| Maximum contaminant level (MCL) | (Environmental Engineering) The maximum allowable concentration of a given constituent in potable water. |
| Metabolism | (Environmental Engineering) The processes which sustain an organism, including energy production, synthesis of proteins for repair and replication. |
| 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. |
| Nitrogen fixation | (Environmental Engineering) The conversion of atmospheric (or dissolved) nitrogen gas into nitrate by microorganisms. |
| Nonpoint source pollution (NPSP) | (Environmental Engineering) Any pollution from a source which cannot be attributed to a particular discharge point, e.g. from agricultural crops, city streets, construction sites, etc. |
| 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. |
| Organic compound | (Environmental Engineering) Any compound containing carbon except for the carbonates (carbon dioxide, the carbonates and bicarbonates), the cyanides, and cyanates. |
| Oxidative phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using a chemical substrate and molecular oxygen. |
| Pathogenic organism | (Environmental Engineering) An organism capable of causing infection. |
| Phenol | (Environmental Engineering) An aromatic benzene ring with a hydroxyl substituted for one hydrogen. |