| 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. |
| Virus | (Environmental Engineering) A submicroscopic genetic constituent which can alternate between two distinct phases. As a virus particle, or virion, it is DNA or RNA enveloped in an organic capsule. As an intracellular virus, it is viral DNA or RNA inserted into the host organisms DNA or RNA. |
| Volatile solids | (Environmental Engineering) (VS) is the amount of matter which volatilizes (or burns) when a water sample is heated to 550EC.Volatile suspended solids |
| Waste minimization | (Environmental Engineering) The elimination or reduction of a waste prior to its generation. This is accomplished by process changes rather than waste treatment methods. |
| Wastewater | (Environmental Engineering) Consumed or used water from a municipality or industry that contains dissolved and/or suspended matter. |
| 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. |