| 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. |
| Synergism is the act of working together. | (Environmental Engineering) Two chemicals which are synergistic have a greater effect together than the sum of their individual effects. The effect can be either positive or negative. |
| System | (Environmental Engineering) An arbitrarily defined area or volume surrounded by a boundary and possessing specific inputs, outputs, and reactions. |
| Total suspended solids | (Environmental Engineering) (TSS) is the amount of suspended (filterable) matter in a water. |
| Trickling filter | (Environmental Engineering) An attached growth biological process in which the microbial film is attached to non-moving rock or plastic media. |
| 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. |
| 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 |