by Brian Tomasik
First published: 28 Sep. 2017; last update: 17 Dec. 2017
Small-scale aerobic composting bins are often full of worms, flies, and/or other invertebrates. These tiny animals are born without their consent into short lives that soon end with possibly painful deaths. For this reason, I dispose of my food scraps into sealed plastic containers that prevent invertebrates from decomposing the waste.
Perhaps the best argument against my method of food-scrap disposal is that anaerobic decomposition in sealed containers and landfills generates methane, a potent greenhouse gas. The net impact of climate change on global invertebrate populations is unclear, but there's at least some chance that climate change will increase the number of invertebrates in the world (Tomasik "Climate ..."). If climate change does end up increasing invertebrate populations, is it possible that anaerobic decomposition of food scraps actually increases invertebrate populations more than aerobic, bug-filled composting does? That's the question I explore in this piece.
A very rough and uncertain calculation suggests that we can't tell whether the number of invertebrate-years created directly by composting organic matter is larger or smaller than the size of the "error bars" for anaerobic decomposition's net impact on global invertebrate-years via climate change. However, because the sign of the net impact of climate change on global invertebrate populations is very unclear, while it's very clear that aerobic composting increases invertebrate populations in compost piles, we should for now continue to favor anaerobic decomposition from the perspective of reducing invertebrate suffering, pending better calculations on this matter.
Food scraps disposed of in sealed containers may generate methane within those containers or when decomposing anaerobically in a landfill. Wikipedia ("Methanogenesis") explains that methanogenesis "is the primary pathway whereby most organic matter disposed of via landfill is broken down."
In contrast, I'll assume in this calculation that aerobic composting of food waste produces only carbon dioxide with no methane. This is unrealistic because in practice compost piles may contain some anaerobic regionsa, but ignoring any methane produced by bug-filled composting only makes my calculation conservative, because I'm overstating the climate-change difference between composting vs. landfilling of food waste.
Banks (2009) gives a helpful example calculation of methane production from household waste (pp. 9-10).b Banks (2009) supposes that we have 322 kg of organic dry matter that has "50% carbon content". Thus, the mass of C in the waste is 0.5 * 322 kg = 161 kg. Banks (2009) assumes that only 60% of carbon is biodegraded, but it seems to me that more than this might eventually be biodegraded over very long time horizons? To be cautious and overestimate methane production, I'll assume that 100% of carbon is biodegraded.c Then, based on the Buswell equation, Banks (2009) estimates that 55% of the biogas produced will be methane and 45% will be CO2.d Thus, 0.55 * 161 = 89 kg of C gets turned into methane. Since the molecular weight of methane is 16, compared with 12 for C, 89 kg of C turns into 89 * 16/12 = 120 kg of methane. Meanwhile, 0.45 * 161 = 72 kg of C turns into CO2. The molecular weight of CO2 is 12 + 2 * 16 = 44, so we have 72 * 44/12 = 270 kg of CO2.
How about for aerobic decomposition? Here we'll assume that the 161 kg of C in the waste is all converted to CO2: 161 * 44/12 = 590. kg of CO2.
Tomasik ("Scenarios ...") estimates (in the subsection titled "How many insect-years?") that 1 metric ton of CO2 causes (over the very long term) a change in expected invertebrate-years of suffering of somewhere between -6 million and +6 million. The impact for one metric ton of methane (over the very long term) is ~3.7 times that amount (see the end of the "Putting it all together" section).e
So, the 590. kg = 0.590 metric tons of CO2 from aerobic decomposition would produce something in the range of ±(0.590 * 6) million ≈ ± 4 million invertebrate-years of suffering.
How about anaerobic decomposition? The 270 kg of CO2 would produce ±(0.27 * 6) million ≈ ± 2 million invertebrate-years of suffering, and the 120 kg of methane would produce ±(0.12 * 3.7 * 6) million ≈ ± 3 million invertebrate-years of suffering. The total is roughly ± 5 million.
So anaerobic decomposition increases the "error bars" on invertebrate-years of suffering by ~1 million, from ± 4 million to ± 5 million.
Rough impact of directly creating invertebrate decomposers
How many invertebrate-years of suffering would be supported by food scraps decomposed aerobically?
Tomasik ("How Many Springtails ...") estimates "that on a typical grassland, each gram of dry matter of vegetation creates roughly 3 to 6 springtail-years, as well as life for many other invertebrates (mites, earthworms, etc.)." To be conservative, let's take this as 3 springtail-years per gram = 3000 per kg. I'll assume the same number of springtails would be produced by food scraps.f (This may be conservative, since household food waste may often be more calorific per gram of dry matter than grass is.)
