C25: Water and Waste
Potable water, water treatment and eutrophication
Water And Waste
📋 Potable Water
Potable water is water that is safe to drink. It is NOT the same as pure water — potable water contains dissolved substances. It must have low enough levels of dissolved salts and microbes to be safe for human consumption.
| Term | Definition |
|---|---|
| Pure water | Contains only H₂O molecules (no dissolved substances); boils at exactly 100°C |
| Potable water | Safe to drink; contains some dissolved substances but at safe levels; no harmful microbes |
| Distilled water | Pure water produced by distillation — no dissolved substances at all |
📝 Water Treatment
In the UK, fresh water from lakes, rivers and underground aquifers is treated to make it potable. The process involves: filtration, sedimentation, and chlorination.
- Filtration — water passes through filters (sand and gravel beds) to remove solid particles.
- Sedimentation — aluminium sulfate is added, which causes fine particles to clump together (flocculation) and settle to the bottom as sediment.
- Chlorination — chlorine is added to kill microbes (bacteria and other harmful microorganisms).
📝 Desalination
Desalination removes salt from sea water to produce potable water. Methods include reverse osmosis and distillation. Both are expensive because they require a lot of energy. Desalination is used in dry countries where fresh water is scarce.
| Method | How It Works | Advantages | Disadvantages |
|---|---|---|---|
| Distillation | Boil sea water, condense the pure water vapour | Produces very pure water | Very energy-intensive and expensive |
| Reverse osmosis | Sea water forced through a semi-permeable membrane under high pressure; salt left behind | Does not require heating | Requires high pressure (energy); membrane can become blocked |
📝 Waste Water Treatment
Waste water (sewage) must be treated before being released into the environment. Treatment involves: screening, sedimentation, biological (aerobic) treatment, and anaerobic digestion.
- Screening — removes large solid objects (e.g. sticks, rags) using grids.
- Sedimentation — solids settle to the bottom forming sludge; the liquid above is effluent.
- Biological treatment (aerobic) — aerobic bacteria digest dissolved organic matter in the effluent. Air is bubbled through to supply oxygen.
- Anaerobic sludge digestion — the sludge is broken down by anaerobic bacteria in the absence of oxygen. This produces methane gas, which can be burned to generate electricity or for heating. The remaining digested sludge can be used as fertiliser.
📝 Fertilisers and Eutrophication
NPK fertilisers provide plants with the essential elements nitrogen (N), phosphorus (P) and potassium (K). They increase crop yields but can cause eutrophication if they enter waterways.
Eutrophication sequence:
- Fertilisers run off into rivers and lakes.
- Excess nutrients cause rapid growth of algae (algal bloom) on the water surface.
- Algae block sunlight, so plants below the surface die.
- When the algae die, bacteria decompose them using aerobic respiration.
- Bacteria use up dissolved oxygen in the water.
- Fish and other aquatic organisms die due to lack of oxygen.
❓ Practice Questions
Q1: Explain the difference between potable water and pure water.
Q2: Describe the three stages of water treatment used to produce potable water from fresh water sources.
Q3: Higher Compare distillation and reverse osmosis as methods of desalination. Explain why desalination is more expensive than treating fresh water.
Q4: Describe the stages of waste water treatment. How can energy be produced during the process?
Q5: Explain how the use of fertilisers can lead to eutrophication and the death of fish in a lake.
✅ Answers
- Potable water is safe to drink but is not pure — it contains dissolved substances at safe levels and no harmful microbes. Pure water contains only H₂O molecules with no dissolved substances; it boils at exactly 100°C.
- (1) Filtration — water passes through sand/gravel filters to remove solid particles. (2) Sedimentation — aluminium sulfate is added so fine particles clump and settle out. (3) Chlorination — chlorine is added to kill microbes, making the water safe to drink.
- Distillation involves boiling sea water and condensing the vapour; reverse osmosis forces water through a semi-permeable membrane at high pressure, leaving salt behind. Both are expensive because they require large amounts of energy — distillation needs heat, reverse osmosis needs high pressure. Treating fresh water is cheaper because filtration, sedimentation and chlorination use much less energy.
- (1) Screening removes large solids. (2) Sedimentation separates sludge from effluent. (3) Aerobic bacteria digest organic matter in the effluent. (4) Anaerobic bacteria digest the sludge, producing methane gas. The methane can be burned to generate electricity or heat.
- Fertilisers run off farmland into lakes. The excess nutrients cause rapid growth of algae (algal bloom). The algae block sunlight so underwater plants die. When the algae die, bacteria decompose them, using aerobic respiration which consumes dissolved oxygen. The oxygen levels fall so low that fish die.
🎯 Exam Tips
- Don't confuse potable water with pure water — potable water is safe to drink but NOT pure
- For water treatment, learn the three stages in order: filtration → sedimentation → chlorination
- For eutrophication, use the full sequence — don't skip steps, examiners look for the complete chain
- Desalination is expensive because of ENERGY requirements — always mention this
- Waste water treatment produces METHANE from anaerobic digestion — this can be used as fuel
🔬 Required Practical
Required Practical: Water Purification
Aim: Purify a water sample by distillation and test the purity of the water produced.
