| Hypolimnion | (Environmental Engineering) The lower layer of a lake. |
| In situ treatment | (Environmental Engineering) Treatment of a waste in place, as opposed to pumping or digging the waste up and then treating it. |
| Infectious disease | (Environmental Engineering) A disease caused by pathogenic organisms. |
| Influent | (Environmental Engineering) The fluid entering a system, process, tank, etc. An effluent from one process can be an influent to another process. See effluent. |
| Ion exchange | (Environmental Engineering) An adsorption process in which one ion is exchanged for another ion of like charge. There is an equivalence of exchanged charge. |
| Irreversible reaction | (Environmental Engineering) A reaction in which the reactant(s) proceed to product(s), but the products react at an appreciable rate to reform reactant(s). |
| Isomers | (Environmental Engineering) Two or more different compounds with the same chemical formula but different structure and characteristics. |
| Kerogen | (Environmental Engineering) A fossilized organic material present in oil shale and some other sedimentary rocks. |
| Ketones | (Environmental Engineering) Organic compounds with two hydrocarbon groups bonded to a carbonyl group. |
| Landfill | (Environmental Engineering) A legal and controlled area for the placement of wastes into the ground. |