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.
3. Outcome:
   Produces “polished effluent”, often safe enough for reuse in irrigation, industrial processes, or replenishing aquifers.

Sludge Treatment and Disposal

1. Sludge collected from earlier stages is thickened, anaerobically digested, and dewatered.
2. The resulting material, called biosolids, may be used as fertilizer, incinerated, or sent to landfills.
3. Anaerobic digestion produces methane gas, which can be used to generate electricity, an example of waste-to-energy technology.

Challenges in Implementing Sewage Treatment

Economic and Infrastructure Barriers

1. HICs (High-Income Countries)

1. Well-developed sewage infrastructure and centralized treatment plants.
2. Around 70–80% of wastewater is treated safely.
3. Advanced technologies used for nutrient removal and recycling.
4. Ongoing challenges include high operational costs and aging infrastructure.

2. LICs (Low-Income Countries)

1. Limited infrastructure, many rural areas lack access to sewage treatment.
2. Only about 4–8% of sewage is treated.
3. Wastewater is often discharged directly into rivers or oceans.
4. Major health risks: outbreaks of cholera, dysentery, and typhoid.
5. Dependence on NGOs and international aid for sanitation projects.

Environmental and Social Inequality

1. Wealthier urban areas tend to have full sewage coverage, while rural and informal settlements lack basic sanitation.
2. The gap between treated and untreated waste creates inequities in environmental health.

Energy and Resource Demands

1. Sewage treatment plants require continuous energy input and chemical usage.
2. This raises concerns about sustainability, particularly in areas with unreliable electricity supply.

Equity and Sustainability in Sewage Management

1. Decentralized Systems: Small-scale or community-based systems (e.g., septic tanks, bio-digesters) can serve rural areas where centralized systems are not feasible.
2. Reuse and Recycling: Treated wastewater can be reused for irrigation or industrial processes, reducing freshwater demand.
3. Public Awareness: Education on sanitation and water conservation is crucial to ensure sustainable sewage management.