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Symbols and Acronyms

nappe   The sheet or curtain of water overflowing a weir or dam. When freely overflowing any given structure, it has a well-defined upper and lower surface.

National Pollutant Discharge Elimination System (NPDES)  A permit that is the basis for the monthly monitoring reports required by most states in the United States.

negative head  (1) The loss of head in excess of the static head (a partial vacuum). (2) A condition of negative pressure produced by clogging of rapid sand filters near the end of a filter run.

negative pressure  A pressure less than the local atmospheric pressure at a given point.

nematode  Member of the phylum (Nematoda) of elongated cylindrical worms parasitic in animals or plants or free-living in soil or water.

nephelometer  An instrument for comparing turbidities of solutions by passing a beam of light through a transparent tube and measuring the ratio of the intensity of the shattered light to that of the incident light.

nephelometric turbidity unit (NTU)  Units of a turbidity measurement using a nephelometer.

net available head  The difference in pressure between the water in a power conduit before it enters the water wheel and the first free water surface in the conduit below the water wheel.

n factor  Values of the roughness coefficient used in Manning formula or Kutter formula. See also roughness coefficient, Manning formula.

nitrate (NO3)  An oxygenated form of nitrogen.

nitrification  The oxidation of ammonia nitrogen to nitrate nitrogen in wastewater by biological or chemical reactions. See also denitrification.

nitrifying bacteria  Bacteria capable of oxidizing nitrogenous material.

nitrite (NO2)  An intermediate oxygenated form of nitrogen.

nitrogen (N)  An essential nutrient that is often present in wastewater as ammonia, nitrate, nitrite, and organic nitrogen. The concentrations of each form and the sum (total nitrogen) are expressed as milligrams per liter (mg/L) elemental nitrogen. Also present in some groundwater as nitrate and in some polluted groundwater in other forms. See also nutrient.

nitrogen cycle  A graphical presentation of the conservation of matter in nature showing the chemical transformation of nitrogen through various stages of decomposition and assimilation. The various chemical forms of nitrogen as it moves among living and nonliving matter are used to illustrate general biological principles that are applicable to wastewater and sludge treatment.

nitrogenous oxygen demand (NOD)  A quantitative measure of the amount of oxygen required for the biological oxidation of nitrogenous material, such as ammonia nitrogen and organic nitrogen, in wastewater; usually measured after the carbonaceous oxygen demand has been satisfied. See also biochemical oxygen demand, nitrification, second-stage BOD.

nitrogen removal  The removal of nitrogen from wastewater through physical, chemical, or biological processes, or by some combination of these.

Nitrosomonas  A genus of bacteria that oxidize ammonia to nitrate.

Nocardia  Irregularly bent, short filamentous organisms that are characterized in an activated-sludge system when a dark chocolate mousse foam is present.

nonclogging impeller  An impeller of the open, closed, or semiclosed type designed with large passages for passing large solids.

nonsettleable solids  Suspended matter that will stay in suspension for an extended period of time. Such a period may be arbitrarily taken for testing purposes as 1 hour. See also suspended solids.

nonuniform flow  A flow in which the slope, cross-sectional area, and velocity change from section to section in the channel.

nozzle  (1) A short, cone-shaped tube used as an outlet for a hose or pipe. The velocity of the emerging stream of water is increased by the reduction in cross-sectional area of the nozzle. (2) A short piece of pipe with a flange on one end and a saddle flange on the other end. (3) A side outlet attached to a pipe by riveting, brazing, or welding.

nozzle aerator  An aerator consisting of a pressure nozzle through which water is propelled into the air in a fine spray. Also called spray aerator.
nutrient  Any substance that is assimilated by organisms and promotes growth; generally applied to nitrogen and phosphorus in wastewater, but also to other essential and trace elements.