Waste Water Treatment
Master Waste Water Treatment for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Sewage treatment involves screening, then sedimentation, then aerobic biological treatment of the liquid effluent.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Waste water from urban sources, agriculture and industry requires treatment before being released into the environment.
Sewage treatment has four stages. Screening and grit removal takes out large solids. Sedimentation allows the water to settle, producing sewage sludge at the bottom and effluent at the top. The sludge is digested anaerobically by bacteria, and the effluent is treated aerobically by bacteria.
The two biological stages differ in oxygen. Anaerobic digestion of the sludge produces methane, which can be used as a fuel, and a digested waste usable as fertiliser. Aerobic treatment of the effluent needs air pumped through it so bacteria can break down organic matter and harmful microbes.
Revision notes
The physical stages
Screening and grit removal takes out large solid objects and grit.
Sedimentation in tanks allows the water to settle. Heavier solids sink to form sewage sludge; the lighter effluent remains at the top.
Anaerobic digestion of sludge
The sewage sludge is digested anaerobically by bacteria, without oxygen.
This produces methane gas, which can be used as a fuel, and a digested waste that can be used as fertiliser on farmland.
Aerobic treatment of effluent
The effluent is treated aerobically, with air pumped through it.
Bacteria use the oxygen to break down organic matter and harmful microorganisms. The treated effluent can then be safely released into rivers or the sea.
Key points
- Waste water needs treatment before release.
- Screening removes large solids and grit.
- Sedimentation separates sludge from effluent.
- Sludge is digested anaerobically.
- Anaerobic digestion produces methane and fertiliser.
- Effluent is treated aerobically with air pumped through.
Worked examples
Example 1
Describe what happens during sedimentation in sewage treatment. [2 marks]
Model answer
The water is allowed to settle in tanks (1 mark). Heavier solids sink to the bottom forming sewage sludge, while the lighter effluent remains at the top (1 mark).
Example 2
State two useful products of the anaerobic digestion of sewage sludge. [2 marks]
Model answer
Methane gas, which can be used as a fuel (1 mark). A digested waste, which can be used as fertiliser on farmland (1 mark).
Example 3
Explain why air is pumped through the effluent during treatment. [2 marks]
Model answer
The bacteria treating the effluent respire aerobically and need oxygen (1 mark), which allows them to break down organic matter and harmful microorganisms (1 mark).
Common mistakes
Confusing sludge and effluent.
Sludge is the heavy solid that sinks; effluent is the lighter liquid above.
Saying both biological stages are the same.
Sludge is treated anaerobically; effluent is treated aerobically.
Forgetting the useful products.
Methane and fertiliser are both examinable outputs.
Saying screening removes microbes.
Screening removes large solids only; microbes are dealt with biologically.
Exam tips
- Learn the four stages in order.
- Keep anaerobic sludge and aerobic effluent clearly separate.
- Name both useful products of anaerobic digestion.
- Link the air supply to aerobic respiration by bacteria.
Key terms
- Sewage sludge
- The solid material settling at the bottom of the tank.
- Effluent
- The liquid remaining above the sludge.
- Anaerobic digestion
- Breakdown by bacteria without oxygen.
- Aerobic treatment
- Breakdown by bacteria using oxygen.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.