BioCycle July 2013, Vol. 54, No. 7, p. 60
Last month’s frequently asked questions column (“Climate Calculations,” June 2013) was all about fugitive emissions from food waste that is either composted or landfilled. In summary, at a relatively well-run landfill, each dry ton of food scraps would emit about 0.75 tons CO2 equivalent of methane. The green energy from that facility provides about 0.25 tons of CO2 as green energy, which results in net emissions of 0.5 tons of CO2 per dry ton of food scraps. The net CO2 emissions from food scraps processed at a well-run composting facility would be 0.02 tons or a 0.48 tons CO2 savings for each ton diverted. Still concerns exist about emissions from trucking the food waste around. This month’s FAQ calculations provide tools to understand transport emissions in relation to methane avoidance emissions.
Transport emissions can be broken down into two parts: 1) Smaller trucks that pick stuff up from homes, restaurants and supermarkets and take them to a centralized location; and 2) Loads put into long-haul trucks for the trip to the landfill.
Let’s start with the long-haul trucks. To calculate the carbon footprint of collecting and hauling food waste, the first step is to figure out the quantity of CO2 released when food scraps are transported in long-haul trucks. Similar to last month, I’m going to use Vermont as the example, posing and answering the frequently asked questions (FAQs).
FAQ: How much wet food waste is generated annually in Vermont?
Every year the people in Vermont produce about 50 kg of wet food waste per person. (I know I used 40 kg/person last month, but 50 kg makes for easier multiplication.) There are 627,000 people in the state, therefore multiply the total number of people by the quantity of food waste per person:
627,000 people * 50 kg food waste per person = 31.35 million kg of wet food waste; 1 ton = 1,000 kg so dividing by (÷) 1,000 = 31, 350 tons of wet food waste per year in Vermont. (By the way, that 31,350 tons of wet food waste, assuming 20% solids, is equal to 6,270 tons of dry food scraps. Using the calculations from last month yields 3,135 tons of CO2 emissions when it is landfilled.)
FAQ: How many truckloads are needed to transport the food waste in Vermont for one year?
A long-haul truck can carry about 25 wet tons (5 dry tons) of waste. Divide the total quantity of food waste by the amount each truck can carry: 1 truck ÷ 25 tons * 31,350 tons = 1,254 truckloads/year.
FAQ: How many gallons of fuel does it take to get to the landfill and back for each truck?
The next step is figuring out how far each truck has to travel to bring food scraps to the landfill. An approximation is good enough for this exercise; you don’t need to get the exact route of each truck. Approximations are also sufficient to calculate this for your own community. To understand the big picture, it doesn’t matter whether your food scraps are hauled to Columbus or Cleveland if they are coming from New York City.
Vermont has two landfills located pretty close to each other near the center of the state. Vermont is about 160 miles long (it is much longer than it is wide). Let’s say that each truck drove 160 miles round trip to bring food waste to the landfill. Average truck mileage is 5 miles/gallon. Step 1 is to figure out how much diesel is used for each truck: 160 miles round trip ÷ 5 miles/gallon = 32 gallons.
FAQ: How much CO2 is released each year by taking food waste to the landfills?
To calculate the amount of CO2 released, convert the number of gallons of diesel to the CO2 equivalent. Diesel emits about 10 kg CO2/gallon. Therefore: 32 gallons * 10 kg CO2 /gallon = 320 kg CO2 per truck.
Next, figure out how much total CO2 is emitted taking food waste to the landfill. The number of truckloads is 1,254, and the amount of CO2 each truckload emits is 320 kg or 0.32 tons of CO2: 1,254 trucks per year * 0.32 tons CO2 per truck = 401.3 tons CO2. This is a little more than 10 percent of the CO2 emitted from the scraps once they are actually in the landfill.
When figuring out the amount of CO2 released for transport to a composting facility instead of to a landfill, 401.3 tons is your baseline number. If the total hauling distance to a composting facility is less than to a landfill, not only will you have credits for reduced methane emissions, you will also have credits for reduced transport emissions.
For fun, and remember here that for some of us, these types of calculations are fun, you can figure out how much transport emissions would be reduced if a number of smaller composting facilities were set up across the state. Let’s say that instead of a 160 mile round trip haul distance, the composting facilities were set up so that no one truck had to travel more than 20 miles one way (40 miles round trip). Assuming we still are looking at full trucks, the transport emissions would be reduced by 75 percent (160 ÷ 40 = 0.25 or one-quarter of what the distance had been).
So the total emissions from hauling to a landfill — 401.3 tons of CO2 — would be reduced by one-quarter to 100.3 tons of CO2 to transport food scraps to local composting facilities. This is a great reduction — but again the big bang for the buck comes from taking the food scraps out of the landfill to begin with, i.e., avoiding the methane generation. In fact, you would have to drive those very same food scraps close to 500 miles (1,000 miles round trip) to get the transport emissions close to the landfill emissions.
The other part of the transport picture is the local collection from households and businesses. It is also possible to understand how much CO2 is being emitted by those trucks and to come up with creative ways to reduce those emissions. For those calculations, you will have to look at the on-line version of this month’s column.
Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at email@example.com.