Atmospheric Pollutants and Water Treatment

Burning fuels powers our cars, homes and industry, but it also releases harmful substances into the air. And the clean water we drink — and the dirty water we throw away — has to be treated before it is safe. Both are major global challenges, and both come down to chemistry. This lesson, part of Topic C6 of OCR Gateway Combined Science A, brings together the two topics: first the pollutants released when fuels burn — carbon monoxide, particulates, sulfur dioxide and oxides of nitrogen — and then how potable (drinking) water is produced and how waste water is treated.

By the end of this lesson you should be able to compare complete and incomplete combustion, name the main atmospheric pollutants and state the source and effect of each, define potable water and distinguish it from pure water, describe how potable water is produced from fresh and salt water, and describe the treatment of waste water.

This lesson is mainly AO1 (recalling each pollutant's source and effect and the stages of water treatment), with AO2 where you apply the idea of incomplete combustion to explain which pollutant forms and why.

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Complete and Incomplete Combustion

When a hydrocarbon fuel burns in plenty of oxygen, complete combustion occurs and the only products are carbon dioxide and water:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$CH4​+2O2​→CO2​+2H2​O

But when there is a limited supply of oxygen — for example in a poorly ventilated boiler or an engine — incomplete combustion occurs. The carbon in the fuel is not fully oxidised, so as well as water the products include carbon monoxide (CO) and carbon (soot/particulates):

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$2CH4​+3O2​→2CO+4H2​O

Incomplete combustion also releases less energy than complete combustion, so it is wasteful as well as dangerous.

	Oxygen supply	Carbon-containing products
Complete combustion	Plenty	Carbon dioxide only
Incomplete combustion	Limited	Carbon monoxide and carbon (soot)

Exam Tip: Tie the products to the oxygen: plenty of oxygen → carbon dioxide (complete); limited oxygen → carbon monoxide and soot (incomplete). A common misconception is that carbon monoxide comes from complete combustion — it comes from incomplete combustion.

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The Main Atmospheric Pollutants

Carbon monoxide ($\text{CO}$CO) is produced by the incomplete combustion of carbon-based fuels. It is dangerous because it is toxic (poisonous) and both colourless and odourless, so it gives no warning. It is toxic because it binds to haemoglobin in red blood cells in place of oxygen, so the blood can carry less oxygen — which can cause drowsiness, unconsciousness and death.

Particulates are tiny solid particles of carbon (soot) and unburnt hydrocarbons, also produced by incomplete combustion (especially of diesel). They cause global dimming (particulates in the air reflect sunlight back into space, reducing the sunlight reaching the surface) and respiratory problems (breathing them in damages the lungs).

Sulfur dioxide ($\text{SO}_2$SO2​) is produced when fuels containing sulfur impurities are burned; the sulfur is oxidised to sulfur dioxide:

$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$S+O2​→SO2​

Sulfur dioxide dissolves in water in the atmosphere and is oxidised to form acidic solutions, which fall as acid rain (damaging buildings, trees and aquatic life); it also causes respiratory problems.

Oxides of nitrogen (NOx) form inside engines. The high temperature gives the nitrogen and oxygen in the air enough energy to react together:

$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$N2​+O2​→2NO

(Nitrogen is normally unreactive, but the very high engine temperatures make it react.) Oxides of nitrogen cause acid rain, respiratory problems and photochemical smog over cities.

Pollutant	Source	Problem(s)
Carbon monoxide (CO)	Incomplete combustion	Toxic — binds to haemoglobin, so blood carries less oxygen
Particulates (soot)	Incomplete combustion	Global dimming and respiratory problems
Sulfur dioxide ($\text{SO}_2$SO2​)	Sulfur impurities in the fuel	Acid rain and respiratory problems
Oxides of nitrogen (NOx)	Nitrogen + oxygen reacting at high temperature in engines	Acid rain, respiratory problems and smog

Exam Tip: Build a mental grid: CO = toxic (haemoglobin); particulates = global dimming + lungs; SO₂ = acid rain (from fuel); NOx = acid rain + smog (from air in engine). A common misconception is that carbon dioxide causes acid rain — it does not; sulfur dioxide and oxides of nitrogen do.

Worked Example 1 — Identifying a pollutant

A power station burns a fuel that contains sulfur. Lakes downwind are becoming acidic and fish are dying. Name the pollutant responsible, explain how it forms, and state how the problem could be reduced.

Step 1 — link the source to the pollutant: the fuel contains sulfur, so burning it produces sulfur dioxide ($\text{S} + \text{O}_2 \rightarrow \text{SO}_2$S+O2​→SO2​).

Step 2 — explain the problem: sulfur dioxide dissolves in water in the atmosphere to form an acidic solution that falls as acid rain, making the lakes acidic and killing fish.

Step 3 — suggest a reduction: remove the sulfur from the fuel before burning, or remove the sulfur dioxide from the waste gases (flue-gas desulfurisation).

Answer: the pollutant is sulfur dioxide, formed from the sulfur impurity in the fuel; it causes acid rain, and can be reduced by removing sulfur from the fuel or scrubbing it from the flue gases.

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Potable Water Is Not Pure Water

Now we turn to water. Potable water is water that is safe to drink. It contains only low levels of dissolved salts and low levels of microbes (microorganisms), so it will not make you ill. Importantly, potable water is not the same as pure water:

• Pure water (in the chemical sense) contains only $\text{H}_2\text{O}$H2​O — nothing dissolved in it.
• Potable water is safe to drink but still contains small amounts of dissolved substances (such as harmless minerals).

So tap water is potable but not pure: it has dissolved minerals in it, which is perfectly safe and often gives water its taste. To produce potable water we need to reduce the dissolved salts and microbes to safe levels — not remove everything.

Exam Tip: Be precise: potable = safe to drink (low dissolved salts and microbes); pure = only $\text{H}_2\text{O}$H2​O. A common misconception is that "potable water is pure water" — it is not.

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Producing Potable Water

In a country like the UK, which has plenty of rain, potable water is usually produced from fresh water (water with low levels of dissolved salts) from sources such as rivers, lakes and aquifers. The steps are:

flowchart LR
  A["Choose a source<br/>(river, lake or aquifer)"] --> B["Filter<br/>to remove solids"]
  B --> C["Sterilise<br/>to kill microbes"]
  C --> D["Potable water<br/>(safe to drink)"]

1. Choose an appropriate source of fresh water.
2. Filter the water to remove insoluble solids such as grit and leaves (often by passing it through filter beds of sand and gravel).
3. Sterilise the water to kill microbes, using chlorine, ozone or ultraviolet (UV) light.

Filtering removes bits of solid; it does not kill microbes. Sterilising kills the microbes. In countries where there is little fresh water, potable water has to be made from sea water by desalination, either by distillation (boiling the water to make steam, leaving the salts behind, then condensing the steam) or by reverse osmosis (forcing the water at high pressure through a partially permeable membrane that holds back the dissolved salts). Both methods use a lot of energy, so desalination is expensive and is used only where fresh water is scarce.

Method	How it works	Drawback
Distillation	Boil → condense the steam	Lots of energy to boil the water
Reverse osmosis	High pressure through a membrane	Lots of energy to pump at high pressure