| Amine | (Environmental Engineering) A functional group consisting of "-NH2." |
| Amino acid | (Environmental Engineering) A functional group which consists of a carbon with a carboxylic acid, "-COOH" and an amine, "-NH2." These compounds are the building blocks for proteins. |
| Anabolism | (Environmental Engineering) Biosynthesis, the production of new cellular materials from other organic or inorganic chemicals. |
| Anaerobes | (Environmental Engineering) A group of organisms that do not require molecular oxygen. These organisms, as well as all known life forms, require oxygen. These organisms obtain their oxygen from inorganic ions such as nitrate or sulfate or from protein. |
| 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. |