Stage 2: Primary Treatment

This stage focuses on removing settleable and floatable solids. It primarily involves:

• Primary sedimentation: Large tanks allow heavy solids to settle out, while lighter materials like oil and grease rise to the surface for removal.

Stage 3: Secondary Treatment

This crucial stage removes dissolved organic matter and biological contaminants like bacteria. Two common methods are:

• Activated sludge: Bacteria in large tanks break down organic matter through an aeration process, requiring oxygen and a controlled environment.
• Trickling filters: Wastewater trickles over a media bed colonized by bacteria, facilitating the breakdown of organic matter.

Stage 4: Tertiary Treatment (Optional)

This final stage removes additional pollutants for specific purposes, like:

• Disinfection: Eliminates harmful bacteria and viruses using chlorine, ultraviolet light, or ozone.
• Nutrient removal: Removes excess nitrogen and phosphorus to prevent algal blooms in receiving bodies of water.
• Advanced filtration: Utilizes membranes or filters to remove further contaminants like heavy metals or pharmaceuticals.