Stop and wonder

Did you know that?

• Salt in water is a result of the rocks being eroded and dissolved in water for millions of years.
• The rivers carry about 4 billion of tons of salt every year at sea.
• Salinity[/glossary] of water depends on the location? It is higher in tropical areas (due to intense evaporation) and lower in the poles due to ice melting and rainfalls; it is relatively low near the continents due to freshwater springs.
• The Mediterranean is considered a rather “salty sea” with high salinity.
• Although desalination is an energy and cost demanding technology, it provides a reliable supply of water regardless of the weather conditions or season (i.e. during high demand tourist/summer seasons) for water scarce areas.
• The Mediterranean region holds about 17% of global seawater desalination capacity.
• Desalinated water can supplement municipal water supplies and it is used also in industry and irrigation.

Solo activity

From salt to fresh water

1. Collect about 100 mL of seawater or brackish water.
2. Add a small quantity of this water to a Petri dish (see the image) or any shallow glass container. Make sure that the dish is clear!
3. Place the dish in a sunny spot for  2-3 days to let water evaporate.
4. What do you observe on the dish ?

Water-readings

Reverse Osmosis

The basic steps followed in a Reverse Osmosis (desalination) unit to produce freshwater from sea (or brakish) water are the following:

1. Intake: Seawater is transferred to the plant through pipes, and thus the units are built near the coast.
2. Pre-treatment: During pre-treatment water is filtered to remove debris, floating particles, etc.
3. Salt removal process: The saltwater passes through a series of semi-permeable membranes with high pressure, and due to “reverse osmosis” the salts are retained on the membranes.
4. Post-treatment: Desalinated water is further treated to avoid infection by microorganisms (disinfection) and to improve its taste. In some cases, it is mixed with freshwater (groundwater).
5. The desalinated water enters the water supply network. Usually, the volume of the desalinated water is about half the initial (intake) volume.
6. The remaining water of high salinity /brine is discharged back to the sea. Before that it operates (turns) an energy turbine, and this way, it reclaims some of the energy consumed in the desalination unit.

Group activity

Visiting a desalination plant

Organise a visit to a desalination plant to collect information  about the processes followed.

Before your visit

• Observe on the map the area where the plant is located. Why do you think the particular area was chosen?

During your visit

• Keep notes and take photos of all steps/processes.
• Ask the staff what they do with the resulting brine.
• Ask the staff what is main energy resource used for the plant’s function.
• Ask the staff what if the plant applies any measures to reduce its climate footprint.

Way forward

Dive in…

if you want to know about other non-conventional water resources.

or go back to the homepage!