycle. Atmospheric vapor condenses and falls to Earth as precipitation.


Single-celled organisms that lack a cell nucleus. They are found in all living things and in all environments. Some bacteria can cause disease.
Bag and cartridge filters
See Technology
Biosand Filter
See Technology
Long shaft drilled into the earth for the construction of water wells.
Bottled Water
Water that is packaged and sold in individual bottles.
Slightly salty water. Brackish water at the Earth’s surface often occurs in estuaries and lagoons where freshwater and seawater mix.


A geographical area, defined by topography, from which all runoff water will drain into a single river system or reservoir. Often used as a synonym for a watershed or a river basin.
Ceramic Filtration
See Technology
Chemical Disinfection
See Process
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An element that occurs naturally in salts. Chlorine is often employed in a gaseous, liquid, or solid form to purify water.
Chlorine dioxide
See Technology
An acute infection caused by the bacteria Vibrio cholerae, which may be found in water contaminated by human feces. Cholera causes severe diarrhea and can be fatal.
Climate change
Alterations in the Earth’s temperature and weather patterns through time. The current scientific consensus links recent warming with human activities such as the burning of fossil fuels.
A substance that, when introduced to water, induces particulates to lump together for easier removal.
See Process
The process by which a vapor becomes a liquid.
Confined aquifers
An aquifer that is located, or “confined,” between impermeable layers of rock or clay.
Conjunctive use
The joint management of surface water and ground water resources.
An eye infection commonly known as “pink eye.” It is an inflammation of the clear membrane that covers the white part of the eye and the inner surface of the eye lid. It may be caused by a variety of viruses and bacteria.
Any harmful or undesirable substance found in water. Contaminants include microorganisms, dissolved naturally occurring minerals, human-generated chemicals, and radiological materials.
The presence of pollutants or other unwanted materials in water.
Conventional Coagulation-Flocculation-Sedimentation
See Technology
Conventional Filtration
See Technology
A waterborne microorganism (protozoa) that causes gastrointestinal illness (cryptosporidiosis) including diarrhea and vomiting. These tiny pathogens are found in surface water sources like reservoirs, lakes, and rivers.


Delayed runoff
A management process that slows water runoff to protect stream organisms or so that more may be collected for human use.
The removal of salts from brackish water or seawater.
Diarrheal diseases
Intestinal diseases, such as cholera, that may cause dangerous dehydration. Diarrhea may be caused by viruses, bacteria, or parasites.
Diatomaceous Earth Filtration
See Technology
Diffused Air
See Technology
Direct Filtration
See Technology
Removal of water from an aquifer, typically by flow into a river or lake. Also the volume of flow in a river per unit time.
The treatment of water to remove or inactivate viruses, bacteria, and other pathogenic organisms.
Dissolved Air Flotation
See Technology
The purification of liquids by boiling. Distilled vapor is collected and condensed into a pure liquid.
Distribution system
Any system by which water is moved from a source to its consumers.
An intestinal disease typically caused by certain bacteria (e.g., Shigella sonnei) or parasites (e.g., Entamoeba histolytica). It is characterized by severe diarrhea that may be accompanied by blood or mucous.


A community of interacting organisms and the environment in which they live.
El Niño and La Niña
Oscillations in the Pacific’s ocean-atmosphere system that dramatically affect weather patterns, including rainfall, around the world. In general, El Niño and La Niña produce the opposite climatic effects.
Electrodialysis/electrodialysis reversal treatment
See Technology
Electromagnetic spectrum
The entire range of electromagnetic radiation at all wavelengths including the visible spectrum, radio waves, and x-rays.
Electrostatic process
Processes involving a force exerted by an unchanging (“static”) electric field upon a charged object such as a dissolved ion.
The mechanical process by which wind and water wear away the Earth’s surface.
A process by which water is converted from a liquid state to a vapor.


