ExpertHard6 marksExtended Response
AQA GCSE · Question 03.4 · Using resources
Water collected from rivers is used in the home for drinking and flushing toilets. Water used in the home must be potable. Potable water is safe to drink. Waste water produced after use in the home is called sewage. Figure 3 shows how water is collected from rivers and returned to rivers after use.
Explain what happens to water in Process A and in Process B in Figure 3. Do not refer to use of water in the home.
Water collected from rivers is used in the home for drinking and flushing toilets. Water used in the home must be potable. Potable water is safe to drink. Waste water produced after use in the home is called sewage. Figure 3 shows how water is collected from rivers and returned to rivers after use.
Explain what happens to water in Process A and in Process B in Figure 3. Do not refer to use of water in the home.
How to approach this question
This is a 6-mark question, so you need to provide a detailed, structured explanation. Break your answer into two parts: Process A and Process B.
**For Process A (making potable water):**
- What is the starting water source? (Ground water)
- What needs to be removed? (Insoluble solids, harmful microbes)
- Describe the key stages: filtration and sterilisation. What do they do?
**For Process B (treating sewage):**
- What is being treated? (Sewage)
- What needs to be removed? (Large objects, organic waste, harmful microbes)
- Describe the key stages: screening, sedimentation, and biological treatment (aerobic digestion). What happens at each stage?
Full Answer
**Process A: Producing potable water**
Process A takes ground water and treats it to make it potable (safe to drink). This involves several stages. First is **sedimentation**, where large solids are allowed to settle out. Then **filtration** is used to remove smaller insoluble particles, often by passing the water through beds of sand and gravel. Finally, the water is **sterilised** to kill harmful microbes (bacteria and viruses). This is usually done by adding small amounts of chlorine, or by using ozone or ultraviolet light.
**Process B: Treating sewage**
Process B treats sewage before it is returned to the river. This also involves multiple stages. First is **screening**, where large objects like grit and twigs are removed by a metal grid. Then, in **sedimentation**, the sewage is allowed to stand in a settlement tank. The heavier solids (sludge) sink to the bottom, while the lighter liquid (effluent) stays on top. The effluent is then treated with **aerobic digestion**, where air is bubbled through it to encourage useful bacteria to break down the organic matter and harmful microbes. The treated effluent can then be safely discharged into the river.
This question assesses knowledge of water treatment processes.
**Process A: Treatment of ground water to make it potable.**
The aim is to make water safe for human consumption.
1. **Filtration:** The water is passed through filter beds, typically made of sand and gravel. This removes insoluble particles like leaves, grit, and other suspended solids.
2. **Sterilisation:** Although the water is now clear, it may still contain harmful microorganisms (pathogens) like bacteria and viruses. To kill these, a sterilising agent is added. Chlorine is most commonly used because it is effective and has a residual effect, keeping the water safe in the pipes. Ozone or ultraviolet (UV) light are alternative methods of sterilisation.
**Process B: Treatment of sewage.**
The aim is to remove waste materials so the water can be safely returned to the environment (e.g., a river).
1. **Screening:** The incoming sewage is passed through a large metal grid (a screen) to remove large objects such as twigs, plastic bags, and grit.
2. **Primary Treatment (Sedimentation):** The screened sewage is then held in a large settlement tank. Solid organic matter, being denser, sinks to the bottom to form a layer of sludge. The less dense liquid part, called effluent, remains on top.
3. **Secondary Treatment (Aerobic Digestion):** The effluent is transferred to another tank where air is bubbled through it. This provides oxygen for aerobic bacteria to thrive. These useful bacteria feed on the organic waste and harmful microbes in the effluent, breaking them down into harmless substances like carbon dioxide and water. The treated effluent is now safe to be discharged.
Common mistakes
✗ Confusing the processes for potable water and sewage treatment.
✗ Missing key stages (e.g., forgetting sterilisation for potable water, or aerobic digestion for sewage).
✗ Using vague terms like "cleaning the water" without specifying the method (filtration, sterilisation etc.) and what is being removed (solids, microbes etc.).
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
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