Which Food-Waste Disposal Options Create the Least Invertebrate Suffering?

By Brian Tomasik

First published: 27 Feb 2018. Last nontrivial update: 2 Mar 2018.

Summary

Disposing of food scraps is a daily activity for most of us. Different methods of food-waste disposal create different amounts of invertebrate suffering. I tentatively suggest the following ranking for how to eliminate food waste, as viewed from the perspective of reducing the number of invertebrates created:

1. (best) Waste less food to begin with(?)

2. Throw out food in airtight bags/containers that will go to an incinerator (if that's how your trash is disposed of)

3. (tied for third place; don't know enough to rank among them)

4. Sink garbage disposal unit (such as InSinkErator units)

5. Disposal in a composting collection bina from which the food scraps will be sent to a large-scale composting facility

6. Home compost pile, home compost barrel, etc.

7. (worst) Worm-bin composting

If food scraps are already infested by bugs (e.g., a melon skin filled with fruit-fly larvae), it's best to kill the bugs in a minimally painful way first, such as by freezing them for a long time. That way the bugs won't be alive to endure crushing, shredding, drowning, or other similarly painful fates when you dispose of the food scraps.

Contents

Introduction

Compost piles/bins are full of worms and other invertebrates who have short lives and possibly painful deaths. These animals produce lots of offspring, many of whom may die soon after birth. Therefore, it causes suffering to those organisms if you have a compost pile/bin.

An aerobic worm-composting bin consuming the food scraps of just one person brings into existence a population of perhaps hundreds of thousands of invertebrates (worms, springtails, mites, beetles, etc.), who are born without their consent and who will soon die painfully. We can prevent this suffering by not using a composting bin. But what are the alternatives? This piece summarizes my current best guesses on this question.

Disposal methods

Not wasting food

Once organic matter exists, someone will eventually eat it (whether that "someone" is humans, other animals, fungi, or bacteria). If we care to some degree about the suffering of all life forms, then the most certain way to reduce biological suffering is to prevent food energy from being created in the first place, i.e., reduce primary productivity. Crop cultivation in Western countries on average may increase net primary productivity, and if so, it might be net bad, although there are many other considerations when assessing this question overall.

Consumer Food WasteIf crop cultivation is not net beneficial, then it seems best to reduce food consumption via wasting less food in the first place.b That said, I have high uncertainty about whether crop cultivation is net good or bad, and probably it varies from one crop type to another, so the recommendation to avoid wasting food is quite tentative.

When you don't waste food, the food doesn't need to be decomposed outside of your digestive system (except for whatever remains in your feces). Of course, some parts of foods are inadvisable to eat, so you typically can't completely avoid generating food scraps.

Incineration

Fire is one exception to the principle that "if organic matter exists, some life form will eventually eat it". Combustion is an abiological way to release the stored energy in plant food. Thus, burning food scraps should in principle entail even less suffering than allowing bacteria to decompose food scraps, ignoring the severe suffering of whatever invertebrates and microorganisms may be immolated during the burning process.

Incinerators are, in general, more popular in Europe than the USA. If your local waste-management system uses incineration, this seems like a good option, although I wonder how many invertebrates get burned to death.

Lobbying for more incineration of garbage (rather than landfilling, etc.) might reduce invertebrate suffering and is arguably environmentally friendly, though environmentalists disagree with one another about whether incineration is better than alternative waste-disposal methods. That said, I would intuitively guess that lobbying for incineration is less cost-effective at reducing invertebrate suffering than lobbying for, e.g., land-use changes that reduce net primary productivity, because my hunch is that the number of invertebrates whose births would be prevented by a change from landfilling to incineration is not huge relative to the total number of invertebrates in existence? My impression is that most organic-matter decomposition in landfills is already done by bacteria rather than invertebrates.

Burn barrels

One might hope to emulate incinerators at home, such as by using burn barrels. However, burn barrels are often illegal: "Burning household trash, whether in an open pit, burn barrel or a wood stove, is illegal, unhealthy, unneighborly and unnecessary."

Washington State Department of Ecology (2004) says that in Washington state, it's illegal to burn "Any substance other than natural vegetation which, when burned, releases toxic emissions, dense smoke or obnoxious odors". (I'm uncertain whether food scraps count as "natural vegetation".) In addition, "burning anything in burn barrels is illegal (WAC 173-425-050(5))."

