| Ketones | (Environmental Engineering) Organic compounds with two hydrocarbon groups bonded to a carbonyl group. |
| Limnology | (Environmental Engineering) The study of freshwater ecosystems. |
| Mass balance | (Environmental Engineering) An organized accounting of all inputs and outputs to an arbitrary but defined system. Stated in other terms, the rate of mass accumulation within a system is equal to the rate of mass input less the rate of mass output plus the rate of mass generation within the system. |
| Metabolism | (Environmental Engineering) The processes which sustain an organism, including energy production, synthesis of proteins for repair and replication. |
| Metalimnion | (Environmental Engineering) The middle layer of a lake. |
| 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. |
| Nitrogenous oxygen demand (NOD) | (Environmental Engineering) The amount of oxygen required to oxidize any ammonia present in a water. |
| 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. |