How Much Difference Does Composting at Home Make?

Smita C. Thomas April 22, 2018 0

By Smita Thomas and Catherine Powell

Earth Day, April 22, 2018

 

What can I compost?

All compostables big and small

Have you been thinking about composting this Earth Day? Do you know how much of a difference it makes? At Energy Shrink, we like to put numbers to things. With some research followed by calculations, this is what we found out. Composting all the organic waste from one US household equates to removing 1.5 cars from the roads or 75% of an average homes’ energy use per year. By composting food scraps alone, 4 households could effectively remove a car from the roads, and 8 households could offset an average homes’ energy use per year.

The big picture information is in the bold headings, with the details provided in each paragraph.  Skip to the end to find out the options for composting.

Food is the most obvious thing to compost. The average household produces 450 lbs. (200 kg.) of food waste per year.  

This is estimated based on a recent US study across three cities which found that the average household wastes 8.7 lbs. of food per week[1]. This jibes with the roughly 6 lbs. per week we seem to be putting into the compost bin at our home. Although, our compostable waste includes not just food but also paper towels and compostable packaging. In fact, food scraps make up only about 12% of the municipal solid waste (MSW). About 70% of US MSW is made up of biomass, and therefore compostable. See table below[2].

The composition of Municipal Solid Waste (MSW) as presented in the “Methane generation in Landfills” paper[2]

The average US household produces 2 tons of Municipal Solid Waste (MSW) per year.

Here is how we arrived at this figure. An average person in the US. produces 4.4 lbs. of solid waste per day[3]. For a typical household size of 2.58 persons[4], this equates to roughly 4150 lbs. (4143 lbs. to be exact) or 1880 kg. of MSW per year. This is about 2.07 US tons (1.88 Metric tons). Of this, 69.5% (see table above) is compostable.  Which is to say that each household produces 2 tons of MSW and 2880 lbs. (1300 kg.) of compostable waste per year.

Each ton of MSW produces 50 Nm3 of CO2 and 50 Nm3 of methane per ton of MSW.

Each ton of MSW produces about 100 Nm3 of Landfill Gas [2] which is about 50% Carbon dioxide and 50% Methane with some other gases in trace amounts[5]. Carbon Dioxide represents the gases that cause global warming. But it doesn’t act alone. In fact, global warming is measured in CO2 equivalent (CO2eq), with the impact of other gases being equated to CO2. The Global Warming Potential of methane is about 30 times that of CO2[6]. [7] A 100-year reference period is the industry standard for comparing the global warming potential (GWP) of gases, which stretches out this effect and makes it seem smaller. But the fact is that methane lasts for only about a decade in the atmosphere on average (8-12 years) compared to 100s of years for CO2[1].  So even though its effect doesn’t last as long as that of CO2, pound for pound, methane is contributing more significantly to melting the glaciers today[8].

Each US household produces 0.3 tons of Methane and 0.2 tons of CO2 per year.

As stated above, each US household produces 2 tons of MSW, and every ton of MSW produces 100 Nm3 of landfill gases. So, 2 tons of solid waste equates to 200 Nm3 of landfill gases, that is 100 Nm3 of CO2 and 100 Nm3 of Methane. Several US landfills report capturing as much as 100 Nm3 of methane per ton of MSW landfilled in a given year[2]. But the Columbia paper[2] takes a conservative approach and uses half of that. We are using the same conservative figure in our calculations here. As per that paper (p. 1248), each tonne of MSW would generate 0.15 tonnes (0.149 to be exact) of methane. So, 2 tons of MSW would produce roughly 0.3 tons of methane. As for CO2, with each ton of MSW producing 50 Nm3 CO2, 2 tons of annual waste would produce 100 Nm3 or 200 kg of COthat is 0.2 tons. (1 Nm3 of CO2 equals 1.97 kg CO2.)

Emissions from the total compostable waste for a US household equate to 1.5 cars driven per year or 75% of an average homes’ energy use.

Now that we know how much greenhouse gas an average household is emitting, we can have some fun measuring the impact of this using EPA’s handy Greenhouse Gas Equivalencies Calculator [https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator, Accessed April 22, 2018]. The results show that a household could offset nearly the equivalent of its annual emissions from driving or energy use at home by composting. It also equates to 181 tree seedlings grown for 10 years! This is the best case scenario if every organic scrap was composted. Even if homes were to compost only the food waste (450 lbs. or 200 kg or 0.2 tons per year, as seen in the first paragraph above), it would be 15% of the total 2,880 lbs. of the total compostable waste per year. Thus, by composting food scraps alone, 4 households could effectively remove a car from the roads, and 8 households could offset an average homes’ energy use.


Our neighborhood of 530 homes could offset the emissions from 400 homes or nearly 800 cars in the best case scenario.

530 homes would produce 159 tons of methane and 106 tons of CO2 per year. The EPA calculator shows that these emissions are equivalent to 793 passenger vehicles driven for one year, or 400 homes’ energy use. The results with composting food scraps only would be 15% of that.

Fairfax County could offset 3.1 Million tons of CO2eq emissions every year by composting the waste from all its households, in the best case scenario. This equates to emissions from 605,000 cars or 305,000 homes.

Fairfax County in Virginia, where we are based, has 405,000 households. The compostable waste from these households would collectively produce 1.2 Million tons of methane (121,500 tons), and 81,000 tons of CO2. The EPA calculator shows that these emissions are equivalent to 605,793 cars driven for one year, or 305, 481 homes’ energy use. With composting only the food waste, 15% of this could be mitigated every year. Our neighboring Falls Church City has initiated a composting program for its residents. Such programs can help a city or county meet its greenhouse emissions goals.

