| NPDES | (Environmental Engineering) The National Pollutant Discharge Elimination System. The discharge criteria and permitting system established by the U.S. EPA as a result of the Clean Water Act and its subsequent amendments or the permit required by each discharger as a result of the Clean Water Act. |
| 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. |
| Pathogenic organism | (Environmental Engineering) An organism capable of causing infection. |
| Phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP). |
| Photophosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) using solar energy. |
| Phototroph | (Environmental Engineering) Organisms which obtain energy from light using photooxidation. |
| 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. |
| 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. |
| 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. |