| 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. |
| 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. |
| Trickling filter | (Environmental Engineering) An attached growth biological process in which the microbial film is attached to non-moving rock or plastic media. |
| 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. |
| 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. |
| Ultimate disposal | (Environmental Engineering) The process of returning residuals back to the environment in a form which will have the minimal or reduced negative environmental impacts. |
| Virion | (Environmental Engineering) A virus particle. Viral DNA or RNA enclosed in an organic capsule. See virus. |