| Pathogenic organism | (Environmental Engineering) An organism capable of causing infection. |
| Phenol | (Environmental Engineering) An aromatic benzene ring with a hydroxyl substituted for one hydrogen. |
| Phenyl- | (Environmental Engineering) A benzene ring named as a constituent group, C6H5-. |
| Phosphorylation | (Environmental Engineering) The synthesis of the energy storage compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP). |
| Photoautotrophic | (Environmental Engineering) Organisms which utilize inorganic carbon dioxide for protoplasm synthesis and light for an energy source. See autotrophic and chemoautotrophic. |
| Photochemical pollutants | (Environmental Engineering) Chemicals which react photochemically (in the presence of sunlight) to destroy ozone in the stratosphere. |
| 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. |
| POTW | (Environmental Engineering) or Publicly Owned Treatment Works Any municipally owned wastewater treatment facility. |
| 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. |