Chemistry • Year 12 • Module 8 • Lesson 10
Water Treatment Processes
Build HSC Band 5–6 extended-response technique on water treatment chemistry, disinfection trade-offs, and desalination evaluation using real Australian data scenarios.
1. Data + scenario — disinfection strategy for the Prospect Water Treatment Plant
8 marks Band 5–6
Scenario. Sydney Water’s Prospect Water Treatment Plant (WTP) in Western Sydney receives water from Warragamba Dam. In an average year the plant processes about 600 billion litres of water for the Greater Sydney region. In spring 2022, following an extended wet period, turbidity readings at the intake spiked to 120 NTU (normal <5 NTU) and dissolved organic carbon (DOC) jumped to 12 mg L⁻¹ (normal <4 mg L⁻¹). The plant’s chemistry team recorded the following disinfection performance data over the following 30-day period.
| Disinfection method | THM concentration in treated water (μg L⁻¹) | E. coli reduction (log units) | Residual at 50 km in network (mg L⁻¹) | Operating cost index (1 = cheapest) |
|---|---|---|---|---|
| Free chlorine (standard dose) | 220 | 4.0 | 0.55 | 1.0 |
| Reduced chlorine + chloramines | 68 | 3.8 | 0.48 | 1.3 |
| UV + chloramines (combined) | 44 | 5.2 | 0.43 | 2.1 |
| Ozone + free chlorine (combined) | 31 | 5.4 | 0.51 | 2.8 |
Note: NHMRC Australian Drinking Water Guidelines (ADWG 2022) recommend THM maximum of 250 μg L⁻¹; WHO guideline 100 μg L⁻¹. All methods shown achieve adequate E. coli reduction (≥3 log units required).
Q1. Evaluate which disinfection strategy Sydney Water should adopt at the Prospect WTP during periods of elevated turbidity and DOC. In your response you must:
- Explain the chemistry of DBP formation and why elevated DOC increases THM risk during chlorination.
- Compare all four strategies against at least three criteria drawn from the data table (e.g. THM level, E. coli reduction, residual protection, cost).
- Reference the ADWG and WHO guideline values when discussing THM safety.
- Reach an evidence-based recommendation that explicitly acknowledges the trade-offs involved.
2. Data + scenario — Perth seawater desalination vs river water treatment
7 marks Band 5–6
Scenario. Perth’s Water Corporation operates the Southern Seawater Desalination Plant (SSDP) at Binningup, commissioned 2013, with a capacity of 50 GL per year. It uses two-stage reverse osmosis (RO) to convert Indian Ocean seawater (total dissolved solids ~35,000 mg L⁻¹, equivalent to ~3.5% salinity) into drinking water (TDS <100 mg L⁻¹). For comparison, the Mundaring Weir catchment — Perth’s main historical surface-water source — now supplies diminishing volumes. The table below summarises key operational indicators for each source in 2023.
| Parameter | SSDP (RO desalination) | Mundaring Weir surface water treatment |
|---|---|---|
| Energy use (kWh per kL produced) | 3.5–4.2 | 0.3–0.6 |
| Climate dependence | None (ocean source) | High (depends on rainfall over catchment) |
| Brine waste produced (volume ratio, brine:product) | ~1.5:1 | Negligible |
| TDS of feed water (mg L⁻¹) | ~35,000 | ~100–400 |
| Coagulation/chlorination steps required? | Pre-treatment + post-disinfection | Full conventional treatment train |
| DBP risk from chlorination | Low (post-RO water is very low in NOM) | Moderate to high during high-flow events |
| Water security in a dry year | High (climate-independent) | Low (dam levels fall) |
Q2. Analyse and evaluate the two water-supply strategies for Perth using the data provided. In your response you must:
- Explain the chemistry of reverse osmosis and why it is the appropriate technology for treating seawater, but not standard for a low-TDS freshwater source.
- Compare the two strategies on at least three quantitative criteria from the table above, using actual data values in your answer.
