AA—atomic absorption spectroscopy.
AAMI grade water—Water meeting quality standards established by the Association for the Advancement of Medical Instrumentation and used mainly for hemodialysis systems.
absolute filter rating—Filter rating meaning that 99.9 percent (or essentially all) of the particles larger than a specified micron rating will be trapped on or within the filter. Standard test procedures, such as those listed in the ANSI/NSF standards for particulate and cysts reduction by drinking water treatment units, should be used to determine and substantiate absolute filter ratings.
absorbed dose—The amount of a chemical that enters the body of an exposed organism.
absorption—The process of one substance actually penetrating into the structure of another substance. This is different from adsorption in which one substance adheres to the surface of another. SEE ALSO adsorption.
absorption factor—The fraction of a chemical making contact with an organism that is absorbed by the organism.
acceptable daily intake (ADI)—Estimate of the largest amount of chemical to which a person can be exposed on a daily basis that is not anticipated to result in adverse effects (usually expressed in mg/kg/day). Same as RfD.
accessible equipment—Water treatment equipment that when installed and operating is exposable and available for proper and thorough cleaning and inspection using only simple tools such as a screwdriver, pliers, or an open-end wrench. Equipment which is easily available for cleaning and inspection without having to use any tools at all is often referred to as readily accessible equipment.
accumulation tank—A vessel or tank which receives and stores product water for use on demand.
accuracy—How closely an instrument measures the true or actual value of the process variable being measured or sensed.
acid—A substance which releases hydrogen ions when dissolved in water. Most acids will dissolve the common metals and will react with a base to form a neutral salt and water. An acid is the opposite of a base, has a pH rating lower than 7.0, will turn litmus paper red, and has a sour taste.
acid mine drainage—Drainage of water from areas that have been mined for coal or other mineral ores; the water has low pH, sometimes less than 2.0 (is acid), because of its contact with sulfur-bearing material; acid drainage is harmful because it often kills aquatic organisms.
acid rain—Precipitation which has been rendered (made) acidic by airborne pollutants.
acidic—The condition of water or soil which contains a sufficient amount of acid substances to lower the pH below 7.0.
acidification—The addition of an acid (usually nitric or sulfuric) to a sample to lower the pH below 2.0. The purpose of acidification is to “fix” a water sample so it won’t change until it is analyzed. The sample is then said to be “acidified.”
acidity—The quantitative capacity of a water or solution to neutralize a base. Acidity is usually measured by titration with a standard solution of sodium hydroxide and expressed in ppm or mg/L of calcium carbonate equivalent.
acre-foot—The volume of water which would cover an area of one acre to a depth of one foot. It is equal to 43,560 cubic feet or 325,851 gallons.
acrylic—Ion exchange resin base made of polyacrylate rather than the more common polystyrene.
action level—The concentration of lead or copper in water specified at Code of Federal Regulations 141.80(c) which determines, in some cases, the treatment requirements contained in subpart I of this code that a water system is required to complete.
activated alumina—A medium made by treating aluminum ore so that it becomes porous and highly adsorptive. Activated alumina will remove several contaminants including fluoride, arsenic, and selenium. Activated alumina medium requires periodic cleaning with an appropriate regenerant such as alum, acid, and/or caustic.
activated carbon—A water treatment medium, found in block, granulated, or powdered form, which is produced by heating carbonaceous substances (bituminous coal or cellulose-based substances such as wood or coconut shell) to 700ºC or less in the absence of air to form a carbonized char, and then activating (oxidizing) at 800ºC to 1000ºC with oxidizing gases such as steam and carbon dioxide (oxygen is never used as the oxidizing gas because its reaction with the carbon surface is too rapid and violent) to form pores, thus creating a highly porous adsorbent material.
Activated carbon is commonly used for dechlorination and for reducing trace and soluble materials such as organic chemicals and radon from water.
activated carbon block filter—Activated carbon block is a blend of fine activated carbon (e.g., 80 X 325 mesh activated carbon), water, and a suitable binder (such as polyethylene or a similar material) that is mixed and molded and hardened or extruded to a cartridge filter of any size and shape. Sometimes specialized media are added along with activated carbon to provide customized performances for specific contaminants. The binder is particularly designed and chosen to hold the carbon and other media in a fixed solid matrix, yet, not to plug up the pores of the activated carbon.
