Science · Level 2 · 178 words

How Water Is Made Safe

Original passage © Studio AM, written for Fluency.

Water leaving a reservoir is not yet drinkable. It carries clay, leaves, and living things too small to see. Between the reservoir and the tap sits a treatment plant, and its work happens in a fixed order.

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. Stillness does most of the cleaning here, and stillness is free.

Next comes the filter, usually a deep bed of sand and gravel. Water passes down through it and leaves behind whatever the settling missed.

The water is now clear. Clear is not the same as safe. Some bacteria are small enough to pass through sand, so the last step kills them, with chlorine or with ultraviolet light.

Each step is cheap because the step before it did its share. Send raw river water straight to a filter and the filter clogs within an hour.

Comprehension questions

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4 questions
1. What is the main point of the passage?

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C. A treatment plant cleans water in a fixed order, each step preparing the next.
Each paragraph is one step, and the last paragraph explains why the steps cannot be reordered. The order is the point, not any single step.

2. According to the passage, what makes the tiny particles sink?

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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.

3. In this passage, "settle" means

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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.

4. Why would a filter clog within an hour if it were fed raw river water?

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D. The settling step normally removes most of the material first.
The passage says each step is cheap because the one before did its share. Without settling, everything that would have sunk arrives at the filter instead.

Source: Written for Fluency. Original passage © Studio AM, written for Fluency.