- Identify the environmental trade-off associated with brine waste from the SSDP and explain the chemistry behind why brine is produced.
- Reach a justified, context-sensitive conclusion about which strategy best supports Perth’s long-term water security and under what conditions the trade-offs change.
3. Source critique — evaluate this claim about water treatment
6 marks Band 5–6
“Modern water treatment plants are so effective that once water has been through coagulation, flocculation, and sedimentation, any remaining risk to public health is essentially eliminated. Chlorination is therefore just a precautionary formality — the real work happens in the physical settling stages. Furthermore, because UV disinfection does not produce any chemical by-products, it is always the safest and most preferable disinfection technology for any urban water supply.”
— Source: hypothetical environmental interest group website, 2023.
Q3. Evaluate this claim. Identify the scientifically flawed elements, explain the correct chemistry, and reformulate the claim about UV into a more defensible statement that acknowledges the real trade-offs of disinfection technologies.
Q1 — Prospect WTP disinfection strategy (8 marks)
Marking criteria:
- 1 mark — Explains DBP chemistry correctly: dissolved organic matter (NOM) reacts with HOCl/Cl₂ to form halogenated by-products including trihalomethanes (e.g. chloroform CHCl₃). Higher DOC → more precursor material → higher THM formation at any given chlorine dose.
- 1 mark — References the data: at standard Cl₂, THM = 220 μg L⁻¹, which exceeds the WHO guideline of 100 μg L⁻¹ (though below the ADWG limit of 250 μg L⁻¹).
- 1 mark — Compares E. coli reduction: all methods achieve ≥3 log units required; UV+chloramines and ozone+chlorine achieve 5.2 and 5.4 log units — higher pathogen inactivation.
- 1 mark — Compares residual protection: free chlorine 0.55, chloramines strategies ~0.43–0.48 mg L⁻¹ — all adequate; chloramines maintain useful residual in large network.
- 1 mark — Compares cost: UV+chloramines (2.1) and ozone+chlorine (2.8) are significantly more expensive than free chlorine (1.0) or reduced chlorine+chloramines (1.3).
- 1 mark — Reaches a defensible recommendation. Accept: “Reduced chlorine + chloramines” as best compromise during high-DOC events — THMs fall to 68 μg L⁻¹ (below WHO guideline), E. coli reduction adequate (3.8 log), residual maintained (0.48 mg L⁻¹), cost only 30% higher than free chlorine. UV+chloramines is even safer on THMs but at 2.1× the cost.
- 1 mark — Explicit acknowledgement of trade-offs: reducing DBPs requires either lower chlorine dose (reducing disinfection speed) or higher cost for UV/ozone; no single option optimises all criteria.
- 1 mark — Uses precise chemical terminology throughout: HOCl, NOM, THMs, residual disinfectant, log reduction, ADWG, WHO.
Sample high-band response: When DOC rises to 12 mg L⁻¹, free chlorine reacts extensively with NOM to form trihalomethanes (e.g. CHCl₃); at standard dose, THMs reach 220 μg L⁻¹, which exceeds the WHO guideline of 100 μg L⁻¹ and approaches the ADWG limit. This creates a public health risk from potential carcinogens in the distribution network. Switching to reduced chlorine + chloramines drops THMs to 68 μg L⁻¹ because chloramines react far less vigorously with NOM and so generate fewer halogenated by-products. E. coli reduction falls slightly (3.8 vs 4.0 log units) but remains well above the 3-log minimum; residual at 50 km (0.48 mg L⁻¹) is adequate for the Sydney network. Cost is only 1.3 times the baseline. UV+chloramines achieves even lower THMs (44 μg L⁻¹) and better pathogen inactivation (5.2 log), but at 2.1 times the cost — this may be justified in short-term crisis events. The recommendation is to adopt reduced chlorine + chloramines as the standard strategy during high-DOC periods, with UV+chloramines available as a contingency for extreme turbidity events. The key trade-off is that no strategy simultaneously minimises DBPs, maximises E. coli reduction, maximises residual, and minimises cost.
