| 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. |
| Complexation | (Environmental Engineering) The ionic bonding of one or more central ions or molecules by one or more surrounding ions or molecules. |
| Composting | (Environmental Engineering) The controlled aerobic degradation of organic wastes into a material which can be used for landscaping, landfill cover, or soil conditioning. |
| Compound | (Environmental Engineering) A substance composed of two or more elements. |
| Compression settling | (Environmental Engineering) Settling which occurs in the lower reaches of clarifiers where particle concentrations are highest. Particles can settle only by compressing the mass of particles below. |
| 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. |