Waste Water Treatment

Every day, homes, farms and factories produce vast quantities of waste water that must be treated before it can be safely returned to the environment. This lesson covers the stages of sewage treatment and the different sources and types of waste water, as required by the AQA GCSE Chemistry specification.

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Sources of Waste Water

Waste water comes from several different sources, each containing different types of contaminants:

Source	Type of Waste Water	Main Contaminants
Domestic (homes)	Sewage	Human waste, detergents, food waste, microorganisms
Agricultural (farms)	Agricultural run-off	Fertilisers (nitrates, phosphates), pesticides, animal waste
Industrial (factories)	Industrial effluent	Heavy metals, organic chemicals, solvents, acids, alkalis

All of these must be treated before the water can be released into rivers, lakes or the sea. If untreated waste water enters waterways, it can:

• Cause eutrophication (excess nutrients lead to algal blooms that deplete oxygen)
• Spread waterborne diseases (cholera, typhoid, dysentery)
• Poison aquatic organisms with toxic chemicals or heavy metals
• Damage ecosystems and reduce biodiversity

Exam Tip: AQA may ask why waste water must be treated. Always mention the specific risks: disease from microorganisms, eutrophication from excess nutrients, and poisoning from toxic chemicals. Generic answers like "it's bad for the environment" will not score full marks.

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Stages of Sewage Treatment

Sewage treatment takes place at a sewage treatment works (also called a water treatment plant). The process involves several stages:

graph TD
    A[Raw Sewage In] --> B["Screening<br>Remove large solids"]
    B --> C["Primary Treatment<br>Sedimentation"]
    C --> D[Sludge settles out]
    C --> E[Effluent liquid]
    E --> F["Secondary Treatment<br>Biological / Aerobic"]
    D --> G["Sludge Treatment<br>Anaerobic digestion"]
    F --> H["Final Treatment<br>Optional tertiary"]
    H --> I["Clean Water<br>Released to rivers"]
    G --> J["Biogas<br>Methane for fuel"]
    G --> K["Fertiliser<br>Treated sludge"]
    style A fill:#ef9a9a,stroke:#c62828
    style B fill:#ffcc80,stroke:#e65100
    style C fill:#fff59d,stroke:#f57f17
    style F fill:#a5d6a7,stroke:#2e7d32
    style G fill:#ce93d8,stroke:#6a1b9a
    style I fill:#90caf9,stroke:#1565c0
    style J fill:#ffab91,stroke:#bf360c
    style K fill:#a5d6a7,stroke:#2e7d32

Stage 1: Screening

The raw sewage first passes through screens (metal grids or meshes) that physically remove large solid objects such as sticks, rags, plastic items and grit. This protects pumps and pipes in later stages.

Stage 2: Primary Treatment — Sedimentation

The screened sewage flows into large sedimentation tanks where it is allowed to stand. Heavy solid particles settle to the bottom under gravity, forming sludge. The liquid that remains on top is called the effluent.

Component	What It Is	What Happens Next
Sludge	Solid material that settles to the bottom of sedimentation tanks	Removed for further treatment (anaerobic digestion)
Effluent	The liquid remaining after sludge has been removed	Passes to secondary treatment

Stage 3: Secondary Treatment — Biological (Aerobic)

The effluent from primary treatment still contains dissolved organic matter and harmful microorganisms. In secondary treatment, aerobic bacteria are used to break down this organic matter.

This is achieved by:

1. Passing the effluent through filter beds containing rocks or plastic media coated with bacteria
2. Or placing the effluent in aeration tanks where air is pumped in to provide oxygen for the bacteria

The bacteria feed on the dissolved organic waste, breaking it down through aerobic respiration. This dramatically reduces the biological oxygen demand (BOD) of the water.

Process	How It Works	Key Feature
Filter beds	Effluent trickles over beds of rocks or plastic; biofilm of bacteria on the surface breaks down organic matter	Simple, low energy, but slower
Aeration tanks	Air is pumped into tanks containing effluent and bacteria; bacteria break down organic matter aerobically	Faster, but requires energy to pump air

Exam Tip: The secondary treatment stage uses aerobic bacteria (they need oxygen). This is why air is pumped into aeration tanks. Make sure you can explain why oxygen is needed — the bacteria carry out aerobic respiration to obtain energy from the organic waste.

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Sludge Treatment — Anaerobic Digestion

The sludge from primary treatment is rich in organic matter and microorganisms. It is treated separately by anaerobic digestion:

1. Sludge is transferred to large sealed tanks called digesters
2. Anaerobic bacteria (bacteria that work without oxygen) break down the organic matter
3. This produces biogas — a mixture of mainly methane (CH4) and carbon dioxide (CO2)
4. The remaining solid material (called digestate) can be dried and used as fertiliser

Product of Anaerobic Digestion	Use
Biogas (mainly methane)	Burned as a fuel to generate electricity or heat at the treatment works
Digestate / treated sludge	Used as fertiliser on farmland (after meeting safety standards)

Exam Tip: Do not confuse aerobic and anaerobic stages. Secondary treatment uses aerobic bacteria (with oxygen) to treat the liquid effluent. Sludge treatment uses anaerobic bacteria (without oxygen) in sealed digesters. This is a common source of confusion in exams.

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Tertiary Treatment (Higher Tier)

Some treatment works include an additional tertiary treatment stage to further improve water quality before releasing it into the environment:

Tertiary Method	What It Does
UV treatment	Kills remaining microorganisms
Membrane filtration	Removes very fine particles and some dissolved substances
Chemical treatment	Removes specific pollutants such as phosphates or heavy metals

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Comparing Water Treatment Methods

Feature	Potable Water Treatment	Sewage Treatment
Source	Fresh water (rivers, lakes, reservoirs)	Waste water from homes, farms, industry
Main aim	Make water safe to drink	Remove contaminants before releasing water to the environment
Key stages	Sedimentation, filtration, sterilisation	Screening, sedimentation, biological treatment, sludge digestion
Energy cost	Moderate	Moderate to high

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Summary