A parasitic disease caused by thread-like worms. The worms may damage the lymph system causing swelling which can lead to elephantiasis. Filariasis is spread by mosquitoes.
The physical removal of solid material from water (or air) by passing it through a porous material.
Filtration Systems
See Process
The agglomeration (i.e., clumping) of particles suspended in water into larger particles (“flocs”) that can be removed by sedimentation or flotation.
See Technology
A charged form of the element fluorine commonly found in rocks and consequently groundwater. Small amounts of fluoride may be added to water to protect against tooth decay, but in larger doses it can cause fluorosis.
A disease caused by the consumption of excess fluoride. Fluorosis can cause bone and tooth damage.
Fossil water
Groundwater supplies that were created during ancient climate conditions and are not renewable under current conditions. Fossil water is also known as “paleowater.”
Water that does not contain significant levels of dissolved minerals or salt.


An inflammation of the membrane lining the stomach and intestines that can cause diarrhea and vomiting. Gastroenteritis may be caused by food or water contaminated by viruses, bacteria or parasites.
Giardia lamblia: A microorganism (protozoa), sometimes found in drinking water, which may cause diarrhea, cramps, and illness (giardiasis). Giardia are commonly found in surface water sources like reservoirs, lakes, and rivers.
Granular Activated Carbon
See Technology
Grid cell
A two-dimensional object that represents the smallest nondivisible element of a grid. A single value for a parameter such as temperature, precipitation, or population is assigned to the entire cell, typically based on the results of mathematical modeling.
Water found below the Earth’s surface in geological reservoirs known as aquifers. Groundwater flows out of the ground naturally in springs and seeps, and can also be pumped out by wells.
Groundwater protection zones
Geographic areas designated for special limits on acceptable levels of potential groundwater pollutants.
Guinea worm
A parasitic worm that matures in the human abdomen then burrows out through painful blisters on the skin. These worms are ingested from drinking contaminated water.


Liver inflammation; it is commonly caused by one of 5 viruses called hepatitis A, B, C, D, and E. Hepatitis A and E can be transmitted by contaminated water.
A parasitic worm that infects the small intestine. Severe cases can result in anemia and stunted growth in children. Hookworm larvae enter the body through the skin, often via the feet. Spread by unsanitary conditions, hookworms infect about one billion people worldwide.
Hydrological cycle
The Sun-driven process of evaporation, condensation, and precipitation that moves water from the oceans and Earth to the atmosphere and back again. Also called the water cycle.
Sanitary and cleanliness practices, like hand washing, that promote good health and help prevent illness.


Infectious diseases
Disease caused by the entrance, growth and multiplication of organisms, like viruses, bacteria or parasites.
The process by which water moves downward from the land surface into the soil.
The facilities, equipment, and materials necessary for the operation of a water supply or sanitation system. Infrastructure includes storage systems such as dams and reservoirs as well as distribution and treatment systems.
Inorganic contaminant
Metals and other non-carbon based pollutants found in water. Inorganic contaminants are sometimes naturally-occurring, but they may also result from human pollution.
Ion Exchange
See Technology
Ion Exchange Systems
See Process
Exposing a substance to electromagnetic waves, typically those with wavelengths shorter than those of visible light such as X-rays, gamma rays, and ultra-violet rays. Irradiation may be used to purify food or water.


A yellowing condition that affects the skin and the whites of the eyes. Jaundice is caused by an excess of bile pigments in the blood. In adults it may signal liver disease.


