Water Treatment Processes
In 2014, Flint, Michigan switched its water source to the Flint River but the treatment plant omitted orthophosphate corrosion inhibitor — a standard step costing less than $100/day. Without it, the iron and lead pipes corroded, and tap water lead levels reached 27 times the EPA safe limit of 15 ppb within months, triggering a federal emergency.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
Raw dam water enters a treatment plant after heavy rain. The sample is cloudy, contains organic matter from the catchment, and may contain microorganisms.
- Which treatment steps would need to happen before the water could be called safe drinking water?
- Why might a treatment plant choose chloramines or UV in some situations instead of relying only on chlorine gas?
Know
- The main stages of drinking water treatment in NSW facilities
- The reagents and processes involved in coagulation and disinfection
- The meaning of DBPs and the basic idea of reverse osmosis desalination
Understand
- How alum forms Al(OH)3 and removes suspended particles
- Why HOCl is the active chlorinating species and why pH matters
- Why different disinfectants involve trade-offs between speed, by-products and residual protection
Can Do
- Sort treatment stages by their chemical purpose
- Classify disinfection methods by strengths and limitations
- Evaluate suitable treatment strategies for realistic NSW water scenarios
Core Content
A multi-barrier system rather than a single magic step
Modern drinking water treatment works because different stages solve different chemical problems. Clear-looking water can still contain fine particles, dissolved organics, and microorganisms.
In NSW treatment facilities, the major stages commonly include coagulation, flocculation, sedimentation, filtration and disinfection. Each stage removes a different class of risk.
NSW drinking water treatment train: coagulation → flocculation → sedimentation → filtration → disinfection. Each stage targets a different problem: particles (coagulation/floc), settling, fine removal, and microbes (disinfection). Coagulant used: alum Al₂(SO₄)₃ — forms Al(OH)₃ colloid that adsorbs particles.
Pause — copy the highlighted treatment train stages into your book.
Drinking-water treatment is a train, not a single step. Each stage removes a different problem before the water is finally disinfected and sent into distribution.
Why alum helps tiny particles come out of suspension
We just saw the treatment train overview. That raises the question: what is the actual chemistry behind coagulation with alum? This card answers it → how Al(OH)₃ forms and why it helps particles flocculate and settle.
Many suspended particles in raw water are too small and too stable to settle by themselves. Coagulation changes that chemistry.
Alum, Al2(SO4)3, provides Al3+ ions in water. These ions hydrolyse to form gelatinous Al(OH)3. The aluminium hydroxide colloid adsorbs suspended particles and helps destabilise them.
Coagulation: Al²⁺ + 3H₂O ⇌ Al(OH)₃(s/colloid) + 3H⁺. Al(OH)₃ colloid adsorbs and destabilises suspended particles. Flocculation: gentle mixing allows destabilised particles to form larger flocs. Sedimentation: flocs settle by gravity. Coagulation is about particles, not microbe killing.
Pause — copy the highlighted coagulation chemistry into your book.
During flocculation, gentle mixing encourages the small destabilised particles to collide and combine into larger flocs. During sedimentation, those larger flocs settle out under gravity.
Physical removal plus activated carbon support
We just saw how coagulation and sedimentation remove most suspended solids. That raises the question: what happens to the fine particles and dissolved organics that remain? This card answers it → filtration through sand, gravel and activated carbon.
Sedimentation removes a lot, but not everything. Filtration acts as the next barrier by removing the smaller particles left behind.
Water may pass through layers such as sand and gravel, which trap remaining particulate matter. Activated carbon can also be used because its large surface area helps adsorb some dissolved organic compounds that affect taste, odour or treatment performance.
Filtration removes remaining particles using sand and gravel. Activated carbon adsorbs dissolved organic compounds affecting taste and odour. Dissolved organic matter can react with chlorine to form disinfection by-products (DBPs) — reducing organics before disinfection is chemically important.
Pause — copy the highlighted filtration points into your book before the check below.
This matters chemically because dissolved organic matter is not only an aesthetic issue. It can also react later during disinfection and contribute to by-product formation.
