| Landfill | (Environmental Engineering) A legal and controlled area for the placement of wastes into the ground. |
| Landfilling | (Environmental Engineering) The placement of wastes into the land under controlled conditions to minimize their migration or effect on the surrounding environment. |
| Ligand | (Environmental Engineering) The ion or molecule which surrounds or complexes with the central atom or ion. |
| 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. |
| 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. |
| Metalimnion | (Environmental Engineering) The middle layer of a lake. |
| Mixed liquor suspended solids (MLSS) | (Environmental Engineering) The total suspended solids concentration in the activated sludge tank. |
| Mixed liquor volatile suspended solids (MLVSS) | (Environmental Engineering) The volatile suspended solids concentration in the activated sludge tank. |