Burning food waste can release carbon monoxide and small particulates (EEK! n.d., p. 7).

Another downside is that if there are any bugs or other critters trapped in the flames, they may endure one of the worst deaths imaginable. Freezing the food waste for a long period (weeks? months?) before burning it would hopefully kill most of the invertebrates it contains in a less painful way than death by burning.

Throwing out food in airtight containers

Main article: "Invertebrates Created by Throwing Out Food Scraps"

Garbage disposal unit

Main article: "Microorganisms Created by Wastewater-Treatment Systems"

Putting food down the sink drain is a convenient way to prevent bugs from eating it in your garbage can. Hickman (2006) reports on a New York City study that, unsurprisingly, found that allowing garbage disposal units would result in "fewer 'disease vectors' (foxes, rats, flies, cockroaches etc) in the city". Diggelman and Ham (2003): "requiring citizens to use [garbage disposal units] can contribute to reducing public health issues (odour, rodents and insects) related to rotting food waste" (p. 513).

Unfortunately, after organic matter is flushed down the drain, it may feed some invertebrates in sewage-treatment plants. However, my impression is that for common sewage-treatment methods like activated sludge, most of the decomposition is done by unicellular organisms, especially bacteria, rather than by invertebrate animals.

Paster (2009): "According to InSinkErator, 70% of food scraps are water, but some of the remaining 30% are solids that are screened out at the entrance to your waste water treatment plant. In most cases this material is also sent to the landfill". And WBUR (2014): "many waste water treatment plants dispose their bio-solid end product into landfills; thus, the food waste ends up in the landfill even if consumers send it down their drains." So, to a greater or lesser degree depending on your wastewater-treatment system, putting food scraps down the drain has some of the same impacts as landfilling. That said, even if both screened-out pre-treatment scraps and post-treatment sewage sludge go to landfills, wastewater treatment still decomposes some fraction of the input organic matter within the treatment plant itself. For example, dissolved and small suspended organic matter presumably passes through screens and enters the treatment plant. So I infer that there's less total organic-matter decomposition in landfills when food scraps go down the drain than when the food scraps are sent directly to the landfills.

In the "Summary" of this piece, I tentatively ranked garbage disposal units as worse than landfilling for a few reasons that will be mentioned later: zooplankton killed by using tap water to run garbage disposal units, greater eutrophication potential with garbage disposal units, more metabolic energy extractable from aerobic decomposition than anaerobic, and greater potential for nutrient and organic-matter recycling with sewage treatment than with landfilled waste. However, these considerations are not the only relevant variables, and I remain very uncertain on the relative ranking of landfilling vs. garbage disposal units.

Composting

Main article: "Invertebrates Created by Composting"

In the "Summary" of this piece, I ranked large-scale composting as somewhat worse than using a garbage disposal unit because my very rough impression is that large-scale composting operations may give rise to more invertebrates per unit of food decomposed than sewage-treatment plants do (?), although I don't have hard data on this. One reason for thinking composting supports more invertebrates is that post-thermophilic compost piles presumably allow for longer lifespans of the decomposer organisms than activated-sludge sewage-treatment plants do. Larger, non-bacterial organisms would seem to have more of a chance to grow in post-thermophilic stages of compost than in activated-sludge tanks.

I suspect that home compost piles are generally worse than large-scale composting systems because home compost piles are less likely to be as hot for as long as large-scale compost windrows. Therefore, I presume that most home compost piles contain more total invertebrates per unit of food decomposed than large-scale compost windrows do?

I suspect worm-bin composting is worse than a regular compost pile because worm bins don't have an thermophilic phase during which few invertebrates are present. And worm bins by design contain lots of invertebrate biomass in the form of earthworms.

Death by overheating intuitively sounds extremely painful, and when compost goes thermophilic, many of the invertebrates already present in the pile or windrow probably suffer awful deaths. If you think these deaths are sufficiently painful, then you might question whether thermophilic composting systems are actually better than non-thermophilic ones in terms of creating less total invertebrate suffering per unit of food decomposed.

Climate change

Another big consideration in this analysis is the greenhouse-gas emissions created by different methods of breaking down food waste. Tomasik ("Invertebrate ...") does a back-of-the-envelope calculation for how much climate change might matter compared with the invertebrates directly produced or averted by different decomposition approaches. The conclusion:

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.

Other considerations

Methods of food-waste disposal differ along a few other dimensions beyond those discussed above.

