What is the passage mainly about?

Wastewater is cleaned mostly by bacteria and then returned to a river others will drink from. (The passage names the method, which is a fed population of microbes, and then the reason it must work, which is that the water goes downstream to somebody's reservoir.)

According to the passage, what can the screens and settling tanks not remove?

Whatever has dissolved in the water. (The second paragraph says they take out the solids but cannot touch what has dissolved. That limit is what forces the next step.)

In this passage, material that is "dissolved" is

mixed into the water so completely that it cannot be strained out (The word is defined by what defeats it: screens and settling tanks cannot touch it, so it is not floating, sitting, or catchable.)

Why does the passage say the standard is "set by the people who will drink it"?

The river carries the treated water into another city's supply. (The water leaves into a river, and downstream another city's reservoir is filling. What one plant releases becomes the next city's raw supply.)

After the Drain: Level 3 Reading Comprehension Questions

Half of a water system runs the other way. Everything that leaves a sink, a shower, or a toilet enters a second network of pipes, sloping downhill toward a plant on the low edge of the city.

What arrives is mostly water. The trouble is what is dissolved in it. Screens and settling tanks take out the solids in the first hour, much as a drinking-water plant does, but they cannot touch what has dissolved.

For that, the plant hires bacteria. Air is blown up through a long tank of wastewater, and a dense population of microbes feeds on the dissolved waste, turning it into more microbes. The plant is not a machine that strips the waste out; it is a place where something is kept alive and fed well. The living sludge is then left to settle, and most of it is pumped back to the head of the tank to keep the population strong.

The water that leaves goes into a river. Somewhere downstream, another city's reservoir is filling. That is the part of the loop nobody sees, and it explains why the standard for what leaves a plant is set by the people who will drink it.

Comprehension questions