Most homes, businesses, and institutions are connected to a sewer system that conveys their wastewater to a public wastewater treatment plant. Sanitary sewer systems carry only domestic and industrial wastewater, while combined sewer systems also carry stormwater runoff. At the plant, the wastewater is cleaned and returned to the environment to be used over and over again.
Wastewater flows by gravity with occasional help from pumps until it reaches the treatment plant. What happens in a wastewater treatment plant is essentially the same as what occurs naturally in a lake or stream. The function of a wastewater treatment plant is to speed up the process by which water is cleaned naturally. Treatment plants are operated 24 hours a day by a treatment team that is committed to protecting public health and the environment.
Wastewater is typically treated through a series of five major steps followed by processes to reuse or to dispose of the remaining products. This treatment requires an intricate balance of physical, biological, and chemical processes. They include:
- Preliminary Treatment includes screening to remove large objects (such as sticks, rags, leaves, and trash) and the settling of grit (heavy, sandy, abrasive matter). The material is collected and discarded, and the remaining flow moves on to primary treatment.
- Primary Treatment involves the reduction of the wastewater flow to remove easily settleable and floatable solids using primary tanks known as clarifiers. Solids removed from this process are often sent to the solids handling portion of the plant.
- Secondary Treatment is designed to grow naturally occurring microorganisms to digest organic material, sometimes remove nutrients, and then to settle to the bottom of a secondary sedimentation basin. After secondary treatment, 85% to 90% of solids have been removed from the wastewater.
- Tertiary (or Advanced) Treatment is used to improve the quality of the water even more, especially if the plant’s permit requires more stringent effluent limits. Usually this entails lower effluent solids and nutrients, such as nitrogen and phosphorus levels.
- Disinfection destroys pathogenic organisms in the effluent before it is discharged into the receiving water body to help protect the public from exposure to pathogens. Alternatives for disinfection include chlorination followed by dechlorination, exposure to ultraviolet light, and the infusion of ozone.
- Solids Handling involves the treatment of the solids removed from the water treatment processes for beneficial use or to be made acceptable for landfills.