For perspective:

  • Annual emissions at Fairfax County were 15 million Tons of CO2eq in 2005  (10% lower in 2017) [Source Fairfax County, 2018, https://www.fairfaxcounty.gov/news2/county-cuts-greenhouse-gas-emissions-by-10-percent-study-reports/].
  • The annual CO2eq emissions of the entire US are about 7 billion tons. The US has 4% of the global population and emits about 15% of global emissions. [Source: EPA. (2016, August). Climate Change Indicators: U.S. Greenhouse Gas Emissions. Retrieved April 6, 2018, from https://www.epa.gov/climate-indicators/climate-change-indicators-us-greenhouse-gas-emissions]

What are the options for Composting?

6 lbs of compostable waste goes out from our home in a typical week. (The bucket sitting on the weighing scale weights 2.25 lbs.) Have you weighed your compost lately?

Even a 3-year old can turn the composting bin

Even a 3-year old can turn this composting bin installed at the Haycock Elementary School in McLean, VA.

  • You can participate in local composting programs, including services that regularly collect food scraps and organic waste from your home. Some of these organizations offer composted fertilizer in return. Locally, we have the options of Veteran Compost (our current provider) or Compost Crew for rates averaging about $30/month. Falls Church City residents enjoy composting for a low rate of $6/month thanks to a city subsidy and an agreement with Compost Crew. [Prices current as of December 2017]
  • Some parks, such as the Potomac Overlook Park, offer a composting place in the Community Garden for residents to deposit their organic waste for free which is then taken care of by park staff and volunteers.
  • Some shops, such as the Mom’s Organic Store in Arlington, offer bins for shoppers to deposit their compostable waste for free. Hint: Trader Joe’s now offers compostable vegetable bags, which can be re-used effectively for transporting organic waste from home.
  • You can also set up a little operation in your own backyard in the form of a compost pile or pit, where you place scraps and waste in a small area that can be rotated or turned over.
  • You could even buy a special compost tumbler, a device which automatically or manually rotates compost.

 

Q. Doesn’t storing food waste make the house smell?

A. It can if you leave it around for days. We keep a nicer-looking countertop compost container at the kitchen counter right above our waste bin. This helps to not get lazy about separating the food waste and putting it in its proper place. Once a day, we transfer the waste into our main compost bucket which is kept in the unconditioned garage. This has been practically freezing and hasn’t caused a problem yet. But we are yet to go through a summer week with it. So we’ll report back on that in a  few months.

Q. Why does organic waste produce methane in a landfill, but not in at a composting bin or pit?

A. Isn’t food bio-degradable after all? So throwing food scraps and paper products into a landfill should be harmless. However, the key difference from composting is that when these materials break down in a landfill they get buried and rot “anaerobically” (without oxygen) and produce methane and carbon dioxide.

Composting is a slow and natural process in which organic waste (waste derived from living matter) from food and the yard is piled up, and allowed to decompose naturally[9] by earthworms, bacteria and other organisms that live in soil. The compost pile needs to be turned frequently, adding oxygen and water to the mix (an “aerobic” process). Over several months, this decomposed material forms ‘humus’ which is a rich, nutrient-dense fertilizer, a great alternative to chemical fertilizers. Synthetic nitrogen used in chemical fertilizers is one of the most significant sources of N2O, a powerful greenhouse gas that is about 310 times more potent than CO2[10]. A layer of well-aerated compost on the soil also reduces soil erosion and helps reduce stormwater run-off. Composting also helps keep food and yard waste out of the landfill. So it has many benefits besides just reduction of greenhouse gases.

 


References:

[1] L. Moreno, D. Hoover, “Estimating Quantities and Types of Food Waste at the City Level”, NRDC Report, October 2017, p.7. Retrieved April 22, 2018, from https://www.nrdc.org/sites/default/files/food-waste-city-level-report.pdf

[2] N.J. Themelis, P.A. Ulloa / Renewable Energy 32 (2007) 1243–1257, “Methane generation in landfills”, Table 3, p.1246. Retrieved on April 22, 2018, from http://www.seas.columbia.edu/earth/wtert/newwtert/Research/sofos/Themelis_Ulloa_Landfill.pdf

[3] “Municipal Solid Waste”, US Environmental Protection Agency (EPA) Archive, Retrieved on April 22, 2018, from https://archive.epa.gov/epawaste/nonhaz/municipal/web/html/

[4] US Census of 2010, Retrieved on April 22, 2018, from https://www.census.gov/prod/cen2010/briefs/c2010br-14.pdf

[5] “Basic information about landfill Gas”, EPA, Retrieved April 22, 2018, from https://www.epa.gov/lmop/basic-information-about-landfill-gas

[6] Understanding Global Warming Potentials, EPA, Retrieved on April 22, 2018, from https://www.epa.gov/ghgemissions/understanding-global-warming-potentials

[7] G. Vaidyanathan, Climate Wire, Dec. 22, 2015. Retrieved on April 22, 2018, from Scientific American https://www.scientificamerican.com/article/how-bad-of-a-greenhouse-gas-is-methane/

[8] https://www.fcrn.org.uk/fcrn-blogs/umpersson/livestock%E2%80%99s-carbon-footprint-importance-comparing-greenhouse-gases

[9] https://study.com/academy/lesson/what-is-composting-definition-and-examples.html

[10] http://www.lhpowerandlight.org/benefits-of-composting.html

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About The Author

Smita Chandra Thomas is a U.S.-based sustainability consultant and a LEED AP. Ms. Thomas has been focused on enabling energy efficiency in buildings through building science for lesser waste, greater economy, better health, and a sustainable environment for future generations. After spending two years in Beijing where she did freelance consulting, learned Mandarin, and blogged, Ms. Thomas currently runs her private consulting practice Energy Shrink in Washington DC. She can be reached at thomas@energy-shrink.com.

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