Wastewater Treatment Process

Wastewater treatment is a process used to convert wastewater into an effluent (outflowing of water to a receiving body of water) that can be returned to the water cycle with minimal impact on the environment or directly reused.

The treatment process varies widely depending on the source and the quality of the wastewater. However, most wastewater treatment plants follow a series of common steps:


  • Screening: Large objects such as sticks, leaves, and plastics are removed using screens or bar racks.
  • Grit Removal: Heavy, inorganic particles like sand and gravel are settled out, often using sedimentation tanks or grit chambers.
  • Primary Sedimentation: The wastewater is allowed to sit in large tanks where heavier solids settle to the bottom as sludge.


  • Activated Sludge Process: Wastewater is mixed with microorganisms (activated sludge) in aeration tanks. These microorganisms consume organic matter, converting it into biomass and carbon dioxide.
  • Trickling Filters: Wastewater is passed over a bed of rocks or plastic media covered with a biofilm of microorganisms that break down organic matter.
  • Rotating Biological Contactors (RBCs): Rotating discs with attached microorganisms are partially submerged in wastewater to facilitate biological treatment.


  • Tertiary treatment is employed when further purification is required to meet stringent water quality standards or to remove specific contaminants.
  • Methods may include chemical coagulation, flocculation, sand filtration, advanced oxidation processes, and membrane filtration.


  • After biological treatment, the wastewater is disinfected to kill harmful pathogens such as bacteria and viruses.
  • Common disinfection methods include chlorination, ultraviolet (UV) radiation, and ozonation.

After these processes, the treated wastewater can be safely discharged into rivers, lakes, or oceans, or used for non-potable purposes like irrigation or industrial processes. It’s important to note that the specific processes and their order can vary based on the design of the wastewater treatment plant and the quality standards required for the treated water.