Wastewater Treatment

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: state that wastewater is treated in seven stages, starting with screening and ending in the river, across two rows.
2. Paragraph 2: describe the first four stages (screening, grit removal, settling and aeration), noting what is removed at each point.
3. Paragraph 3: describe the last three stages (second settling, disinfection and discharge to the river).
4. End with a sentence that makes clear the treated water is returned to the river at the end of the process.

Key features to spot

• Stage 1 is screening, where a set of inclined metal bars catches leaves, sticks and other large objects before the water moves on.
• Stage 2 is grit removal in a shallow channel, where sand and gravel settle to the bottom and are collected.
• Stage 3 is a circular settling tank with a rotating scraper, where solid matter sinks and is removed.
• Stage 4 is aeration, where air bubbles are released from diffusers in a rectangular tank to support the breakdown of waste.
• Stage 5 is a second circular settling tank, which removes the remaining solids before disinfection.
• Stage 6 is disinfection, shown as three spray heads that spray the water, and stage 7 is the river, which receives the treated water.

Useful vocabulary

• is screened to: The wastewater is screened to remove large solid objects.
• is allowed to settle: The water is allowed to settle in a circular tank so that solids sink.
• are removed from: Grit and sand are removed from the channel by a scraper.
• is aerated in: The water is aerated in a tank before the second settling stage.
• is treated with: The water is treated with disinfectant before it is released.
• is discharged into: The treated water is discharged into the river.
• is then passed to: After screening, the water is then passed to the grit channel.
• the final stage: The final stage is the release of treated water into the river.
• before it is returned to: The water is treated before it is returned to the river.
• is followed by: Aeration is followed by a second settling stage.

Common mistakes

• Describing both settling tanks as identical stages, when the task requires them to be listed separately as stages 3 and 5.
• Calling the aeration tank a 'mixing tank' rather than naming the bubbles that come from the diffusers.
• Stating that the river is a treatment stage; it is the final destination, and the overview should say so.
• Missing the second row and therefore failing to show that the process continues from stage 4 down to stage 5.
• Explaining the chemistry of disinfection in detail, which goes beyond what the diagram shows and wastes words.

Examiner tip: Name the first and last stages in the overview and describe each tank by its shape and function so that the two settling stages are clearly distinguished.