Revision notes for AQA GCSE Chemistry Waste water treatment. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Waste water treatment
What you'll learn
- Why waste water must be treated before it is released into the environment.
- The main stages of sewage treatment: screening, sedimentation, digestion and biological treatment.
- The difference between sewage sludge and effluent.
- How to compare the difficulty of obtaining potable water from ground water, waste water and salt water.
Why do we need to treat waste water?
Modern towns, cities, farms and factories produce huge amounts of waste water. This water cannot usually be released straight into rivers or the sea because it may contain substances that harm living organisms or spread disease.
Definition
Waste water
Waste water is water that has been contaminated by human activity, such as domestic use, agriculture or industrial processes.
Common sources include:
- Urban waste water from homes, schools, shops and hospitals.
- Sewage, which includes toilet waste and dirty water from sinks, baths and washing machines.
- Agricultural waste water from farms, which may contain animal waste, fertilisers and soil.
- Industrial waste water from factories, which may contain chemicals as well as organic waste.
Definition
Organic matter
Organic matter means carbon-based material from living things or their waste, such as food waste, faeces, dead plants and animal waste. Microorganisms can decompose it.
If untreated waste water enters a river, microorganisms may break down the organic matter using oxygen dissolved in the water. This can reduce the oxygen available for fish and other aquatic organisms. Waste water can also contain harmful microbes, also called pathogens, which are microorganisms that can cause disease.
Key Idea
Why treatment matters
Waste water is treated to remove organic matter, harmful microbes and harmful chemicals before the water is released back into the environment.
Potable water is not the same as pure water
Before looking at sewage treatment, it helps to remember what we mean by potable water.
Definition
Potable water
Potable water is water that is safe to drink. It is not pure water: it contains low levels of dissolved substances and very low levels of microbes.
Pure water would contain only H₂O molecules. Potable water does not have to be pure; it just has to be safe.
Common Mistake
Potable does not mean pure
Do not write that potable water contains “nothing else”. Drinking water often contains dissolved minerals, but these must be present at safe levels.
Sewage treatment: the big picture
Sewage treatment is the process used to clean sewage before the water is returned to the environment. It separates solids from liquids and uses microorganisms to break down waste.
The diagram shows the main route: large solids are removed first, then smaller suspended solids settle, then the solid sludge and liquid effluent are treated in different ways.
Stage 1: screening and grit removal
The first stage is mainly physical separation.
Screening
Screening removes large objects from sewage. These might include rags, wet wipes, plastics and other large debris.
The sewage passes through metal bars or mesh screens. Large solids are trapped, while the liquid and smaller particles pass through.
Grit removal
Grit means small, heavy particles such as sand, gravel and small stones. These are removed because they can damage pumps and machinery.
Grit removal usually works because these particles are dense and settle out when the flow of sewage is slowed down.
Key Idea
First remove the obvious solids
Screening and grit removal do not make water safe to drink. They simply remove large debris and heavy particles so later treatment stages can work properly.
Stage 2: sedimentation
After screening and grit removal, the sewage enters a large settlement tank. Here, the flow is kept slow so that suspended solids can settle.
Definition
Sedimentation
Sedimentation is the process where suspended solid particles settle to the bottom of a liquid due to gravity.
Sedimentation produces two important products:
- Sewage sludge: the thick, semi-solid material that settles at the bottom.
- Effluent: the liquid that remains above the sludge and flows on for further treatment.
Definition
Effluent
Effluent is liquid waste or treated liquid that flows out of a treatment stage.
Definition
Sewage sludge
Sewage sludge is the semi-solid mixture of organic matter and other solids that settles out during sewage treatment.
Common Mistake
Mixing up sludge and effluent
Sludge is the solid material that settles at the bottom. Effluent is the liquid that leaves the sedimentation tank for further treatment.
Stage 3: anaerobic digestion of sewage sludge
The sewage sludge still contains lots of organic matter. It is treated by anaerobic digestion.
Definition
Anaerobic digestion
Anaerobic digestion is the breakdown of organic matter by microorganisms in the absence of oxygen.
During anaerobic digestion:
- Microorganisms break down organic matter in the sludge.
- The volume of solid waste is reduced.
- Biogas is produced, which contains methane and can be used as a fuel.
- The remaining digested sludge may be disposed of or sometimes used as fertiliser if it is safe.
Definition
Biogas
Biogas is a mixture of gases produced when microorganisms break down organic matter anaerobically. It contains methane, which can be burned as a fuel.
Key Idea
Sludge is treated without oxygen
Sewage sludge is treated by anaerobic digestion because microorganisms can break down the organic matter without oxygen, producing useful biogas.
Stage 4: aerobic biological treatment of effluent
The liquid effluent from sedimentation still contains dissolved and tiny suspended organic matter. It needs further treatment before release.
Definition
Aerobic biological treatment
Aerobic biological treatment uses microorganisms in the presence of oxygen to break down organic matter in effluent.
