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Symbols and Acronyms

tapered aeration   The method of supplying varying quantities of air into the different parts of an aeration tank in the activated-sludge process, more at the inlet, less near the outlet, in approximate proportion to the oxygen demand of the mixed liquor under aeration.

tee  A pipe fitting, either cast or wrought, that has one side outlet at right angles to the run. A single-outlet branch pipe.

temperature  (1) The thermal state of a substance with respect to its ability to transmit heat to its environment. (2) The measure of the thermal state on some arbitrarily chosen numerical scale. See also Celsius, centigrade, Fahrenheit.

temporary hardness  Hardness that can be removed by boiling; more properly called carbonate hardness. See also carbonate hardness, hardness.

tertiary effluent  The liquid portion of wastewater leaving tertiary treatment.

tertiary treatment  The treatment of wastewater beyond the secondary or biological stage; term normally implies the removal of nutrients, such as phosphorus and nitrogen, and a high percentage of suspended solids; term now being replaced by advanced waste treatment. See also advanced waste treatment.

thermal stratification  The formation of layers of different temperatures in bodies of water.

thermophilic digestion  Digestion occurring at a temperature approaching or within the thermophilic range, generally between 43 and 60 °C (110 and 140 °F).

thermophilic range  That temperature range most conducive to maintenance of optimum digestion by thermophilic bacteria, generally accepted as between 49 and 57 °C (120 and 135 °F). See also thermophilic digestion.

thickeners  Any equipment or process, after gravity sedimentation, that increases the concentration of solids in sludges with or without the use of chemical flocculents.

threshold odor  The minimum odor of the water sample that can barely be detected after successive dilutions with odorless water. Also called odor threshold.

threshold odor number  The greatest dilution of a sample with odor-free water that yields a definitely perceptible odor.

titration  The determination of a constituent in a known volume of solution by the measured addition of a solution of known strength to completion of the reaction as signalled by observation of an end point.

tolerance  The ability of an organism to withstand exposure to a specific compound; a tolerance level may be defined as a period of exposure or a level of exposure (concentration) that is withstood.

total carbon (TC)  A quantitative measure of both total inorganic and total organic carbon as determined instrumentally by chemical oxidation to carbon dioxide and subsequent infrared detection in a carbon analyzer. See also total organic carbon.

total dissolved solids (TDS)  The sum of all dissolved solids (volatile and nonvolatile).

total dynamic discharge head  Total dynamic head plus the dynamic suction head or minus the dynamic suction lift.

total dynamic head (TDH)  The difference between the elevation corresponding to the pressure at the discharge flange of a pump and the elevation corresponding to the vacuum or pressure at the suction flange of the pump, corrected to the same datum plane, plus the velocity head at the discharge flange of the pump minus the velocity head at the suction flange of the pump.

total head  (1) The sum of the pressure, velocity, and position heads above a datum. The height of the energy line above a datum. (2) The difference in elevation between the surface of the water at the source of supply and the elevation of the water at the outlet, plus velocity head and lost head. (3) The high distance of the energy line above the datum; energy head. (4) In open channel flow, the depth plus the velocity head.

total organic carbon (TOC)  The amount of carbon bound in organic compounds in a sample. Because all organic compounds have carbon as the common element, total organic carbon measurements provide a fundamental means of assessing the degree of organic pollution.

total oxygen demand (TOD)  A quantitative measure of all oxidizable material in a sample water or wastewater as determined instrumentally by measuring the depletion of oxygen after high-temperature combustion. See also chemical oxygen demand, total organic carbon.

total pumping head  The measure of the energy increase imparted to each pound of liquid as it is pumped, and therefore, the algebraic difference between the total discharge head and the total suction head.

total solids (TS)  The sum of dissolved and suspended solid constituents in water or wastewater.

total suspended solids (TSS)  The amount of insoluble solids floating and in suspension in the wastewater. Also referred to as total nonfilterable residue.

toxicant  A substance that kills or injures an organism through chemical, physical, or biological action; examples include cyanides, pesticides, and heavy metals.

toxicity  The adverse effect that a biologically active substance has, at some concentration, on a living entity.

toxic wastes  Wastes that can cause an adverse response when they come in contact with a biological entity.

trace nutrients  Substances vital to bacterial growth. Trace nutrients are defined in this text as nitrogen, phosphorus, and iron.

transformer  An electromagnetic device for changing the voltage of alternating current electricity.

trap  (1) A device used to prevent a material flowing or carried through a conduit from reversing its direction of flow or movement, or from passing a given point. (2) A device to prevent the escape of air from sewers through a plumbing fixture or catch basin.

trash  Debris that may be removed from reservoirs, combined sewers, and storm sewers by coarse racks.

trash rack  A grid or screen placed across a waterway to catch floating debris.

trickling filter  Secondary treatment process where wastewater trickles over rock or honeycombed-shaped plastic media. Biomass and slimes containing microorganisms form on the media and utilize the organic matter for growth and energy.

tri-halomethanes (THM)  Derivatives of methane (CH4) in which three halogen atoms (chlorine, bromine, or iodine) are substituted for three of the hydrogen atoms.

trough  A structure, usually with a length several times its transverse dimensions, used to hold or transport water or other liquids.

tube settler  A series of tubes, about 2 in. in diameter or 2-in. square, placed in a sedimentation tank to improve the solids removal efficiency.

tubing  (1) Flexible pipe of small diameter, usually less than 2 in. (2) A special grade of high-test pipe fitted with couplings and fittings of special design.

turbidimeter  An instrument for measurement of turbidity in which a standard suspension is used for reference.

turbidity  (1) A condition in water or wastewater caused by the presence of suspended matter and resulting in the scattering and absorption of light. (2) Any suspended solids imparting a visible haze or cloudiness to water that can be removed by filtration. (3) An analytical quantity usually reported in turbidity units determined by measurements of light scattering. See also formazine turbidity unit, nephelometric turbidity unit.

turbine pump  A centrifugal pump in which fixed guide vanes partially convert the velocity energy of the water to pressure head as the water leaves the impeller.

turbulence  (1) The fluid property that is characterized by irregular variation in the speed and direction of movement of individual particles or elements of the flow. (2) A state of flow of water in which the water is agitated by cross currents and eddies, as opposed to laminar, streamline, or viscous flow. See also turbulent flow.

turbulent flow  (1) The flow of a liquid past an object such that the velocity at any fixed point in the fluid varies irregularly. (2) A type of fluid flow in which there is an unsteady motion of the particles and the motion at a fixed point varies in no definite manner. Also called eddy flow or sinuous flow.

turnover  The phenomenon of vertical circulation that occurs in large bodies of water. It results from the increase in density of water above and below 39.2 °F (4 °C), the temperature of minimum density. In the spring, as the surface of the water warms above the freezing point, the water increases in density and tends to sink, producing vertical currents; in the fall, as the surface water becomes colder, it also tends to sink. Wind may also create such vertical currents. Also called overturn.

two-staged digestion  The biological decomposition of organic matter in sludge followed by solids–liquid separation of the digested sludge. Two-stage digestion uses two compartments or two tanks to separate the violent initial digestion period from the slower final period to enhance both the digestion and the solids–liquid separation after digestion.