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First teaching 2024

First exams 2026

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

Revision Note

Alistair Marjot

Written by: Alistair Marjot

Reviewed by: Jacque Cartwright

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

Flowchart illustrating sewage treatment. Raw sewage passes through screens and grit chamber, then primary and secondary treatment, ending in effluent and sludge disposal.
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

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.