| 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. |
| Respiration | (Environmental Engineering) Energy production in which oxygen is the terminal electron acceptor, i.e. oxidation to produce energy where oxygen is the oxidizing agent. See fermentation. |
| 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). |
| Runoff | (Environmental Engineering) The water that flows overland to lakes or streams during and shortly after a precipitation event. |
| Saltwater intrusion | (Environmental Engineering) The gradual replacement of freshwater by saltwater in coastal areas where excessive pumping of groundwater occurs. |
| Secondary standards | (Environmental Engineering) Recommended drinking water quality standards which relate to aesthetics and/or health. These standards are recommended, not required. See primary standards. |
| 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. |
| Secured landfill | (Environmental Engineering) A landfill which has containment measures such as liners and a leachate collection system so that materials placed in the landfill will not migrate into the surrounding soil, air and water. |