Water Treatment

Summary: Potable water production: sedimentation, filtration, chlorination (or ozone/UV). Desalination by distillation or reverse osmosis. Water is tested for purity by checking boiling point. Tags: igcse chemistry environmental-chem Created: 2026-07-14 Last Updated: 2026-07-16

Potable Water vs Pure Water

A critical distinction in water treatment is between potable water and pure water:

Potable water must be free from:

• Harmful microorganisms (bacteria, viruses, protozoa)
• Toxic chemicals (heavy metals, pesticides)
• Excessive dissolved salts (brackish or saline water)
• Suspended solids (silt, clay, organic debris)

However, potable water typically contains dissolved mineral ions such as Ca²⁺, Mg²⁺, Na⁺, Cl⁻, SO₄²⁻, and HCO₃⁻ at safe concentrations. These give water its characteristic taste and some nutritional value.

Pure water is only required for specialised applications: laboratory analysis, pharmaceutical manufacturing, and in cooling systems where mineral deposits (scale) would cause problems.

Sources of Water

Surface Water

Rivers, lakes, and reservoirs are the most common sources of municipal water. Surface water is relatively easy to access but contains:

• Suspended solids (silt, clay, organic matter)
• Microorganisms (bacteria, viruses, algae)
• Dissolved organic compounds from decaying vegetation
• Agricultural runoff (fertilisers, pesticides)

Surface water typically requires full treatment before it is potable.

Groundwater

Water from underground aquifers (accessed by boreholes and wells) is generally cleaner than surface water because the ground acts as a natural filter. Groundwater may contain:

• Dissolved minerals leached from rocks (Ca²⁺, Mg²⁺ causing hardness; Fe²⁺ causing discolouration)
• Fewer microorganisms than surface water
• Possible contamination from leaking septic tanks or industrial spills

Groundwater often requires less treatment than surface water — sometimes only disinfection.

Seawater

Seawater contains approximately 3.5% dissolved salts (35 g per dm³), making it undrinkable without desalination. It is an abundant but energy-intensive water source, mainly used in arid regions with access to the sea and cheap energy (e.g., the Middle East).

Water Treatment Stages

The treatment of fresh water (from rivers or reservoirs) to produce potable water typically involves the following stages:

Stage 1: Screening

Water passes through metal screens (bars with gaps of several centimetres) to remove large debris such as branches, leaves, plastic waste, and other floating objects. This is a purely physical process that protects downstream equipment from damage or clogging.

Stage 2: Sedimentation

Water flows slowly through large sedimentation tanks (also called settling tanks or clarifiers). The reduced flow velocity allows suspended solid particles — silt, sand, clay — to settle to the bottom under gravity, forming a layer of sludge. The clearer water above the sludge is drawn off for further treatment.

Chemicals called coagulants (typically aluminium sulfate, Al₂(SO₄)₃, or iron(III) sulfate) are often added at this stage. Coagulants cause fine, colloidal particles that would not settle on their own to clump together into larger aggregates (flocs) that settle more readily.

Aluminium sulfate hydrolyses in water:

Al₂(SO₄)₃ + 6H₂O → 2Al(OH)₃(s) + 3H₂SO₄

The gelatinous aluminium hydroxide precipitate entraps fine particles and microorganisms as it settles, enhancing the effectiveness of sedimentation.

Stage 3: Filtration

The clarified water from sedimentation passes through sand and gravel filters:

• Gravel layer (coarse, at the bottom): Supports the filter bed and prevents sand from being washed through
• Fine sand layer (top): Traps remaining fine suspended particles that did not settle out

This is a physical process — solid particles are trapped in the spaces between sand grains. The filtered water emerging from the bottom is clear and free of visible suspended matter.

In modern treatment plants, filters may also include a layer of activated carbon (activated charcoal), which adsorbs dissolved organic compounds that cause unpleasant tastes and odours. Some plants use membrane filtration (microfiltration or ultrafiltration) instead of, or in addition to, sand filtration.

Stage 4: Chlorination (Disinfection)

Filtration removes suspended solids but does not remove bacteria, viruses, and other microorganisms. Disinfection is essential to destroy pathogens that cause diseases such as cholera, typhoid, and dysentery.

Chlorine gas (Cl₂) is the most common disinfectant. When added to water, chlorine reacts to form hypochlorous acid:

Cl₂(g) + H₂O(l) → HOCl(aq) + HCl(aq)

Hypochlorous acid (HOCl) is the active disinfectant. It is a powerful oxidising agent that kills bacteria and viruses by disrupting their cell membranes and destroying essential enzymes.

The key advantage of chlorine is that it provides residual protection — a small amount of chlorine remains in the water after treatment, protecting it from recontamination as it travels through pipes to consumers.

A typical residual chlorine level in tap water is 0.2–0.5 mg per dm³ — enough to be effective against microorganisms but not high enough to cause taste or health concerns.

