Composting, Diversion Rates, and Flat Landfill Tonnage
In 2021, the city of Millbrook added curbside food-scrap composting to its existing recycling program. Over two years, the share of residential waste diverted from landfill rose from 28% to 36%. However, the tonnage sent to landfill remained almost flat: 41,000 tons in 2021 and 40,600 tons in 2022. The city's sustainability director concludes that 'composting has not reduced landfill use' and recommends canceling the program. A consultant notes that the city's population grew by 6% over the period, that recycling markets collapsed for mixed paper (a major stream), and that the composting contractor missed 11 collection days in 2022. The director's memo reports only the diversion percentage and the landfill tonnage, not per-household participation or contamination rates. The program costs $2.3 million annually. The city's contract with the composting contractor has a two-year cancellation penalty. The city council asks whether the program should continue.
1. Evaluate the director's conclusion. Does flat total landfill tonnage show that composting failed? Use the population and recycling-market details.
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Flat total tonnage does not by itself show failure. The population grew 6%, so per-capita landfill tonnage fell: 41,000/P in 2021 versus 40,600/(1.06P) ≈ 38,302/P in 2022, a decline of about 6.6%. Holding total tonnage nearly flat while absorbing 6% more residents is consistent with the program diverting some waste. The recycling-market collapse for mixed paper is a rival explanation for why total tonnage did not fall more: paper that used to be recycled may have gone to landfill, masking compost savings. The 11 missed collection days also reduced the program's opportunity to work. The director's conclusion compares the wrong counterfactual-what would landfill tonnage have been without composting?-and the memo lacks participation and contamination data. The conclusion is premature, though not proven wrong.
2. Explain why the diversion rate rose while landfill tonnage stayed flat. What does this reveal about using a percentage as an outcome measure?
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Diversion rate is a ratio: diverted tonnage divided by total waste generated. If population and total waste grow, both diverted and landfilled tonnage can rise while the diversion rate also rises. Conversely, a collapse in mixed-paper recycling can move paper from the diverted category into landfill, pushing the rate down even if composting is removing food scraps. Here the rate rose from 28% to 36%, yet landfill tonnage was almost flat; that is possible if total waste grew and the composition shifted. A percentage hides absolute quantities, so it cannot answer whether landfill use fell. The council needs absolute tonnages by material, per-household participation, contamination rates, and per-capita trends.
3. What would be the strongest evidence for canceling the program?
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The strongest case for canceling would be evidence that the program is not responsible for any landfill reduction and is costly. For example: per-household participation is very low; contamination is so high that collected scraps are rejected and landfilled; and after controlling for population growth and the paper-market collapse, landfill tonnage is no lower than a matched counterfactual. Add a high cost per ton diverted ($2.3 million annually) and the cancellation penalty. That package would show the program's measured benefit is illusory or smaller than alternatives. Flat raw tonnage alone is weak because it ignores population and the paper-market confounder.