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The Current - Residential Composting Impact Computed

By Smita Chandra Thomas and Jennifer Jang

In Brief
The average US household can have a significant impact on the environment by diverting food scraps alone from landfill to composting. By composting, four households could effectively remove a car from the roads. Eight households composting food scraps could offset an average home's electricity use per year.

Average US Figures
Food is the most obvious item to compost. The average US household produces 450 lbs. of food waste per year. Food scraps make up about 12% of the US municipal solid waste (MSW).

In all, about 70% of US municipal solid waste is made up of biomass and is therefore compostable. See Figure 1 below.

Figure 1. The composition of Municipal Solid Waste

Yearly, the average US household produces about 2 tons of municipal solid waste including food amongst other organic and non-organic garbage. Where does all this garbage go? A majority goes to landfill, some is incinerated, some recycled and some composted. Each ton of municipal solid waste in a landfill produces 0.3 tons of methane and 0.2 tons of CO2 per year with some other gases in trace amounts.

Global Warming Potential of Food Waste
Carbon dioxide (CO2), is the primary gas that causes global warming, but many other gases are also responsible. Methane gas is one of the biggest culprits. The global warming potential (GWP) of methane is several times that of CO2. A 100-year reference period is the industry standard for comparing the GWP of gases, downplaying methane's overall impact by averaging it over 100 years. Methane lingers in the atmosphere for only a decade on average (8-12 years) compared to hundreds of years for CO2. Consequently, even though its duration in the atmosphere is shorter than CO2's, pound for pound, methane's contribution to melting the glaciers is actually much greater.

The equivalent impact of these greenhouse gas emissions can be estimated using the EPA's Greenhouse Gas Equivalencies Calculator. As seen in Figure 1, about 70% of the total municipal solid waste is compostable. In the best-case scenario, where every organic scrap is composted, a household's composting could offset nearly the equivalent of its annual emissions from driving or home energy use. (Detailed calculations are available in the Energy Shrink blogpost.) By composting food scraps alone, 4 households could effectively remove a car from the roads, and 8 households could offset an average home's electricity use.

Case Study, Washington D.C.
Washington DC is comprised of 281,475 households. In DC, 49% of the city's trash goes to landfills, 43% is incinerated, and the remaining 7% is recycled and only 1% is composted. The compostable waste in this trash collectively produces more than 42,000 tons of methane and 28,000 tons of CO2. The EPA calculator shows that these emissions are equivalent to about 29,000 cars driven for one year or the energy use of 130,000 homes. By composting only the food waste, 15% of this result could be mitigated every year. Table 1 shows these conservative estimates for the half of the food waste that would head to landfill.

Table 1. Potential impact of composting half of the residential food scraps in the DMV region

Potential impact from composting half of the residential food scraps in the DMV region
Region Number of Households Tons of Methane Tons of CO2 Equivalent Emissions from: Passenger vehicles driven for one year Equivalent Emissions from: Homes' electricity use for one year
Washington D.C. 281,475
as of Dec 7, 2018
6,328 4,219 34,000 19,000
Fairfax County, Virginia 405,000 9,113 6,075 50,000 28,000
Montgomery County, Maryland 373,346
as of Dec 7, 2018
8,400 5,600 32,000 18,000

The 43% waste headed for incineration goes to a waste-to-energy facility in Lorton, Virginia. Most recent figures show that this facility produces about 80MW of electricity, roughly a sixth of what an average coal plant would produce. This is commendable, but the efficiency of waste to energy systems is not 100%. A large part of it still goes up the smoke stack and causes additional emissions.

Looking Ahead
More recently, DC has "adopted a bold vision to divert 80% of all solid waste generated in the District through source reduction, reuse, recycling, composting, and anaerobic digestion." We can all do our part by not wasting food in the first place and diverting our food scraps to composting instead of trash.

Smita Chandra Thomas is the Principal for Energy Shrink, a consultancy for high-performance buildings. With nearly two decades of professional experience, she is a subject matter expert in energy efficiency in buildings at the cutting-edge of green technology and energy management strategy.

Jennifer Jang is a business manager at Energy Shrink with a background in professional writing and social technologies.

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