A sewer treatment plant, or wastewater treatment plant (WWTP), is a facility that treats wastewater from homes, businesses, and industry using physical, biological, and chemical processes to remove pollutants and contaminants, producing treated effluent (clean water) that can be safely discharged back into the environment or reused. The treatment process typically involves three main stages: primary treatment (removing large solids), secondary treatment (using bacteria to break down organic matter), and tertiary treatment (further purifying the water for safe reuse).  
Why are sewer treatment plants necessary?

• Public Health:
  They prevent harmful bacteria and viruses from contaminating water sources and spreading diseases. 

• Environmental Protection:
  They remove pollutants from wastewater before it enters rivers, lakes, and oceans, protecting ecosystems and wildlife. 

• Water Reuse:
  The treated water, or effluent, can be reused for irrigation, industrial purposes, and even in some cases for drinking water, which supports water conservation efforts. 

The main stages of treatment:

• 1. Primary Treatment (Physical/Mechanical):
  • Screening: Large objects like sticks, rags, and debris are removed using screens. 
  • Grit Removal: The wastewater passes through a grit chamber, where heavier particles like sand and gravel settle out due to a decrease in water velocity. 
  • Primary Sedimentation: The water flows into large tanks where gravity causes smaller suspended solids to settle to the bottom, forming sludge, while lighter materials like grease are skimmed from the top. 

• 2. Secondary Treatment (Biological):
  • Aeration/Biological Reactors: The wastewater is mixed with activated sludge, which contains microorganisms (bacteria) that consume organic matter and nutrients. 
  • Clarification: The water then moves to secondary clarifiers, where the activated sludge settles to the bottom, allowing the now-cleaner water to move on to the next stage. 

• 3. Tertiary Treatment (Advanced):
  • Filtration: Further filtration removes any remaining suspended particles. 
  • Disinfection: The water is disinfected using chemicals (like bleach) or other methods to kill any remaining harmful pathogens. 
  • Nutrient Removal: Specific biological or chemical processes can be used to remove additional nutrients, such as phosphorus and nitrogen, to prevent excessive growth of algae in receiving waters. 

After these stages, the fully treated and disinfected water, or effluent, is safe to be discharged into the environment or to be reused.