| Species | (Environmental Engineering) In chemistry, an ion or molecule in solution. |
| Sterilization | (Environmental Engineering) The destruction or inactivation of all microorganisms. See Disinfection. |
| Stratosphere | (Environmental Engineering) The atmosphere from approximately 12 km to 70 km. The temperature of the atmosphere increases in this region.Strong acid |
| 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. |
| 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. |
| Toxicity | (Environmental Engineering) A U.S. EPA hazardous waste characteristic defined with a rigorous test procedure, the TCLP (for Toxicity Characteristic Leaching Procedure). In the procedure, a waste is extracted for 24 hours with an acetic acid solution. The acid extract is then analyzed for the presence of any of the contaminants listed in the procedure. |
| Trace contaminants | (Environmental Engineering) Contamination found in trace (very low) levels. |
| Troposphere | (Environmental Engineering) The lower atmosphere, from the earth's surface to approximately 12 km. This portion of the earth's atmosphere contains about 95 percent of the atmospheric gases. The temperature gradually declines through this region. |