GCSE Chemistry revision

Waste water treatment

Using the Earth's resources and obtaining potable water

AQA 4.10.1.3 Foundation & Higher

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Domestic waste water comes from household sources such as showers, sinks, and toilets, and is directed to sewage treatment plants via the sewer system.
Agricultural waste water includes nutrient runoff from fields and animal waste from farms, which can pollute natural water sources.
Industrial waste water originates from factories that manufacture or use chemicals, often containing harmful chemical substances that require additional treatment.

1. Sources of Waste Water AQA

Domestic waste water comes from household sources such as showers, sinks, and toilets, and is directed to sewage treatment plants via the sewer system.
Agricultural waste water includes nutrient runoff from fields and animal waste from farms, which can pollute natural water sources.
Industrial waste water originates from factories that manufacture or use chemicals, often containing harmful chemical substances that require additional treatment.

All waste water must be treated before it can be safely disposed of or returned to natural water sources such as rivers or the ocean.
Domestic and agricultural sewage requires the removal of organic matter and harmful microbes to prevent pollution and health risks.
Industrial waste water often contains toxic chemicals, requiring extra treatment steps beyond standard sewage processing.

2. Why Waste Water Must Be Treated AQA

All waste water must be treated before it can be safely disposed of or returned to natural water sources such as rivers or the ocean.
Domestic and agricultural sewage requires the removal of organic matter and harmful microbes to prevent pollution and health risks.
Industrial waste water often contains toxic chemicals, requiring extra treatment steps beyond standard sewage processing.

The first stage of sewage treatment is screening, where large objects such as twigs and plastic bottles are removed from the waste water.
Screening is typically achieved by passing the sewage through a mesh that only allows the liquid sewage to pass through.

The screened sewage is transferred to a settlement tank where sedimentation occurs, allowing heavier solid particles to sink to the bottom.
The dense solid material that collects at the bottom of the settlement tank is called sludge, while the lighter liquid layer above it is called the effluent.
The sludge and effluent are separated into two different tanks for further treatment.

4. Step 2: Sedimentation AQA

The screened sewage is transferred to a settlement tank where sedimentation occurs, allowing heavier solid particles to sink to the bottom.
The dense solid material that collects at the bottom of the settlement tank is called sludge, while the lighter liquid layer above it is called the effluent.
The sludge and effluent are separated into two different tanks for further treatment.

Air is pumped into the effluent tank to provide oxygen, creating aerobic conditions for microorganisms to break down organic matter.
Aerobic digestion by microorganisms, including the breakdown of harmful microbes, continues until the water is safe to release back into the environment.

5. Step 3: Biological Breakdown of Effluent AQA

Air is pumped into the effluent tank to provide oxygen, creating aerobic conditions for microorganisms to break down organic matter.
Aerobic digestion by microorganisms, including the breakdown of harmful microbes, continues until the water is safe to release back into the environment.

The sludge tank is kept sealed with little or no oxygen, creating anaerobic conditions for microorganisms to break down the organic matter.
Anaerobic digestion produces methane gas, which can be captured and burned as a renewable energy source.
The remaining digested sludge is rich in nutrients and can be used as a fertiliser on agricultural land.

6. Step 3: Anaerobic Digestion of Sludge AQA

The sludge tank is kept sealed with little or no oxygen, creating anaerobic conditions for microorganisms to break down the organic matter.
Anaerobic digestion produces methane gas, which can be captured and burned as a renewable energy source.
The remaining digested sludge is rich in nutrients and can be used as a fertiliser on agricultural land.

Waste water containing toxic substances requires additional treatment stages beyond the standard three-step process.
Chemicals can be added to cause toxic metals to precipitate out of the water, making them easier to remove.
Ultraviolet (UV) radiation can be used to break down certain harmful substances in industrial waste water.

7. Treating Industrial and Toxic Waste Water AQA

Waste water containing toxic substances requires additional treatment stages beyond the standard three-step process.
Chemicals can be added to cause toxic metals to precipitate out of the water, making them easier to remove.
Ultraviolet (UV) radiation can be used to break down certain harmful substances in industrial waste water.

Treating waste water requires significantly more effort than treating fresh water, but is considerably easier than desalinating salt water.
Some countries with limited fresh water supplies, such as Singapore, rely on treated waste water as a source of drinking water.