Even though the binder does occlude a portion of the adsorption sites, the finer mesh size gives activated carbon block filters faster adsorption kinetics and generally two to four times greater adsorption capacity than equivalent volumes of loose granular activated carbon. Activated carbon block filters typically have a 0.5 to one micron filtration capability, making it also helpful for particulate filtration, insoluble lead reduction, and demonstrating, in some cases, removal of Giardia and Cryptosporidium.
activated silica—A negatively-charged colloidal substance generally formed by combining a dilute sodium silicate solution with a dilute acidic solution (or other activant). Generally used as a coagulant aid.
active transport—An energy-expending mechanism by which a cell moves a chemical across the cell membrane from a point of lower concentration to a point of higher concentration, against the diffusion gradient.
acute—Occurring over a short period of time; used to describe brief exposures and effects which appear promptly after exposure.
acute exposure—A single exposure to a toxic substance which results in severe biological harm or death. Acute exposures are usually characterized as lasting no longer than a day.
acute toxicity—The ability of a substance to cause poisonous effects resulting in severe biological harm or death soon after a single exposure or dose. Also, any severe poisonous effect resulting from a single short-term exposure to a toxic substance.
additive effect—Combined effect of two or more chemicals equal to the sum of their individual effects.
adsorbate—The liquid, gas, or solid substance which is adsorbed as molecules, atoms, or ions.
adsorbent—A water treatment medium, usually solid, capable of the adsorption of liquids, gases, and/or suspended matter. Activated alumina and activated carbon are common adsorbents used in water processing.
adsorption—The physical process occurring when liquids, gases, or suspended matter adhere to the surfaces of, or in the pores of, an adsorbent medium. Adsorption is a physical process which occurs without chemical reaction. SEE ALSO absorption.
aeration—The process whereby water is brought into intimate contact with air by spraying or cascading, or air is brought into intimate contact with water by an air aspirator or by bubbling compressed air through the body of water. Both pressure (closed) aerators and open (gravity) aerators are used. Closed aeration is used chiefly for oxidation; open aeration for degassing.
aerobic—An action or process conducted in the presence of air, such as aerobic digestion of organic matter by bacteria.
aesthetic contaminants—Characteristics of water which affect its taste, odor, color, and appearance (and may affect the objects touched by the water) but which do not in themselves have any adverse health effects in otherwise potable water. Suggested Maximum Contaminant Levels (MCLs) for various aesthetic contaminants in drinking water are part of the National Secondary Drinking Water Regulations issued by the USEPA. These aesthetic standards are advisory only, not enforceable by the USEPA. SEE ALSO drinking water standards.
age tank—A tank used to store a chemical solution of known concentration for feed to a chemical feeder. Also called a day tank.
agglomeration—The collecting or coalescence of dispersed suspended matter into larger masses or flocs which can settle and be filtered from water.
aggregate—A mass or cluster of soil particles, often having a characteristic shape.
agrochemical—Synthetic chemicals (pesticides and fertilizers) used in agricultural production.
air binding—A situation where air enters the filter media. Air is harmful to both the filtration and backwash processes. Air can prevent the passage of water during the filtration process and can cause the loss of filter media during the backwash process.
air check—A device which allows water, but not air, to pass through it. An air check is a typical component of a treatment system using a regenerant eductor.
air dryer—A device used to dry out (desiccate) air by removing the water vapor. An air dryer may be used in ozone generation, for example, to produce higher concentrations of ozone and lessen the production of corrosive nitrous oxides.
air gap—A clear vertical space between the end of the water treatment device’s drain line and the flood level rim of a receptacle which holds water. An air gap is used to prevent cross connection between the water treatment device and a possible source of waste water thereby preventing a reverse flow of water from the sewer into the water supply system. Without an air gap, such reverse flow could occur due to an increase in the pressure in the sewer system or the creation of a negative pressure in the water supply line. Local plumbing codes indicate how wide the air gap needs to be.
air padding—Pumping dry air into a container to assist with the withdrawal of a liquid or to force a liquefied gas such as chlorine out of a container.
air stripping—A technique of aeration for the removal of dissolved gases and volatile substances, often pesticides or hydrocarbon products in water supplies. Large volumes of air are bubbled through the water to remove (strip out) the dissolved gases and volatile substances. The equipment used is a tall, open column with baffling or rock-like materials with air forced upward through a descending sprayed stream of water to be treated. SEE ALSO packed tower aeration.
