| Reversible reaction | (Environmental Engineering) A reaction in which the reactant(s) proceed to product(s), but the products react at an appreciable rate to reform reactant(s). |
| Runoff | (Environmental Engineering) The water that flows overland to lakes or streams during and shortly after a precipitation event. |
| Saltwater intrusion | (Environmental Engineering) The gradual replacement of freshwater by saltwater in coastal areas where excessive pumping of groundwater occurs. |
| 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. |
| Substrate level phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using organic substrates without molecular oxygen. |
| Surface water | (Environmental Engineering) Water which is contained in lakes, rivers, and oceans. |
| Synergism is the act of working together. | (Environmental Engineering) Two chemicals which are synergistic have a greater effect together than the sum of their individual effects. The effect can be either positive or negative. |
| Total dissolved solids | (Environmental Engineering) (TDS) is the amount of dissolved matter in the water. |
| Trace contaminants | (Environmental Engineering) Contamination found in trace (very low) levels. |