| Anaerobic process | (Environmental Engineering) A process which only occurs in the absence of molecular oxygen. |
| Anoxic process | (Environmental Engineering) A process which occurs only at very low levels of molecular oxygen or in the absence of molecular oxygen. |
| Anthropogenic | (Environmental Engineering) Of, made, or caused by human activity or actions. |
| Aromatic | (Environmental Engineering) A form of bonding in which ring compounds share electrons over more than two atoms. The electrons are delocalized. This leads to unusual ring stability. |
| Attached growth reactor | (Environmental Engineering) A reactor in which the microorganisms are attached to engineered surfaces within the reactor. Examples of attached growth reactors are the trickling filter and the rotating biological contactor. See suspended growth reactor. |
| Autotrophic | (Environmental Engineering) Organisms which utilize inorganic carbon for synthesis of protoplasm. Ecologists narrow the definition further by requiring that autotrophs obtain their energy from the sun. In microbiologist parlance, this would be a photoautotroph. See photoautotrophic and chemoautotrophic. |
| Autotrophs | (Environmental Engineering) A group of organisms capable of obtaining carbon for synthesis from inorganic carbon sources such as carbon dioxide and its dissolved species (the carbonates). This group includes plants and algae. |
| Bacteria | (Environmental Engineering) One celled microorganisms which do not have a nuclear membrane. |
| Baghouse filter | (Environmental Engineering) A fabric filter device used to remove particulate air pollutants. |
| Biochemical oxygen demand (BOD) | (Environmental Engineering) The amount of oxygen required to oxidize any organic matter present in a water during a specified period of time, usually 5 days. It is an indirect measure of the amount of organic matter present in a water. |