Technical Resources

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Symbols and Acronyms

data   Records of observations and measurements of physical facts, occurrences, and conditions reduced to written, graphical, or tabular form.

debris  Generally, solid wastes from natural and man-made sources deposited indiscriminately on land and water.

decantation  Separation of a liquid from solids or from a liquid of higher density by drawing off the upper layer after the heavier material has settled.

dechlorination  The partial or complete reduction of residual chlorine by any chemical or physical process. Sulfur dioxide is frequently used for this purpose.

declining growth phase  Period of time between the log-growth phase and the endogenous phase, where the amount of food is in short supply, leading to ever-slowing bacterial growth rates.

decomposition  The breakdown of complex material into simpler substances by chemical or biological processes.

decomposition of wastewater  (1) The breakdown of organic matter in wastewater by bacterial action, either aerobic or anaerobic. (2) Chemical or biological transformation of the organic or inorganic materials contained in wastewater.

defoamer  A material having low compatibility with foam and a low surface tension. Defoamers are used to control, prevent, or destroy various types of foam, the most widely used being silicone defoamers. A droplet of silicone defoamer contacting a bubble of foam will cause the bubble to undergo a local and drastic reduction in film strength, thereby breaking the film. Unchanged, the defoamer continues to contact other bubbles, thus breaking up the foam. A valuable property of most defoamers is their effectiveness in extremely low concentration. In addition to silicones, defoamers for special purposes are based on polyamides, vegetable oils, and stearic acid.

degasification  (1) The removal of a gas from a liquid medium. (2) In water treatment, the removal of oxygen from water to inhibit corrosion. It may be accomplished by mechanical methods, chemical methods, or a combination of both.

degreasing  (1) The process of removing greases and oils from waste, wastewater, sludge, or solids. (2) The industrial process of removing grease and oils from machine parts or iron products.

degree  (1) On the centigrade or Celsius thermometer scale, 1/100 of the interval from the freezing point to the boiling point of water under standard conditions; on the Fahrenheit scale, 1/180 of this interval. (2) A unit of angular measure; the central angle subtended by 1/360 of the circumference of a circle.

demand  The rate at which electrical energy is delivered to a piece of power-consuming equipment or system.

demand average  The demand on an electrical system or any of its parts over an interval of time, as determined by dividing the total number of watt-hours by the number of hours (units of time) in the interval.

demand coincident  The sum of two or more demands that occurs in the same demand interval.

demand factor  The ratio of the maximum demand of the system or part of a system to the total connected load of the system or part of the system under consideration.

demand instantaneous peak  The maximum demand at the instant of greatest load.

demand interval  The period of time that electrical energy flows is averaged to determine demand, such as 60 minutes, 15 minutes, or instantaneous.

demand maximum  The greatest of all demands of the load under consideration that occurs during a specified period of time.

demand noncoincident  The sum of two or more individual demands that do not occur in the same demand interval, which is meaningful only when considering demands within a limited period of time, such as a day, week, month, and a heating or cooling season.

denitrification  The anaerobic biological reduction of nitrate nitrogen to nitrogen gas; also, removal of total nitrogen from a system. See also nitrification.

density current  A flow of water through a large body of water that retains its unmixed identity because of a difference in density.

deoxygenation  The depletion of the dissolved oxygen in a liquid either under natural conditions associated with the biochemical oxidation of the organic matter present or by addition of chemical reducing agents.

deoxygenation constant  A constant that expresses the rate of the biochemical oxidation of organic matter under aerobic conditions. Its value depends on the time unit involved (usually 1 day) and varies with temperature and other test conditions.

departure  The difference between any single observation and the normal.

deposition  The act or process of settling solid material from a fluid suspension.

depth of blanket  Level of sludge in the bottom of a secondary clarifier, typically measured in feet.

design criteria  (1) Engineering guidelines specifying construction details and materials. (2) Objectives, results, or limits that must be met by a facility, structure, or process in performance of its intended functions.

design flow  Engineering guidelines that typically specify the amount of influent flow that can be expected on a daily basis over the course of a year. Other design flows can be set for monthly or peak flows.

design loadings  Flowrates and constituent concentrations that determine the design of a process unit or facility necessary for proper operation.

design voltage  The nominal voltage for which a line or piece of equipment is designed. This is a reference level of voltage for identification and not necessarily the precise level at which it operates.

detention time  The period of time that a water or wastewater flow is retained in a basin, tank, or reservoir for storage or completion of physical, chemical, or biological reaction. See also contact time, retention time.

detergent  (1) Any of a group of synthetic, organic, liquid, or water-soluble cleaning agents that are inactivated by hard water and have wetting and emulsifying properties but, unlike soap, are not prepared from fats and oils. (2) A substance that reduces the surface tension of water.

