| 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. |
| 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. |
| 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. |
| Covalent bond | (Environmental Engineering) A bond in which electrons are shared approximately equally by two atoms. |