| Adsorption | (Environmental Engineering) A surface phenomena in which a solute (soluble material) concentrates or collects at a surface (the adsorbent). |
| 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. |
| Aerobic process | (Environmental Engineering) A process which requires molecular oxygen. |
| Alcohol | (Environmental Engineering) An organic compound with one or more hydroxyl "-OH" groups. |
| 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. |