My climate-change calculations above were based on 322 kg of organic dry matter. That same amount of food waste implies 322 * 3000 = 1 million springtail-years created by composting. This figure of 1 million invertebrate-years is the same as the increase in error bars that we found in the previous climate-change calculation. (This is interesting, because I didn't rig the input numbers to come out so close.)
Thus, it remains an open question whether climate-change side effects might exceed in importance the harm of directly feeding lots of invertebrates in aerobic compost. However, because the climate-change impacts could go either way, while aerobic composting clearly creates extra invertebrate-years, it seems that aerobic composting is net worse in expectation than sealing food scraps and landfilling them, relative to my current estimates.
Also, my springtail calculation omits mites, earthworms, flies, beetles, and other bugs that decompose organic matter.
What if we give moral weight to very small organisms?
Tomasik ("Scenarios ...") focused on insects (rather than invertebrates in general) and assumed that there are 1018 insects on Earth at any given time (see the section "How many insect-years?"). However, there may be ~1019 to ~1022 nematodes on Earth (Tomasik "How Many Wild ..."). So shouldn't the climate-change numbers potentially be much higher?
Maybe, but the composting numbers should be higher to a similar degree. I roughly estimate that there are ~1018 springtails on Earthg—the same as the number of insects assumed in Tomasik ("Scenarios ..."). So if nematodes outnumber springtails by some multiple M, then the numbers of invertebrates created/prevented by climate change and created by composting should also be multiplied by M. For example, if there are, say, 1020 nematodes on Earth, then the climate-change numbers should be multiplied by M = 1020 / 1018 = 102 if we want to include nematodes. And the composting numbers should also be multiplied by ~102 if we want to include nematodes, because if there are 1020 / 1018 = 102 nematodes for every springtail on Earth, I assume there are ~102 nematodes for every springtail in compost.h This would leave the relative comparison of the magnitudes of these impacts the same as before.
If you care somewhat about bacteria and other non-animal microorganisms, then anaerobic decomposition of organic matter still creates some suffering. However, as long as you think that decomposition done by bacteria gives rise to somewhat less direct suffering (ignoring climate change and other side effects) than decomposition that also includes invertebrates, then the order of magnitude of my numbers above won't change. For example, if aerobic, invertebrate-dense composting of a given quantity of food waste creates X units of direct suffering (ignoring climate change and other side effects), while anaerobic degradation of that food waste creates 0.6 * X units, then the increase in suffering due to aerobic composting is 0.4 * X, which is of the same order of magnitude as X. The magnitude of uncertainty on my calculations in this piece may be several orders of magnitude, so small considerations like this don't change the big picture, which is that it's very unclear whether direct suffering or indirect climate-change effects (whether net good or net bad) dominate the calculation.
Alternate methane estimates
Lundie and Peters (2005) examine the environmental impacts of various methods of food-waste disposal. One disposal method is amateur home composting in which the compost bin is anaerobic: "there is a strong risk that residential composters do not properly mix and aerate their compost bins, nor have the oxygenating assistance of earthworms" (p. 278). The authors estimate that anaerobic composting of food scraps could create 273 kg CO2-eq. per functional unit (Table 1, p. 280), where "For this study, the functional unit is defined as the management of the average amount of food waste produced by a household in 1 year. In the Waverley Council area, this amounts to 182 kg (wet) per annum" (p. 277).
Diggelman and Ham (2003), citing a source called Morgan (1995) that I can't find online, say (p. 503) that "food waste is 30% solids and 70% water". So 182 kg of wet mass should be roughly 182 * 0.3 = 50 kg of dry matter.
What is 273 kg CO2-eq. in terms of methane? I'm not exactly sure, but Lundie and Peters (2005) say that their estimate of "Climate change is based on  using a 100 year timescale" (p. 279), where source "" is a 1995 Intergovernmental Panel on Climate Change (IPCC) report, 584 pages long. I haven't read that source itself, but I assume that it uses global warming potentials similar to those in Wikipedia ("Global"). In particular, Wikipedia ("Global") reports that in 2001, IPCC used a 100-year global warming potential for methane of 23, which means 273 kg CO2-eq. translates to about 273/23 = 12 kg methane.
My calculation earlier in this piece suggested that 322 kg of organic dry matter would give rise to 120 kg of methane (as well as some CO2), which is 0.37 kg of methane per kg of organic dry matter. The estimate based on Lundie and Peters (2005) suggests that 50 kg of dry matter would give rise to 12 kg of methane, which implies 0.2 kg of methane per kg of dry matter. It's reassuring that these numbers are pretty close (0.37 vs. 0.2). The Lundie and Peters (2005) number is slightly more conservative than mine, which would make the climate-change estimates a bit lower (but not enough to affect the basic conclusion of this piece, which is that it's uncertain whether climate-change impacts are more significant or less significant than direct creation of invertebrates by aerobic composting).
