How Water Is Made Safe
Written by Studio AM.
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Water taken from a reservoir may need treatment before it is safe to drink. It carries clay, leaves, and living things too small to see. Between the reservoir and the tap sits a treatment plant, and many plants use the following sequence.
First the water is made to drop what it carries. A chemical is stirred in that makes the tiny particles stick to one another in clumps heavy enough to sink. The water then sits still in a wide tank while those clumps settle to the bottom. This removes much of the suspended material before filtration.
Next comes the filter, usually a deep bed of sand and gravel. Water passes down through it and traps many particles that settling missed.
The water is now clear. Clear is not the same as safe. Some bacteria are small enough to pass through sand, so a disinfection step treats remaining germs, using methods such as chlorine or ultraviolet light.
Removing particles before filtration helps prevent filters from clogging too quickly. The exact treatment depends on the water source, and operators test the water to check that the process is working.
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Questions
Choose an answer. The explanation appears after you answer.
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Question 1 of 4
What is the main point of the passage?
The answer is C: A common treatment sequence uses several steps that support one another.
The passage follows settling, filtration, and disinfection, while noting that the exact treatment depends on the source.
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Question 2 of 4
According to the passage, what makes the tiny particles sink?
The answer is A: A chemical stirred in that makes them clump together.
The second paragraph says a chemical is stirred in that makes the particles stick to one another in clumps heavy enough to sink. Chlorine comes last, and heat is never mentioned.
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Question 3 of 4
In this passage, "settle" means
The answer is B: sink slowly to the bottom and stay there
The clumps are heavy enough to sink, and they settle while the water sits still in a tank. The everyday senses of the word do not fit what is happening.
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Question 4 of 4
Why can removing particles before filtration help a filter last longer?
The answer is D: The settling step normally removes most of the material first.
The settling step removes material that would otherwise reach and potentially clog the filter.
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