| Equivalent | (Environmental Engineering) The mass of the compound which will produce one mole of available reacting substance. Thus, for an acid, this would be the mass of acid which will produce one mole of H+, for a base, one mole of OH-. |
| 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. |
| Ion exchange | (Environmental Engineering) An adsorption process in which one ion is exchanged for another ion of like charge. There is an equivalence of exchanged charge. |
| 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. |
| 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. |
| 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. |
| 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. |