Q2 — Perth desalination vs surface water (7 marks)
Marking criteria:
- 1 mark — Explains RO chemistry: a semi-permeable membrane allows water molecules to pass under high applied pressure but rejects dissolved ions (Na⁺, Cl⁻, Mg²⁺, Ca²⁺, SO₄²⁻ etc.) by size/charge exclusion. This reduces TDS from ~35,000 to <100 mg L⁻¹. Not suitable for low-TDS freshwater because energy cost is disproportionate to the dissolved-solids load.
- 1 mark — Quantitative comparison of energy: SSDP uses 3.5–4.2 kWh/kL vs 0.3–0.6 kWh/kL for surface water — RO requires 6–14 times more energy per kilolitre produced.
- 1 mark — Climate independence: SSDP is climate-independent (ocean source does not vary with rainfall); Mundaring supply is highly climate-dependent and declining under reduced-rainfall trends in the south-west of WA.
- 1 mark — DBP comparison: post-RO water is very low in NOM, so chlorination produces minimal THMs. Surface water treatment carries moderate to high DBP risk during storm events. Correct chemistry link to why (NOM drives THMs).
- 1 mark — Brine trade-off: RO produces ~1.5 L of concentrated brine per litre of product water. Brine contains elevated Na⁺, Cl⁻, Mg²⁺, Ca²⁺, SO₄²⁻ — if discharged without dilution it raises local salinity, threatening marine benthic communities near the outlet. Correctly describes brine as the reject side of the membrane separation.
- 1 mark — Context-sensitive conclusion: under current declining rainfall, SSDP is essential for water security — the energy cost is the price of climate resilience. Surface water treatment is much cheaper and lower in DBP risk when the catchment delivers adequate flow, so the optimal strategy blends both sources. Trade-offs change: if electricity from renewables becomes cheaper, RO trade-offs improve; if rainfall recovers, catchment supply can be prioritised.
- 1 mark — Uses precise terminology throughout: semi-permeable membrane, TDS, NOM, brine, specific energy (kWh/kL), climate resilience.
Q3 — Source critique (6 marks)
Marking criteria:
- 1 mark — Overall evaluative judgement: the claim contains two significant scientific flaws.
- 1 mark — Flaw 1: Coagulation, flocculation and sedimentation remove suspended particles, not microorganisms. While large protozoa may be partially removed, bacteria and viruses remain in suspension after sedimentation. Disinfection (chlorination, UV etc.) is not a formality — it is the primary microbial kill step and is essential for public health protection. Without it, pathogens such as Cryptosporidium, Giardia and enteric viruses would reach the consumer.
- 1 mark — Correct chemistry for why disinfection is essential: HOCl (from Cl₂ + H₂O) and UV radiation are the active agents for pathogen inactivation; physical settling cannot disinfect water.
- 1 mark — Flaw 2: UV leaving no residual disinfectant in distribution. UV disinfects water at the treatment plant but leaves no disinfectant residual as the water travels through hundreds of kilometres of pipes. Any microbial contamination introduced after UV treatment (e.g. biofilm in old pipes, cross-connections) cannot be controlled. Free chlorine or chloramines are essential to maintain residual protection throughout a large distribution network.
- 1 mark — Concedes the one valid element: UV does not produce chlorine-based DBPs such as THMs, which is a genuine advantage when DBP minimisation is the priority and distribution runs are short.
- 1 mark — Defensible reformulation: “UV disinfection avoids the formation of chlorinated DBPs including trihalomethanes, making it advantageous for small facilities or closed systems with short distribution runs. However, because UV provides no residual protection in the distribution network, it cannot replace chlorine-based disinfection as the primary technology for large urban water systems with extended pipe networks. For large systems, UV is best used as a supplementary pre-disinfection step upstream of a residual-forming disinfectant such as chloramines.”