Connections: CO2 Math For Compost Benefits

Sally Brown

Sally Brown
BioCycle August 2013, Vol. 54, No. 8, p. 45

If you’ve followed the calculations from the first two columns in this series (Connections, June and July, 2013) you have likely realized the benefits of food waste diversion from landfills, i.e., landfill methane avoidance and potential for shorter hauling distances. The magnitude of methane emissions is high enough that even with much longer haul distances, landfill diversion to composting or anaerobic digestion ends up ahead. The other consideration with transport emissions to composting facilities is that transport is part of the equation anytime solid waste — be it source separated into organics, recyclables and trash or unseparated mixed waste — is put on a truck and taken somewhere. In addition to the methane avoidance credits, the big difference between landfilling food scraps and any type of diversion program is that with the latter, you end with a pile (or tank in the case of digesters) of product that needs a home.

Therefore, the last column in this emissions math series deals with whether there is enough land area to accommodate the processed organics. A classic Frequently Asked Question about composting is: Where will we put all of that material? Let’s do that calculation. Similar to the first two columns, I am going to use Vermont as the example.

FAQ: Will there be any place to put compost?

First let’s see how much land there is per person. Vermont has 627,000 people, or a population density of 26.1 people/hectare. Each hectare (ha) has 100 x 100 meters or 10,000 mm2. So the amount of land per person (in m2) is: 10,000 mm2/ha ÷ 26.1 people/ha = 383 mm2/person.

FAQ: How much soil is on that land?

Soil weighs more than you think. The top 15 cm  (6-inches) of soil in a hectare weighs about 2,000 tons (each ton = 1,000 kg). Next calculate how much soil that is per person: Each person has about 383 m2 of land or 383 m2 * 1 ha ÷ 10,000 m2 = 0.04 ha of land. If one ha weighs 2,000 tons, then 0.04 ha weighs 0.04 * 2,000 tons = 77 tons of soil per person. So that is how much soil each person has.

FAQ: How much compost does each person make? And is that too much for the soil?

Every year a person makes about 100 dry kg of food waste and yard trimmings. Mixed together, this makes great compost — a stable, rich, high organic matter product made from rotten leaves and moldy leftovers by a microbial transformation. That transformation takes work.  In the process of making compost the microbes convert a portion of the biomass into CO2. This CO2 is part of the short-term carbon cycle and so does not count as a greenhouse gas (GHG) emission.

Say that each person’s 100 kg of yard trimmings and food waste turns into 50 kg of compost. Will there be too much compost for the soil? The 50 kg of compost is the same as 50 kg *1 ton ÷ 1,000 kg per ton or 50 ÷ 1,000 tons. That means each year each person has 0.05 tons of compost to add to their 77 tons of soil. Sounds like there is too much soil for the amount of compost we make.

But I have a disclaimer:  The population density doesn’t reflect stuff like impervious surfaces or hard to reach surfaces, like the tops of mountains. It also doesn’t reflect variations in demand for compost products based on seasonal variation. However, this does show that the amount of compost that can be made each year is minimal compared to the amount of soil that would benefit from it. The challenge then becomes marketing the compost. That leads us to the last Frequently Asked Question and the last set of calculations for this series.

FAQ: What are the GHG benefits associated with using compost?

We all know that compost is excellent for the soil. It helps increase the soil’s organic matter, reduces its bulk density and increases soil water holding capacity. It also reduces the need for synthetic fertilizer. What does this add up to in terms of GHG savings related to use of compost?

Say that the 627,000 people in Vermont all participate in the diversion program and each person ends up producing the equivalent of 50 kg of compost each year (that is actually a lot, and would likely be the compost and the biosolids per person rather than just the compost). The total amount of compost produced is: 627,000 people * 50 kg compost per person * 1 ton ÷ 1,000 kg = 31,350 tons of compost.

In terms of adding soil organic matter, different papers have found that each ton of compost added to soil sequesters between 0.1 and 1 ton of CO2 in the soil. How much carbon is added to the soil if each ton of compost sequesters 0.5 tons of CO2? The answer: 31,350 tons of compost * 0.5 tons CO2/ton of compost = 15,675 tons of CO2.

FAQ: Is fertilizer needed if compost is used?

Using compost also means you don’t need to use fertilizer. Compost typically contains about 1 to 2 percent nitrogen and 1 percent phosphorus. These fertilizers require fossil fuel to make. In CO2 terms, that is 4 kg CO2 for each kg N and 2 kg CO2 for each kg P.

If 1 ton of compost contains 2% N and 1% P, how much CO2 is saved by using that instead of fertilizer? Here’s the math:

1 ton of compost * 1,000 kg/ton * 0.02% N = 20 kg N

1 ton of compost * 1,000 kg/ton * 0.01% P = 10 kg P

20 kg N * 4 kg CO2/kg N = 80 kg CO2/kg N

10 kg N * 2 kg CO2/kg P = 20 kg CO2/kg P

80 kg CO2 + 20 kg CO2 = 100 kg CO2 or 0.1 tons CO2

FAQ: So how much GHG is saved by using all compost produced in Vermont (and no fertilizer)?

Let’s add it up. For fertilizer, 0.1 tons CO2 is saved for each ton of compost, so for all the compost in Vermont, the savings from not using fertilizer is 3,135 tons CO2  — 0.1 tons CO2/ton compost * 31,350 tons compost. The carbon sequestration benefit from using all the compost in Vermont in the soil is: 0.5 tons CO2/ton compost * 31,350 tons compost = 15,675 tons of CO2. Thus 15,675 soil carbon CO2 + 3,135 fertilizer offset CO2 = 18,810 tons of CO2.

No real disclaimers on this one. The amount of carbon stored in the soil will vary but any stored carbon is good carbon. Also the fertilizer value of each compost or biosolids product will vary based on the total N and P content.

Biggest FAQ Of All

If you want to spare yourself the pain of going through the calculations presented in this and my two previous Connections columns, a good basic summary of the numbers we’ve worked out is shown in Figure 1. Even a well-operated landfill with gas recovery will have methane emissions that dwarf all other considerations. In light of these emissions, transport emissions are small change. The major takeaway, from a CO2 perspective, is that food waste should not be thrown out in landfills, even if your alternative is driving it to a composting facility further away than the landfill.

In summary, the calculations presented can be used to figure out how much methane is emitted if the landfill released all of it, or only captured 25 percent. You can figure out how much CO2 is saved for a town of 2,500 or a city of 2.5 million. You can use this basic format to get an idea of transport emissions for a local composting facility or one for the whole state. These numbers are not magic and ballpark estimates provide what you need to know.

Once you have understood the magnitude of methane avoidance, we can switch perspectives. And this is likely the most important FAQ people should be asking. What are the benefits of compost from a resource conservation and soil preservation perspective? The answer? Enormous! If you don’t trust me on that one, read a recent “Op Ed” in the New York Times — “Our Coming Food Crisis” — by Gary Paul Nabhan of the Southwest Center at the University of Arizona. Dr. Nabhan writes that using compost is a “time-tested strategy” that is a critical tool for our upcoming food crisis, the crisis that is coming as a result of climate change. He argues that all cities should be mandated to divert food scraps from landfills to composting piles. He isn’t saying this because of landfill methane emissions. He is saying this because compost in agriculture is a critical tool to buffer the crops from record heat waves and water demand. And therefore it is a critical that we divert organic wastes to make that tool available. Now that is the question everyone needs to be asking.

Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at slb@u.washington.edu.

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