4.4F Sewage treatment (HL) Notes

1. Sewage treatment is the process of removing contaminants from wastewater, primarily from domestic, industrial, and agricultural sources, to make it safe for release back into the environment or for reuse.
2. The primary goals are to:
   1. Protect public health by removing pathogens and toxic substances.
   2. Prevent water pollution and eutrophication in aquatic systems.
   3. Recycle water for agricultural, industrial, or even domestic use
3. It typically occurs in three main stages: Primary (physical), Secondary (biological), and Tertiary (chemical or advanced) treatment.

Stages of Sewage Treatment

Primary Treatment (Physical Separation of Solids)

1. Purpose: To remove large debris, grit, and suspended solids through mechanical and gravitational processes.
2. Process:
   1. Screening: Metal grids or mesh filters trap large materials such as rags, plastics, and wood.
   2. Grit Removal: Water passes through grit chambers where heavier particles like sand and stones settle out.
   3. Sedimentation: Wastewater flows into large tanks (called primary clarifiers) where smaller suspended solids settle at the bottom as sludge.
   4. Skimming: Floating materials like grease and oil rise to the top and are skimmed off.
3. Outcome: Removes up to 60-70% of suspended solids and 30-40% of organic matter, producing primary sludge and partially clarified effluent.

Secondary Treatment (Biological Decomposition of Organic Matter)

1. Purpose: To degrade dissolved and colloidal organic matter using microorganisms that feed on organic waste.
2. Processes involved:
   1. Aeration: Air or oxygen is pumped into tanks to support aerobic bacteria, which consume organic matter.
   2. Activated Sludge Process: A mixture of wastewater and bacteria (sludge) is aerated, allowing bacteria to metabolize organic compounds.
   3. Trickling Filter System: Wastewater is sprayed over beds of stones or plastic media where microorganisms grow as biofilms and degrade organic pollutants.
   4. Sewage Lagoons: Shallow ponds where natural microbial communities and sunlight break down waste.
3. Outcome: Removes up to 85-90% of organic matter and reduces pathogens significantly.

Tertiary Treatment (Chemical and Advanced Filtration)

1. Purpose: To remove remaining nutrients, pathogens, and chemical pollutants for high-quality effluent.
2. Processes involved:
   1. Chemical Precipitation: Chemicals (e.g., aluminum or iron salts) are added to remove phosphates.
   2. Filtration: Fine filters or activated carbon beds remove micro-particles and dissolved contaminants.
   3. Disinfection:
      1. Chlorination: common and inexpensive, but may form harmful byproducts.
      2. Ultraviolet (UV) Radiation: effective without chemicals.
      3. Ozonation: highly efficient and leaves no residue, but costly.
   4. Biological Nutrient Removal (BNR): Certain bacteria remove nitrates and phosphates to prevent eutrophication.