| Cell | (Environmental Engineering) A unit of varying dimensions in a landfill which is isolated from the environment by 6 to 12 inches of soil cover. A cell is one day's waste or less. A cell is covered with soil at the end of each day. |
| Chemical fixation (or stabilization/solidification) | (Environmental Engineering) A term for several different methods of chemically immobilizing hazardous materials into a cement, plastic, or other matrix. |
| 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. |
| 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. |
| Consumers | (Environmental Engineering) Organisms which consume protoplasm produced from photosynthesis or consume organisms from higher levels which indirectly consume protoplasm from photosynthesis. |
| Conversion | (Environmental Engineering) The fraction of a species entering a system which is converted to product. |
| Corrosive waste | (Environmental Engineering) A waste that is outside the pH range of 2 to 12.5 or a waste that corrodes steel at a rate greater than 6.35 mm (0.25 in) per year. One of EPA's four hazardous waste properties. |