Water Treatment and Potable Water

This lesson covers the production of potable water, waste water treatment and desalination, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You need to understand the difference between potable water and pure water, describe the steps in water treatment, and know when and how desalination is used.

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Potable Water vs Pure Water

It is important to understand the distinction between potable water and pure water:

Term	Definition
Pure water	Contains only H\u2082O molecules and nothing else. Has a boiling point of exactly 100 \u00b0C and a melting point of exactly 0 \u00b0C.
Potable water	Water that is safe to drink. It is not pure — it contains dissolved minerals and other substances, but the levels of these are low enough to be safe for human consumption.

• Pure water is very difficult and expensive to produce. It requires distillation.
• Potable water is what comes from the tap. It contains low levels of dissolved salts and microorganisms have been killed, but it still contains dissolved substances.

Exam Tip: Do not confuse "potable" with "pure." Potable water is safe to drink but is NOT chemically pure. If the exam asks for the definition of potable water, state: "water that has sufficiently low levels of dissolved substances and microorganisms to be safe to drink." Do NOT say it is pure.

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Sources of Fresh Water

In the UK, most fresh water comes from:

• Rain that collects in rivers and reservoirs (surface water).
• Lakes.
• Aquifers — underground stores of water in permeable rock. Water from aquifers is called groundwater and is accessed through wells or boreholes.

The choice of water source depends on availability. In the UK, there is usually sufficient rainfall to provide adequate fresh water. In some countries with less rainfall, alternative methods (such as desalination) are necessary.

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Steps in Water Treatment

In the UK, fresh water from rivers, reservoirs and aquifers is treated at a water treatment works before it is piped to homes. The process involves three main stages:

Stage 1: Sedimentation

• The water is passed into large settlement tanks.
• Heavy, insoluble particles (such as grit, sand, soil and debris) settle to the bottom under gravity.
• Chemicals called flocculants (e.g. aluminium sulfate) may be added. These cause small suspended particles to clump together (flocculate) into larger particles that settle more easily.
• The settled material is called sludge and is removed.

Stage 2: Filtration

• The water passes through filter beds made of sand and gravel.
• These filter beds trap remaining small, solid particles that did not settle during sedimentation.
• The water that passes through is much clearer.

Stage 3: Sterilisation (Chlorination)

• Chlorine is added to the water to kill harmful bacteria and other microorganisms.
• This ensures the water is safe to drink (potable).
• The chlorine remains in the water at very low levels as it travels through the pipe network, providing continued protection against contamination.

Summary of the Three Stages

Stage	What Happens	What Is Removed
Sedimentation	Heavy particles settle to the bottom	Large insoluble solids (grit, sand, debris)
Filtration	Water passes through sand/gravel filters	Smaller solid particles
Sterilisation	Chlorine is added	Bacteria and microorganisms

Exam Tip: When describing water treatment, always give the three stages in order: sedimentation, filtration, chlorination. For each stage, state what happens and what it removes. This is a common 4–6 mark question.

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Alternative Sterilisation Methods

While chlorine is the most common sterilisation agent in the UK, other methods can also be used:

Method	How It Works	Advantages	Disadvantages
Chlorine	Chemical added to water; kills bacteria	Cheap, effective, provides residual protection in pipes	Can form harmful by-products; slight taste
Ozone (O\u2083)	Ozone gas is bubbled through water; strong oxidising agent kills microorganisms	Very effective disinfectant; leaves no taste	Expensive; no residual protection (breaks down quickly)
Ultraviolet (UV) light	UV radiation damages the DNA of microorganisms, preventing reproduction	No chemicals added; effective	No residual protection; water must be clear (turbid water blocks UV)

Exam Tip: If asked about sterilisation methods, know at least two and be able to compare them. Chlorine provides "residual protection" (it remains in the water as it travels through pipes), which is an advantage over ozone and UV light.

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Desalination

Desalination is the process of removing dissolved salts (and other substances) from seawater or brackish water to produce potable water.

When Is Desalination Used?

Desalination is used in regions where there is insufficient fresh water from rivers, lakes or aquifers. This includes:

• Arid countries (e.g. Saudi Arabia, UAE, Kuwait).
• Islands with limited fresh water supplies.
• During droughts or water shortages.

