1. Preliminary Treatment:

Purpose: To remove large debris and grit that could damage equipment or interfere with downstream processes.

• Screening removing large objects like rags and trash
• Grit removal using chambers or channels to settle out sand and gravel

2. Primary Treatment:

Purpose: To separate settleable solids and floatable materials.

• Sedimentation: Wastewater is held in large tanks (clarifiers) where gravity causes solids to settle to the bottom forming sludge and lighter materials like oil and grease float to the top.
• Skimming: Floating materials are skimmed off the surface.
• Sludge Removal: Settled sludge is collected and sent for further treatment.

3. Secondary Treatment:

Purpose: To remove dissolved and suspended organic matter using biological processes.

• Aeration: Wastewater is mixed with air to encourage microorganisms to consume organic pollutants.
• Biological Reactors: Systems like activated sludge or trickling filters support biological degradation.
• Secondary Clarification: Microorganisms and solids settle out forming activated sludge.

4. Tertiary Treatment:

Purpose: To further purify water, including nutrient removal and disinfection.

• Filtration: Water passes through sand or membrane filters to remove remaining solids.
• Disinfection: Chlorine, UV light, or other methods kill remaining pathogens.
• Nutrient Removal: Nitrogen and phosphorus are removed through chemical or biological processes.

5. Sludge Treatment:

Purpose: To stabilize and reduce the volume of sludge from primary and secondary processes.

• Digestion: Sludge is treated with anaerobic bacteria to break down organic matter and produce biogas.
• Dewatering: Excess water is removed using centrifuges, belt presses, or drying beds.
• Disposal/Reuse: Treated sludge can be used as fertilizer, composted, or disposed of in landfills.