Why HOCl matters more than just "chlorine in water"
We just saw how filtration removes organics that could react with chlorine. That raises the question: what is the actual chemistry of chlorination, and why does pH matter so much? This card answers it → the HOCl/OCl⁻ equilibrium and how pH shifts the balance.
When chlorine is added to water, the important question is not just how much chlorine was dosed. The real question is which chemical species are present.
HOCl is the more effective disinfecting species. As pH rises, more of the chlorine is present as OCl-, which is less effective as a disinfectant. That means pH influences how strongly chlorination works.
Cl₂ + H₂O ⇌ HOCl + HCl; then HOCl ⇌ H⁺ + OCl⁻. HOCl is the more effective disinfectant. Lower pH favours HOCl (stronger disinfection); higher pH favours OCl⁻ (weaker disinfection). pH management matters even after chlorine is dosed.
Add the chlorination equilibria and the pH rule to your notes before the check below.
Chlorination effectiveness depends on equilibrium chemistry, not just dose. Lower pH favours HOCl, which is the more active disinfecting species, while higher pH shifts more chlorine into OCl⁻.
Choosing the safest overall system, not just the fastest kill step
We just saw that chlorination chemistry depends on pH. That raises the question: what about the problems chlorine itself can create — and are there better alternatives? This card answers it → DBPs, alternative disinfectants and desalination trade-offs.
A treatment plant adds chlorine to kill pathogens — textbook procedure. But the source water contains natural organic matter from decaying leaves. The chlorine reacts with that organic matter to form trihalomethanes (THMs) including chloroform, a Group 2B carcinogen. Safe water in, carcinogen-containing water out. That is the disinfection by-product problem — and it is why choosing a disinfection method is not just about killing microbes; it is about what else the disinfectant might react with.
Disinfection by-products (DBPs), including trihalomethanes (THMs), can form when chlorine reacts with natural organic matter in water. That is why treatment plants try to reduce organic material before chlorination and why by-product risk matters in treatment design.
DBPs (trihalomethanes) form when chlorine reacts with natural organic matter. UV: no DBPs, but no residual; Ozone: powerful, no residual, high energy; Chloramines: fewer DBPs, has residual, but slower. Reverse osmosis (RO) removes dissolved salts but is energy-intensive.
Pause — copy the highlighted comparison of disinfection methods into your book.
In some NSW contexts, desalination is also part of water supply strategy. Reverse osmosis (RO) forces water through a membrane that removes salts and many dissolved substances, but it is energy-intensive. That energy cost is one of the major trade-offs of desalination.
Classify each stage by what problem it solves
This table shows why treatment is best understood as a classification task. Different chemical problems require different treatment stages, and no single method solves all of them well.
Complete the Learn phase to unlock Practice.
Activities
Place each process into the correct functional category and explain the chemistry briefly.
1. Coagulation with alum
2. Activated carbon filtration
3. Reverse osmosis
For each situation, choose the most suitable disinfection approach and justify it using the trade-offs in the lesson.
1. A treatment plant wants strong final disinfection but also wants to reduce DBP formation compared with free chlorine.
2. A small treatment step needs fast pathogen inactivation but the treated water will not be stored long or sent through a large pipe network.
3. A coastal city needs a freshwater supply from seawater, but planners are worried about cost and energy use.
Check Your Understanding
Understand Band 3
1. What is the main purpose of coagulation in water treatment?
Understand Band 4
2. Which species is the more active disinfectant in chlorinated water?
Apply Band 4
3. As pH rises, chlorinated water generally contains more OCl⁻. What is the main implication?
Analyse Band 5
4. Why is removal of organic matter before chlorination chemically important?
Analyse Band 5
5. Which statement best compares alternative disinfection methods?
Apply Band 4
1. Describe the major stages of drinking water treatment in a NSW water treatment facility, from coagulation to disinfection. (4 marks)
Analyse Band 5
2. Explain the chemistry of both coagulation with alum and chlorination with chlorine. In your answer, refer to Al(OH)3, HOCl and the effect of pH. (5 marks)
Evaluate Band 5–6
3. Evaluate the most suitable disinfection strategy for a large Sydney water supply network that wants strong public-health protection while limiting DBP formation. In your answer, compare free chlorine with at least one alternative method. (5 marks)
Show All Answers
Activity 1
1. Coagulation with alum belongs in particle removal. Al³⁺ hydrolyses to Al(OH)₃, which adsorbs suspended particles and helps them form larger flocs.
