| 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. |
| Conversion | (Environmental Engineering) The fraction of a species entering a system which is converted to product. |
| 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. |
| Discrete settling | (Environmental Engineering) Settling in which individual particles settle independently, neither agglomerating or interfering with the settling of the other particles present. This occurs in waters with a low concentration of particles. |
| Disease | (Environmental Engineering) Any impairment of the normal function of an organism. |
| Disinfection | (Environmental Engineering) The destruction or inactivation of pathogenic microorganisms. See sterilization. |
| Effluent based standards | (Environmental Engineering) Standards which set concentration or mass per time limits on the effluent being discharged to a receiving water. |
| Electronegativity | (Environmental Engineering) The potential of an atom to attract electrons when the atom is bonded in a compound. The scale is 0 to 4 with 0 being the most electropositive (low attraction) and 4 being the most electronegative (high attraction). |
| Elementary reaction | (Environmental Engineering) A reaction in which the rate expression corresponds to the stoichiometric equation. |