| Groundwater | (Environmental Engineering) Water which is contained in geologic strata. Also properly written as two words, ground water. |
| 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. |
| 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 treatment | (Environmental Engineering) Treatment which includes all operation prior to and including primary treatment, e.g., bar screening, grit removal, comminution, and primary sedimentation. |
| 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. |
| Secondary treatment | (Environmental Engineering) In wastewater treatment, the conversion of the suspended, colloidal and dissolved organics remaining after primary treatment into a microbial mass with is then removed in a second sedimentation process. Secondary treatment included both the biological process and the associated sedimentation process. |
| Siting | (Environmental Engineering) Obtaining government (federal, state, and local) permission to construct an environmental processing, treatment, or disposal facility at a given site. |
| Species | (Environmental Engineering) In chemistry, an ion or molecule in solution. |
| Stratosphere | (Environmental Engineering) The atmosphere from approximately 12 km to 70 km. The temperature of the atmosphere increases in this region.Strong acid |