Fertilizer

Composting recycles nutrients, and if compost is applied to farms or lawns, it increases plant growth, which creates more food energy that will, regrettably, feed more invertebrates down the road. Likewise, biosolids from sewage may be used to fertilize crop fields.

I would guess that organic wastes in landfills don't appreciably increase subsequent plant growth because they're buried far underground? So on the dimension of "reducing nutrients available for future plant growth", landfilling seems to be the best option. Consistent with this point, Wikipedia ("Biosolids") explains: "Encouraging agricultural use of biosolids is intended to prevent filling landfills with nutrient-rich organic materials from the treatment of domestic sewage that might be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth."

It's not clear how much the fertilizer consideration matters, because farmers would probably use synthetic fertilizers if they didn't use other fertilizers. ReFED (2016) reports that "It is difficult for compost to compete on price with synthetic fertilizer, which benefits from cheap oil and large production economies of scale driven by industrial agriculture." Of course, compost and biosolids are not just fertilizer but also serve as soil conditioner. And the existence of fertilizers derived from food scraps may slightly increase total fertilizer use, by lowering prices or at least increasing farmers' options. Kansas City's chief agronomist, interviewed in NPR (2013), estimates that the biosolids he receives for free are "worth nearly $200,000 year. And he'd use more of it, if he could get it." Diggelman and Ham (2003), citing other sources, report: "The Madison Metropolitan Sewerage District [in Wisconsin, USA] estimates an average fertilizer value of $15/dry ton; at MMSD the demand for sludge by farmers exceeds the supply" (p. 512).

Eutrophication

Even if farmers would just use synthetic fertilizers to replace biosolids, one can't make a similar argument about eutrophication; it wouldn't just happen to the same degree in the absence of sewage treatment. Sewage-treatment plants can release nutrients into receiving water bodies, increasing algae growth and perhaps invertebrate populations. Lundie and Peters (2005) found that disposing of food waste down the drain had about 3 times the eutrophication potential of landfilling (Table 3, p. 284), which is one way in which landfilling seems plausibly better than using garbage disposal units.

Covering over land

One other, probably minor, consideration when evaluating food-disposal methods is whether and how much natural primary production they prevent by covering land area.

Water use by garbage disposal units

When I got an apartment that had a garbage disposal unit, the landlord told me that when using it, I should turn the water on before turning the unit on, and I should keep the water running until the food is shredded. Other sources agree. Plumbing Services Inc. (2016) says: "Disposals are designed to work with wet substances so run COLD water briefly before you turn on the disposal and while it’s running.[...] Let the disposal, and water, run for a minutec after the food is done grinding up to ensure that any small debris have made it out of the system." "Garbage Disposals" (2010) says: "Run cold water whenever running the garbage disposal, as it keeps the motor and other operating parts from overheating, as well as assisting the waste to go down the drain pipe easier."

Hickman (2006) reports on a study finding that garbage disposal units "use one litre per day on average, but others say this can be as high as 16 litres [...] per use." Lundie and Peters (2005)d report that garbage disposal units require "12.4 L water/kg of food waste" to operate (p. 280). This seems to be kg in terms of wet weight.e Diggelman and Ham (2003)f report a similar number.g

Many foods are mostly water. Berkeley Wellness (2011) reports water fractions between 75% and 96% for some common high-water-content foods. Diggelman and Ham (2003) report (p. 503) that "food waste is 30% solids and 70% water (Morgan 1995)". Using this last estimate, 1/0.3 ≈ 3 kg of wet-weight food waste would contain about 1 kg of dry-weight food waste. Given 12.4 L of water used to grind up a kg of wet-weight food waste, 12.4 / 0.3 ≈ 40 L of water are required to put 1 kg of dry-weight food waste down the drain.

Tomasik ("Water ...") estimates that typical withdrawal of surface water for home use kills roughly ~10 crustacean zooplankton per L. This is a somewhat conservative estimate, so let's bump it up to 100 zooplankton per L, especially if we include rotifers in addition to crustaceans. Thus, disposing of 1 kg of dry-weight food waste down the drain would kill (40 L) * (100 zooplankton per L) = 4000 zooplankton.

