| Clarifier (sedimentation basin) | (Environmental Engineering) A tank in which quiescent settling occurs, allowing solid particles suspended in the water to agglomerate and settle to the bottom of the tank. The solids resulting from the settling being removed as a sludge. |
| Closure | (Environmental Engineering) The act of preparing a landfill for long term inactivity, including placement of a cover over the landfill to prevent infiltration of surface water. |
| Colloids | (Environmental Engineering) Small particles which have a negligible settling velocity. These particles have a very small mass so gravitational force is low compared to surface frictional forces. Typical colloidal sizes range from 10-3 mm to 1 mm. |
| Composting | (Environmental Engineering) The controlled aerobic degradation of organic wastes into a material which can be used for landscaping, landfill cover, or soil conditioning. |
| Covalent bond | (Environmental Engineering) A bond in which electrons are shared approximately equally by two atoms. |
| Denitrification | (Environmental Engineering) The anoxic biological conversion of nitrate to nitrogen gas. It occurs naturally in surface waters low in oxygen, and it can be engineered in wastewater treatment systems. |
| Dump | (Environmental Engineering) An illegal and uncontrolled area where wastes have been placed on or in the ground. See Landfill. |
| Electrostatic precipitator | (Environmental Engineering) A device which uses an electric field to trap particulate pollutants. |
| Equivalent | (Environmental Engineering) The mass of the compound which will produce one mole of available reacting substance. Thus, for an acid, this would be the mass of acid which will produce one mole of H+, for a base, one mole of OH-. |
| Eucaryotic organisms | (Environmental Engineering) Organisms which possess a nuclear membrane. This includes all known organisms except viruses and bacteria. |