2. Activated carbon filtration belongs in removal of remaining fine particles and some dissolved organics. Its large surface area helps adsorb compounds affecting water quality.
3. Reverse osmosis belongs in dissolved salt removal. It uses a membrane to separate water from salts, but it has a significant energy cost.
Activity 2
1. Chloramines are a strong choice because they generally form fewer DBPs than free chlorine while still providing a residual disinfectant in distribution, although they act more slowly.
2. UV is a strong option when rapid disinfection is needed but long-term residual protection is not essential, because UV leaves no residual disinfectant in the water.
3. Reverse osmosis is the main process for desalination. The major concern is its high energy demand and therefore higher operating cost.
Multiple Choice
1. B — coagulation destabilises suspended particles so larger flocs can form.
2. A — HOCl is the more active disinfectant species.
3. D — higher pH shifts chlorine chemistry toward OCl⁻, so disinfection becomes less effective.
4. C — removing organics helps reduce DBP formation such as trihalomethanes.
5. B — chloramines usually reduce DBP formation compared with free chlorine, but they disinfect more slowly.
Short Answer Model Answers
Q1 (4 marks): Major drinking water treatment stages include coagulation, flocculation, sedimentation, filtration and disinfection. In coagulation, alum helps destabilise fine suspended particles. Flocculation brings these together into larger flocs. Sedimentation allows the flocs to settle. Filtration through media such as sand, gravel and activated carbon removes remaining particles and some dissolved organics. Disinfection then reduces pathogen risk before the water enters supply.
Q2 (5 marks): In coagulation, alum provides Al³⁺ ions that hydrolyse to form Al(OH)₃. This colloidal aluminium hydroxide adsorbs suspended particles and helps them combine into flocs that can later settle. In chlorination, chlorine reacts with water to form HOCl and HCl. HOCl is the active disinfectant species. HOCl is also in equilibrium with OCl⁻, and as pH rises a greater proportion becomes OCl⁻. Because OCl⁻ is a weaker disinfectant, higher pH reduces disinfection effectiveness.
Q3 (5 marks): For a large Sydney distribution network, free chlorine provides strong disinfection and leaves a residual, but it can form more DBPs when organic matter is present. Chloramines are often a better compromise when the goal is to maintain residual protection while reducing DBP formation. However, chloramines disinfect more slowly than free chlorine. UV is useful because it avoids chlorine-based DBPs, but it leaves no residual disinfectant in the network, so by itself it is less suitable for long pipe systems. Overall, a strategy that reduces organics first and then uses chloramines for residual protection is often the most suitable balance for this scenario.
Return to the 2014 Flint, Michigan water crisis. Now that you understand the full treatment train, explain precisely which step was omitted and why its chemistry mattered.
- Which chemical stage was missing from Flint's treatment process — and what specific corrosion chemistry does orthophosphate inhibitor actually prevent in iron and lead pipes?
- Why does pH matter even after chlorine has been added — and how could an incorrect pH have worsened Flint's corrosion problem further?
- Why might a water treatment plant in a system with old lead pipes choose chloramines instead of free chlorine as the disinfectant?
Review
Name the five main stages of drinking water treatment in a NSW facility, in order.
Write the equation showing how alum forms a colloidal species that helps remove particles.
Which chlorine species is the more effective disinfectant — HOCl or OCl⁻ — and which pH favours it?
What are disinfection by-products (DBPs), and what is the main precaution taken to reduce them?
Compare chloramines with UV disinfection on two criteria: DBP formation and residual protection.