| Chemical oxygen demand (COD) | (Environmental Engineering) The amount of oxygen required to oxidize any organic matter in the water using harsh chemical conditions. |
| Chemoautotrophic | (Environmental Engineering) Organisms which utilize inorganic carbon (carbon dioxide or carbonates) for synthesis and inorganic chemicals for energy. See autotrophic and photoautotrophic. |
| Chemotroph | (Environmental Engineering) Organisms which obtain energy from the metabolism of chemicals, either organic or inorganic. |
| Chlorofluorocarbons | (Environmental Engineering) Synthetic organic compounds used for refrigerants, aerosol propellants (prohibited in the U.S.), and blowing agents in plastic foams. CFCs migrate to the upper atmosphere destroying ozone and increasing global warming. Typical atmospheric residence times are 50 to 200 years. |
| 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. |
| Climatology | (Environmental Engineering) The study of the climate, how the earth's atmosphere performs over long periods of time. |
| 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. |
| Coagulation | (Environmental Engineering) Particle destabilization to enhance agglomeration. |
| 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. |
| Complexation | (Environmental Engineering) The ionic bonding of one or more central ions or molecules by one or more surrounding ions or molecules. |