alarm contact—A switch that operates when some preset low, high, or abnormal condition exists.
alcohol—A class of organic compounds containing one or more hydroxyl (OH-) groups.
algae—Single-celled or simple multicelled organisms, commonly found in surface water, which produce their own food through photosynthesis. Excessive algae growth may cause the water to have undesirable odors or tastes, and decay of algae can deplete the oxygen in the water.
algal bloom—Sudden, massive growths of microscopic and macroscopic plant life, such as green or blue-green algae, which develop in lakes and reservoirs.
algicide—Any substance or chemical specifically formulated to kill or control algae.
aliphatic—A type of organic compound in which the characteristic chemical groups are linked to a straight or branched carbon chain, as opposed to a carbon ring structure. In complex structures, the chains may also be cross-linked. SEE ALSO aromatic; heterocyclic.
aliphatic hydroxy acids—Organic acids with carbon atoms arranged in branched or unbranched open chains rather than in rings.
aliquot—Portion of a sample.
alkaline—The condition of water or soil which contains a sufficient amount of alkali substances to raise the pH above 7.0.
alkalinity— A measure of concentration of all species in water capable of neutralizing an acid by binding a hydrogen ion, expressed as mg/L or ppm of calcium carbonate equivalents. Alkalinity is measured by titration and is not equivalent to pH.
The most common alkalinity components in naturally occurring surface and groundwater are bicarbonate, carbonate ions and hydroxide ions. As bicarbonate and carbonate ions typically occur in the greatest concentration, the contribution by other species to the measured alkalinity is negligible and often ignored for the purposes of sizing water treatment equipment. SEE ALSO alkalinity tests; pH.
alkalinity tests—There are three different tests used for testing alkalinity. They are usually done in this order:
1. The pH alkalinity test will indicate the presence or absence of hydroxide alkalinity. A pH reading of 8.3 or above indicates the presence of hydroxide alkalinity.
2. The phenolphthalein test measures “P alkalinity” which is made up of all hydroxide alkalinity plus 1/2 of the carbonate alkalinity. P alkalinity is that portion of alkalinity above pH 8.3. Phenolphthalein in water changes color from pink above pH 8.3 to clear at pH 8.3.
3. The Methyl Orange test measures the “M alkalinity” which is the remaining ½ of the carbonate alkalinity plus all of the bicarbonate alkalinity. Methyl orange alkalinity measures total alkalinity of water. The methyl orange endpoint is pH 4.5; no alkalinity exists in water below pH 4.5.
Note: Free carbon dioxide (CO2) gas can exist dissolved in water between pH 4.5 and pH 8.3. Below pH 4.5, CO2 escapes quickly into the environment and above pH 8.3 CO2 becomes carbonate (CO32-) alkalinity. SEE ALSO alkalinity.
alkylaryl sulfonate—A generic term covering a wide range of anionic surfactants and detergent processing aids. The alkylaryl sulfonates of primary interest to the detergent industry are the surfactants classified as linear alkylate sulfonates, usually sodium salts, and the processing aids ammonium, potassium, or sodium salts of toluene, xylene, or cumene sulfonates. They are used to solubilize the active ingredients in some liquid detergents. SEE ALSO alkylbenzene sulfonate; linear alkylate sulfonate; surfactant.
alkylbenzene sulfonate (ABS)—A major class of aklylaryl sulfonate surfactants used in detergents; usually a sodium salt. ABS is anionic and high sudsing. Prior to the mid-1960s, the form of ABS most widely used in detergent formulations resisted biodegradation. In 1965, detergent manufacturers voluntarily replaced ABS nationally in household laundry products by a more rapidly biodegradable variety of ABS called linear alkylate sulfonate, or LAS.
