Chem 3 - Water - Water Treatment

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Water Treatment

Water is a vital and useful resource. It would be impossible to describe all of the ways in which people all around the world use water. Water is an important raw material, which we use in many industrial processes and as a solvent too. Many substances react with water to form solutions which are either acidic or alkaline, and these too have many industrial uses.

Other uses of water worldwide are for washing and cleaning, and of course for drinking. Providing people with clean drinking water which is uncontaminated by disease, sewage or chemicals, is a major issue all over the world.

Water which comes from boreholes is usually fairly clean because it has been filtered as it travels through the rocks around the borehole. Normally we just need to disinfect water like this with Chlorine to make it safe to drink.

When we take water from rivers and reservoirs we usually need to give it more treatment than this. This treatment involves a number of physical and chemical processes. To start with, the water source is chosen so that it contains as few dissolved chemicals as possible. The water then passes through the 5 stages.

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Water Treatment 2

The 5 stages:

- As the water enters the water treatment works, it passes through a screen. This is made from the bars of metal placed close together. These catch large objects such as leaves and twigs.

- Aluminium sulfate and lime are added to the water. Small particles of dirt clump together so that they sink to the bottom of the water. The sludge that collects like this is dumped in a landfill where it forms mud.

- The water is passed through a special filter made of fine sand. This removes any remaining particles of mud or grit, so the water is sparkling clean.

- Although the water now looks clean, it may contain bacteria that is harmful. A small amount of chlorine is added to the water to kill any bacteria in the water.

- The pH of the water is checked and corrected so that it is neutral. It is then stored in large tanks and service reservoirs ready to be pumped to homes, schools, offices and factories etc.

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Water Treatment 3

Some people use filter jugs in their homes. These usually have a top part into which you put tap water.

As the water goes from the top part of the jug to the lower part, it passes through a filter cartridge. This normally contains activated Carbon, and ion-exchange resin, and silver.

- The Carbon in the filter reduces the levels of Chlorine, Pesticides and other organic impurities in the water

- The ion-exchange resin removes Calcium, Magnesium, Lead, Copper and Aluminium ions.

- Some filter cartridges may contain Silver, which discourages the growth of bacteria within the filter.

In most jugs, the filter cartridge needs to be changed every few weeks.

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Water Treatment 4

Even water that has been treated and then passed through a jug filter is not pure. It will still contain many substances dissolved in it. But despite this, it is definitely fit to drink. We can get pure water by distilling it - turing impure water into steam an then condensing it - or de ionising it.

We use distilled water in chemistry practical work because it is pure, and so contains nothing but water. But with nothing else in it, distilled water is not very interesting to drink!

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