Larva (plural, larvae)
The immature form of an animal or insect. Larvae undergo a metamorphosis during development into their adult form.
An infection of the skin and nerves caused by the bacterium Mycobactrium leprae. Infection leads to ulceration of skin, bones and visera and can result in loss of sensation and deformation. Also called Hansen’s disease.
Lime Softening
See Technology


A tropical infectious disease that caused by a protozoan parasite that is spread by mosquitoes. Malaria is characterized by cycles of chills and fever. Severe cases result in anemia and death. Malaria kills more than 1 million people each year—most of them children under 5 years old.
Mechanical Aeration
See Technology
Water produced by melting snow or ice.
A thin, flexible sheet of material.
Membrane Processes
See Process
Microscopic organisms including bacteria, fungi, protozoa, viruses and algae. Some microbes are beneficial while others pose risks to human health.
See Technology
Plant or animal life so small that it can only be seen through a microscope such as bacteria, yeasts, algae, and protozoa. Some microorganisms are beneficial while others pose risks to human health.


See Technology
Nitrate (NO3)
A common anion in water. Common sources of nitrates are fertilizers, septic tanks, and untreated or incompletely treated sewage. Nitrate is highly soluble under most conditions and is therefore difficult to remove from water. At high levels, nitrate in drinking water can cause methemoglobinemia, which is commonly known as "blue baby syndrome."
Nominal Molecular Weight Cutoff (NMWC)
The molecular weight of a particle that typically cannot pass through the membrane. This is an alternative to specifying the average pore size of a membrane.
A group of viruses responsible for severe stomach flu (gastroenteritis). These viruses are also known as “Norwalk-like viruses.”


Organic contaminant
An undesirable concentration of a chemical compound made up of mostly carbon and hydrogen; some of these compounds come from life processes, and some are synthetic.
The process by which water passes across a semi-permeable membrane from an area of higher water concentration (i.e., a more dilute solution) to one of lower water concentration (i.e., a more saline solution).
Oxidant (or oxidizing agent)
A substance that removes electrons from another substance. Oxidants like chlorine are used in water purification because they interfere with many biological processes in pathogens. They are also useful in changing dissolved iron and manganese into an insoluble form that can be filtered out of the water.
See Technology
Ozone (O3)
A molecule consisting of three oxygen atoms. Ozone is naturally-occurring, but for the purpose of water treatment (see ozonation) it is typically synthesized on site.


Packed Tower Aeration
See Technology
Non-renewable groundwater that was created during ancient climactic conditions. Paleowater is also known as “fossil water.”
An organism that lives on or inside another “host” organism. Parasites do not benefit their hosts, but instead feed at their expense.
Small, solid particles that are suspended in water.
An organism that triggers acute or chronic disease. Bacteria, viruses, and parasites are common pathogens.
A foreign substance that adversely affects water quality.
Polyamide membranes
Synthetic water filtration membranes that function even at extremes of pH and water temperature.
Large-molecule chemicals used for many synthetic purposes. In water treatment polymers are often used as coagulants.
Polysulfone membranes
Synthetic water filtration membranes that are resistant to extreme temperatures and pH, and heavy chlorine exposure.
This term describes a substance full of tiny holes (“pores”) that allow water to pass through.
Powdered Activated Carbon
See Technology
The processes by which atmospheric water vapor falls to Earth including rain, snow, hail, sleet, dew and frost.
Single-celled eukaryotic organisms often present in water. Protozoa may be parasitic and some, like Giardia and Cryptosporidium, are pathogenic.


Rainwater harvesting
The process of collecting rainwater and storing it for use in drier seasons.
water added to an aquifer, typically by rainfall or snowmelt that seeps into the ground, but sometimes by humans through wells or infiltration ponds.
A large body of water used to store supplies for human use.
A semisolid substance secreted by plants or produced artificially by combining molecules to form polymers. Used for water treatment in ion exchange systems.
Reverse Osmosis
See Technology
This term describes the environment found on the banks of a river or stream.
River basin
The total land area that is drained by a river and all of its tributaries. Often used as a synonym for a watershed or a catchment.
River blindness
A major cause of preventable blindness. The disease (onchocerciasis) is caused by parasitic worms which are transmitted by the bite of the blackfly.
A group of parasitic invertebrates that pass through human skin or are ingested as eggs or larvae. Roundworms typically infect the intestines and cause a number of diseases.
Run off
Water from precipitation or snowmelt that flows across or just under the land surface to enter streams, rivers and other surface waters.


