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Reading passage Grade 6 English Language Arts

The Microbes at Windlass Bay: A Grade 6 Reading Passage

A grade 6 informational passage on the living microbes that clean wastewater at a fictional plant, with seven comprehension questions and a full answer key.

Objectives

  1. Students will identify the stated role of microbes in cleaning wastewater at a treatment plant.
  2. Students will infer why an operator manages the number of microbes rather than only the machines.
  3. Students will cite two details from the passage to explain a consequence the passage does not state directly.

Vocabulary

Floc
Floc is the fluffy clumps of dirt and treatment chemicals that form in the water during cleaning at a treatment plant. Floc looks like tiny snowflakes drifting in the water.
Clarifier
A clarifier is a big round or rectangular tank where water sits calmly so solids can sink to the bottom and be scraped away. Treatment plants use clarifiers at more than one stage of cleaning.

Also: microbe, aeration tank, return sludge, bulking

Teacher overview

Wastewater treatment is one of the few places in daily life where a public utility keeps a population of living organisms alive on purpose, which is the part of the process a description of tanks and pumps leaves out. This passage follows the microbes at the fictional Windlass Bay Water Reclamation Plant, explains why operators feed them air and return some of them to the tank, and shows what happens when the population is damaged. Teachers may want to add the regulatory context the passage leaves out: under the EPA secondary treatment regulation, a plant's monthly average for BOD and suspended solids must reach 30 mg/L or less, which is roughly 85 percent removal.

The Microbes at Windlass Bay: A Grade 6 Reading Passage — Grade 6 English Language Arts
Name: ____________________________ Date: ______________

Passage

The Windlass Bay Water Reclamation Plant sits at the edge of a fictional coastal town. Every drain in town leads to it. Water arrives carrying soap, food scraps, and human waste. The plant has to take those things out before the water goes back to the bay. Visitors often expect machines to do that work. Machines move the water and stir it, yet the cleaning itself is done by living things.

The heart of the plant is a long tank called an aeration tank. The water there looks brown and it churns like a pot on a stove. Billions of tiny living things live in that water. Most are bacteria, and some are larger single-celled organisms called protozoa. Together they are known as microbes. The waste in the water is food to them. As they eat, they take the waste out of the water.

Microbes need air the way people do. Blowers push air through pipes in the floor of the tank. The air rises as fine bubbles, and the bubbles are what make the tank churn. Without that air, the useful microbes would die off within hours. Operators watch the air supply as closely as they watch any pump.

As the microbes eat, they stick to one another. They form small brown flakes called floc. Floc is heavy enough to sink, which is the whole point. The water flows on into a wide, quiet round tank called a clarifier. Nothing stirs there. The floc drifts down to the sloped bottom, and clear water slides off the top edge.

Some of that settled floc is pumped straight back to the aeration tank. Operators call it return sludge. Returning it keeps the number of microbes steady. If too many microbes are removed, the tank cleans poorly. If too few are removed, the tank grows crowded and the floc stops settling well. Deciding how much to return and how much to take away is a daily judgment call.

Operators also look at the microbes under a microscope. Shapes tell them how the tank is doing. Stalked protozoa waving in the water are a good sign. Thin threads that stretch between flakes are a warning. When threads take over, they hold the flakes apart, and the floc stays light and settles too slowly. Operators call that problem bulking, and it can send solids out of the plant with the clean water.

The population needs steady food, steady air, and mild temperatures. Cold weather slows the microbes down, so cleaning takes longer in winter than in summer. A strong chemical poured into a sewer can kill a large share of them at once. Rebuilding the population can take weeks, and the water leaving the plant is dirtier the whole time. That is the reason towns ask people to keep paint, solvent, and drain cleaner out of the sewer.

Operators at Windlass Bay describe their job as keeping a population alive rather than running a machine. They feed it, watch its health, and hold its conditions steady day after day. When those conditions hold, the water that leaves the plant is clean enough to return to the bay.

Questions

  1. According to the passage, what do the microbes in the aeration tank use as food?
  2. Why do blowers push air through pipes in the floor of the aeration tank?
  3. What is floc, and why does it matter that floc is heavy?
  4. What happens to the floc after the water reaches the clarifier?
  5. Why do operators pump some of the settled floc back into the aeration tank?
  6. The passage says thin threads between flakes are a warning sign. Explain why, using the passage.
  7. Explain why pouring drain cleaner into a sewer could hurt the plant. Use two details from the passage.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: The waste in the water, including soap, food scraps, and human waste.
    The passage states that the waste arriving from the town's drains is food to the microbes, and that they take it out of the water as they eat.
  2. Answer: Because the microbes need air to live, and without it the useful ones would die off within hours.
    The passage adds that the rising bubbles are also what make the tank churn.
  3. Answer: Floc is the small brown flakes microbes form when they stick together, and it matters because floc is heavy enough to sink.
    Sinking is how the solids are separated from the water in the next tank.
  4. Answer: The floc drifts down to the sloped bottom of the clarifier while clear water slides off the top edge.
    The passage notes that nothing stirs the clarifier, which is why settling can happen there.
  5. Answer: To keep the number of microbes in the tank steady.
    The passage explains that removing too many leaves the tank cleaning poorly, while removing too few leaves it crowded so the floc stops settling well.
  6. Answer: Because threads hold the flakes apart so the floc settles too slowly, a problem the passage calls bulking.
    Students should connect floc that does not settle to solids leaving the plant with the clean water.
  7. Answer: Accept any answer that uses two details, such as the statement that a strong chemical can kill a large share of the microbes at once and the statement that rebuilding the population can take weeks.
    Strong answers add that the water leaving the plant is dirtier during that whole rebuilding period.

Differentiation

Support

Provide a labeled flow drawing of the four stages named in the passage, which are the arriving water, the aeration tank, the clarifier, and the return line, and let students point to the stage before answering questions 2 through 5.

Extension

Ask students to write a short set of instructions a plant could mail to residents, using three details from the passage to explain why certain products must not go down a drain.

ELL supports

Pre-teach "microbe," "aeration," "floc," and "clarifier" with a picture for each and a one-line definition. Allow students to answer questions 1 through 4 with short phrases, and provide the frame "The threads are a warning because ___." for question 6.

Extensions

Have students compare this passage with any drinking water treatment text the class has read, then list two jobs done by living things here that are done by chemicals or filters there.