| Alkalinity | (Environmental Engineering) The capacity of a water to neutralize acids. |
| 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. |
| 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. |