Classroom activity Grade 11 English Language Arts
Explain It Backward: The Tamsleigh Digester in Six Steps
A grade 11 activity in which teams build a process explanation from its end product back to its input, then audit another team's version for assumed steps.
Objectives
- Students will separate the steps of a process from true statements that lie outside it.
- Students will order an explanation so that no step depends on an unstated earlier step.
- Students will identify assumed steps in another writer's explanation.
Vocabulary
- Biosolids
- Biosolids are the nutrient-rich solid material left over after sewage sludge has been treated and made safe. Many farms use treated biosolids as fertilizer for their fields.
Also: anaerobic, digester, biogas, stabilize, assumed step
Teacher overview
This is a card-sorting and writing activity for teams of three or four. Teams start from the end of a process, which is an engine running on gas and a truck hauling away stabilized solids, and work backward to the sludge that entered the tank, then write the explanation forward and hand it to another team for an audit. Building backward is what exposes an assumed step, because a writer who starts at the beginning can skip a stage without noticing while a writer moving backward has to ask what produced each thing in turn. The water content carries a misconception that decides one of the cards: the microbes in a digester are routinely assumed to need oxygen, since the microbes usually described first are those in an aeration basin, where air is blown through the water. A digester works only because oxygen is kept out, and the methane in the gas exists for that reason.
Activity Directions
- Set up, five minutes. Form teams of three or four and give each team a cut card set from the Student Handout, chart paper, and markers. Tell the class that the Tamsleigh Water Reclamation Facility and every figure in the packet are fictional. State the end point aloud: an engine at the plant runs on gas from a sealed tank, and a truck hauls away a stabilized solid.
- Round 1, backward build, twelve minutes. Working from the end point, each team selects the six cards that are steps in the chain and lays them out in reverse order, ending with the material that entered the tank. The eight remaining cards go in a discard pile, and the team writes one short reason on each discarded card.
- Round 2, forward write, twelve minutes. Teams flip the sequence and write the explanation forward on chart paper, in no more than 150 words, using one sentence per step. The rule is that a reader may not need to know anything the previous sentence did not supply.
- Round 3, the assumed-step audit, ten minutes. Teams trade chart paper. The auditing team draws a caret between any two sentences where a step is missing and writes the question a reader would have at that point. Auditors mark at most three carets, so they must choose the gaps that matter most.
- Round 4, repair, six minutes. Teams take their chart paper back and repair one gap only, staying within the 150-word limit, which forces a cut somewhere else. The activity ends when every team has posted a repaired version and named the sentence it shortened.
- Debrief, five minutes. Ask the five questions on the Student Handout. Question 4 is the one to slow down on, since the card about where the tank's heat comes from is defensible as a step and defensible as an aside, and teams should be able to argue either position from the writing rule in Round 2 rather than from the science.
Student Handout
The Tamsleigh Water Reclamation Facility is fictional, and so is every figure below.
The end point. At the Tamsleigh plant, an engine runs on gas drawn from a sealed tank, and a truck hauls away a stabilized solid from the same tank. Your job is to explain how both of those things came to exist.
Gas log, one week at the Tamsleigh digester.
| Day | Gas produced | Methane share | Engine status |
|---|---|---|---|
| Monday | 41,000 cubic feet | 62 percent | Running |
| Tuesday | 39,500 cubic feet | 61 percent | Running |
| Wednesday | 40,200 cubic feet | 62 percent | Down for service, gas flared |
| Thursday | 40,800 cubic feet | 63 percent | Running |
| Friday | 38,900 cubic feet | 61 percent | Running |
Card set. Six of these fourteen statements are steps in the chain. Eight are not.
- Card A. Thickened sludge from the settling tanks is pumped into a sealed, heated tank.
- Card B. Air is blown through the tank so the microbes inside have the oxygen they need.
- Card C. The tank is held near the temperature of a warm bath, kept free of oxygen, and stirred so the contents stay mixed.
- Card D. Chlorine is added at the end to kill any bacteria that remain.
- Card E. One group of microbes breaks the solids down into simple acids.
- Card F. Grit and rags are screened out before anything reaches the settling tanks.
