| 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. |
| Shock load | (Environmental Engineering) Influent wastewater entering the plant which has an unusually high organic content and/or high flow rate. |
| Site remediation | (Environmental Engineering) The process of cleaning up a hazardous waste disposal site that has either been abandoned or that those responsible either refuse to cleanup or are financially unable to cleanup. |
| Source reduction | (Environmental Engineering) The elimination or reduction of the waste at the source by modification of the actual process which produces the waste. |
| Surface water | (Environmental Engineering) Water which is contained in lakes, rivers, and oceans. |
| Suspended growth reactor | (Environmental Engineering) A reactor in which the microorganisms are suspended in the wastewater. Examples of suspended growth reactors are activated sludge reactors and anaerobic digesters. See attached growth reactor. |
| Total dissolved solids | (Environmental Engineering) (TDS) is the amount of dissolved matter in the water. |
| Total solids | (Environmental Engineering) (TS) is the amount of organic and inorganic matter which is contained in a water. |