| Catabolism | (Environmental Engineering) The production of energy by the degradation of organic compounds. |
| 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. |
| Composting | (Environmental Engineering) The controlled aerobic degradation of organic wastes into a material which can be used for landscaping, landfill cover, or soil conditioning. |
| Consumers | (Environmental Engineering) Organisms which consume protoplasm produced from photosynthesis or consume organisms from higher levels which indirectly consume protoplasm from photosynthesis. |
| Decomposers | (Environmental Engineering) Organisms which utilize energy from wastes or dead organisms. Decomposers complete the cycle by returning nutrients to the soil or water and carbon dioxide to the air or water. |
| 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. |
| Deoxygenation | (Environmental Engineering) The consumption of oxygen by the different aquatic organisms as they oxidized materials in the aquatic environment. |