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From seawater to drinking water: How desalination works

About this lesson

Students will learn all about desalination, the different processes and conduct their own experiment to discover first hand how solar desalination works.


Year level: 7

Theme: Water supply


Learning objectives

Students can:
  • describe what desalination is
  • explain the process of desalination
  • make a solar desalination model.

Curriculum links

Science

  • Mixtures, including solutions, contain a combination of pure substances that can be separated using a range of techniques ACSSU113
  • Collaboratively and individually plan and conduct a range of investigation types, including fieldwork and experiments, ensuring safety and ethical guidelines are followed ACSIS125 

Sustainability

  • OI.8 – Designing action for sustainability requires an evaluation of past practices, the assessment of scientific and technological developments, and balanced judgements based on projected future economic, social and environmental impacts.

Things you will need

Lesson description

  • Using the activity sheet have students investigate how desalination works:
    • Pour water into the ice-cream container and stir in the salt.
    • Put the small container in the middle of the ice-cream container.
    • Place a plastic sheet over the ice-cream container and fix it to the sides with sticky tape.
    • Place the weight on top of the plastic sheet (in the middle) and place in the sun for 10 minutes and see what happens. Check again after 30 minutes, 1 hour and 2 hours.
  • Have students make a table to record their observations for each of the various times. What differences do they observe?
  • View the video Crystal clear – the story of desalination (12:14).

Reflect & summarise

  • Revisit what desalination is and have students write a paragraph in their own words about the desalination process.
  • Each group reports to the class on what happened in their experiment.

Extension activities

  • Research the history of desalination (including the different desalination processes used over time). Students could make up a timeline to show the history, and find out how different desalination processes work.
  • Have students research information about the Perth Seawater Desalination Plant (e.g. where it is located, the desalination process used, and associated environmental and health aspects). They could then write a report using some of these points as headings.
  • Book a desalination school talk for your class.
  • Learn about the desalination treatment process in The water cycle e-Learning activity.

Teacher background information

Desalination

There are different types of desalination. The main processes are evaporative (thermal) desalination, which is used in this experiment and reverse osmosis (pressure) desalination, which is used in the Perth Seawater Desalination Plant and Southern Seawater Desalination Plant.

Perth Seawater Desalination Plant

In response to our drying climate, Water Corporation built a seawater desalination plant on the coast near Kwinana in 2006. This has now become an important part of the Integrated Water Supply Scheme. The plant treats water from the ocean to produce freshwater to supplement our existing water supply. The by-product of the treatment process (a salty concentrate) is returned to the ocean. Safeguards have been put in place to make sure that the environment is not adversely affected by the plant’s discharge into Cockburn Sound.
Energy for the plant is purchased from the Emu Downs Wind Farm. Reverse osmosis technology was chosen for Perth’s desalination plant because it is the most economical and environmentally friendly process.

The reverse osmosis process uses high pressure and a very fine membrane to extract fresh water from salty water. The membrane acts like a strainer, allowing water molecules to pass through and retaining salt and other impurities. Small desalination plants on fishing boats, yachts and large cruise ships and in some households use reverse osmosis and the process has been used for many years to provide fresh drinking water to the remote towns of Denham and Ravensthorpe.

Southern Seawater Desalination Plant

Located in Binningup in the South West, this plant is capable of producing 100 billion litres of drinking water a year – that’s 1/3 of Perth’s water supply!

The plant’s energy requirements are offset by the entire output from 2 renewable energy farms near Geraldton. Water Corporation purchase 100% of the output from these renewable energy sources.

Resources

Take a virtual tour

Did you know?

The Perth Seawater Desalination Plant, located in Kwinana, was Australia’s first large-scale seawater desalination plant.

Key vocabulary

  • Desalination: Several processes that remove salt and minerals from saline water 
  • Fresh water: Water with a low salt content: generally less than 1000mg per litre
  • Osmosis: Circulation of molecules through a semipermeable membrane from a place of high concentration to a place of low concentration until the concentration on both sides is equal
  • Reverse osmosis: A water purifying technique where pressure is applied to force liquid through a semipermeable membrane in the opposite direction to normal osmosis
  • Solar desalination: A technique for desalinating water using solar energy