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Lesson plan Grade 10 Social Studies & Civics

Culverdon's Biosolids Decision: Landfill, Fields, or Digester

A grade 10 lesson in which students cost three disposal options for a fictional plant's biosolids, then argue the choice at a simulated district board hearing.

Objectives

  1. Students will calculate the annual cost of three biosolids management options from stated unit costs.
  2. Students will state the marginal cost of the digester option and identify the benefits that cost buys.
  3. Students will present and defend a position at a simulated district board hearing using cost evidence.

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.
Secondary treatment
Secondary treatment uses billions of helpful microbes that eat the dissolved waste in the water. The plant keeps the microbes supplied with air, then settles them out, leaving much cleaner water.
Anaerobic digester
An anaerobic digester is a sealed tank where bacteria break down sewage sludge without any oxygen, shrinking its volume and producing methane gas as a byproduct. Some treatment plants capture that gas and burn it to generate their own electricity or heat.

Also: land application, tipping fee, marginal cost, nutrient recovery

Teacher overview

Federal secondary treatment rules require a wastewater plant to discharge effluent averaging 30 mg/L or less of BOD and suspended solids, down from the roughly 100-300 mg/L of BOD typical of raw sewage. Meeting that standard does not make the removed material vanish. It concentrates the solids into biosolids that a utility then has to haul, spread, burn, or digest, and paying for that handling is a recurring line in every sewer budget. The misconception this lesson corrects is the belief that treatment destroys what it removes, which leads students to assume disposal is free; the arithmetic below shows it is one of the larger controllable costs a fictional district faces. Students cost three options for the fictional Culverdon Water Reclamation District and then argue the choice at a simulated board hearing.

Procedure

Framing the problem (7 min): Ask students where the solids removed from sewage go after treatment. Take three or four answers without correcting them, then state the secondary treatment standard of 30 mg/L and explain that what leaves the water becomes biosolids. Tell students the fictional Culverdon district produces 2,400 wet tons of biosolids a year and must choose how to handle them.

Reading the three options (10 min): Distribute the handout. Read the three options aloud and have students mark which costs are one-time capital costs and which recur every year. Confirm as a class that the storage lagoon and the digester are capital costs spread across twenty years, while hauling is annual.

Handout calculations (22 min): Students complete questions 1 through 6 in pairs with calculators. Watch for two predictable errors: forgetting to subtract the biogas value before totaling the digester's annual cost, and comparing the digester's capital cost directly against another option's annual cost.

Board hearing (16 min): Assign four roles and give each three minutes of testimony followed by one question from the board: the district engineer, a farmer who would receive the biosolids, a resident of the road the trucks use, and the district's finance officer. The rest of the class sits as the board, hears two minutes of public comment, and takes a recorded vote on which option to fund. Require every speaker to cite one figure from the handout.

Exit ticket (5 min): Students answer question 7 individually and hand it in. If fewer than half name a benefit that the dollar figures do not capture, revisit the difference between costs a budget records and costs it does not.

Culverdon's Biosolids Decision: Landfill, Fields, or Digester — Grade 10 Social Studies & Civics
Name: ____________________________ Date: ______________

Student Handout

The Culverdon Water Reclamation District (a fictional utility). Culverdon's plant serves the town of Culverdon and produces 2,400 wet tons of biosolids each year. The district board must choose among three options.

Option A, landfill. Hauling and tipping cost $68 per wet ton. No new construction is needed.

Option B, land application on nearby farm fields. Hauling and spreading cost $41 per wet ton, but the district must first build a storage lagoon costing $310,000, which the district would pay for evenly over 20 years.

Option C, anaerobic digester. A digester costing $4,600,000, paid for evenly over 20 years, would reduce the volume of biosolids by 40 percent. The remaining material would still be spread on fields at $41 per wet ton. The digester's biogas would replace $47,000 of purchased energy each year.

  1. What does Option A cost Culverdon per year?
  2. What does Option B cost Culverdon per year, counting both the hauling and the storage lagoon paid for evenly over 20 years?
  3. Under Option C, the digester cuts the 2,400 wet tons by 40 percent. How many wet tons remain to be hauled each year?
  4. Using your answer to question 3, what is Option C's yearly hauling cost at $41 per wet ton after the $47,000 of avoided energy purchases is subtracted?
  5. The digester costs $4,600,000 paid evenly over 20 years. Adding that yearly amount to your answer from question 4, what does Option C cost per year in total?
  6. How much more does Option C cost each year than Option B?
  7. Option C costs the most per year of the three. Name one benefit of the digester that the dollar figures above do not capture, and explain why a board member might still vote for it.

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: $163,200 per year. Multiplying 2,400 wet tons by $68 per ton gives $163,200.
  2. Answer: $113,900 per year. Hauling costs 2,400 times $41, or $98,400, and the $310,000 lagoon divided across 20 years adds $15,500, for a total of $113,900.
  3. Answer: 1,440 wet tons per year. A 40 percent reduction leaves 60 percent of the material, and 2,400 times 0.60 is 1,440.
  4. Answer: $12,040 per year. Hauling 1,440 tons at $41 per ton costs $59,040, and subtracting the $47,000 of avoided energy purchases leaves $12,040.
  5. Answer: $242,040 per year. The digester's $4,600,000 divided across 20 years is $230,000 a year, and adding the $12,040 from question 4 gives $242,040.
  6. Answer: $128,140 per year. Subtracting Option B's $113,900 from Option C's $242,040 leaves $128,140.
  7. Answer: Accept any well-reasoned benefit that the cost table leaves out, such as far fewer truck trips past homes on the haul road, reduced odor complaints, a smaller volume to find fields for if a neighboring county stops accepting biosolids, or protection against future rises in tipping fees or energy prices. A strong answer explains that the board is buying reduced risk and reduced nuisance, neither of which appears as a line in the annual budget.

Differentiation

Support

Provide a completed cost table for Option A and the first line of Option B, so students begin with a worked model of separating annual hauling from capital spread across twenty years. Allow calculators throughout and let students answer question 7 orally to a partner before writing.

Extension

Ask students to recompute Option C on the assumption that the district borrows the capital and pays interest, using a total repayment figure the teacher supplies, and then state how large the interest would have to be before the board should reject the digester outright.

ELL supports

Pre-teach "biosolids," "hauling," "tipping fee," and "capital cost" with a labeled diagram showing solids leaving the plant by truck. Provide a two-column chart headed "paid once" and "paid every year" for students to sort the figures into before calculating, and offer sentence frames for the hearing testimony.