Sewage Treatment (HL) (DP IB Environmental Systems & Societies (ESS))

Sewage Treatment

• Sewage treatment is the process of removing harmful substances from wastewater so it can be:

  • Safely returned to the environment

  • Used for domestic purposes (e.g. for toilets, showers, drinking water)

• It prevents pollution, protects public health, and helps conserve water by recycling treated water

The three stages of sewage treatment

Primary treatment

• Purpose: Removes large solid materials (e.g. debris) and some suspended solids

• Process:

  • Screening: Large objects like sticks, rubbish, and plastic are filtered out using screens (usually made from metal bars)

  • Comminution: Any remaining large solids are ground up by a device called a comminutor to prevent pipe blockages

  • Grit removal: Sand and gravel settle to the bottom of the grit chamber

  • Sedimentation: Wastewater flows into sedimentation tanks (also known as primary clarifier) where smaller suspended solid particles settle at the bottom as sludge

  • Skimming: Floating materials like grease and oil are skimmed from the surface

• Result: Water is partially cleaned but still contains dissolved and smaller particles

Secondary treatment

• Purpose: Breaks down organic matter using biological processes

• Process:

  • Aeration: Oxygen is pumped into the wastewater to encourage bacteria to break down organic pollutants (e.g. human waste, food)

  • Biological degradation: Microorganisms (mainly bacteria) consume the organic waste, converting it into harmless by-products like carbon dioxide, water, and 'activated sludge'

    • Activated sludge contains aerobic bacteria that decompose organic matter

  • Clarification: Water is again left to settle (in a secondary clarifier) so that any remaining sludge can be removed

    • A portion of this sludge is reused as activated sludge in the aeration tank to continue the process of organic matter decomposition

• Result: Water is significantly cleaner but may still contain some nutrients (e.g. nitrates, phosphates) and pathogens

Tertiary treatment

• Purpose: Removes remaining nutrients, chemicals, and pathogens for maximum water quality

• Process:

  • Chemical treatment: Chlorine or ultraviolet light (UV) is used to disinfect the water and kill pathogens

  • Nutrient removal: Processes like chemical precipitation are used to remove excess nitrogen and phosphorus, which can cause eutrophication if released into water bodies

  • Filtration: Water may be filtered through sand, activated carbon, or other materials to remove any final impurities

• Result: Effluent (treated water) is now clean enough to be safely discharged into rivers, lakes, or reused

Awaiting image: Sewage treatment

Image caption: Primary and secondary water treatment stages

Challenges in implementing sewage treatment

High income countries (HICs)

• Advanced infrastructure:

  • HICs usually have well-established sewage treatment facilities that cover most urban and rural areas

• High costs:

  • Although advanced, maintaining and upgrading sewage treatment plants is expensive

  • Continuous investment is needed to meet stricter environmental regulations and improve efficiency

Low income countries (LICs)

• Limited infrastructure:

  • In many LICs, especially in rural or densely populated areas, sewage treatment infrastructure is either insufficient or non-existent

• Health risks:

  • Untreated sewage can lead to serious public health problems such as waterborne diseases like cholera, typhoid, and dysentery

• Cost barriers:

  • Building and maintaining sewage treatment facilities is costly, and many LICs lack the financial resources to develop this infrastructure

  • This limits access to proper sanitation, especially in rural areas

Social and economic inequality

• Wealth gaps:

  • In many countries, wealthier communities have access to better sanitation and sewage treatment

  • Poorer or marginalised groups often live without proper facilities

• International aid and partnerships:

  • Many LICs rely on international organisations and NGOs to provide funding and expertise to build or improve sewage systems