| 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). |
| Maximum contaminant level (MCL) | (Environmental Engineering) The maximum allowable concentration of a given constituent in potable water. |
| Nitrogen fixation | (Environmental Engineering) The conversion of atmospheric (or dissolved) nitrogen gas into nitrate by microorganisms. |
| Oxidative phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using a chemical substrate and molecular oxygen. |
| Procaryotic organisms | (Environmental Engineering) Organisms which do not have a cellular membrane. |
| Reactive waste | (Environmental Engineering) A waste which; 1) reacts violently with water, 2) forms potentially explosive mixtures with water, 3) is normally unstable, 4) contains cyanide or sulfide in sufficient quantity to evolve toxic fumes at high or low pH, 5) is capable of exploding if heated under pressure, or 6) is an explosive compound listed in Department of Transportation (DoT) regulations. One of EPA's four hazardous waste properties. |
| Receiving water | (Environmental Engineering) A water which receives wastewater (treated or otherwise) discharges. |
| Receiving water quality standards | (Environmental Engineering) Standards which require a discharger to maintain a certain quality level in the receiving water. |
| Recycling | (Environmental Engineering) The recovery and reuse of a product which would otherwise be thrown away. |
| Refuse derived fuel (RDF) | (Environmental Engineering) A fuel derived from the combustible portion of municipal solid waste. The fuel is often processed into small briquettes, similar in size to charcoal. |