| Primary standards | (Environmental Engineering) Required drinking water quality standards related directly to human health. These standards are required and enforceable by the U.S. EPA. See secondary standards. |
| Primary treatment | (Environmental Engineering) Treatment which includes all operation prior to and including primary treatment, e.g., bar screening, grit removal, comminution, and primary sedimentation. |
| Procaryotic organisms | (Environmental Engineering) Organisms which do not have a cellular membrane. |
| Producers | (Environmental Engineering) Autotrophic organisms which produce protoplasm using inorganic carbon and energy from the sun. |
| Publicly owned treatment works (POTW) | (Environmental Engineering) A municipal or domestic wastewater treatment facility. |
| 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. |
| Reaeration | (Environmental Engineering) The dissolving of molecular oxygen from the atmosphere into the water. |
| 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. |
| 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. |