Methods of Desalination

1. Distillation

• Seawater is heated until it boils.
• The water vapour rises, leaving the dissolved salts behind.
• The vapour is condensed to produce pure water.
• This is essentially simple distillation on a large scale.

Disadvantage: Requires a large amount of energy to heat the water, making it expensive.

2. Reverse Osmosis

• Seawater is forced through a semi-permeable membrane at high pressure.
• The membrane has very tiny pores that allow water molecules to pass through but block the larger salt ions and other dissolved substances.
• Pure water passes through the membrane; the concentrated salt solution is left behind.

Advantage: Uses less energy than distillation. Disadvantage: The membranes are expensive and need regular replacement; high-pressure pumps are needed.

Comparing Desalination Methods

Feature	Distillation	Reverse Osmosis
Energy required	Very high (heating)	High (pumping) but less than distillation
Cost	Expensive	Expensive (membranes + pumps)
Quality of water	Very pure	Very pure
Main limitation	Energy cost	Membrane maintenance and cost

Exam Tip: Both desalination methods require a lot of energy, which makes them expensive. If the exam asks why desalination is not used more widely, the answer is the high energy cost. In the UK, it is cheaper to treat fresh water from rivers and reservoirs.

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Waste Water Treatment

Water used in homes (from toilets, sinks, baths and washing machines) and in industry becomes waste water (sewage and industrial effluent). This must be treated before it can be safely released back into rivers or the sea.

Stages of Waste Water Treatment

Stage 1: Screening

• Large objects (rags, plastic, debris) are removed by passing the waste water through screens (metal grids).
• Grit and sand are also removed in a grit chamber where the flow slows and heavy particles settle.

Stage 2: Sedimentation (Primary Treatment)

• The waste water enters large settlement tanks.
• Solid organic matter settles to the bottom as sludge.
• The sludge is removed for further treatment.
• Oils and grease float to the surface and are skimmed off.

Stage 3: Biological Treatment (Secondary Treatment)

• The liquid from sedimentation is passed through aeration tanks or trickling filter beds.
• Aerobic bacteria (bacteria that require oxygen) break down dissolved organic matter and remaining suspended particles.
• Air is bubbled through to provide oxygen for the bacteria.
• This is a biological process — microorganisms decompose the organic waste.

Stage 4: Final Treatment

• The treated water may undergo further sedimentation to remove any remaining suspended material.
• It may also be sterilised (e.g. with UV light or chlorine) before being discharged into rivers.
• The treated water is now clean enough to be safely released into the environment.

Sludge Treatment

The sludge collected during sedimentation is treated separately:

• It is placed in large tanks called digesters.
• Anaerobic bacteria (bacteria that do not require oxygen) break down the organic matter in the sludge.
• This process produces methane gas (biogas), which can be used as a fuel.
• The remaining solid can be used as fertiliser on farmland (once tested for safety).

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Industrial Waste Water

Waste water from industry may contain additional harmful substances:

• Heavy metals (e.g. lead, mercury) from metal processing.
• Organic chemicals (e.g. solvents, pesticides) from manufacturing.
• Acids or alkalis from chemical processes.

Industrial waste water requires additional treatment to remove these hazardous substances before it can be discharged. Regulations require industries to treat their waste water to specific standards.

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The Water Cycle and Sustainability

The water cycle naturally recycles water through evaporation, condensation and precipitation. Water treatment processes work alongside the natural water cycle:

• Water is taken from rivers/reservoirs \u2192 treated \u2192 supplied to homes/industry \u2192 used \u2192 becomes waste water \u2192 treated at waste water works \u2192 returned to rivers \u2192 naturally purified and recycled.

Sustainable water management involves:

• Reducing water consumption.
• Fixing leaks in the pipe network.
• Recycling and treating waste water effectively.
• Using desalination only where necessary (due to energy cost).

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Required Practical: Investigating the Composition of Drinking Water

This is Edexcel Core Practical 14 (1CH0). It applies the separation techniques from earlier in the topic — evaporation, testing with silver nitrate, and pH measurement — to compare different samples of drinking water and show that potable water is not chemically pure.

Aim: To investigate what is dissolved in samples of distilled water, tap water and mineral water, and to determine which sample is closest to pure water.

Hypothesis / prediction: Distilled water will leave no residue and have negligible conductivity, while tap water and mineral water will leave visible residues, conduct electricity and give a positive test for chloride ions, because they contain dissolved mineral salts.