Sodium hypochlorite (NaOCl), sold as bleach, is an alternative to chlorine gas and works by the same mechanism:

NaOCl(aq) + H₂O(l) → HOCl(aq) + NaOH(aq)

Alternative Disinfection Methods

Ozone (O₃)

Ozone is a powerful oxidising agent and effective disinfectant. It is generated on-site by passing an electrical discharge through oxygen:

3O₂(g) → 2O₃(g)

Advantages:

• More effective than chlorine against certain resistant pathogens (e.g., Cryptosporidium)
• Does not produce chlorinated organic by-products (some of which are potentially harmful)
• Removes tastes and odours

Disadvantages:

• Ozone decomposes quickly and provides no residual protection — water can be recontaminated after treatment
• More expensive than chlorination (requires on-site electricity generation)
• Equipment and operating costs are higher

Ozone is often used in combination with a small chlorine dose to provide residual protection.

Ultraviolet (UV) Light

Water is passed through tubes containing UV lamps. UV radiation (specifically UV-C, wavelength ~254 nm) damages the DNA of microorganisms, preventing them from reproducing.

Advantages:

• No chemicals added to the water
• Effective against chlorine-resistant pathogens
• No taste or odour issues

Disadvantages:

• Provides no residual protection — water can be recontaminated after treatment
• Requires clear water (turbidity blocks UV penetration)
• Lamps require regular replacement and consume electricity

Comparison of Disinfection Methods

Additional Treatment Steps (Optional)

Fluoridation

In some regions, fluoride ions (usually as sodium fluoride, NaF, or hexafluorosilicic acid, H₂SiF₆) are added to water at ~1 mg per dm³ to reduce tooth decay. Fluoridation is controversial in some countries and is not practised everywhere. The chemistry involves fluoride ions being incorporated into tooth enamel, forming fluorapatite which is more resistant to acid attack than natural hydroxyapatite.

pH Adjustment

If water is too acidic (low pH), it can corrode metal pipes, potentially leaching toxic metals such as lead and copper into the water. Lime (calcium oxide, CaO) or slaked lime (calcium hydroxide, Ca(OH)₂) is added to raise the pH:

CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Ca(OH)₂(aq) + 2H⁺(aq) → Ca²⁺(aq) + 2H₂O(l)

If water is too alkaline (high pH), carbon dioxide or a mild acid may be added to lower the pH.

Desalination

In regions where freshwater is scarce but seawater is abundant, desalination is used to produce potable water. The two main methods are:

Distillation

Seawater is boiled, and the steam is condensed to produce pure water, leaving the dissolved salts behind.

Process:

1. Seawater is heated to boiling (~100 °C)
2. Water vapour rises, leaving behind dissolved salts and minerals
3. The steam is passed through a condenser (cooled pipes) where it condenses to liquid water
4. The distillate is pure water (too pure for drinking — minerals may be added back for taste)

Energy considerations: Distillation is energy-intensive because water has a high specific heat capacity and a high latent heat of vaporisation. Every kilogram of water requires ~2260 kJ to vaporise. For this reason, distillation is primarily used in countries with abundant, cheap energy — notably the oil-rich, water-poor nations of the Middle East (Saudi Arabia, UAE, Kuwait).

Multi-stage flash distillation (MSF) improves efficiency: seawater is heated under pressure, then passed through a series of chambers at progressively lower pressures, causing it to flash-boil repeatedly. The steam from each stage condenses, and the heat released preheats incoming seawater, recovering energy.

Reverse Osmosis

Reverse osmosis (RO) uses high pressure to force water through a semi-permeable membrane that allows water molecules to pass but blocks dissolved salts and other impurities.

Normal osmosis: If pure water and salt water are separated by a semi-permeable membrane, water naturally flows from the pure side to the salt side, attempting to equalise concentrations.

Reverse osmosis: External pressure (typically 50–70 atm for seawater) is applied to the salt water side, reversing the natural osmotic flow. Pure water is forced through the membrane, leaving concentrated brine behind.

Comparison with distillation:

Reverse osmosis is more energy-efficient than distillation because it does not require a phase change (liquid water remains liquid throughout). It has become the dominant desalination technology globally.

Testing Water Purity

Boiling Point Test

Pure water boils at exactly 100 °C at standard atmospheric pressure (1 atm / 101.3 kPa). The presence of dissolved impurities raises the boiling point (boiling point elevation) and lowers the freezing point (freezing point depression). These are colligative properties — they depend on the number of dissolved particles, not their identity.

A sample that boils at exactly 100 °C is pure water. A sample that boils over a range of temperatures (e.g., 100–103 °C) contains dissolved impurities.

Freezing Point Test

Pure water freezes at exactly 0 °C at 1 atm. If dissolved impurities are present, the freezing point is depressed below 0 °C.

