| Equilibrium: | (english) An object is in equilibrium if the resultant of the system of forces acting on it has zero magnitude. See static equilibrium and dynamic equilibrium. |
| External force: | (english) A surface force or body force acting on an object. External forces are sometimes called applied forces. |
| Ecology | (Environmental Engineering) The study of living organisms and their environment or habitat. |
| Ecosystem | (Environmental Engineering) An organism or group of organisms and their surroundings. The boundary of an ecosystem may be arbitrarily chosen to suit the area of interest or study. |
| 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). |
| Electrostatic precipitator | (Environmental Engineering) A device which uses an electric field to trap particulate pollutants. |
| Elementary reaction | (Environmental Engineering) A reaction in which the rate expression corresponds to the stoichiometric equation. |
| Epilimnion | (Environmental Engineering) The top layer of a lake. |
| Equivalent | (Environmental Engineering) The mass of the compound which will produce one mole of available reacting substance. Thus, for an acid, this would be the mass of acid which will produce one mole of H+, for a base, one mole of OH-. |