Nonpoint source – A source of pollutant discharge that is not traceable to a discrete location.
Non-potable water – Water considered unsafe and/or unpalatable for drinking (see also potable water).
Nutrient – Any element or compound that is essential to the life and growth of plants or animals, either as such or as transformed by chemical or enzymatic reactions; nitrogen and phosphorous are the major nutrients of concern in polluted waters.
Ocean water (seawater) – A uniform solution contained approximately 96.5% water and 3.5% ionized salts; ingestion of substantial amounts will create a bodily chloride imbalance with harmful effects.
Oceanic – A term used to describe the parts of the oceans deeper than 200 meters.
Oceanography – The science dealing with oceans, including their form, physical and chemical features, and related phenomena.
Open channel – Any natural or artificial waterway or conduit in which a fluid flows with a free surface exposed to atmospheric pressure.
Osmosis – The passage of a pure liquid (usually water) through a semipermeable membrane from a solution of low concentration into a solution of a higher concentration (e.g. the flow of pure water into a solution of salt and water); see also reverse osmosis.
Osmotic pressure – The pressure that results from osmosis.
Outfall – The place where an effluent is discharged into a receiving water.
Outfall sewer – A pipe or conduit that transports wastewater effluent to a final point of discharge.
Oxygenation – The increase of the dissolved oxygen content within water through aeration.
Ozonation – The addition of ozone to a water supply to reduce taste and odor problems and for disinfection.
Ozonator – A device that creates ozone from oxygen and adds it to water.
Parts per billion (ppb) – The fraction (ppbm for mass fraction and ppbv for volume fraction) multiplied by 109; it is a unit used to measure extremely small concentrations of a substance; equivalent to units of µg/L in water.
Parts per million (ppm) –The fraction (ppmm for mass fraction and ppmv for volume fraction) multiplied by 106; it is a unit used to measure small concentrations of a substance; equivalent to units of mg/L in water.
Pathogenic waste – A discarded waste that contains organisms capable of causing disease.
Percolation – The flow of a liquid downward through a filtering medium or soil layer.
Permafrost – The portion of the Earth which is permanently frozen, such as the Artic regions and portions of Alaska.
Permeability – The degree to which a liquid can move freely through soils.
Physical quality – The physical characteristics possessed by a material; it includes temperature, color, odor, and turbidity.
Physical treatment – A water or wastewater treatment process that utilizes physical means for pollutant removal; processes include screening, grinding, settling, filtration, and centrifugation.
Physicochemical – A term used to describe processes that involve both physical and chemical characteristics; adsorption is one such process that involves both physical and chemical attraction of an adsorbate to an adsorption site.
Point source of pollution – Pollution originating from a discrete source, such as the outflow from a pipe, ditch, tunnel, concentrated animal-feeding operation, or floating craft.
Pollutant – Any harmful substance present in air, water, or soil.
Pollution – The direct or indirect alteration of the physical, thermal, biological, or radioactive properties of any part of the environment in such a way as to create a hazard or potential hazard to the health, safety, or welfare of any living species.
Potable water – Water that is safe for human consumption.
POTW – Publicly owned treatment works, or municipal wastewater treatment plants.
Pre-aeration – A water treatment process in which the removal of dissolved gases, and the addition of oxygen are performed; typically, in aeration towers or aeration tanks.
Precipitate – A solid that separates out from a liquid due to some physical or chemical change in the liquid.
Pretreatment – Any process employed to partially remove pollutants from water prior to any subsequent treatment process.
Psychrometric chart – A chart employed to determine the properties of moist air as a function of temperature.
Public water system (PWS) – Any system that provides piped water for human consumption to at least 15 service connections or regularly serves 25 individuals.
Purging – A cleansing or removal of impurities, foreign matter, or undesirable contaminants from a process through periodic withdrawals of liquids or solids.
Purification – The removal of undesirable constituents from a substance by one or more separation techniques.
Rank – The stage reached by coal in the course of its carbonation; the chief ranks, in order of increasing carbon content, are lignite, subbituminous coal, bituminous coal, and anthracite.
Raw water – The untreated water that enters the first treatment unit of a water treatment plant.
Receiving water – Any body of water (e.g. river, lake, ocean, stream, etc.) into which treated wastewater is discharged to.
Recirculating cooling water – The recycling of cooling water to greatly reduce water use by reusing the water to perform several cooling operations.
Reclaimed water – Treated wastewater that is reused for generally non-potable uses to supplement or replace other raw water supplies.
Recycled water – Wastewater that has been treated for reuse and is recycled, generally for non-potable uses within a home or industrial facility.
Red tide – A proliferation and accumulation of certain microscopic algae, predominantly dinoflagellates, in coastal waters; some species produce toxins that are labeled harmful algae blooms, or Harmful Algal Blooms (HABs), that pose a serious and recurring threat to human health, wildlife, marine ecosystems, fisheries, and coastal aesthetics.
Red water – A rust-colored water, usually resulting from the presence of precipitated ferric iron salts.
Release – Any spilling, leaking, pumping, pouring, emitting, emptying, discharging, injecting, escaping, leaching, dumping, or disposing into the environment.
Renewable water supply – The rate of supply of water (volume per unit time) potentially or theoretically available for use in a region on an essentially permanent basis.
Reproducibility – The ability to repeat an experiment, reaction, measurement, or process and produce the same results.
Reservoir – Any body of water employed for the storage, control, or regulation of water.
Reverse osmosis – A water treatment process employed to separate water from pollutants by the application of pressure to force the water through a semipermeable membrane.
Rinse – The removal