Primary treatment physically removes large and suspended solids, secondary treatment uses microorganisms to decompose organic matter, and tertiary treatment removes remaining nutrients, pathogens, and other pollutants. The stages progressively improve water quality before discharge or reuse.
How the Three Stages Work
This is HL-only content in IB ESS subtopic 4.4 Water pollution, although biochemical oxygen demand (BOD) and eutrophication are studied by both SL and HL students.
| Treatment stage | Main processes | Main result |
|---|---|---|
| Primary treatment | Screens remove large objects; grit chambers remove sand and gravel; sedimentation allows suspended solids to settle as sludge, while oils may be skimmed from the surface. | Removes much of the solid material but leaves dissolved organic matter, nutrients, and microorganisms. |
| Secondary treatment | Aerobic microorganisms decompose organic matter in activated-sludge tanks or biological filters. Further settling separates microbial biomass from the treated water. | Reduces organic pollution and therefore lowers BOD, leaving more dissolved oxygen available in the receiving ecosystem. |
| Tertiary treatment | Advanced filtration and chemical or biological processes remove nitrates and phosphates. Disinfection using chlorine, ozone, or ultraviolet radiation reduces pathogens. | Produces higher-quality effluent suitable for safer discharge or, after sufficiently advanced processing, reuse. |
The mechanism matters because untreated organic material supports rapid microbial decomposition. This raises BOD, decreases dissolved oxygen, and may cause hypoxia and the death of aquatic organisms. Removing nitrates and phosphates during tertiary treatment also reduces the risk of eutrophication.
A common misconception is that tertiary treatment automatically produces drinking water. It produces highly treated effluent, but potable reuse requires additional treatment, monitoring, and strict standards. Singapore's NEWater system demonstrates how advanced membrane filtration and ultraviolet disinfection can support water reuse, although high costs and technological requirements may limit access elsewhere, creating an environmental justice issue.
Exam Technique
For an explain question, identify the process at each stage and link it to its environmental effect. Do not merely state that each stage “cleans the water”; use terms such as sedimentation, microbial decomposition, BOD, nutrient removal, and disinfection.