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Reading passage Grade 9 English Language Arts

Knowing When to Backwash a Filter at the Tarnwell Plant

A grade 9 informational passage following a fictional plant operator's decision about when to backwash a filter, with literal, inferential, and procedure questions.

Objectives

  1. Students will explain how headloss and turbidity together signal that a filter needs backwashing.
  2. Students will follow the passage's backwash procedure and apply its stated exception.
  3. Students will write a short informative paragraph explaining a technical decision to a new reader.

Vocabulary

Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
Backwash
Backwash is when a treatment plant reverses the flow of water through a filter to flush out the dirt and particles it has trapped. It is like rinsing out a dirty sponge so the filter can keep working well.

Also: headloss, filter run, ripening

Teacher overview

Filtration is easy to picture as a screen that either works or clogs, which leaves out the judgment an operator actually has to make. This original passage follows a fictional operator through one shift at an imagined plant, where the real question is when to stop the filter and wash it out. The passage teaches two competing signals, headloss and turbidity, and gives the exception that overrides the normal schedule. A procedure question and a short writing question let students both follow the steps and explain the reasoning in their own words.

Knowing When to Backwash a Filter at the Tarnwell Plant — Grade 9 English Language Arts
Name: ____________________________ Date: ______________

Passage

Marisol Verne starts every shift at the Tarnwell Filtration Plant the same way. She reads four numbers off the panel for each of the plant's six filters, writes them on a clipboard, and then walks out to look at the filters themselves. The panel readings are the plant's written record, and the walk gives her what the panel cannot show.

A filter here is a concrete box about the size of a classroom. Inside it, water passes down through a layer of anthracite coal and a layer of fine sand. That water has already been through coagulation and settling, so most of the dirt is gone before it ever arrives. The filter catches what is left. Particles wedge into the narrow spaces between grains, and clean water leaves through pipes in the floor.

The filter keeps every particle it catches. Over hours, the spaces between the grains fill in. Water has to push harder to get through, and the plant measures that resistance as headloss. Headloss is simply the difference in water level above and below the filter bed. When it climbs past the design limit, the bed is effectively full.

Turbidity is the second signal. Turbidity measures how cloudy water is, and a meter on each filter's outlet reports it continuously. Federal rules for plants like Tarnwell require finished water to stay at or below 0.3 NTU in 95 percent of monthly samples, and never above 1 NTU. Tarnwell holds itself to a tighter internal target, because waiting for the legal limit leaves no margin for a reading that is still climbing.

Verne explains the two signals to trainees as a race. Usually headloss wins. The filter fills up, the resistance climbs, the run ends, and nothing ever appears in the treated water. Sometimes, though, turbidity wins instead. A filter that has been running a long time can begin to release particles it caught earlier, and the outlet reading starts to creep up while headloss still looks fine. Operators call that breakthrough, and it is the situation the rules are written for.

The backwash itself reverses the flow. Clean water is pumped up through the floor of the filter, so the bed lifts and the grains tumble against one another. The trapped dirt washes free and leaves over a trough at the top. Air is often blown in first to loosen the bed. Then the water rate is raised, held, and lowered gradually, because a sudden stop can pack the bed unevenly.

There is a cost to washing a filter, which is why Verne does not simply wash them all every morning. A backwash uses treated water the plant has already paid to clean. It also puts the filter through a ripening period. For the first minutes after a wash, a clean bed actually filters slightly worse than a partly loaded one, since the fine spaces that do the work have just been rinsed open. Good plants send that first water to waste rather than to town.

The decision follows a set order. Verne backwashes when headloss reaches the design limit, or when the filter run reaches the maximum hours the plant allows, whichever comes first. There is one exception, and it overrides both. If the outlet turbidity climbs above the internal target, she backwashes immediately, no matter what the headloss gauge says and no matter how many hours are left in the run. A filter that is releasing what it caught is no longer doing its job.

On this particular morning, filter three shows headloss well under its limit and eleven hours left in its allowed run. Its outlet turbidity, though, has climbed for forty minutes straight and has just passed the internal target. Verne does not check the schedule. She calls the control room, takes filter three out of service, and starts the wash.

Her clipboard note is four words long: early wash, turbidity rising. A supervisor reading it next week will know exactly what happened and why, which is the other half of an operator's job.

Questions

  1. What two signals does Verne watch to decide when a filter needs backwashing, and what does each one measure?
  2. According to the passage, what turbidity limits do federal rules set for finished water at plants like Tarnwell?
  3. Explain what breakthrough is and why it worries operators more than high headloss does.
  4. List, in order, the steps of the backwash the passage describes.
  5. Filter five shows headloss below its design limit, six hours left in its allowed run, and outlet turbidity that has just passed the plant's internal target. According to the passage, what should the operator do, and which rule applies?
  6. The passage says a freshly washed filter goes through a ripening period. Using the surrounding sentences, explain what ripening means and why plants send the first water to waste.
  7. Write a short informative paragraph explaining to a new trainee why Tarnwell does not simply backwash every filter each morning. Use at least 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: Headloss, which measures the resistance the filter puts up as its spaces fill with trapped particles, and turbidity, which measures how cloudy the water leaving the filter is. Both appear on the plant's panel, and a meter on each filter's outlet reports turbidity continuously.
  2. Answer: Finished water must stay at or below 0.3 NTU in 95 percent of monthly samples and must never go above 1 NTU. The passage adds that Tarnwell holds itself to a tighter internal target.
  3. Answer: Breakthrough is a filter releasing particles it caught earlier, so outlet turbidity rises while headloss still looks normal. It worries operators because the dirty water is heading toward town rather than staying in the filter.
  4. Answer: Blow air in first to loosen the bed; raise the rate of clean water pumped up through the filter floor so the bed lifts and the grains tumble; hold that rate while the dirt washes out over the trough at the top; then lower the rate gradually rather than stopping suddenly.
  5. Answer: The operator should backwash filter five immediately. The turbidity exception overrides both the headloss limit and the hours remaining in the run, because a filter releasing particles is no longer doing its job.
  6. Answer: Ripening is the first period after a wash, when a clean bed filters slightly worse than a partly loaded one because the fine spaces that trap particles have just been rinsed open. Plants send that first water to waste so the poorer-quality water never reaches customers.
  7. Answer: Accept any informative paragraph using at least two passage details, such as that a backwash spends treated water the plant already paid to clean, that a freshly washed bed filters worse during ripening, and that a filter with low headloss and normal turbidity is still doing useful work.

Differentiation

Support

Give these students a two-column organizer labeled headloss and turbidity, with three blanks under each for what it measures, what it signals, and what the operator does. Let question 4 be answered by ordering four pre-written step cards, and allow question 2 to be answered by underlining the sentence that states the rule.

Extension

Ask students to write the clipboard note Verne would leave if headloss had reached its limit while turbidity stayed steady, then explain in two sentences how a supervisor would read that note differently from the note in the passage.

ELL supports

Pre-teach turbidity, headloss, backwash, filter run, and ripening with a simple labeled drawing of a filter box showing water entering at the top, the coal and sand layers, and the outlet pipes below. Provide sentence starters for question 7, such as "Tarnwell does not wash every filter each morning because ____."

Extensions

Have students turn the decision in this passage into a one-page flowchart with two inputs and one exception, then trade charts and test a partner's chart against the situation in question 5. Any chart that sends filter five back to service has missed the exception.