Meanwhile, how many invertebrates are affected by preventing the food waste sent down the drain from being composted? Tomasik ("How ...") 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.)." I'll assume 3 springtail-years to be conservative. Tomasik ("How ...") further suggests that a springtail-year might contain on the order of ~10 springtail lifetimes and deaths. Assuming that home composting of food waste creates similar numbers of springtails as decomposition on grassland per gram of organic dry matter decomposed, then 1 kg of food-waste dry matter would create (3 springtail-years per g) * (1000 g per kg) * (10 springtail deaths per springtail-year) = 30,000 springtail deaths, not counting other decomposer animals. To be (overly) conservative, I'll assume that the number is only 30,000 invertebrates of any type, not just springtails.

This is the number of invertebrate deaths caused by home composting of the food. Of course, wastewater disposal of food also creates some invertebrates. If we assume that wastewater disposal creates, say, 50% fewer invertebrates than home composting (just to make up a number), then wastewater disposal prevents 0.5 * 30,000 = 15,000 invertebrate births and deaths per kg of dry matter relative to home composting. This is only a few times more than the 4000 zooplankton killed by the flushing water of the garbage disposal unit. Given the high uncertainty in these estimates (15,000 and 4000), these two quantities seem potentially within each other's margins of error.

Still, I still suspect the garbage disposal unit wins because it's not clear if killing zooplankton through water use is net bad, given that the zooplankton would have died in some other way later on. Meanwhile, the extra invertebrates created by home composting relative to wastewater treatment wouldn't have existed at all if the food had been sent down the drain rather than composted, which means their deaths would have been completely averted.

That said, this consideration makes garbage disposals look slightly worse than landfilling along the dimension of killing zooplankton, although I don't know what the full water footprint of landfilling is.

If you live in an area where tap water comes from groundwater, then garbage-disposal water use probably kills considerably fewer zooplankton than what I calculated here.

Other environmental impacts

Lundie and Peters (2005) examined 8 environmental impacts of methods of food-waste disposal, including eco-toxicity, acidification potential, and so on. In principle, these effects should be considered as part of a complete evaluation. However, all the methods of food-waste disposal had pretty small impacts with respect to eco-toxicity and acidification potential (Table 4, p. 284).

Energy extractable from aerobic vs. anaerobic respiration

So far I've been assuming that decomposition by bacteria alone is preferable to decomposition by some invertebrates + some bacteria on the assumption that bacteria suffer less per joule of food energy burned than invertebrates do on account of the cognitive simplicity of bacteria. However, this is a debatable proposition, since one might think that the immense numerosity of bacterial decomposers compensates for the simplicity of any given one of them.

However, there may be another argument in favor of anaerobic bacterial decomposition: less total energy can be extracted from food through anaerobic respiration. So even if bacteria matter morally the same amount as invertebrate animals per joule of energy used, anaerobic bacteria presumably extract fewer joules of energy from a given amount of organic matter? (This is my best guess based on preliminary reading, but I'd like to learn more about this topic.) To the extent this is true, it seemingly favors landfilling food waste (where decomposition is mostly anaerobic) over aerobic composting and garbage disposal units. (Municipal wastewater treatment is typically aerobic, though residual sewage sludge is sometimes digested anaerobically.)

Aggie Horticulture (2009): "much less heat is generated in anaerobic decomposition than in aerobic decomposition."

Wolfe (2001): "the anaerobic respiration pathways release less net energy than the aerobic approach" (p. 47). Wikipedia ("Cellular respiration") explains that aerobic respiration generates ~29-30 ATP molecules per glucose molecule, while anaerobic respiration generates 2 ATP molecules per glucose molecule, which makes aerobic respiration ~15 times more efficient. That's a huge difference in the amount of energy extracted. So even if one cared slightly more about bacteria than about invertebrate animals per joule of food energy burned, anaerobic respiration done by bacteria might still entail less suffering than aerobic respiration done by bacteria and larger critters? (This is especially so given that invertebrates have lots of bacteria within their digestive systems, so some organic matter eaten by invertebrates still ends up being eaten by bacteria.) Of course, I wonder if the end products of anaerobic respiration, such as ethanol, can themselves be used to power other microorganisms? Also, as Wikipedia ("Cellular respiration") notes, "some anaerobic organisms, such as methanogens are able to continue with anaerobic respiration, yielding more ATP" than just 2 per glucose molecule.

Methanogenesis is a form of anaerobic respiration. As far as scientists know, only archaea can perform methanogenesis. (When I say "bacteria" in this piece, I mean "bacteria and/or archaea".) Methane has more energy than CO2 (indeed, methane can be burned to generate CO2), which suggests that less energy is released by methanogenesis of a given organic molecule than by aerobic respiration.

