IELTS Writing Task 1 Process Diagram: How Drinking Water Is Treated

The diagram below shows how raw water is treated at a water treatment plant before it reaches homes and businesses. Summarise the information by selecting and reporting the main features, and make comparisons where relevant. Write at least 150 words.

The process is linear, beginning with the intake of raw water and ending with distribution to consumers, and consists of seven sequential stages:

Raw water is drawn from a river or reservoir and passed through a screen, which removes large debris such as leaves and branches.
Coagulant chemicals, such as aluminium sulphate, are added to the water, causing small suspended particles to clump together into larger masses known as floc.
The water flows into a sedimentation tank, where the floc settles to the bottom as sludge, leaving clearer water above it.
The clarified water is passed through layers of sand and gravel filters, which trap any remaining fine particles.
The filtered water is disinfected with chlorine to eliminate any bacteria and other pathogens that are still present.
The disinfected water is pumped into a covered clean-water reservoir for storage.
The stored water is pumped from the reservoir through underground pipes to reach homes and businesses.

Examiner notes: why this is a band 9 response

Task Achievement

The overview identifies both the number of stages and the general trend (water becoming progressively cleaner) before any stage is described in detail, and all seven stages (screening, coagulation, sedimentation, filtration, disinfection, storage and distribution) are reported accurately and in the correct order.

Coherence & Cohesion

The seven stages are grouped into three paragraphs rather than presented as a flat list, with each paragraph covering a coherent phase of treatment (raw water intake and clarification; filtration and disinfection; storage and distribution), linked by markers such as “subsequently”, “next”, “once” and “finally”.

Lexical Resource

Technical process vocabulary is used precisely and without over-explanation (“coagulant”, “floc”, “sedimentation tank”, “clean-water reservoir”), and the response varies its reporting verbs (“drawn”, “passed through”, “added”, “flows”, “trap”, “pumped”) rather than repeating a single generic verb.

Grammatical Range & Accuracy

The passive voice is applied consistently and accurately throughout, since no single human agent performs these industrial stages (“is drawn”, “are added”, “is disinfected”, “is distributed”), and relative and participle clauses (“causing the small suspended particles… to clump together”, “leaving clearer water above it”) add grammatical range without error.

Common pitfalls specific to this task

See commonMistakes above: the most damaging is merging sedimentation and filtration into one stage, since the diagram clearly separates settling in a tank from passing through a sand and gravel filter bed, and collapsing the two loses a full stage of the process.