| Producers | (Environmental Engineering) Autotrophic organisms which produce protoplasm using inorganic carbon and energy from the sun. |
| 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. |
| Shock load | (Environmental Engineering) Influent wastewater entering the plant which has an unusually high organic content and/or high flow rate. |
| Sterilization | (Environmental Engineering) The destruction or inactivation of all microorganisms. See Disinfection. |
| 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. |
| 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. |
| Thiols | (Environmental Engineering) Organic compounds which contain the "-SH" functional group. Also called mercaptans. |
| Total solids | (Environmental Engineering) (TS) is the amount of organic and inorganic matter which is contained in a water. |
| Trophic level | (Environmental Engineering) A level in the food chain. The first trophic level consists of the primary producers, autotrophs. The second trophic level is vegetarians which consume autotrophic organisms. |