Of course, since methane has more energy than CO2, other organisms (methanotrophs) might later be able to extract some of that energy. Indeed, Lessner (2009) says that most methane is consumed by other organisms:

The anaerobic decomposition of biomass is an integral component of the global carbon cycle, producing approximately one billion tonnes of methane annually, most of which is oxidized to carbon dioxide separately by strictly aerobic methanotrophic bacteria and anaerobic methanotrophic microbes. However, a significant amount of methane escapes into the atmosphere where it is a potent greenhouse gas that contributes to global warming.

Some of that atmospheric methane will get converted to CO2 through abiotich (and hence not very sentient) processes, in which case the stored energy in methane won't power life forms.

In addition, I would conjecture that methanogenesis + methanotrophy using a given amount of food waste produces less total biologically usable energy than aerobic respiration would (since otherwise, why would we have evolved to use aerobic respiration?), but I haven't done enough reading to verify this conjecture.

Anyway, there are other metabolic processes besides methanogenesis, such as fermentation, which Bokashi composting uses. Again, I would guess that these are less efficient than aerobic respiration, but I don't know for sure.

Methane capture

Environmentalists typically prefer for wastewater-treatment facilities and landfills to capture methane, so that it can be used for electricity and heat rather than being emitted into the atmosphere.

American Biogas Council (n.d.) has a map of biogas landfills, wastewater plants, etc. in the USA. Stevens (2013) says that "According to biogasdata.org, more than 1,200 U.S. wastewater treatment plants have anaerobic digesters producing biogas". Leibrock (2013) says that while "43 percent of the wastewater treatment plants in the United States are currently using anaerobic digestion in some way, according to biogasdata.org", "Only 104 of those plants[...] use it to generate energy".

Laumer (2008) says:

Only a very small fraction of sewered kitchens in North America or Europe discharge to wastewater treatment plants (WWTPs) with operating, full scale, bio-gas capture facilities. And, we see no evidence of a strong movement toward municipalities installing bio-gas fired generators on central wastewater treatment plants. Conversely, there is a very strong movement toward capture of landfill-produced bio-gas in North America.

Paster (2009), who opposes putting food scraps down the drain, says:

The EPA requires that new and modified landfills designed to hold 2.5 million cubic meters install gas collection and control systems and California has this requirement for all new landfills. Some landfills even use the methane to generate electricity or send it to natural gas pipelines (Methane = Natural Gas). Landfills that are not legally required to do so can earn greenhouse gas emission reduction credits for capturing emissions. So far no state (that I have been able to find) requires the capture and destruction of methane gas emitted by waste water treatment plants.

EPA (2009): "Of the 2,300 or so currently operating or recently closed [municipal solid waste] landfills in the United States, more than 420 have [landfill gas] utilization projects." Wikipedia ("Landfill"): "In some countries, landfill gas recovery is extensive; in the United States, for example, more than 850 landfills have active landfill gas recovery systems." Wikipedia ("Landfill gas ..."): "The number of landfill gas projects, which convert the gas into power, went from 399 in 2005 to 519 in 2009 in the United Kingdom, according to the Environment Agency."

It's worth keeping in mind that a lot of the decomposition in activated-sludge wastewater-treatment processes is aerobic (i.e., non-methane-producing), while I think most decomposition in landfills is anaerobic (methane-producing). Diggelman and Ham (2003) estimate that "The mass of food waste in primary and attributable to food waste in secondary sludge sent to [anaerobic] digesters is used to determine that 4.8 kg of methane is produced from 100 kg of food waste" (p. 508). Meanwhile, Diggelman and Ham (2003) estimate "6.8 kg of methane produced anaerobically from 100 kg food waste in the landfill, two-thirds recovered" (p. 509). So about (1/3) * (6.8 kg) ≈ 2 kg of methane are released from a methane-capture landfill. Even if the methane from sludge digestion were entirely released, it would only be a bit more than twice as much as is released by the methane-capture landfill. (In fact, the particular sludge digester that Diggelman and Ham (2003) examined captured its methane (p. 508), but I'm making this point regarding anaerobic sludge digesters that don't capture their methane.)

