C3 2.3 Water Treatment

Learning Objectives To discuss how we make drinking water safe to drink. To identify the differences between pure water and water that is fit to drink. To deconstruct and explain the process of water treatment. To evaluate methods of water treatment.

Recap Activity Hardwater Scale Soap Heating Calcium ions Scum Soften Make an explanatory sentence by choosing a pair of words from below and linking them together. (use as much chemistry as you can) Do this twice! Hardwater Scale Soap Heating Calcium ions Scum Soften

Borehole Water The borehole is a common technique for water extraction. Soil Rock Aquifer Aquifer water has been naturally filtered by porous rocks.

Borehole Water Sterilisation Sterilisation can be performed by treating the water with chlorine. Borehole water still requires sterilisation to remove any microbes.

River & Reservoir Water Surface water from rivers and reservoirs requires additional treatment to filter out particulates. This treatment involves several chemical and physical processes.

Water Treatment Process 1. Upon exiting the river or reservoir, water passes through a series of grated bars to remove any large objects such as leaves and twigs. River/Reservoir 1 2. The water then enters a settlement tank – sand and soil settle out. 2

3. Aluminium sulfate and lime are added to the water 3. Aluminium sulfate and lime are added to the water. Small particles of dirt clump together and sink to the bottom. It forms a sludge which is taken to landfill. 3 Al2(SO4)3 CaO Al2(SO4)3 CaO 4. The water is passed through a filter made of fine sand. This removes any remaining particles of mud or grit, so the water is clear. 4 5. Although the water may look clean, it could still contain deadly microbes. It is therefore sterilised with small amounts of chlorine. 5 Cl2 Cl2 Cl2

6 7 7. The water is stored in large tanks until it is needed. 6. The pH of the water is checked and corrected to ensure neutrality. 8 8. The water is pumped to homes, schools, offices and factories etc.

Filter Jugs The top section of a filter jug contains a filter cartridge. The cartridge contains activated carbon, an ion exchange resin and silver. The activated carbon in the filter reduces the levels of chlorine, pesticides and other organic impurities. The ion exchange resin removes calcium, magnesium, lead, copper and aluminium ions. Some filters may contain silver, this discourages the growth of bacteria inside the filter. Q. What are the advantages and disadvantages of filter jugs? Draw a table, make bullet point answers, use chemistry! 5 min

“Pure” or “fit to drink”? Pure water contains only water molecules. Water can only be fully purified by distillation. Chemists use distilled water in the lab because of its purity. This is a water distillation apparatus. Our technicians have one operating regularly if you want to see it.

“Pure” or “fit to drink”? Water “purified” by filter jugs is not actually pure. It still contains some dissolved ions. Some water companies market bottled water on the fact that it is impure. E.g. it is full of ions that supposedly add to the flavour – mineral water.

Why don’t we purify all water? To purify water completely it must be distilled. This means boiling it, then condensing the steam back to liquid water. To boil the water a lot of energy is required. Also, if we drank pure water all the time, our bodies may become deficient in certain minerals - (metal ions).

Desalinisation UAE is mostly desert, they have to distil sea-water as a water supply. This is remove the dissolved sodium chloride (salt). The process of removing salt is called desalinisation. Desalinisation uses vast amounts of energy. Why do you think the Saudis are not too bothered about that fact? Is it sustainable?

Summary Water for drinking should only contain low levels of dissolved substances and microbes. Water is made fit to drink by filtering it to remove solids and adding chlorine to reduce the number of microbes. We can make pure water by distillation but this requires large amounts of energy which makes it expensive.