Testing for Dissolved Solids

Evaporation test:

1. Measure a known volume of the water sample
2. Evaporate the water to dryness in a pre-weighed evaporating dish
3. Weigh the dish again — any increase in mass is due to the dissolved solids that remain as residue
4. Calculate: dissolved solids (g/dm³) = mass of residue (g) / volume of water (dm³)

Conductivity test: Pure water is a very poor conductor of electricity because it contains negligible concentrations of ions (the self-ionisation of water produces only 10⁻⁷ mol/dm³ each of H⁺ and OH⁻). If a water sample shows significant electrical conductivity, it contains dissolved ionic substances (salts, acids, or bases).

Tap water conducts electricity slightly due to dissolved Ca²⁺, Mg²⁺, Na⁺, Cl⁻, and HCO₃⁻ ions. Seawater is a good conductor due to its high salt concentration (~0.6 mol/dm³ NaCl plus other ions).

Wastewater Treatment

Water that has been used domestically or industrially (sewage) must be treated before it can be released into rivers or the sea. Treatment prevents pollution, eutrophication, and the spread of water-borne diseases.

Primary Treatment (Physical)

1. Screening: Large objects (rags, plastics, debris) are removed by metal screens
2. Sedimentation: Sewage flows into large tanks where solid organic matter settles as primary sludge and grease/oil rises to the surface for skimming

Secondary Treatment (Biological)

The liquid from primary treatment still contains dissolved and suspended organic matter. In secondary treatment:

• The liquid is aerated (air bubbled through) to encourage the growth of aerobic bacteria
• These bacteria digest (oxidise) the organic pollutants, converting them to CO₂, water, and more bacterial cells
• The mixture then passes to a secondary sedimentation tank where the bacterial biomass settles as secondary sludge

This is essentially an accelerated, controlled version of the natural decomposition that would occur in a river.

Tertiary Treatment (Advanced)

If the treated water is to be released into sensitive environments or reused:

• Filtration: Sand or membrane filtration removes remaining fine particles
• Disinfection: Chlorination, UV, or ozone destroys remaining pathogens
• Nutrient removal: Chemical or biological processes remove nitrates and phosphates to prevent eutrophication of receiving waters

Sludge Treatment

The sludge collected from primary and secondary treatment is further processed:

• Anaerobic digestion: Bacteria break down the organic matter in the absence of oxygen, producing methane gas (which can be burned to generate electricity for the plant) and a stabilised solid residue
• The stabilised sludge (biosolids) can be used as agricultural fertiliser or soil conditioner, or disposed of in landfill

Key Concepts from Past Papers

• carbon dioxide
• mark each for any 2 of: carbon monoxide, carbon, water
• calcium oxide carbon dioxide
• carbon dioxide water
• mark each for any two of: plastics, sewage, microbes
• carbon carbon dioxide
• mark each for any two of: catalytic converters, low sulfur fuels, flue gas desulfurisation
• mark each for any 2 of:

Keywords from Past Papers

carbon, dioxide, monoxide, values, water, gas, between, inclusive, these, catalytic, climate, change, global, warming, increased

Related Notes

• Composition of Air
• Air Pollution
• Climate Change and Greenhouse Gases
• Sulfur and Compounds

Sources

• OpenStax Chemistry 2e, Chapter 11 (Solutions and Colloids) — Section 11.4 on Colligative Properties; Chapter 14 (Acid-Base Equilibria) — openstax.org/books/chemistry-2e/
• BBC Bitesize GCSE Chemistry, Water Treatment and Purification — bbc.co.uk/bitesize/topics/
• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 10: Chemistry of the Environment
• CK-12 Chemistry, Water Treatment and Desalination — ck12.org/chemistry/

Past Paper Sources

• 0620/32 Feb/March 2015: Q66(c)(i) (2m), Q11(a)(ii) (1m)
• 0620/32 Feb/March 2017: Q33(b)(iii) (1m), Q22(e)(ii) (0m)
• 0620/32 Feb/March 2018: Q11(a)(iv) (1m)
• 0620/32 Feb/March 2019: Q77(b)(ii) (2m)
• 0620/32 Feb/March 2021: Q33(a)(i) (2m), Q33(a)(i) (2m)
• 0620/32 Feb/March 2022: Q55(b)(ii) (4m)
• 0620/32 May/June 2018: Q11(a)(iv) (1m)
• 0620/32 May/June 2020: Q11(a)(i) (1m)
• 0620/33 May/June 2016: Q88(c)(i) (1m), Q55(b)(ii) (0m)
• 0620/33 May/June 2019: Q33(b)(i) (1m)
• 0620/33 May/June 2022: Q22(d)(ii) (2m), Q44(c)(i) (1m)
• 0620/33 May/June 2024: Q44(a)(ii) (2m), Q66(e)(ii) (1m), Q88(b)(i) (0m)

Common Misconceptions