| Saltwater intrusion | (Environmental Engineering) The gradual replacement of freshwater by saltwater in coastal areas where excessive pumping of groundwater occurs. |
| Softening | (Environmental Engineering) The removal of divalent cations by precipitation or ion exchange. |
| 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. |
| 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. |
| 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. |
| 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. |
| Ultimate biochemical oxygen demand (BODu) | (Environmental Engineering) The total amount of oxygen required to oxidize any organic matter present in a water, i.e. after an extended period, such as 20 or 30 days. |
| Weak acid | (Environmental Engineering) An acid that does not ionize completely under the conditions of interest. Examples include acetic acid, carbonic acid, and hypochlorous acid. See strong acid. |
| Adaptive maintenance - | (Software Engineering) activity associate with changing an application to make it conform to changes in its external environment |