Technical Resources

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Symbols and Acronyms


Imhoff cone   A cone-shaped graduated vessel used to measure the volume of settleable solids in various liquids of wastewater origin during various settling times.

impedance, resistance, and reactance  The relationship between impedance, resistance, and reactance is given by the following equation:

Z = R + jX


      = impedance (Ω),

      R = resistance (Ω),

      X = reactance (Ω), and

      j is the imaginary unit .

impeller  A rotating set of vanes designed to impel rotation of a mass of fluid.

incineration  Combustion or controlled burning of volatile organic matter in sludge and solid waste reducing the volume of the material while producing heat, dry inorganic ash, and gaseous emissions.

incinerator  A furnace or apparatus for incineration.

index  (1) An indicator, usually numerically expressed, of the relation of one phenomenon to another. (2) An indicating part of an instrument.

indicator  (1) A device that shows by an index, pointer, or dial the instantaneous value of such quantities as depth, pressure, velocity, stage, or the movements or positions of water-controlling devices; a gauge. See also recorder. (2) A substance giving a visible change, usually of color, at a desired point in a chemical reaction, generally at a prescribed end point.

indicator gauge  A gauge that shows, by means of an index, pointer, or dial the instantaneous value of such characteristics as depth, pressure, velocity, stage, discharge, or the movements or positions of waste-controlling devices. See also indicator, recorder.

industrial wastewater  Wastewater derived from industrial sources or processes.

infectious hepatitis  An acute viral inflammation of the liver characterized by jaundice, fever, nausea, vomiting, and abdominal discomfort; may be waterborne.

infiltration  (1) The flow or movement of water through the interstices or pores of a soil or other porous medium. (2) The quantity of groundwater that leaks into a pipe through joints, porous walls, or breaks. (3) The entrance of water from the ground into a gallery. (4) The absorption of liquid by the soil, either as it falls as precipitation or from a stream flowing over the surface.

inflow  In relation to sanitary sewers, the extraneous flow that enters a sanitary sewer from sources other than infiltration, such as roof leaders, basement drains, land drains, and manhole covers. See also ­infiltration.

influent  Water, wastewater, or other liquid flowing into a reservoir, basin, treatment plant, or treatment process. See also effluent.

inlet  (1) A surface connection to a drain pipe. (2) A structure at the diversion end of a conduit. (3) The upstream end of any structure through which water may flow. (4) A form of connection between the surface of the ground and a drain or sewer for the admission of surface or stormwater. (5) An intake.

inlet control  Control of the relationship between headwater elevation and discharge by the inlet or upstream end of any structure through which water may flow.

inorganic  All those combinations of elements that do not include organic carbon.

inorganic matter  Mineral-type compounds that are generally nonvolatile, not combustible, and not biodegradable. Most inorganic-type compounds or reactions are ionic in nature; therefore, rapid reactions are characteristic.

instrumentation  Use of technology to control, monitor, or analyze physical, chemical, or biological ­parameters.

interlock  A means of tying one or more circuits to another. It may be mechanical in nature but more likely involves the use of relays or solid state components.

intermittent chlorination  A technique of noncontinuous chlorination used to control biological fouling of surfaces in freshwater circuits, particularly those used for heat transfer.

intermittent filter  A natural or artificial bed of sand or other fine-grained material to the surface of which wastewater is applied intermittently in flooding doses and through which it passes; filtration is accomplished under aerobic conditions.

inventory  A detailed list showing quantities, descriptions, and values of property. It may also include units of measure and unit prices. The term is often confined to consumable materials but may also cover fixed assets. When an inventory covers all property of the enterprise and is priced as of a certain date, it is known as an appraisal.

ion  A charged atom, molecule, or radical that affects the transport of electricity through an electrolyte or, to a certain extent, through a gas. An atom or molecule that has lost or gained one or more electrons.

ion exchange  (1) A chemical process involving reversible interchange of ions between a liquid and a solid, but no radical change in structure of the solid. (2) A chemical process in which ions from two different molecules are exchanged. (3) The reversible transfer or sorption of ions from a liquid to a solid phase by replacement with other ions from the solid to the liquid. See also regeneration.

irrigation  The artificial application of water to lands to meet the water needs of growing plants not met by rainfall.

irrigation requirement  The quantity of water, exclusive of precipitation, that is required for crop production. It includes surface evaporation and other economically unavoidable water waste.

irrigation return water  Drainage water from irrigated farmlands, generally containing high concentrations of dissolved salts and other materials that have been leached out of the upper layers of the soil.