| Advanced wastewater treatment | (Environmental Engineering) The removal of any dissolved or suspended contaminants beyond secondary treatment, often this is the removal of the nutrients nitrogen and/or phosphorus. |
| Aerobes | (Environmental Engineering) Organisms which require molecular oxygen as an electron acceptor for energy production. See anaerobes. |
| Aldehyde | (Environmental Engineering) An organic compound with a carbonyl at one end of a hydrocarbon group. |
| Alkalinity | (Environmental Engineering) The capacity of a water to neutralize acids. |
| 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. |