detoxification  Treatment to modify or remove a toxic material.

dewater  (1) To extract a portion of the water present in a sludge or slurry. (2) To drain or remove water from an enclosure. A river bed may be dewatered so that a dam can be built; a structure may be dewatered so that it can be inspected or repaired.

dewatered sludge  The solid residue remaining after removal of water from a wet sludge by draining or filtering. Dewatering is distinguished from thickening in that dewatered sludge may be transported by solids handling procedures.

dewatering  The process of partially removing water; may refer to removal of water from a basin, tank, reservoir, or other storage unit, or the separation of water from solid material.

dewpoint  The temperature to which air with a given concentration of water vapor must be cooled to cause condensation of the vapor.

dialysis  The selective separation of dissolved or colloidal solids on the basis of molecular size by diffusion through a semipermeable membrane. See also reverse osmosis.

differential plunger pump  A reciprocating pump with a plunger so designed that it draws the liquid into the cylinder on the upward stroke but is double-acting on the discharge stroke.

diffused aeration  Injection of air under pressure through submerged porous plates, perforated pipes, or other devices to form small air bubbles from which oxygen is transferred to the liquid as the bubbles rise to the water surface.

diffused air  Small air bubbles formed below the surface of a liquid to transfer oxygen to the liquid.

diffuser  A porous plate, tube, or other device through which air is forced and divided into minute bubbles for diffusion in liquids. In the activated sludge process, it is a device for dissolving air into mixed liquor. It is also used to mix chemicals such as chlorine through perforated holes.

diffusion  (1) The transfer of mass from one fluid phase to another across an interface, for example liquid to solid or gas to liquid. (2) The spatial equalization of one material throughout another.

diffusion aerator  An aerator that blows air under low pressure through submerged porous plates, perforated pipes, or other devices so that small air bubbles rise continuously through the water or wastewater.

digested solids  Solids digested under either aerobic or anaerobic conditions until the volatile content has been reduced to the point at which the solids are relatively nonputrescible and inoffensive.

digester  A tank or other vessel for the storage and anaerobic or aerobic decomposition of organic matter present in the sludge. See also anaerobic digestion.

digester coils  A system of hot water or steam pipes installed in a digestion tank to heat the digester contents.

digestion  (1) The biological decomposition of the organic matter in sludge, resulting in partial liquefaction, mineralization, and volume reduction. (2) The process carried out in a digester.

discharge  The flow or rate of flow from a canal, conduit, pump, stack, tank, or treatment process. See also effluent.

discharge area  The cross-sectional area of a waterway. Used to compute the discharge of a stream, pipe, conduit, or other carrying system.

discharge capacity  The maximum rate of flow that a conduit, channel, or other hydraulic structure is capable of passing.

discharge head  A measure of the pressure exerted by a fluid at the point of discharge, usually from a pump.

discharge rate  (1) The determination of the quantity of water flowing per unit of time in a stream channel, conduit, or orifice at a given point by means of a current meter, rod float, weir, pitot tube, or other measuring device or method. The operation includes not only the measurement of velocity of water and the cross-sectional area of the stream of water, but also the necessary subsequent computations. (2) The numerical results of a measurement of discharge, expressed in appropriate units.

disconnecting means  A device, group of devices, or other means whereby the conductors of a circuit can be disconnected from the source of power.

discrete sedimentation  Sedimentation in which removal of suspended solids is a function of terminal settling velocity.

disinfectant  A substance used for disinfection and in which disinfection has been accomplished.

disinfected wastewater  Wastewater to which a disinfecting agent has been added.

disinfection  (1) The killing of waterborne fecal and pathogenic bacteria and viruses in potable water supplies or wastewater effluents with a disinfectant; an operational term that must be defined within limits, such as achieving an effluent with no more than 200 colonies fecal coliform/100 mL. (2) The killing of the larger portion of micro­organisms, excluding bacterial spores, in or on a substance with the probability that all pathogenic forms are killed, inactivated, or otherwise rendered nonvirulent.

dispersion  (1) Scattering and mixing. (2) The mixing of polluted fluids with a large volume of water in a stream or other body of water. (3) The repelling action of an electric potential on fine particles in suspension in water, as in a stream carrying clay. This dispersion usually is ended by contact with ocean water causing flocculation and precipitation of the clay, a common cause of shoaling in harbors. (4) In a continuous-flow treatment unit, the phenomenon of short-circuiting.

displacement pump  A type of pump in which the water is induced to flow from the source of supply through an inlet pipe and valve and into the pump chamber by a vacuum created therein by the withdrawal of a piston or piston-like device which, on its return, displaces a certain volume of the water contained in the chamber and forces it to flow through the discharge valves and discharge pipes.

disposal  Release to the environment. See also ultimate disposal.

