Education: water purification
The survival of life on Earth is dependent on a precious resource
WITHOUT water people, animals and plants can’t survive. We have a variety of water sources: rivers, dams, streams and even the ocean. But the water often isn’t clean enough to drink as many things can dissolve or float in it. That’s why it easily becomes contaminated, brackish or undrinkable. So where does the clean drinking water in our taps come from?
Nature has ways of purifying water, such as wetlands that act as natural purifiers. Though wetlands are effective, water purification in them happens too slowly to provide fresh water for everyone on Earth.
That’s why humans have devised ways of purification, a process that happens in water-treatment plants.
THERE ARE FIVE STEPS IN WATER PURIFICATION
1 Screening Water flows through a large screen or sieve to remove solid objects such as rocks, fish, trash, plants and even raw sewage. 2 Binding and sedimentation Water flows into large tanks where chemicals are added to cause coagulation and flocculation (binding processes) which cause dirt particles to stick together and become heavy. All medium-size particles and solid materials (silt) then sink to the bottom (a process called sedimentation). 3 Aeration Any remaining solid particles are now small enough to be eaten by bacteria. Bacteria need oxygen to thrive and that’s why air is now pumped through the water. Many water-treatment plants use chemicals instead of bacteria for this step. 4 Filtration Next the water flows through a special filter made of layers of sand (each about 1m thick) and gravel (30cm thick) to remove any remaining microscopic particles and bacteria. But bacteria and germs are tiny organisms so there might still be some of these left in the water. 5 Disinfection In the final step, chemicals such as chlorine, which are safe for humans, are added to kill any remaining germs.
DESALINATION
About 70% of Earth’s surface is covered in water but only 3% of this is fresh – the rest is salty seawater. Scientists have done a lot of research into how to remove salt from sea- and brackish water so people and animals can drink it.
But desalination – as this process is known – is expensive, so we can’t use it to meet all our water needs.
One method of desalination is called thermal distillation. Seawater is heated and pumped into low-pressure tanks. The water quickly evaporates and becomes steam, while the salt stays behind. When the steam condenses again it’s channelled away as fresh water.
Another desalination method is to freeze seawater. Fresh and salt water freeze at different temperatures. When the fresh water in seawater freezes and forms ice crystals, the crystals are removed from the salt water (which is still liquid because it freezes at a lower temperature).
The saltiness on the outside of the ice is rinsed off and when the ice melts it’s fresh water.
Electrodialysis can be used to desalinate brackish water (but not seawater, as that contains too much salt).
An electric current is sent through the water, breaking up the salt into positive and negative ions. These ions are then removed in a process called ion exchange, leaving the water saltless.
One of the main desalination methods is reverse osmosis. Water is passed at high pressure through a thin membrane that lets fresh water through but stops minerals such as salt from passing through. A good reverse osmosis system can remove nitrates, sodium, copper, lead, organic chemicals and fluoride from water.