| 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. |
| Suspended growth reactor | (Environmental Engineering) A reactor in which the microorganisms are suspended in the wastewater. Examples of suspended growth reactors are activated sludge reactors and anaerobic digesters. See attached growth reactor. |
| 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. |
| System | (Environmental Engineering) An arbitrarily defined area or volume surrounded by a boundary and possessing specific inputs, outputs, and reactions. |
| Thermocline | (Environmental Engineering) The depth at which an inflection point occurs in a lake temperature profile. |
| Thiols | (Environmental Engineering) Organic compounds which contain the "-SH" functional group. Also called mercaptans. |
| Total dissolved solids | (Environmental Engineering) (TDS) is the amount of dissolved matter in the water. |
| Total solids | (Environmental Engineering) (TS) is the amount of organic and inorganic matter which is contained in a water. |
| Total suspended solids | (Environmental Engineering) (TSS) is the amount of suspended (filterable) matter in a water. |