SEE ALSO alkylaryl sulfonate; linear alkylate sulfonate; surfactant.
allotrope—One of several possible forms of a substance. Ozone (O3) is a triatomic allotrope of oxygen (O2).
alluvial—Relating to mud and/or sand deposited by flowing water. Alluvial deposits may occur after a heavy rainstorm.
alpha decay—A type of radioactivity where the nucleus of an atomic species changes or disintegrates (decays) via an energetic helium ion (alpha particle) being ejected. Alpha decay produces a daughter nucleus of atomic number two less than the parent and of atomic mass number four less than the parent. SEE ALSO beta-minus decay; beta-plus decay; gamma decay; radioactivity.
alternating current (AC)—An electric current that reverses its direction (positive/ negative values) at regular intervals.
alternating system—A dual automatic softener system functioning with one unit in service and one on standby. When a predetermined gallonage of softened water has passed or when a monitor detects hard water breakthrough, the idle or standby unit automatically goes into service. The spent unit then regenerates and becomes the idle/standby unit.
alum—The common name for aluminum sulfate [Al2(SO4)3 • 14H2O] which is often used as a coagulant in water treatment.
alumino silicate—SEE gel zeolite.
aluminum silicate—SEE pumicite.
ambient—Environmental or surrounding conditions.
ambient temperature—Temperature of the surrounding air (or other medium). For example, temperature of the room where a gas chlorinator is installed.
amide—A group of compounds formed by the reaction of an organic acid with ammonia or an amine.
ammonia—An alkaline gas composed of nitrogen and hydrogen (NH3). Ammonia gas is colorless and lighter than air, and exhibits a sharp, intensely irritating odor. Ammonia is extremely soluble in water, reacting to form ammonium hydroxide (NH4OH). At high pH, free ammonia gas (NH3) may be present in water. But, at the pH of most natural waters, ammonia is almost completely ionized as the ammonium ion (NH4+). A typical concentration of ammonia in fresh water rivers and lakes is 0.1 to 1.0 milligrams per liter, expressed as nitrogen (N). Cation exchange in the hydrogen form and adsorption by certain zeolite clays, such as clinoptilolite, may be used to remove ammonium ion from water. At greater than pH 9, air stripping can be used to remove ammonia (NH3) gas from water. Chlorination readily converts ammonia to chloramines (e.g., NH2Cl, NHCl2, and NCl3) that can then be removed from water, as free chlorine is, with activated carbon, except contact times as much as 10 to 100 times that required for free chlorine removal may be required for chloramine removal.
ammonium—The ionic form of ammonia nitrogen (NH4+) that is usable by plants. SEE ALSO ammonia.
amoeba—A single-celled protozoan that is widely found in fresh and salt water. Some types of amoebas cause diseases such as amoebic dysentery.
amperage—The strength of an electric current measured in amperes. The amount of electric current flow, similar to the flow of water in gallons per minute.
ampere—The unit used to measure current strength. The current produced by an electromotive force of one volt acting through a resistance of one ohm.
amperometric—Based on the electric current that flows between two electrodes in a solution.
amperometric titration—A means of measuring concentrations of certain substances in water (such as strong oxidizers) based on the electric current that flows during a chemical reaction. SEE ALSO titrate.
amphoteric—Partly one and partly the other. Capable of reacting chemically as an acid or a base, or as a cation or as an anion, depending on pH.
anaerobic—A condition in which there is no air or no available free oxygen.
anaerobic organism—An organism that can thrive in the absence of oxygen (air), such as bacteria in a septic tank.
analog—The readout of an instrument by a pointer (or other indicating means) against a dial or scale.
analyzer—A device which conducts periodic or continuous measurement of some factor such as chlorine, fluoride, or turbidity. Analyzers operate by any of several methods including photocells, conductivity, or complex instrumentation.
angstrom unit—A unit of length equal to 10-10 meters or 10-8 centimeters or 3.937 X 10-9 inches or .0001 microns. The symbol for angstrom is Å.
animal studies—Investigations using animals as surrogates for humans on the expectation that results in animals are pertinent to humans.
anion—An ion with a negative charge. An anion [such as chloride (Cl-), nitrate (NO3-), bicarbonate (HCO3-), or sulfate (SO42-)] may result from the dissociation of a salt, acid, or alkali.
anion exchange—An ion exchange process in which anions in solution are exchanged for other anions from an ion exchanger. SEE ALSO ion exchange.
anion membrane—SEE ion exchange membrane.
anion softening – Two components are necessary for the formation of boiler scale: a cation and an anion that can react to form an insoluble salt. Examples are calcium or magnesium bicarbonate, which, when heated, form the insoluble carbonate. Anion softening removes the bicarbonate ion and replaces it with the soluble chloride which does not form scale when combined with calcium or magnesium. This process is also known as “dealkalization”.
