Syllabus Edition
First teaching 2024
First exams 2026
Sewage Treatment (HL) (DP IB Environmental Systems & Societies (ESS))
Revision Note
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
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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