| Hindered (Zone) settling | (Environmental Engineering) Settling in which particle concentrations are sufficient that particles interfere with the settling of other particles. Particles settle together as a body or structure with the water required to traverse the particle interstices. |
| Limnology | (Environmental Engineering) The study of freshwater ecosystems. |
| Maximum contaminant level (MCL) | (Environmental Engineering) The maximum allowable concentration of a given constituent in potable water. |
| Nitrification | (Environmental Engineering) The biological oxidation of ammonia and ammonium sequentially to nitrite and then nitrate. It occurs naturally in surface waters, and can be engineered in wastewater treatment systems. The purpose of nitrification in wastewater treatment systems is a reduction in the oxygen demand resulting from the ammonia. |
| Nitrogenous oxygen demand (NOD) | (Environmental Engineering) The amount of oxygen required to oxidize any ammonia present in a water. |
| NPDES | (Environmental Engineering) The National Pollutant Discharge Elimination System. The discharge criteria and permitting system established by the U.S. EPA as a result of the Clean Water Act and its subsequent amendments or the permit required by each discharger as a result of the Clean Water Act. |
| POTW | (Environmental Engineering) or Publicly Owned Treatment Works Any municipally owned wastewater treatment facility. |
| Primary standards | (Environmental Engineering) Required drinking water quality standards related directly to human health. These standards are required and enforceable by the U.S. EPA. See secondary standards. |
| Publicly owned treatment works (POTW) | (Environmental Engineering) A municipal or domestic wastewater treatment facility. |
| Reactive waste | (Environmental Engineering) A waste which; 1) reacts violently with water, 2) forms potentially explosive mixtures with water, 3) is normally unstable, 4) contains cyanide or sulfide in sufficient quantity to evolve toxic fumes at high or low pH, 5) is capable of exploding if heated under pressure, or 6) is an explosive compound listed in Department of Transportation (DoT) regulations. One of EPA's four hazardous waste properties. |