anionic polymer—A polymer having negatively charged groups of ions; often used as a filter aid and for dewatering sludges.
annular space—A ring-shaped space located between two circular objects, such as two pipes.
anode—The positive pole of an electrolytic system; also the metal which goes into solution in a galvanic cell. Sacrificial anodes of metals such as magnesium, aluminum, or zinc are sometimes installed in water heaters or other tanks to deliberately establish galvanic cells to control corrosion of the tank through the sacrifice of the anode. SEE ALSO sacrificial anode.
antagonism—Interference or inhibition of the effect of one chemical by the action of another chemical.
anthracite—A filter medium produced from crushed anthracite coal and screened to specific mesh sizes.
antibiotic—A substance produced by certain microorganisms or man-made synthetically, that in dilute solutions, can kill or prevent the growth of microorganisms.
antiparticle (antimatter)—Any of several species of subatomic particles that are identical in mass to ordinary particles, but opposite in electrical charge or (in the case of the neutron) in magnetic moment. Thus, a positron is an electron with a positive charge, an antiproton is a proton with a negative charge, and an antineutron has no charge but has a magnetic moment opposite to that of a neutron. A photon is its own antiparticle. When an antiparticle collides with its opposite particle (e.g., a collision of an electron and a positron), both particles are annihilated and their masses are converted to photons of equivalent energy. The same is true of other subatomic particles (neutrinos, mesons, etc.), some of which are fantastically short-lived (of the order of billionths of a second).
antiseptic—anti, against + sepsis, decay. Preventing or inhibiting the growth and multiplication of microorganisms, especially pathogenic microorganisms, without necessarily destroying them. SEE ALSO aseptic; aseptic procedure; disinfect; sanitize; sterilize.
AOP—advanced oxidation process.
appropriative—Water rights to or ownership of a water supply which is acquired for the beneficial use of water by following a specific legal procedure.
appurtenance—Machinery, appliances, structures, and other parts of the main structure necessary to allow it to operate as intended, but not considered part of the main structure.
aquatic—Plants or animal life living in, growing in, or adapted to water.
aqueous—Something made up of, similar to, or containing water; watery.
aquifer—A natural water-bearing geological formation (e.g., sand, gravel, sandstone) which is found below the surface of the earth.
aragonite—A form of calcium carbonate that appears in pearls.
arithmetic mean—An average value of a data set calculated by determining the sum of all the numbers in the data set and dividing by the number of data values within the set.
aromatic—A type of organic compound in which the characteristic chemical groups are linked to a particular type of six-member hexagonal carbon ring which contains three double bonds, typified by benzene. Such rings have peculiar stability and chemical character, and are present in the rather reactive and highly versatile compounds derived from petroleum and cool tar. The name is due to the strong and not unpleasant odor characteristic of most substances of this nature. SEE ALSO aliphatic; heterocyclic.
artesian (aquifer or well)—Water held under pressure in porous rock or soil confined by impermeable geologic formations. An artesian well is free-flowing. SEE ALSO confined aquifer.
“as calcium carbonate”—SEE calcium carbonate equivalent.
aseptic—a, not + sepsis, decay. 1. Free or freed from pathogenic organisms and their toxins. 2. A sterile condition, free from germs, infection, and any form of life. SEE ALSO antiseptic; aseptic procedure; disinfect; sanitize; sterilize.
aseptic procedure—A method used to prevent microbial contamination; equipment and tools are cleaned and sanitized, disinfected, or sterilized; personnel wear sanitary or sterilized gloves and sometimes clean caps and masks. SEE ALSO antiseptic; aseptic; disinfect; sanitize; sterilize.
aspirator—A device which creates movement of air, liquids, and granular substances by suction.
assay—A test for a particular chemical or effect.
