Waste water treatment — AQA GCSE Chemistry
Verified courseTeaching note checked against the official specification.
Waste water treatment explained
Waste water treatment covers 5 specification points, including Urban lifestyles and industrial processes… and Sewage and agricultural waste water require….
Read the full explanation
Open a point below for its explanation and revision guidance.
Urban lifestyles and industrial processes produce large amounts of waste water that require treatment before being released into the environment.
Urban living and industry generate waste water from homes, businesses and factories. Domestic waste water includes water from sinks, baths, toilets and washing machines, while industrial processes may add chemicals, oils or other pollutants. If this water were released untreated, it could harm aquatic life, spread disease and pollute rivers, lakes and groundwater. Treatment removes solids, organic matter and harmful microorganisms before the water is returned to the environment. A typical treatment sequence includes screening to remove large objects, sedimentation to settle suspended solids, and biological or chemical treatment to break down organic matter and reduce harmful microbes. The treated water must meet environmental standards before release.
Sewage and agricultural waste water require removal of organic matter and harmful microbes.
Sewage from homes and agricultural waste water from farms contain organic matter and harmful microbes. Organic matter includes food waste, faeces and plant material; harmful microbes include bacteria and viruses that can cause disease. If released untreated, organic matter is broken down by microorganisms in rivers, which uses up dissolved oxygen and can kill fish. Harmful microbes can contaminate drinking water and cause illness. Treatment therefore focuses on removing or breaking down organic matter and reducing harmful microbes. This can involve sedimentation, biological treatment using bacteria, and disinfection. The treated water is then safe to release, and the removed solids may be processed further.
Industrial waste water may require removal of organic matter and harmful chemicals.
Industrial processes produce waste water that is often more variable and more contaminated than domestic sewage. It may carry organic matter, such as food, blood or solvents from factories, and harmful chemicals, including heavy metal ions, acids, alkalis or toxic compounds. Before this water can be returned to a river or reused, these substances must be removed or reduced to safe levels. Treatment usually combines physical separation, such as screening or settling, with chemical methods, such as neutralisation, precipitation or adsorption onto activated carbon. For example, acidic factory water is neutralised with an alkali, and dissolved metal ions are precipitated as insoluble hydroxides and filtered off. The specific method depends on the pollutant, so a factory must identify its waste before choosing treatment.
Sewage treatment includes: screening and grit removal sedimentation to produce sewage sludge and effluent anaerobic digestion of sewage sludge aerobic biological treatment of effluent.
Sewage treatment is a sequence of stages that removes solids and reduces the organic load before the treated water is released. Screening and grit removal take out large objects, rags, plastics and dense grit that would damage pumps or block pipes. Sedimentation then lets finer suspended solids settle, producing sewage sludge at the bottom and effluent above. The sludge is treated by anaerobic digestion, in which microorganisms break it down without oxygen, reducing its volume and producing biogas that can be used as fuel. The effluent undergoes aerobic biological treatment, where microorganisms with oxygen break down remaining organic matter. The treated effluent can then be discharged safely to a river or sea, provided it meets quality standards.
Students should be able to comment on the relative ease of obtaining potable water from waste, ground and salt water.
Potable water is water that is safe to drink, with low levels of dissolved salts and microorganisms. The ease of obtaining it depends on the source. Ground water, such as water from aquifers, is usually easiest because it is often naturally filtered through rock and may need only screening, aeration and chlorination. Waste water, including sewage and industrial effluent, is harder because it contains organic matter, harmful chemicals and microorganisms, so it needs multiple treatment stages. Salt water is the hardest because dissolved salts must be removed by distillation or reverse osmosis, both of which need significant energy and equipment. When comparing sources, consider the number of treatment steps, the energy required and the cost of the plant needed.
Your focus
- Identify sources of waste water from urban lifestyles and industrial processes.
- Explain why waste water must be treated before release into the environment.
- Describe the main stages of waste water treatment and what each stage removes.
Show all 15 objectives
- Identify organic matter and harmful microbes as the main contaminants in sewage and agricultural waste water.
- Explain the environmental and health consequences of releasing untreated sewage or agricultural waste water.
- Describe treatment processes that remove organic matter and reduce harmful microbes.
- State that industrial waste water may contain organic matter and harmful chemicals.
- Describe at least two treatment methods used to remove organic matter or harmful chemicals from industrial waste water.
- Explain why the treatment chosen depends on the pollutants present in the waste water.
- List the main stages of sewage treatment in the correct order.
- Describe what happens during sedimentation, anaerobic digestion and aerobic biological treatment.
- Explain the purpose of each stage in producing safe effluent and treated sludge.
- Define potable water and state that it must have low levels of dissolved salts and microorganisms.
- Compare the treatment needed for waste, ground and salt water.
- Explain why salt water is generally the most difficult and expensive source of potable water.
Waste water treatment exam tips
Marking Points
- State that urban lifestyles produce domestic waste water from homes and that industrial processes produce waste water containing a range of pollutants.
- Explain that untreated waste water can harm the environment by causing disease, deoxygenating water or introducing toxic substances.
- Describe the purpose of treatment as removing solids, organic matter and harmful microbes before release.
- Outline a treatment sequence such as screening, sedimentation and biological treatment, linking each stage to what it removes.
- Recognise that treated water must meet environmental standards before being released into rivers or the sea.
- State that sewage and agricultural waste water contain organic matter and harmful microbes.
- Explain that organic matter is broken down by microorganisms, which reduces dissolved oxygen in water and harms aquatic life.
- Explain that harmful microbes can cause disease and must be reduced before water is released.
- Describe treatment methods such as sedimentation, biological treatment and disinfection that remove organic matter and harmful microbes.
