Atmospheric concentrations of carbon dioxide are now at or above 400 ppm — the highest levels for over 3 million years. I find it terrifying. To bring these levels down in a sustainable way we are going to have to reinvent ourselves: change how we get and use energy. That will take some time. And this is time we need to buy. While it is beyond the capabilities of most individuals to make efficient solar collectors or finally figure out cold fusion, we can all figure out how to put the food in one bin and the trash in another. We can also figure out how to put compost on the tomato plants instead of fertilizer. Composting won’t bring these concentrations down below 350 ppm, but it will buy us some time. This column and the next two are dedicated to buying more time by giving readers some hard numbers (i.e., crude calculations) that clearly show that organics diversion has to be in the tool box to reduce atmospheric CO2 concentrations.
When discussing the potential for organics diversion, you likely are met with some of those Frequently Asked Questions (FAQ) such as “what about emissions from composting” or ‘”what about all the emissions from the collection trucks.” Based on past experience, I assembled a range of these questions and have shown how to calculate the answers. The calculations contain only minimal amounts of math. They are set up to alter the basics to make them fit individual situations. I road tested this math at a workshop in Vermont this past spring, so am using Vermont as the default for the examples.
The first set of questions focuses on emissions from landfills versus composting facilities. Are landfills really that bad, people ask? If they collect the methane gas, aren’t they green? This question was addressed several years ago in this column (see “Landfill Gas Math,” December 2009 and “Landfill Gas Math, Take 2,” May 2011). For now, I am going to focus on comparing the emissions from each option.
FAQ: Are landfills really all that bad? If they collect the methane gas, aren’t they green?
To figure out if landfill energy is really green, the first step is to determine how much methane food waste will make in a landfill. Next is what portion of that methane is collected and converted to electricity and what portion escapes to the atmosphere. Let’s start with a quick version of that calculation (the full version is available in the web version of this column at www.biocycle.net).
For this approximation, we can say that 1 dry ton of food waste will decompose and produce 65 kg of methane (CH4). If all of this is captured and used to make electricity (100% efficiency) you would end up with 1,000 kWh of power, which is the same as 1 MWh (the capital M stands for 1,000 k). In Vermont, the normal emissions from producing 1 MWh of electricity is 0.47 tons of CO2. This will vary for each state or region as it is based on an average of all various electricity-generating sources (e.g., coal, hydro, solar, wind, etc.). These emissions numbers are found on EPA’s website (www.epa.gov/cleanenergy/energy-resources/egrid/index.html).
What if the landfill captured less of the CH4, e.g., 75 percent? Here’s the math: 75% efficiency = 1 MWh * 0.75 = 0.75 MWh; 0.75 MWh of power @ 0.47 tons of CO2 for each MWh gets you 0.35 tons of CO2 credit. That calculation can be changed for different collection efficiencies, e.g., 50 percent efficiency yields a credit of 0.23 tons of CO2.
Here’s the rub (or the other side of that calculation). Let’s say that the portion of CH4 that didn’t get made into electricity was released to the atmosphere. What is the CO2 equivalent of what was released? (Remember that CH4 is 23 times as potent as CO2). If 75 percent is made into energy, that means 25 percent is released to the atmosphere; 65 kg CH4 * 0.25= 16.25 kg CH4 is released. And 16.25 kg CH4 * 23 (CO2e for CH4)= 374 kg CO2e or just a little bit more than the credit gained with the energy.
And if 50 percent of the CH4 is released?: 65 kg CH4 *0.5 = 32.5 kg CH4; 32.5 kg CH4* 23 (CO2e for CH4)= 748 kg CO2e or a lot more than the credit obtained for energy. It is important to remember that gas generation starts well before gas collection in the majority of landfills. So that means that the 50 percent efficiency number is on the optimistic side of things.
This can be scaled to your city or state. Say that each person makes about 40 kg dry food waste per year. Multiply that by however many people live in your state, city, county, township and you can do the same calculation for your own community. Note that the CO2e for electricity will vary based on where you live. Next month’s column provides calculations for anaerobic digestion of food waste. Anaerobic digesters would not only make more CH4 (because of tightly monitored conditions), it would also have a much higher gas collection efficiency.
FAQ: People have heard that composting releases CH4 — doesn’t that mean it is the same thing as landfilling?
It is true that composting can emit CH4. Let’s figure this out. In a paper I wrote (Brown et al., 2008) we figured that the high end of emissions — a wet, poorly managed pile — would be about 2.5 percent of the initial carbon. On the other end, we figured that for a well-managed sweet smelling pile, those numbers would be closer to 0. Research has shown that covering a windrow with finished compost is an easy way to get to that sweet spot. Enclosed systems with negative aeration are a more expensive but also effective way to get there.
Back to that same one ton of dry food waste. Let’s say that that is about 40 percent carbon (C): 1 ton * 0.4 C per ton = 400 kg of C per dry ton of feedstock. Let’s say 2.5 percent of that turns into methane: 400 kg * 2.5% (0.025) = 10 kg CH4 from 1 ton of food waste in a poorly managed pile. Methane is 23 times more potent than CO2: 10 kg CH4 * 23 = 230 kg CO2.
Bill Horwath from the University of California Davis has measured CH4 on a well behaved pile and a too wet saturated pile using state of the art techniques. He found emissions of CH4 from 1 kg (good pile) to 9 kg (wet smelly pile) per m2. This is really close to our estimate from that paper. I’m going to say that a m2 of feedstocks weighs about 1 dry ton. This will vary but is close enough and makes calculations easy:
1 kg CH4 per dry ton materials * 23 kg CO2 per kg CH4= 23 kg CO2
9 kg CH4 per dry ton materials * 23 kg CO2 per kg CH4 = 207 kg CO2
So you can see that the worst case above is really a worst case — comparable to a landfill. For good piles, you can measure methane but it is negligible.
So here’s the bottom line for landfill energy versus compost pile emissions: A decent landfill with 50 percent collection efficiency will produce enough energy to provide 0.23 tons of CO2 credits. It will emit enough CH4 to yield 0.75 tons of CO2 debits.
You end up coming out about 0.5 tons in the hole for each ton of food waste.
A poorly operated compost pile will release 0.21 tons of CO2 per ton of food waste.
A well-managed pile will release 0.02 tons of CO2 per ton of food waste. This is a very good incentive to keep your pile sufficiently dry to reach temperature quickly.
Remember that for each ton of food waste composted instead of landfilled, you also can take some credit for the CH4 that was not released from the landfill.
Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at slb@u.washington.edu.

Reference

Brown, S. C. Krueger and S. Subler. 2008 Greenhouse Gas Balance for Composting Operations. J. Environ. Qual., 37:1396-1410.