Quantifying the GHG Benefit of Composting
By: Chandu Visweswariah
CURE100 offers a Carbon Tracker software application that allows a household to determine its annual Greenhouse Gas (GHG) impact measured in metric tons of CO2e per year (learn about the Carbon Tracker in a previous blog post in this series and try the Carbon Tracker for yourself here). The Carbon Tracker allows a household to determine its “big hitters” and provides a quantified framework for systematic decarbonization. It also offers suggestions on how to decarbonize. The Carbon Tracker relies on GHG modeling techniques from various sources and studies. For example, what is the carbon impact of using a unit of electricity in different parts of the country? What is the carbon impact of various diets? How much carbon savings result from switching your furnace to biodiesel 20% or B-20 instead of fuel oil? Commuting by train? Recycling? Flying?
The Carbon Tracker “charges” each household an average amount of 0.806 metric tons of CO2e for Food and Waste, and offers “discounts” for recycling and composting. You get a total discount of 0.339 tons if you recycle paper, plastic, metal and glass, still leaving 0.467 tons of emissions which includes carting of Municipal Solid Waste (MSW) and the GHG impact of either incineration or a landfill.
This poses the question: What is a good number for the GHG reduction from composting? Is it worth our time and effort? Project Drawdown ranks composting as either 57th or 62nd (in two different scenarios) in effectiveness at fighting climate change out of 76 decarbonization ideas. We know that 75% of Americans don’t compost their food waste, so should we be on a crusade to change this? Our previous blog post on composting explained why commonly stated reasons for not composting are misinformed and another post gives you step-by-step instructions. If we offered a quantified carbon discount for composting, perhaps more Carbon Tracker users would be encouraged to compost. This article examines how we came up with that quantified number which has been implemented in the latest version of the Carbon Tracker.
To answer this question, we will separately determine the number of tons of food scraps that an average household could compost per year, and then multiply that by the average savings in GHG per ton of food scraps.
The total food waste across the nation is a shocking 25M tons. When divided by 122.8 million households, that works out to 0.204 tons per household per year (or 448 pounds per household per year). To triangulate this number, consider that the EPA says 24% of household waste is organic while this fact sheet from the University of Michigan cites a 15.2% number, leading to an average figure of about 0.3 tons of organic waste per household per year. For our purposes, this is sufficient to establish a range of 0.2 to 0.3 tons of compostable organic waste per household per year.
The Environment Protection Agency (EPA) has provided us with a Waste Reduction Model or WARM to estimate the GHG savings from recycling and composting. Running out the calculations using this handy-dandy calculator gives us a result of 0.8808 metric tons of GHG per ton of food scraps composted, assuming a landfill solution. Note that composting saves about 50% of the GHG compared to sending food scraps to a landfill.
Combining the above two results gives us a total of 0.204 times 0.8808 = 0.180 tons per year of GHG savings. This is clearly an upper bound on the GHG savings per household because food waste occurs on farms, during food transportation, in restaurants and grocery stores in addition to households. Also, not all food waste goes to a landfill, some of it is incinerated in which case the GHG savings of composting is reduced. Taking these two factors into account, we end up with an approximate result of 0.1 tons of CO2e savings per household per year from composting, implying about 20% savings in GHG over recycling alone. We invite our readers to help us improve the accuracy of this estimate.
The “step-down” chart of GHG emissions from recycling and composting for an average household thus looks like the picture here.
Which begs the question: to compost or not to compost? On the one hand, one may make the argument that this is too much trouble to save 0.1 metric tons of GHG in a total average GHG impact of 50 tons per household per year. This is not even a 1% savings! Reducing food waste or making diet modifications will produce much higher carbon savings. On the other hand, composting enthusiasts will remind us that every bit of GHG savings counts, and that GHG is not the only reason to compost. Fewer garbage bags are used when trash is reduced, leading to less plastic pollution. Less fuel is burned transporting garbage. Because municipal trash is picked up only once a week, composting is a more convenient way to dispose of food scraps at our convenience. The resulting soil from composting is part of a regenerative and sustainable cycle since it can be used for your lawns, plants, bushes and trees.
As a family that has composted ever since we moved into our house in 2009, we can add that there’s a visceral sense of satisfaction when food scraps find their way back into enriching our soil instead of being carted to an incinerator or landfill.