14.3 Treating Waste Water

A free GCSE Chemistry lesson for AQA (8462) from OBA School: where waste water comes from, the stages of sewage treatment, and comparing sources of potable water. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the sources of waste water and what must be removed from them
• Describe the stages of sewage treatment: screening, sedimentation, anaerobic digestion of sludge, aerobic treatment and final sedimentation of effluent
• Explain why industrial waste water may need extra treatment
• Compare how easy it is to get potable water from ground water, waste water and salt water
• Carry out calculations using data from a treatment works

Key words

• Sewage: Waste water from homes, farms and industry, containing organic matter and microbes.
• Screening: Passing sewage through a mesh to remove large objects, then letting grit settle out.
• Sedimentation: Leaving sewage to stand in tanks so that heavier solids sink.
• Sludge: The solid layer that settles at the bottom of sedimentation tanks.
• Effluent: The liquid layer above the sludge.
• Anaerobic digestion: Breakdown of organic matter by bacteria in the absence of oxygen. It produces biogas (mainly methane).
• Aerobic treatment: Breakdown of organic matter by bacteria when air (oxygen) is bubbled through.

Where waste water comes from

We produce huge amounts of waste water from homes (urban life), farms and industry. It must be treated before it is released into rivers or the sea.

• Homes (sewage); Main things to remove organic matter (human waste, food), harmful microbes, detergents
• Farms (agricultural); Main things to remove animal waste (organic matter, microbes), fertiliser run-off
• Industry; Main things to remove organic matter and harmful chemicals, e.g. toxic metal ions, which need extra treatment

Key fact: Untreated sewage harms rivers. Microbes can spread disease, and bacteria breaking down organic matter use up the oxygen in the water, so fish die.

The stages of sewage treatment

• 1 Screening and grit removal – a mesh removes large objects such as rags and plastic; grit and sand settle out.
• 2 Sedimentation – in large tanks, heavier solids sink to form sludge; the lighter liquid on top is effluent.
• 3 Anaerobic digestion of sludge – bacteria break down the organic matter without oxygen. This makes biogas (mainly methane), which is burned to generate electricity. The digested sludge can be used as a fertiliser.
• 4 Aerobic biological treatment of effluent – air is bubbled through, and aerobic bacteria break down the remaining organic matter and kill many harmful microbes. This can take hours or days.
• 5 Final (secondary) sedimentation – the bacteria settle out in a last tank. Some are sent back to the aerobic tank to keep it working; the rest join the sludge. The clear water can now be released into a river or the sea.

Extra steps for sensitive rivers. If the river is home to delicate wildlife, the water may be passed through a sand filter and sterilised with ultraviolet light or chlorine before release. UV is often preferred, because chlorine can react with organic matter to form toxic chlorine compounds.

More about the sludge. Digestion is faster in a warm tank, but heating it costs energy. Biogas is mostly methane with carbon dioxide and a little hydrogen sulfide; it can be burned to power the works, or cleaned to methane and added to the gas supply. Drying the digested sludge makes it much smaller and lighter to transport – it can be spread on fields or burned as a fuel.

Exam tip: Learn the two pairs: sludge → anaerobic digestion (no oxygen, makes methane); effluent → aerobic treatment (air bubbled through).

Data from a treatment works

Worked example: percentage of organic matter removed

Start: 320 mg/dm³. After aerobic treatment: 18 mg/dm³.
decrease = 320 − 18 = 302 mg/dm³
percentage removed = 302 ÷ 320 × 100 = 94.375%

Answer: 94.4% (3 s.f.)

Worked example: biogas (Higher tier)

A digester makes 3600 dm³ of methane, CH₄, at room temperature and pressure. What mass is this?
moles = 3600 ÷ 24 = 150 mol. Mr of CH₄ = 12 + 4 × 1 = 16
mass = 150 × 16 = 2400 g

Answer: 2.4 kg of methane

Comparing sources of potable water

• Ground water (and lakes, rivers); What is needed filtering and sterilising (14.2); How easy? easiest and cheapest
• Waste water; What is needed sewage treatment, then further filtering and sterilising; many stages; How easy? harder; more stages and checks
• Salt water (sea); What is needed desalination by distillation or reverse osmosis, then sterilising; How easy? hardest; needs large amounts of energy

Countries short of fresh water may recycle treated waste water into drinking water, using extra stages such as membranes and UV.

Common mistakes

• Mixing up sludge and effluent. Sludge is the solid that sinks; effluent is the liquid on top.
• Swapping aerobic and anaerobic. Sludge is digested without oxygen; effluent is treated with air.
• Saying sewage treatment makes water potable. It makes water safe to return to rivers; more treatment is needed to drink it.
• Forgetting that industrial waste water may contain harmful chemicals that need extra removal.

Revision questions

Try these, then check your answers below each one.

Question 1. Anaerobic digestion of sewage sludge produces a useful gas. Name the main gas in it.

Answer: methane

Working: Biogas is mainly methane, which can be burned to generate electricity.

Question 2. True or false? Industrial waste water may need extra treatment to remove harmful chemicals as well as organic matter.

Answer: True

Working: True. Industrial waste water can contain harmful chemicals that normal sewage treatment does not remove.

Question 3. Organic matter falls from 400 mg/dm³ in raw sewage to 20 mg/dm³ after treatment. What percentage of the organic matter was removed?

Answer: 95 %

Working: Decrease = 380 mg/dm³. 380 ÷ 400 × 100 = 95%.

Related lessons

• Previous lesson: 14.2 Water Safe to Drink
• Next lesson: 14.4 Extracting Metals from Ores
• In this chapter: 14.1 Finite and Renewable Resources
• In this chapter: 14.5–14.6 Life Cycle Assessments and Reduce, Reuse, Recycle
• In this chapter: 14.7 Required Practicals: The Earth's Resources

Continue in the full course

This lesson is section 14.3 (AGCCH1403) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.