Desalination of Seawater

The question

How to approach this process diagram question

A process diagram shows how something is made or how a cycle works. Describe the stages in order, using sequence language and the passive voice.

1. Say what process the diagram shows and how many stages it has.
2. Overview: the number of stages, where it starts and ends, and whether it is a cycle or a straight line.
3. Describe every stage in order (First, Next, After that, Finally) mostly in the passive: the grain is crushed.
4. Do not add opinions or explain why; describe what happens.

How to plan this answer

Structure

1. Overview: say there are six stages in a single row, starting with seawater intake and ending with homes.
2. Paragraph 2: describe the early stages (intake, filtering and pumping) in order, using passive verbs such as is drawn in and is passed through.
3. Paragraph 3: describe the later stages (membrane, freshwater tank and delivery to homes), noting that the water is stored before it reaches consumers.
4. A short closing sentence summarising that the process moves in one direction is sufficient.

Key features to spot

• Stage 1 is a seawater intake structure at the shoreline, which draws water in from the sea before any treatment begins.
• Stage 2 is a filter, a cylindrical vessel with a perforated inner layer, which removes solid particles from the water.
• Stage 3 is a pump with a motor, which pressurises the water so that it can be forced through the membranes.
• Stage 4 shows three membrane units arranged in parallel, which separate the salt from the water.
• Stage 5 is a fresh water tank, shown as a large cylinder with a ladder, where treated water is stored.
• Stage 6 is homes, which means the final output is water for domestic use rather than an industrial product.

Useful vocabulary

• is drawn in from: Seawater is drawn in from the sea through an intake structure.
• is passed through: The water is passed through a filter to remove solid particles.
• is pressurised by: The water is pressurised by a pump before it reaches the membranes.
• is forced through: The pressurised water is forced through the membranes, which remove the salt.
• is stored in: Treated water is stored in a large tank before distribution.
• is then piped to: The fresh water is then piped to homes.
• in a single line: The six stages are arranged in a single line from intake to homes.
• the first stage: The first stage is the seawater intake, located at the coast.
• before it reaches: The water is stored before it reaches the consumers.
• the process begins with: The process begins with the collection of seawater from the sea.

Common mistakes

• Calling the membrane stage a filter, or merging stages 2 and 4 into one description, which loses the two-step treatment shown in the diagram.
• Stating that the pump removes salt; the diagram shows the pump moving the water, while the membrane is the stage that separates the salt.
• Omitting the freshwater tank because it appears to be only storage, although it is a distinct stage with its own position in the sequence.
• Giving the number of membranes as a different figure from the three shown.
• Describing the homes stage as a 'consumer' or 'customer' stage, rather than simply stating that water reaches homes.

Examiner tip: Use the passive voice for each treatment step and keep the six stages in their numbered order; a single clear paragraph for the early stages and one for the later stages is enough.