IB Environmental systems and societies 4.4F Sewage Treatment Notes

Sewage treatment protects health and water by removing contaminants in three stages: physical, biological, then chemical.

1. Sewage treatment removes contaminants from domestic, industrial, and agricultural wastewater so it is safe to release or reuse.
2. It protects public health by removing pathogens and toxic substances.
3. It prevents water pollution and the eutrophication it can trigger (see 4.4C).
4. It recycles water for agricultural, industrial, or domestic use.
5. Treatment runs in three main stages: primary (physical), secondary (biological), and tertiary (chemical or advanced).

The three stages of sewage treatment

Primary treatment (physical separation)

1. Screening traps large materials such as rags, plastics, and wood on metal grids or mesh filters.
2. Grit removal lets heavier particles like sand and stones settle out in grit chambers.
3. Sedimentation in primary clarifiers lets suspended solids settle at the bottom as sludge.
4. Skimming removes floating grease and oil from the top.
5. Primary treatment removes roughly 60-70% of suspended solids and 30-40% of organic matter, leaving primary sludge and partially clarified effluent.

Secondary treatment (biological decomposition)

1. Aeration pumps air or oxygen into tanks to support aerobic bacteria that consume organic matter.
2. The activated sludge process aerates a mixture of wastewater and bacteria so the microbes metabolise organic compounds.
3. Trickling filters spray wastewater over stone or plastic beds where microbes grow as biofilms and degrade pollutants.
4. Sewage lagoons are shallow ponds where natural microbial communities and sunlight break down waste.
5. Secondary treatment removes roughly 85-90% of organic matter and cuts pathogens significantly.