- Card G. A second group of microbes converts those acids into methane and carbon dioxide.
- Card H. The plant's aeration basin uses a different community of microbes that require oxygen.
- Card I. Gas collects at the top of the sealed tank and is drawn off, dried, and piped to the engine.
- Card J. When the engine is down for service, the gas is burned off at a flare instead.
- Card K. What remains in the tank is a stabilized solid, which is dewatered and hauled away as biosolids.
- Card L. Ultraviolet light disinfects the treated water before it leaves the plant.
- Card M. Phosphorus is recovered from the liquid pressed out of the dewatered solids.
- Card N. The heat that keeps the tank warm comes from the engine's exhaust.
Debrief questions.
- Which card did your team argue about longest, and what settled it?
- Card B and Card H both mention oxygen, and neither is a step. Explain why each is left out, and name the card that states the tank's actual oxygen condition.
- Your auditors marked a gap. Name the question a reader would have asked there.
- Card N is true and relevant. Give the strongest case for including it and the strongest case for leaving it out.
- The gas log shows Wednesday's production holding steady while the engine was down. State what that tells a reader about where the gas comes from.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
The six step cards, in forward order: A, C, E, G, I, K. Backward, teams should have laid them out as K and I first, since those are the two things the class was told to explain, then G, E, C, and A.
The eight cards that are not steps. Card B is false, and it is the misconception the activity is built around, since the tank works by excluding oxygen. Card D belongs to disinfection of the treated water rather than to the digester. Card F is true and upstream of the chain, so it explains what reaches the settling tanks rather than what happens in the tank. Card H is true and is the contrast that makes Card C meaningful, yet it is not a step in this chain. Card J is true and the gas log confirms it, though flaring is what happens to the gas afterward rather than a stage in making it. Card L is out of scope. Card M describes a side stream taken after Card K. Card N is true and is the defensible borderline case, since the heat loop supports Card C rather than advancing the chain.
Debrief question 1. Accept any card with a reason. Most teams argue about Card N or Card J. A strong answer names the rule that settled it rather than the fact.
Debrief question 2. Card H is true and Card B is false, and both are outside the chain. Card C covers the tank's actual condition, so a team that keeps Card H instead of Card C has swapped the contrast for the step.
Debrief question 3. Accept any gap the auditors marked. The two most common are between Card A and Card E, where a reader needs Card C to know why anything happens in the tank at all, and between Card G and Card K, where a reader is not told that solids remain after the gas leaves.
Debrief question 4. For including Card N: it answers the question Card C raises, which is where the heat comes from, and it shows the process feeding itself. For leaving it out: the explanation is about how sludge becomes gas and biosolids, and the heat source is a supporting detail that pushes the piece past its word limit. Either case is acceptable when it argues from the audience and the limit.
Debrief question 5. Wednesday's production held near the other days while the engine was down, so the gas is produced by the tank rather than by the engine. The engine is a customer for the gas, not its source, and the flare on Wednesday shows what happens when that customer is unavailable.
Differentiation
Support
Give these teams a card set with the eight non-step cards already removed, so Round 1 becomes ordering rather than sorting. Provide the end point written as two labeled boxes on the chart paper and allow twenty words per step instead of a single sentence. Model the first backward move with the class, showing how the question "what produced this" chooses the next card.
Extension
Ask these teams to write a second version for a reader who has never heard of a treatment plant, keeping the same six steps and the same word limit, and then to list every word they had to define and every word they were able to drop. Strong lists identify one term that could not be avoided and explain why.
ELL supports
Pre-teach "sludge," "sealed," "stabilized," and "dewatered" with a labeled cross-section sketch of a covered tank with gas at the top and solids at the bottom. Give the card set with a small icon on each step card's topic, and allow teams to build the sequence with cards alone before any writing. Provide the sentence frame "Before this can happen, the tank must" for the backward round.
Extensions
Have students apply the backward method to an explanation they are already drafting in another class, building it from its conclusion back to its first claim, and then write a short note on which step in their own draft turned out to be assumed. Ask them to name whether the assumed step was one they knew and skipped or one they had never thought about, since those two failures are repaired differently.