Sewage treatment is a level 2 strategy: the activity carries on, but what leaves the pipe is cleaned first. The technology has existed for over a century. Whether a community actually has it depends far more on money and infrastructure than on science — and that is usually the real point of the exam question.
📚 What you need to know
Sewage treatment removes harmful substances from wastewater so it can be safely returned to the environment or reused.
Primary treatment is physical — screening, grit removal and settling remove solids.
Secondary treatment is biological — aerated bacteria digest the dissolved organic matter.
Tertiary treatment is chemical and physical — nutrients are stripped, water is filtered and pathogens are killed.
Each stage lowers the BOD of the effluent, which is why treated water no longer causes an oxygen crash downstream.
The barriers in low income countries are cost, infrastructure and maintenance, not lack of knowledge.
Why bother?
Untreated sewage does three things at once. It carries a huge organic load that strips oxygen from the receiving water. It carries nutrients that cause eutrophication. And it carries pathogens that spread cholera, typhoid and dysentery. Treatment tackles all three, which is why sanitation is one of the biggest public health wins in history.
The three stages
Secondary treatment removes most of the oxygen demand but leaves nitrates and phosphates behind. Only tertiary treatment takes those out, and it costs the most to run.
Primary treatment — physical
Screening: metal bars catch sticks, rags, plastic and other large objects.
Comminution: a grinder chops up remaining large solids so they cannot block the pipes.
Grit removal: sand and gravel sink to the bottom of a grit chamber.
Sedimentation: in a large settling tank, suspended particles sink and form sludge.
Skimming: grease and oil float and are scraped off the top.
Result: the water looks much better but is still full of dissolved organic matter, so its BOD is still high.
Secondary treatment — biological
Aeration: oxygen is pumped in so aerobic bacteria can thrive.
Biological degradation: the bacteria consume the dissolved organic waste, turning it into carbon dioxide, water and more bacteria, known as activated sludge.
Clarification: the water settles again in a secondary clarifier so the sludge can be removed.
Recycling the sludge: some activated sludge is returned to the aeration tank to keep the bacterial population high.
Result: a big fall in BOD. But nutrients such as nitrate and phosphate remain, and so do some pathogens.
Secondary treatment is doing on purpose, in a tank, exactly what would otherwise happen by accident in the river. The difference is that the oxygen is supplied by a pump instead of being stolen from fish.
Tertiary treatment — chemical and physical polish
Nutrient removal: chemical precipitation takes out excess nitrogen and phosphorus, preventing eutrophication downstream.
Filtration: sand or activated carbon removes remaining fine impurities.
Disinfection: chlorine or ultraviolet light kills pathogens.
Result: effluent clean enough to discharge into rivers and lakes, or to reuse.
Link it back: primary treatment cuts suspended solids and turbidity. Secondary cuts BOD. Tertiary cuts nutrients and pathogens. If a data table shows treated effluent still causing algal blooms, the plant almost certainly has no tertiary stage.
Why not everyone has it
Context
Situation
Main challenge
High income countries
Well-established plants covering most urban and rural areas
Maintaining and upgrading ageing infrastructure is expensive, and standards keep tightening
Low income countries
Treatment often limited or absent, especially in rural and rapidly growing informal settlements
Capital cost of building plants, plus the running cost, trained staff and reliable electricity to keep them working
Within any country
Wealthier areas served, poorer or marginalised communities not
Sanitation follows wealth, so the health burden falls on those least able to avoid it
Two consequences follow, and both are worth marks:
Health. Untreated sewage spreads waterborne disease. This is a direct, measurable cost in illness and lost income.
Dependence on outside funding. Many low income countries rely on international organisations and NGOs to fund and design systems, which raises questions about who sets priorities and who maintains the plant afterwards.
EXAM PRACTICE
A town’s sewage works has primary and secondary treatment only. Downstream, BOD is low but algal blooms are increasing. Explain these observations and suggest one improvement. [4]
Explain the low BOD
Secondary treatment uses aerated bacteria to break down dissolved organic matter, so the effluent has little remaining oxygen demand.
Explain the blooms
Those stages do not remove nitrate and phosphate. The nutrients are still discharged and act as fertiliser for algae.
Suggest the fix
Add a tertiary stage using chemical precipitation, or route the effluent through a constructed wetland to strip nutrients biologically.
Low BOD and high nutrients together point to missing tertiary treatmentConstructed wetlands are a great answer for lower budgets: cheap to run, no chemicals, but they need land.
💡 Exam tip
Tag each stage with its type: physical, biological, chemical. It makes the sequence easy to recall under pressure.
Say what is left after each stage, not just what is removed. That is where the higher marks sit.
Mention constructed wetlands and biofilters as lower-cost alternatives to conventional tertiary treatment.
For HIC and LIC comparisons, talk about running costs and maintenance, not just building costs. Plants fail when nobody can afford to run them.
Connect sewage treatment to the three-level framework: it is level 2, controlling the release.
⚠️ Common mix-up
Thinking secondary treatment removes nutrients. It removes organic matter. Nitrate and phosphate survive it.
Calling primary treatment “cleaning”. It is separation — solids out, dissolved pollutants still in.
Forgetting the aeration step. Without pumped oxygen the aerobic bacteria cannot work fast enough.
Assuming treated effluent is drinking water. It is safe to discharge; drinking water needs further treatment.
Blaming lack of knowledge in low income countries. The technology is well known; the barriers are financial and infrastructural.
Up next: Biotic and Abiotic Water Quality Indicators (HL) — using living organisms as a pollution meter, and when to trust a machine instead.
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