As one final estimate, Diggelman and Ham (2003) assume that "6.8 kg of methane [are] produced anaerobically from 100 kg food waste in the landfill" (p. 509). Again assuming that food waste is 70% water, this implies 6.8 kg of methane for 30 kg of organic dry matter, or 6.8/30 = 0.2 kg of methane per kg of dry matter. Happily, this is the same as what I calculated above based on numbers from Lundie and Peters (2005).
In my methane calculation earlier in this piece, I said: "Banks (2009) assumes that only 60% of carbon is biodegraded, but it seems to me that more than this might eventually be biodegraded over very long time horizons? To be cautious and overestimate methane production, I'll assume that 100% of carbon is biodegraded." If I had stuck with 60%, my "methane per kg of organic dry matter" would have been not 0.37 kg but (0.37 kg) * (0.6) = 0.2 kg, the same as the alternate methane estimates based on Lundie and Peters (2005) and Diggelman and Ham (2003). I don't know if this explains the discrepancy between my number and theirs or if it's just coincidence.
- Footer (2014) explains (pp. 11-12):
One of the dirty little secrets in the composting world is that traditional composting methods inherently generate greenhouse gasses (GHG). This is never mentioned in traditional composting circles, but is actually a pretty big problem. [...]
Even under the best aerobic composting conditions, greenhouse gasses are emitted. But in reality most people don’t maintain perfect aerobic compost piles; they let them go anaerobic, and that is a cause for concern. How many people start actively aerating a compost pile but then give up over time, leaving it untouched and anaerobic? A lot. When organic material composts anaerobically, nitrous oxide, ammonia, and hydrogen sulfide are produced. [...] Methane is also produced in the pile when methane-producing microbes take over. [...] The average home composter isn’t harvesting the methane so the gas goes into the atmosphere.
- I'm unsure whether Banks (2009)'s calculation is about anaerobic digestion of regular garbage (including paper, plastic, metal, etc.) or just of organic wastes. He says that not all the dry matter in the waste is organic dry matter. But this issue is irrelevant for my calculations here, because I focus on the "organic dry matter" portion of Banks (2009)'s numbers. (back)
- Diggelman and Ham (2003), p. 509: "It was assumed that 95% of food waste solids are decomposable [...] of which 84% is decomposed in the landfill during the design life". So it looks like 0.95 * 0.84 = 80% of solids are decomposed during the design life? Of course, I assume more will decompose after the landfill is retired. (back)
- Here's an independent confirmation of this rough estimate. Banks (2009) reports (p. 5) that the Buswell equation is
CcHhOoNnSs + (1/4)*(4c - h - 2o + 3n + 2s)H2O → (1/8)*(4c - h + 2o + 3n + 2s)CO2 + (1/8)*(4c + h - 2o - 3n - 2s)CH4 + nNH3 + sH2S.
C21.53H34.21O12.66N1.00 + 7.40H2O → 10.03CO2 + 11.50CH4 + NH3.
The fraction of methane compared with CO2 is 11.50/(11.50 + 10.03) = 53.41%.
- Why isn't the multiplier for methane relative to CO2 higher? It's because my estimates for the impact of greenhouse gases look at a long time horizon (1 million years), and CO2 lasts in the atmosphere vastly longer than methane. If I were to only look at a shorter time horizon, like 100 years, then methane would have relatively more impact than CO2, but the absolute impact of both greenhouse gases would be smaller than when using a long time horizon. Hence, using a shorter time horizon would make it less likely that climate-change considerations would dominate over the direct invertebrate-creation impact of aerobic composting. (back)
- Of course, some vermicomposting bins may contain few or no springtails, but they may contain other invertebrates of similar size, like mites. I'm focusing on springtails just as an example of a highly abundant but small invertebrate. (back)
- Springtail densities may be ~104 per m2 of soil = 108 per hectare (Fleming 2015), and the area of non-desert and non-mountain land on Earth is 15.77 billion acres = 6.382 billion hectares (Pianka n.d.). Multiplying this by 108 per hectare gives almost 1018 springtails on Earth. (back)
- I'm ignoring the fact that nematodes can live in marine sediments, while I assume springtails can't. But assuming that at least a decent fraction of all nematodes live on land, then the ratio of (total nematodes on Earth) / (total springtails on Earth) should be pretty similar to the ratio (total nematodes in soil) / (total springtails in soil). (back)