| Mixed liquor suspended solids (MLSS) | (Environmental Engineering) The total suspended solids concentration in the activated sludge tank. |
| Mixed liquor volatile suspended solids (MLVSS) | (Environmental Engineering) The volatile suspended solids concentration in the activated sludge tank. |
| 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. |
| Publicly owned treatment works (POTW) | (Environmental Engineering) A municipal or domestic wastewater treatment facility. |
| Runoff | (Environmental Engineering) The water that flows overland to lakes or streams during and shortly after a precipitation event. |
| 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. |
| Shock load | (Environmental Engineering) Influent wastewater entering the plant which has an unusually high organic content and/or high flow rate. |
| Stratosphere | (Environmental Engineering) The atmosphere from approximately 12 km to 70 km. The temperature of the atmosphere increases in this region.Strong acid |
| Surface water | (Environmental Engineering) Water which is contained in lakes, rivers, and oceans. |