While somewhat less favored by environmentalists than methane capture, flaring landfill methane rather than releasing it is also good from the standpoint of reducing suffering because the stored chemical energy in the methane is burned rather than used to power biological creatures like methanotrophs. EPA (2009) explains: "Current EPA regulations under the Clean Air Act require many larger landfills to collect and combust [landfill gas]. There are several compliance options, including flaring the gas". So even if a landfill doesn't capture its methane, it may still combust its methane.

Footnotes

  1. Wikipedia ("Source ..."): "Source Separated Organics (SSO) is the system by which waste generators segregate compostable materials from other waste streams at the source for separate collection. [...] Organic materials collected in SSO programs typically get delivered to composting facilities where the waste is turned into nutrient-rich soil amendments known as compost. Organic feedstock can also be delivered to anaerobic digestion facilities that produce biogas, a source of renewable energy." In the current article I haven't discussed anaerobic digestion of food scraps except in the context of sewage-sludge treatment, but I presume that anaerobic digestion creates many fewer invertebrates per kg of food waste than aerobic composting does.  (back)
  2. Note that the net impact of crop cultivation on total suffering partly depends on how the harvested food is disposed of. There could be some cases where growing crops is

    • slightly worse than leaving potential farmland as native vegetation if the harvested food is composted and feeds lots of bugs, but
    • slightly better than leaving potential farmland as native vegetation if the harvested food is decomposed by bacteria and fungi only.

    Here are some made-up numbers to illustrate the above point. Suppose that over one year, native vegetation creates 50 units of suffering per hectare, farming creates 40 units of suffering per hectare not counting disposal of the harvested human-edible food, disposing of the harvested human-edible food grown on that hectare using only bacteria/fungi would create 5 units of suffering, and invertebrate-filled composting of the harvested human-edible food would create 15 units of suffering.

    However, in some cases I expect that crop cultivation will still be either net good or net bad regardless of how the harvested human-edible food is disposed of. For example, suppose that in one year, native vegetation creates 50 units of suffering per hectare, while farming creates 52 units of suffering per hectare not counting disposal of the harvested human-edible food. Then regardless of the food-disposal method, creating demand for farmed food would be bad.  (back)

  3. I would guess that "a minute" here just means "a moment" rather than a literal minute?  (back)
  4. Note: InSinkErator, a producer of garbage disposal units, played some role in this study (pp. 275-76): "In-Sink-Erator approached the Cooperative Research Centre for Waste Management and Pollution Control (CRCWMPC) for assistance regarding an environmental, technical, economic and social assessment of their product."  (back)
  5. How can I tell? Lundie and Peters (2005) define their "functional unit" as "the management of the average amount of food waste produced by a household in 1 year", adding that "In the Waverley Council area, this amounts to 182 kg (wet) per annum" (p. 277). Later, the authors explain that garbage disposal units require 2335 L of water per functional unit, 97% of which is used during operation of the device. So the water used per wet kg of food waste is 0.97 * 2335 L / 182 kg = 12.4 L/kg.  (back)
  6. A note on possible conflicts of interest: Diggelman and Ham (2003) report that "The partial support for this research project received from the National Association of Plumbing-Heating-Cooling Contractors is gratefully acknowledged, as is help from numerous professionals involved with the various technologies who provided data and reviewed key aspects of the project" (p. 514).  (back)
  7. Diggelman and Ham (2003) say regarding garbage disposal units: "1031 kg [i.e., 1031 L] carrier water is required per 100 kg food waste" (p. 504), which is 10.31 L per kg. I assume this is kg of wet food weight because Diggelman and Ham (2003)'s analysis is based on wet weight (e.g., see p. 501).

    Diggelman and Ham (2003) also note that this water-use estimate "is subject to change depending on household practices. For example, if the [garbage disposal unit] is used only while rinsing dishes, no additional water would be required for [garbage disposal unit] usage. Conversely, if the [garbage disposal unit] is always used separately, more carrier water may be used" (p. 511).  (back)

  8. Wikipedia ("Atmospheric methane"):

    The major removal mechanism of methane from the atmosphere involves radical chemistry; it reacts with the hydroxyl radical (·OH) in the troposphere or stratosphere to create the CH·3 radical and water vapor. [...]

    The methyl radical formed in the above reaction will, during normal daytime conditions in the troposphere, usually react with another hydroxyl radical to form formaldehyde. [...] Formaldehyde can react again with a hydroxyl radical to form carbon dioxide and more water vapor.

      (back)