In this stage:
- Air or oxygen is pumped through the effluent.
- Aerobic microorganisms use the oxygen for respiration.
- They break down remaining organic matter.
- This reduces the amount of organic waste in the water.
This stage is called biological because living microorganisms carry out the treatment.
Key Idea
Effluent is treated with oxygen
Effluent is treated aerobically: oxygen is supplied so microorganisms can break down remaining organic matter more effectively.
Example
Choosing the correct sewage treatment stage
A sample from a sewage works contains liquid waste with small amounts of dissolved organic matter, but most of the solid sludge has already been removed. Which treatment should it go to next?
- The sample is mainly liquid, so it is not the sludge route. Sludge goes to anaerobic digestion.
- The liquid after sedimentation is called effluent, so it needs further treatment to remove remaining organic matter.
- Because the remaining waste is in the effluent, the correct next stage is aerobic biological treatment, where microorganisms use oxygen to break down the organic matter.
Why different waste waters need different treatment
Not all waste water is the same.
Sewage and agricultural waste water
Sewage and agricultural waste water usually contain:
- Organic matter, such as faeces, food waste and animal waste.
- Harmful microbes that may cause disease.
So the main aim is to remove organic matter and harmful microbes.
Industrial waste water
Industrial waste water may contain:
- Organic matter.
- Harmful chemicals, such as toxic metal compounds, acids, alkalis or solvents.
So industrial waste water may need extra chemical treatment, depending on what the factory produces.
Tip
Link treatment to the pollutant
In exam answers, say what needs removing. For example: sewage needs treatment to remove organic matter and harmful microbes; industrial waste water may also need treatment to remove harmful chemicals.
Obtaining potable water: ground water, waste water and salt water
The specification asks you to compare how easy it is to obtain potable water from different sources.
Ground water
Ground water is water found underground in rocks and soil. It is often easier to make potable than waste water or salt water because it has already been filtered naturally as it passed through layers of rock.
It may still need treatment, such as filtration and sterilisation, but it usually contains fewer suspended solids and fewer microbes than sewage.
Waste water
Waste water is harder to turn into potable water because it may contain:
- Organic matter.
- Harmful microbes.
- Suspended solids.
- Harmful chemicals, especially if it comes from industry.
It needs several treatment stages before it can be safely released. Making it directly drinkable would usually require even more careful treatment and testing.
Salt water
Salt water, such as seawater, contains large amounts of dissolved salts. These cannot be removed by simple filtration because the salt particles are dissolved.
To obtain potable water from salt water, you need desalination.
Definition
Desalination
Desalination is the removal of dissolved salts from water, usually by distillation or reverse osmosis.
Desalination can produce potable water, but it is expensive because it requires a lot of energy, especially for distillation or high-pressure reverse osmosis.
Key Idea
Relative ease of obtaining potable water
In general, ground water is usually the easiest to make potable. Waste water needs many treatment stages because it contains organic matter and microbes. Salt water is difficult and expensive because dissolved salts must be removed.
Example
Comparing water sources for potable water
A coastal town can use ground water, treated waste water or seawater as a source of potable water. Which source is likely to need the least treatment?
- Compare the main impurities: ground water may contain some dissolved minerals and microbes, waste water contains organic matter and microbes, and seawater contains high levels of dissolved salts.
- Decide which impurities are easiest to remove. Suspended particles and microbes can often be removed by filtration and sterilisation, but dissolved salts need desalination, which is energy-intensive.
- Ground water is likely to need the least treatment because it has often been naturally filtered through rock and usually contains fewer harmful contaminants than waste water or seawater.
Common Mistake
Do not assume every sample is identical
The easiest water source can depend on local conditions. Polluted ground water may be harder to treat than clean ground water. In GCSE questions, use the information given in the question and compare the contaminants present.
The whole sewage treatment sequence
You should be able to describe the stages in order:
- Screening and grit removal remove large objects and heavy particles.
- Sedimentation allows suspended solids to settle, producing sewage sludge and effluent.
- Anaerobic digestion treats the sewage sludge without oxygen and produces biogas.
- Aerobic biological treatment treats the effluent using oxygen and microorganisms.
Tip
Memory route
Think: big solids out → small solids settle → sludge without oxygen → effluent with oxygen.
Exam technique
In the exam
- If asked to describe sewage treatment, give the stages in the correct order and state what each stage removes or produces.
- Use the words sludge and effluent carefully: sludge is the settled solid; effluent is the liquid.
- When comparing water sources, explain the reason: ground water is usually easier, waste water contains organic matter and microbes, and salt water needs energy-intensive desalination.
Self review
Check yourself
- What are the four main stages of sewage treatment, in order?
- What is the difference between sewage sludge and effluent?
- Why is salt water usually more expensive to turn into potable water than ground water?
Using the Earth's resources and obtaining potable water
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