A sewage treatment plant, also known as a wastewater treatment plant (WWTP), processes sewage water through a series of stages to remove pollutants and make it safe for discharge or reuse. These stages are typically categorized as preliminary, primary, secondary, and tertiary treatment, with a separate process for sludge treatment.
1. Preliminary Treatment:
Purpose: To remove large debris and grit that could damage equipment or interfere with downstream processes.
Methods: 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.
Methods:
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 to float to the top.
Skimming: The floating materials are skimmed off the surface.
Sludge Removal: The settled sludge is collected and sent for further treatment.
3. Secondary Treatment:
Purpose: To remove dissolved and suspended organic matter using biological processes.
Methods:
Aeration: Wastewater is mixed with air (or oxygen) to encourage the growth of microorganisms (bacteria and other microbes) that consume organic pollutants.
Biological Reactors: Various reactor designs (e.g., activated sludge, trickling filters) are used to facilitate the biological degradation process.
Secondary Clarification: The treated wastewater then flows into another set of clarifiers where the microorganisms and remaining solids settle out, forming activated sludge.
4. Tertiary Treatment:
Purpose: To further purify the water, often including nutrient removal and disinfection, before discharge or reuse.
Methods:
Filtration: Wastewater passes through filters (e.g., sand filters, membrane filters) to remove remaining suspended solids and some dissolved pollutants.
Disinfection: Chlorine, UV light, or other methods are used to kill remaining pathogens (bacteria, viruses, etc.).
Nutrient Removal: Processes like chemical precipitation or biological nutrient removal (BNR) can be used to remove nitrogen and phosphorus.
5. Sludge Treatment:
Purpose: To stabilize and reduce the volume of the sludge produced in primary and secondary treatment.
Methods:
Digestion: Sludge is treated with anaerobic bacteria in digesters to break down organic matter and produce biogas (a renewable energy source).
Dewatering: Sludge is dewatered (excess water removed) using methods like centrifuges, belt presses, or drying beds.
Disposal/Reuse: Treated sludge can be disposed of in landfills, used as fertilizer, or composted.