- Recognise that agricultural waste water may also contain nutrients or animal waste, which adds to the organic load.
- Industrial waste water can contain organic matter, such as food residues, oils or solvents, which must be removed because it can lower dissolved oxygen in rivers when decomposed by microorganisms.
- Harmful chemicals in industrial waste water include toxic metal ions, acids, alkalis and other dangerous compounds that can poison aquatic life or contaminate drinking water sources.
- Treatment methods are chosen to match the pollutant: screening removes large solids, sedimentation removes suspended particles, neutralisation treats acids or alkalis, and precipitation removes dissolved metal ions.
- Organic matter may be broken down by aerobic biological treatment, while some dissolved chemicals are removed by adsorption onto activated carbon or by chemical precipitation.
- Treated industrial waste water must meet safe discharge limits before release, so the level of treatment required depends on the original waste and the sensitivity of the receiving environment.
- Screening and grit removal is the first stage and removes large solid objects and dense grit that could damage equipment or block later stages.
- Sedimentation allows suspended solids to settle, producing sewage sludge at the bottom and a liquid effluent above.
- Anaerobic digestion of sewage sludge uses microorganisms in the absence of oxygen to break down organic matter, reducing sludge volume and producing biogas.
- Aerobic biological treatment of effluent uses microorganisms in the presence of oxygen to break down remaining organic matter before the effluent is discharged.
- The stages are ordered so that large solids are removed first, then suspended solids, then dissolved or fine organic matter, making later treatment more effective.
- Ground water is generally the easiest source to treat because it is often already filtered by rock and may need only simple screening, aeration and disinfection.
- Waste water is harder to treat because it contains organic matter, harmful chemicals and microorganisms, so it requires several stages such as screening, sedimentation, biological treatment and disinfection.
- Salt water is the hardest source because dissolved salts must be removed by distillation or reverse osmosis, which require large amounts of energy and specialised equipment.
- The relative ease of obtaining potable water can be compared by considering the number of treatment stages, the energy demand and the cost of the process.
- All sources must be treated to reduce microorganisms and dissolved salts to safe levels before the water is considered potable.
Examiner Tips
- 💡Use the phrase 'before being released into the environment' to show you understand the purpose of treatment.
- 💡When describing treatment, name the stage and state what it removes, for example 'screening removes large solid objects'.
- 💡Link environmental harm to specific effects, such as harmful microbes causing disease or organic matter reducing oxygen levels in water.
- 💡Use the terms 'organic matter' and 'harmful microbes' explicitly in your answers to match the specification.
- 💡Explain the consequence of not removing organic matter, such as oxygen depletion, to show deeper understanding.
- 💡When describing treatment, link each stage to what it removes, for example 'biological treatment uses bacteria to break down organic matter'.
- 💡Link each treatment method to the type of pollutant it removes, rather than listing methods without a reason.
- 💡Use the phrase harmful chemicals for toxic dissolved substances and organic matter for decomposable carbon-based material, so your answers match the specification language.
- 💡When given a data table or flow diagram, identify which stage removes which pollutant before writing your explanation.
- 💡Learn the stages in order and be ready to name the product of each stage, especially sludge and effluent.
- 💡Use the words anaerobic and aerobic accurately, linking anaerobic to digestion of sludge and aerobic to treatment of effluent.
- 💡If asked to explain a stage, state what is removed or broken down and why that helps the next stage or the final discharge.
- 💡Compare sources using clear criteria such as number of stages, energy use and cost, rather than saying one is simply better.
- 💡Use the term potable water correctly and avoid calling it pure water.
- 💡When describing desalination, name distillation or reverse osmosis and state that both require significant energy.
Common Mistakes
- Thinking all waste water is sewage; the correction is to recognise that industrial waste water may contain different pollutants such as chemicals and oils.
- Believing treatment simply dilutes waste; the correction is that treatment removes or breaks down pollutants rather than just diluting them.
- Confusing waste water treatment with potable water treatment; the correction is that waste water treatment aims to make water safe for release, while potable water treatment aims to make water safe to drink.
- Thinking organic matter is always harmless; the correction is that its decomposition removes oxygen from water and damages ecosystems.
- Confusing harmful microbes with all bacteria; the correction is that some bacteria are useful in treatment, while harmful microbes are the disease-causing ones that must be reduced.
- Believing boiling is used to treat large volumes of waste water; the correction is that large-scale treatment uses sedimentation, biological processes and disinfection rather than boiling.
- Treating all waste water as if it were sewage; the correction is to recognise that industrial waste water varies widely and may need specific chemical treatment rather than only biological treatment.
- Assuming dilution alone makes harmful chemicals safe; the correction is that toxic substances must be removed or chemically changed, because dilution does not destroy them.
- Confusing organic matter with harmful chemicals; the correction is that organic matter is carbon-based material that microorganisms can decompose, whereas harmful chemicals include toxic metal ions and other dangerous substances.
- Reversing the roles of sludge and effluent; the correction is that sludge is the settled solid material and effluent is the liquid that remains above it.
- Confusing anaerobic digestion with aerobic biological treatment; the correction is that anaerobic digestion occurs without oxygen and treats sludge, while aerobic treatment occurs with oxygen and treats effluent.
- Thinking screening removes dissolved chemicals; the correction is that screening and grit removal only remove large solid objects and dense particles, not dissolved substances.
- Saying salt water is easy to treat because the sea is large; the correction is that the volume of water does not remove dissolved salts, so energy-intensive desalination is needed.
- Assuming waste water only needs filtering; the correction is that waste water also requires biological and chemical treatment to remove organic matter, harmful chemicals and microorganisms.
- Confusing potable water with pure water; the correction is that potable water is safe to drink and still contains some dissolved substances, whereas pure water contains no dissolved substances.