– A thin sheet of material through which a gaseous or liquid solution may pass.
Membrane hydrolysis – A hydrolysis process that occurs when a colloidal electrolyte is separated from pure water by a membrane.
Membrane selectivity – The ability of a membrane to allow passage of only anions or cations.
Microorganism – An organism of microscopic size generally considered to include bacteria, algae, protozoa, fungi, and rotifers but excluding viruses; they (primarily bacteria) are employed in biological wastewater treatment processes to remove biodegradable organic matter and suspended solids. Some species are human pathogens and are removed by water and wastewater treatment to prevent disease transmission and protect public health.
Mine water – The water encountered in mining operations that, if discharged into surface streams or other bodies of water, often contaminates them and makes them unfit for beneficial use.
Mineral spring – A spring that contains high levels of mineral salts.
Miscibility – The ability of a liquid or gas to dissolve uniformly in another liquid or gas.
Monitoring – A periodic or continuous surveillance or testing to determine the level of compliance with statutory requirements and pollutant levels in various media, or in living things.
Monitoring well – A well drilled to collect groundwater samples for analysis to determine the amounts, types, and distribution of contaminants in the groundwater.
Mother liquor – A concentrated solution that is substantially freed from undissolved matter by filtration, centrifuging, or decantation; the product can be obtained by evaporation and/or crystallization.
Municipality – A city, town, borough, county, parish, district, or other public body.
Natural resources – The land, fish, wildlife, biota, air, water, groundwater, drinking water supplies, and other such resources belonging to, managed by, held in trust by, pertaining to, or otherwise controlled by the United States.
Navigable water – Any stream, lake, arm of the sea, or other natural body of water that is navigable and of sufficient capacity to float watercraft for the purposes of commerce, trade, transportation, or recreation, as indicated under the Clean Water Act.
Nitrates – Oxidized nitrogen that forms a monovalent anion; is the form of nitrogen that is produced from the aerobic oxidation of ammonia (nitrification); and that serves as an electron acceptor for the anaerobic oxidation of organic matter (denitrification); it is a nutrient that can result in eutrophication of lakes and ponds and is the cause of “blue baby” syndrome in humans if in high concentrations in drinking water: its MCL is 10 mg/L in drinking water.
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