Water, like that found in the world’s oceans, that contains significant amounts of dissolved salts such as sodium and chlorine.
The process of maintaining clean, hygienic conditions by the proper disposal of garbage and human waste. Good sanitation practices help prevent disease.
An uncomfortable skin condition, often called “the itch,” caused by mites. Scabies is spread through close contact in unsanitary conditions.
A disease (also called bilharzia) caused by parasitic worms found in freshwater. Schistosomiasis can damage the liver, lungs, intestines and bladder.
The gravity-driven process by which suspended particles settle to the bottom of a body of water.
Slow Sand Filtration
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Solar disinfection
See Technology
Solar distillation
See Technology
Solar treatment
See Process
Source Water
Water that has not been treated, but rather exists in its natural state.
The water vapor produced by sublimation.
The process by which solid water (ice or snow) becomes water vapor without entering a liquid state.
Surface water
All water, fresh and salt, that is direct contact with the atmosphere. Oceans, rivers and lakes are all sources of surface water.


Terraced farming
A farming method in which steep hillsides are fashioned into a step-like series of level terraces. Terraced farming conserves runoff water and prevents soil erosion.
A bacterial eye infection that begins as conjunctivitis, but if left untreated can cause blindness. Trachoma spreads through direct contact and is most common in communities without proper sanitation facilities.
Process by which water that is taken up by plants, typically through their roots, is evaporated from their leaves.
Tray aerators
See Technology
Treatment Technique (technology)
Any process intended to cleanse water of contaminants.
The visual appearance of cloudy water filled with suspended particles. Turbidity, as an optical property, may be measured and used to rate water quality and clarity.
A dangerous bacterial disease often spread by contaminated water, or by food prepared with contaminated water.


See Technology
Ultraviolet light/UV Rays
Ultraviolet (UV) rays are part of the electromagnetic spectrum. They are characterized by wavelengths shorter than visible light but longer than X-rays. UV light is used in water treatment.


Virtual water
The amount of water embedded in food or other products needed for its production. For example, if it takes one cubic meter of water to grow one kilogram of grain, and a country exports one million kilograms, the virtual water outflow would be one million cubic meters.
Extremely tiny microorganisms, often pathogenic, that reproduce within the cells of a host organism. Viruses may be present in human or animal waste and spread through contaminated water.


Wastewater treatment
The purification of waters carrying human and animal waste, or the waste products of industry and agriculture.
Water cycle
The Sun-driven process of evaporation, condensation, and precipitation that moves water from the oceans and Earth to the atmosphere and back again. Also called the hydrological cycle.
Water table
The surface where the water pressure equals atmospheric pressure. It is typically below ground surface, except in wetlands, after heavy precipitation or snowmelt, etc. In sands and gravels, it nearly corresponds to the top of “zone of saturation,” that is, where the earth’s pores are completely filled with water rather than air.
Water-based infections
Diseases caused by aquatic organisms, like parasitic worms.
Water-borne infections
Diseases that are spread when people drink contaminated water or eat food prepared with contaminated water.
Water-related insect vector
A disease spread by insects, such as mosquitoes and flies, that breed in water.
Water-washed infections
Infections of the skin and eyes, like trachoma, and some dysentery that are spread from person-to-person in settings of inadequate water availability resulting in poor personal hygiene.
The entire land area that drains into a specific stream, lake, or other body of water. Often used as a synonym for a catchment or a river basin.


A bacterial skin infection that may also affect bone and cartilage. Yaws is prevalent in communities with poor sanitation services, and spreads easily by skin-to-skin contact through a scrape or cut.
Yellow fever
A viral disease that is transmitted by mosquitoes. Among other ailments, it can cause liver failure which initiates the jaundice from which the disease takes its name.

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