| 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. |
| Reversible 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). |
| Saltwater intrusion | (Environmental Engineering) The gradual replacement of freshwater by saltwater in coastal areas where excessive pumping of groundwater occurs. |
| Secondary treatment | (Environmental Engineering) In wastewater treatment, the conversion of the suspended, colloidal and dissolved organics remaining after primary treatment into a microbial mass with is then removed in a second sedimentation process. Secondary treatment included both the biological process and the associated sedimentation process. |
| Site remediation | (Environmental Engineering) The process of cleaning up a hazardous waste disposal site that has either been abandoned or that those responsible either refuse to cleanup or are financially unable to cleanup. |
| Siting | (Environmental Engineering) Obtaining government (federal, state, and local) permission to construct an environmental processing, treatment, or disposal facility at a given site. |
| Softening | (Environmental Engineering) The removal of divalent cations by precipitation or ion exchange. |
| Source reduction | (Environmental Engineering) The elimination or reduction of the waste at the source by modification of the actual process which produces the waste. |
| Substrate level phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using organic substrates without molecular oxygen. |