| Receiving water quality standards | (Environmental Engineering) Standards which require a discharger to maintain a certain quality level in the receiving water. |
| 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. |
| Secured landfill | (Environmental Engineering) A landfill which has containment measures such as liners and a leachate collection system so that materials placed in the landfill will not migrate into the surrounding soil, air and water. |
| Siting | (Environmental Engineering) Obtaining government (federal, state, and local) permission to construct an environmental processing, treatment, or disposal facility at a given site. |
| Surface water | (Environmental Engineering) Water which is contained in lakes, rivers, and oceans. |
| 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. |
| 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. |
| Wastewater | (Environmental Engineering) Consumed or used water from a municipality or industry that contains dissolved and/or suspended matter. |