Natural Sciences and Technology Grade 6-A - Thunderbolt Kids
Natural Sciences and Technology Grade 6-A - Thunderbolt Kids Natural Sciences and Technology Grade 6-A - Thunderbolt Kids
There are five steps (or processes) in the treatment of water. The five processes commonly used to treat water are screening, sedimentation, aeration, settling, and disinfection. Let us look at them in turn. STEP 1: SCREENING The raw water that arrives at the plant may contain dirt, fish, rubbish, plants and even sewage. These things are screened out as the water flows into the plant. This means that the water passes through a screen (which is very much like a large sieve), and the solid matter stays behind on the screen. After the screening step the water is still dirty, but the large pieces of rubbish have been removed. STEP 2: SETTLING During this step the raw water is allowed to stand in a large tank called a settling tank. QUESTIONS What happens to the dirt when muddy water stands for a long time? When the raw water stands in the settling tank, the medium-sized pieces of solid matter (called sludge) sink to the bottom of the tank. The raw water at the top is still dirty but now it only contains small pieces of solid matter. The bits of solid matter left are small enough for small organisms (such as bacteria) to eat. This is what happens in the next step of the treatment process. 200 Matter and Materials
STEP 3: AERATION The raw water now flows into a special tank that contains bacteria. These are useful bacteria, because they help to break down the last little bits of solid matter as well as breaking down any natural soluble pollutants. Since bacteria need oxygen to stay alive and healthy, air is bubbled through the water. This process is called aeration, because the name comes from the word 'air'. STEP 4: FILTERING Next, the water flows through a special filter made of layers of sand and gravel, just like the one you designed, only much bigger. The gravel layer of the filter is about 30 cm deep and the sand layer is about 1mdeep! The filtering step removes any remaining particles and most of the bacteria left in the water. After this step the water is clear, but some germs and bacteria from STEP 3 may still be in the water. Remember that germs and bacteria are small enough to pass between the gaps between sand and gravel. STEP 5: DISINFECTION During disinfection, chemicals are added to the water to kill any surviving germs. KEY CONCEPTS • Clean water is important for people, plants and animals. • Water can be cleaned by processes such as sieving, filtering, settling, decanting, boiling and adding chemicals to kill germs. • The water we use in our homes is cleaned before and after we use it. Chapter 6. Processes to purify water 201
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There are five steps (or processes) in the treatment of water. The<br />
five processes commonly used to treat water are screening,<br />
sedimentation, aeration, settling, <strong>and</strong> disinfection.<br />
Let us look at them in turn.<br />
STEP 1: SCREENING<br />
The raw water that arrives at the plant may contain dirt, fish,<br />
rubbish, plants <strong>and</strong> even sewage.<br />
These things are screened out as the water flows into the plant.<br />
This means that the water passes through a screen (which is very<br />
much like a large sieve), <strong>and</strong> the solid matter stays behind on the<br />
screen.<br />
After the screening step the water is still dirty, but the large pieces<br />
of rubbish have been removed.<br />
STEP 2: SETTLING<br />
During this step the raw water is allowed to st<strong>and</strong> in a large tank<br />
called a settling tank.<br />
QUESTIONS<br />
What happens to the dirt when muddy water st<strong>and</strong>s for a long<br />
time?<br />
When the raw water st<strong>and</strong>s in the settling tank, the medium-sized<br />
pieces of solid matter (called sludge) sink to the bottom of the<br />
tank.<br />
The raw water at the top is still dirty but now it only contains small<br />
pieces of solid matter. The bits of solid matter left are small<br />
enough for small organisms (such as bacteria) to eat. This is what<br />
happens in the next step of the treatment process.<br />
200 Matter <strong>and</strong> Materials