ASTM grade water—SEE reagent grade water.
asymmetric—Not similar in size, shape, form, or arrangement of parts on opposite sides of a line, point, or plane.
atom—The smallest possible component of an element. An atom is comprised of a nucleus (made up of one or more protons and two or more neutrons—except for hydrogen which may have no neutrons) and one or more electrons which revolve around the nucleus. Atoms come together to form molecules.
atomic absorption spectroscopy (AA)—A spectroscopy chemical analytical technique used for determining the metal elements in water by measuring the well defined characteristic light wave lengths absorbed by each respective element when the element has been thermally excited into an atomic vapor. The sample to be analyzed is atomized into an atomic vapor by either aspirating the sample into a specific flame (in flame AA) or by vaporization with a tube of graphite that is electrically heated to a temperature between 1500 and 2800ºC (in flameless AA). A light beam of specific characteristic wave lengths is directed through the vapor, into a monochromator that further defines the very small range of wave lengths to be analyzed, and into a detector that measures the amount of light absorbed by the atomized element. Identification of the element is possible because each element has its own well-defined characteristic absorption wave length. The amount of absorbance measured is proportional to the concentration of the element in the sample. SEE ALSO emission spectroscopy; spectroscopy.
attrition—The gradual lessening of the capacity or effectiveness of a medium. This may occur due to friction, sacrificial properties of the medium, chemical attack on the medium, or contaminant saturation of the medium.
automatic dishwasher detergent—A cleaning product designed specifically for use in automatic dishwashers. It must produce little or no suds or foam because too much foam can inhibit the washing action. Its important functions include the following:
• Tie up water hardness minerals to permit the detergent to do its cleaning job.
• Make water wetter (reduce surface tension) to penetrate and loosen soil.
• Emulsify greasy or oily soil.
• Suppress foam caused by protein soils such as egg and milk.
• Help water to sheet off surfaces, thus minimizing water spots.
• Protect china patterns and metals from the corrosive effects of heat and water alone.
Basic ingredients in most automatic dishwasher detergents include:
Surfactant (nonionic)—lowers the surface tension of water so that it will more quickly wet out the surfaces and the soils, thus allowing water to sheet off dishes and not dry in spots. The surfactant also helps remove and emulsify fatty soils like butter and cooking fat. Nonionic surfactants are used because they generally have the lowest sudsing characteristics.
Builder (complex phosphates)—combines with water hardness minerals (primarily calcium and magnesium) and holds them in solution so that the minerals cannot combine with food soils and so that neither the minerals themselves nor the mineral/food soil combination will leave insoluble spots or film on dishes.
Corrosion inhibitor (sodium silicate)—helps protect machine parts, prevent the removal of china patterns, and the corrosion of metals such as aluminum.
Fragrance (optional)—covers the chemical odor of the base product and stale food odors.
Oxidizing agent—helps break down protein soils like egg and milk, aids in removing such stains as coffee or tea, and lessens spotting of glassware.
Processing aids—generally inert materials that allow the active ingredients to be combined into a usable form.
Suds suppressor—controls foam from food soils, especially protein soils.
automatic water softener (or automatic filter)—A water softener (or filter) that is equipped with a clock timer which automatically initiates the backwash and/or regeneration process at certain preset intervals of time. All operations, including bypass of treated or untreated water (depending upon design), backwashing, brining, rinsing, and returning the unit to service are performed automatically.
autotrophic—auto, self + trophe, nourishment. Capable of obtaining food or nourishment from simple raw materials. Autotrophic organisms or autotrophs are organic compound producers such as algae, plants, and certain bacteria that use carbon dioxide or carbonates, inorganic nitrogen, water, and an energy source such as photosynthesis from sunlight to make (or synthesize) complex molecules. Opposed to heterotrophic. SEE ALSO heterotrophic.
available chlorine—A measure of the amount of chlorine available in chlorinated lime, hypochlorite compounds, and other materials that are used as a source of chlorine when compared with that of elemental (liquid or gaseous) chlorine.
available expansion—The vertical distance from the sand surface to the underside of a trough in a sand filter. This distance is also called freeboard.
average—SEE arithmetic mean.
Avogadro’s number (Avogadro’s Constant)— The number of atoms or molecules of a substance contained in one gram mole (a value of the molecular weight of the substance expressed in grams) of that substance. Example: one gram mole of carbon (C), MW=12.00, is 12.00 grams and contains 6.022 x 1023 atoms of C. Oxygen, (O2), being a diatomic molecule, would weigh 32 grams and contain the same number of molecules as 12 grams of Carbon.
axial flow—SEE longitudinal flow.
axial to impeller—The direction in which material being pumped flows around the impeller or flow parallel to the impeller shaft.
axis of impeller—An imaginary line running along the center of a shaft (such as an impeller shaft).