Method: 1. Collect a sample of impure water (e.g. salty water). 2. Set up a distillation apparatus: heat the water in a flask, collect the vapour in a condenser, and collect the condensed water (distillate) in a clean beaker. 3. Measure the boiling point of the distilled water using a thermometer — pure water boils at exactly 100°C. 4. Test the original sample and the distilled water by evaporating to dryness — pure water leaves no residue; impure water leaves dissolved solids behind. 5. Compare the results.
Variables: IV: method of purification (distillation), DV: boiling point of water / residue after evaporation, Control: volume of water, heating method
🔢 Maths Skills
Mathematical Skills
You need to interpret data on water quality (e.g. dissolved substance concentrations), calculate percentage purity, and analyse data from desalination experiments.
Maths Example
A water sample has a mass of 200 g. After distillation, the residue left behind has a mass of 2.4 g. Calculate the percentage of dissolved substances in the original water.
Percentage dissolved substances = (2.4 ÷ 200) × 100 = 1.2%
Maths Example 2
A desalination plant processes 1,000,000 litres of sea water per day. Sea water contains 35 g of salt per litre. Calculate the total mass of salt removed per day.
Total salt = 35 × 1,000,000 = 35,000,000 g = 35,000 kg = 35 tonnes per day
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Potable water is pure water Correct: Potable water is SAFE to drink but is NOT pure — it contains dissolved substances at safe levels. Pure water has NOTHING dissolved in it
2. Wrong: Chlorination kills all substances in water Correct: Chlorination only kills MICROBES (bacteria) — it does NOT remove dissolved substances or solid particles. Filtration and sedimentation are needed for those
3. Wrong: In eutrophication, fertilisers directly kill fish Correct: Fertilisers cause algal blooms → algae die → bacteria decompose them → bacteria use up oxygen → fish die from LACK OF OXYGEN. The fertilisers do not directly kill the fish
4. Wrong: Distillation and evaporation are the same process Correct: Evaporation boils off the solvent (losing it). Distillation boils off the solvent AND CONDENSES it back to collect the pure liquid — nothing is lost
✍️ 6-Mark Question
Extended Answer
6 marks: Describe how fresh water is treated to make it potable in the UK. Explain why treating sea water to make it potable is more expensive, and describe one method of desalination.
In the UK, fresh water from rivers, lakes and aquifers is treated in three stages. First, filtration removes solid particles — the water is passed through beds of sand and gravel. Second, sedimentation involves adding aluminium sulfate to make fine particles clump together (flocculation) and settle to the bottom as sediment. Third, chlorination involves adding chlorine to kill harmful microbes (bacteria), making the water safe to drink. Treating sea water (desalination) is more expensive because both methods require large amounts of energy. In distillation, the sea water must be heated to boiling, which requires significant fuel. In reverse osmosis, very high pressure is needed to force water through a semi-permeable membrane, leaving the salt behind. This pressure is generated by pumps that consume large amounts of electricity. Fresh water treatment is cheaper because filtration, sedimentation and chlorination all use relatively little energy compared to boiling water or generating high pressure.
Mark scheme: 1 mark for filtration (removes solids); 1 mark for sedimentation (aluminium sulfate, particles clump); 1 mark for chlorination (kills microbes); 1 mark for explaining why desalination is expensive (energy); 1 mark for describing distillation OR reverse osmosis; 1 mark for comparison with fresh water treatment cost
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A country has limited fresh water but abundant sea water. Two desalination options are considered:
- Distillation plant: costs £500 million to build, uses 15 kWh of energy per m³ of water, produces 10,000 m³/day
- Reverse osmosis plant: costs £300 million to build, uses 4 kWh of energy per m³ of water, produces 10,000 m³/day
The country also has abundant sunshine and could build solar panels to provide energy at 5p per kWh.
(a) Calculate the daily energy cost for each plant if powered by solar energy.
(b) The distillation plant produces water of higher purity than the reverse osmosis plant. Suggest why reverse osmosis might still be the better choice.
(c) A student suggests that the country should use waste water treatment instead of desalination. Evaluate this suggestion.
Answers: (a) Distillation: 15 × 10,000 = 150,000 kWh/day. Cost = 150,000 × 0.05 = £7,500/day. Reverse osmosis: 4 × 10,000 = 40,000 kWh/day. Cost = 40,000 × 0.05 = £2,000/day. (b) Reverse osmosis uses much less energy (4 vs 15 kWh/m³), has lower daily running costs (£2,000 vs £7,500), and costs less to build (£300M vs £500M). The water purity from reverse osmosis may be sufficient for drinking water standards, making the extra purity from distillation unnecessary. (c) Waste water treatment recycles used water, which is cheaper than desalination. However, the country has limited fresh water, meaning there may not be enough waste water to meet demand. A combination approach might work best — treating waste water for non-drinking uses (irrigation, industry) while using reverse osmosis for drinking water, reducing the total desalination needed.
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