dissolved air flotation (DAF)  A separation process in which air bubbles emerging from a supersaturated solution become attached to suspended solids in the liquid undergoing treatment and float them up to the surface. See also diffused air.

dissolved oxygen (DO)  The oxygen dissolved in liquid, usually expressed in milligrams per liter (mg/L) or percent saturation.

dissolved solids  Solids in solution that cannot be removed by filtration; for example, NaCl and other salts that must be removed by evaporation. See also total dissolved solids.

distributor  A device used to apply liquid to the surface of a filter or contact bed. Distributors are of two general types: fixed and movable. The fixed type consists of perforated pipes, notched troughs, sloping boards, or sprinkler nozzles. The movable type consists of rotating, reciprocating, or traveling perforated pipes or troughs applying a spray or a thin sheet of liquid.

diurnal  (1) Occurring during a 24-hour period; diurnal variation. (2) Occurring during the day (as opposed to night). (3) In tidal hydraulics, having a period or cycle of approximately 1 tidal day.

diversity  The characteristic or variety of electrical loads whereby individual maximum demands usually occur at different times. Diversity among equipment loads results in diversity among the loads of transformers, feeders, and substations.

diversity factor  The ratio of the sum of the coincident demands of two or more loads to their maximum demands for the same period.

domestic wastewater  Wastewater derived principally from dwellings, business buildings, institutions, and the like. It may or may not contain groundwater, surface water, or stormwater.

dosing tank  Any tank used in applying a dose; specifically used for intermittent application of wastewater to subsequent processes.

double-suction impeller  An impeller with two suction inlets, one on each side of the impeller.

double-suction pump  A centrifugal pump with suction pipes connected to the casing from both sides.

DPD method  An analytical method for determining chlorine residual using the reagent DPD (n-diethyl-p-phenylenediamine). This is the most commonly and officially recognized test for free chlorine residual.

drag  The resistance offered by a liquid to the settlement or deposition of a suspended particle.

drag coefficient  A measure of the resistance to sedimentation or flotation of a suspended particle as influenced by its size, shape, density, and terminal velocity. It is the ratio of the force per unit area to the stagnation pressure and is dimensionless. See also friction factor.

drain  (1) A conduit or channel constructed to carry off, by gravity, liquids other than wastewater, including surplus underground, storm, or surface water. It may be an open ditch, lined or unlined, or a buried pipe. (2) In plumbing, any pipe that carries water or wastewater in a building drainage system.

drawdown  (1) The magnitude of the change in surface elevation of a body of water as a result of the withdrawal of water. (2) The magnitude of the lowering of the water surface in a well, and of the water table or piezometric surface adjacent to the well, resulting from the withdrawal of water from the well by pumping. (3) In a continuous water surface with accelerating flow, the difference in elevation between downstream and upstream points.

drum screen  A screen in the form of a cylinder or truncated cone that rotates on its axis.

dry-bulb temperature  The temperature of air measured by a conventional thermometer.

dry feeder  A feeder for dispensing a chemical or other fine material to water or wastewater at a rate controlled manually or automatically by the rate of flow. The constant rate may be either volumetric or gravimetric.

drying beds  Confined, shallow layers of sand or gravel on which wet sludge is distributed for draining and air drying; also applied to underdrained, shallow, dyked, earthen structures used for drying sludge.

dry suspended solids  The weight of the suspended matter in a sample after drying for a specified time at a specific temperature.

dry weather flow  (1) The flow of wastewater in a combined sewer during dry weather. Such flow consists mainly of wastewater, with no stormwater included. (2) The flow of water in a stream during dry weather, usually contributed entirely by groundwater.

dual-media filters  Deep-bed filters using discrete layers of dissimilar media, such as anthracite and sand, placed one on top of the other.

duplex pump  A reciprocating pump consisting of two cylinders placed side by side and connected to the same suction and discharge pipe; the pistons move so that one exerts suction while the other exerts pressure resulting in continuous discharge from the pump.

dynamic equilibrium  See population dynamics.

dynamic head  (1) When there is flow, (a) the head at the top of a waterwheel; (b) the height of the hydraulic grade line above the top of a waterwheel; and (c) the head against which a pump works. (2) That head of fluid that would produce statically the pressure of a moving fluid.

dynamic suction head  The reading of a gauge on the suction line of a pump corrected for the distance of the pump below the free surface of the body of liquid being pumped; exists only when the pump is below the free surface. When pumping proceeds at the required capacity, the vertical distance from the source of supply to the center of the pump minus velocity head and entrance and friction losses. Internal pump losses are not subtracted.

dynamic suction lift  When pumping proceeds at the required capacity, the vertical distance from the source of supply to the center of the suction end of a pump, plus velocity head and entrance and friction losses. Internal pump losses are not added.