Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Landfill methane gas emissions

  1. Aug 18, 2015 #1
    I read that biodegradable waste in landfills produces methane, a harmful GHG. But didn't biodegradable waste always exist? We throw paper away, but the trees we produce paper from would have died eventually. Our organic waste comes from plants and animals that would have died eventually too. So wouldn't they have produced methane anyway? What difference does our contribution in landfills make?

    People say composting is environmentally friendly, but isn't it the same? Also, what about waste to energy incineration?

    How do composting, landfills, and WtE compare in terms of GHG emissions?
     
  2. jcsd
  3. Aug 18, 2015 #2
    Yes. Methane is a powerful greenhouse gas. Yes it has always existed. It is a question of how much of it existed.

    Decaying vegetable material has always produced methane, and lots of it. But we add additional methane from plastic degradation and other oil industry sources.

    Understand we have about 40º of greenhouse heating on earth. Yet just 2-3º of extra heating will be devastating to our environment. So we don't need to add much extra to hurt ourselves.

    Methane is a strong, persistent greenhouse gas. CO2 is a weak, persistent gas and water vapor is strong but not persistent. We add lots of all three unnaturally to the atmosphere. That can't be good. (Well, since the water is mostly in the form of crop irrigation for food, perhaps that one is acceptable.)
     
  4. Aug 18, 2015 #3
    I though plastic doesn't biodegrade?

    And how do composting, landfills, and WtE compare in terms of GHG emissions?
     
  5. Aug 18, 2015 #4
    Plastic normally biodegrades very slowly (thousands of years). But there are plastics designed to biodegrade more quickly. They are often used in disposable consumer products.

    For good or ill, almost nothing biodegrades in landfills anyway. Only a small percentage of the mass in a typical landfill gets wet enough for breakdown.
     
  6. Aug 18, 2015 #5
    Okay.

    But is composting or waste to energy more environmentally friendly?
     
  7. Aug 18, 2015 #6

    russ_watters

    User Avatar

    Staff: Mentor

    There is a metric called Global Warming Potential that describes how something compares to carbon dioxide as a greenhouse gas. Over a 20 year period, methane's GWP is 86 (86x CO2). https://en.m.wikipedia.org/wiki/Global-warming_potential

    However in terms of the impact of composting: everything in a compost decays whether you put it in a compost pile or not, so it should be carbon neutral.... Unless it would otherwise be going to a landfill where the methane is to be harvested.
     
  8. Aug 18, 2015 #7
    So whats the difference in composting and landfills for organic waste? Don't both produce methane?

    And what about incinerating waste? In terms of global warming potential?
     
  9. Aug 18, 2015 #8

    russ_watters

    User Avatar

    Staff: Mentor

    As far as I know, the only differences are that you save the landfill space and get to use the compost as fertilizer.
    I would think it is dirtier to burn it directly than to let nature convert it to methane first and then burn it, because the rest of the waste stays in the landfill instead of being thrown into the atmosphere.

    That is a subject of some controversy:

    http://switchboard.nrdc.org/blogs/ngreene/biomass_not_carbon_neutral_and.html
     
  10. Aug 18, 2015 #9

    russ_watters

    User Avatar

    Staff: Mentor

    Biomass is a big issue. It is renewable, which people mistake for "clean", and because of its abundance it is growing rapidly -- it is much bigger than solar, for example, despite the attention and money lavished on solar power and despite being described as "the new coal".
     
  11. Aug 18, 2015 #10
    But biomass is different from waste incineration correct? Biomass is harvesting crops like corn and algae to produce ethanol, which at best is carbon neutral.

    My instincts told me that waste incineration would be worse than carbon neutral, but then I read about Amsterdam's AEB WtE: http://www.aebamsterdam.com, which apparently is deemed renewable. They claim have decreased CO2 emissions and contributed to the city's target on cutting 40% of carbon emissions.

    Also, I read that while landfills undergoe anaerobic digestion, composts have aerobic digestion which is apparently better. But I am not sure about that either because another study says the opposite:www.iswa.org/uploads/tx_iswaknowledgebase/10-302_FP.pdf
     
  12. Aug 18, 2015 #11

    russ_watters

    User Avatar

    Staff: Mentor

    According to the wiki on biomass, it is any plant derived waste, in any form, which encompasses all of what we are discussing. Ie, it includes methane from landfills (from food and wood) as well other crop waste and direct wood waste. There are a variety of ways to utilize it.
    Again, though people often think of the terms as equivalent, "renewable" has nothing to do with pollution ("carbon neutral"). The largest source of clean energy is nuclear power, which is technically not renewable.

    "Renewable" just means we won't run out.
    Dunno. Certainly compost piles have pre-selected waste and can be managed for efficiency.
     
  13. Aug 18, 2015 #12
    Composting has a number of benefits besides being carbon neutral. It provides a friendly bio-reactor that nurtures soil activity and makes things grow. I've seen large (municipal) compost plants that use the resulting insect activity for free range chicken food for example. However composting is labor intensive.

    Utility bio-mass plants often harvest scrub wood for fuel. (Young trees and brush grow much more quickly than mature trees.) The land used provides some sort of natural habitat which is good, but it prevents climax vegetation which is bad. It is of course carbon neutral. My understanding is that this is typical of bio-mass plants. Of course any agricultural waste is happily burned as well.

    Municipal waste incinerators typically use the energy from the incineration to run local plant operations, so the energy isn't all wasted, but they are not typically as efficient as dedicated plants. Even sewer plants do their best to burn the methane they produce to help out.
     
  14. Aug 18, 2015 #13

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Decomposition in a compost pile is (or should be) aerobic. You're supposed to keep your compost pile aerated. Aerobic decomposition produces a lot of solid organic compounds, plus a few gases. Decomposition in a landfill is anaerobic. Anaerobic decomposition produces a lot more gases (particularly methane) and fewer solid organic compounds than does aerobic decomposition.

    A lot of landfills now capture the methane produced by those landfill.


    You have to take that with a slight grain of salt. Suppose you want to paint over a window. The first coat of paint cuts out most of the light. The second coat, not so much. The third coat, even less. There's a lot more CO2 in the atmosphere than there is CH4. The CO2 window is partially closed; adding a bit more CO2 is a bit like painting over that already-painted window. Because there's not so much CH4 in the atmosphere, that window is still fairly open.
     
  15. Aug 18, 2015 #14

    russ_watters

    User Avatar

    Staff: Mentor

    I think you're saying that like the commonly misunderstood SPF of suntan lotion, the number represents and inverse proportion, such that the difference between a medium-sized number and a big number isn't very much. IE, for SPF, an SPF of 4 is a transmittance of 1/4 or a blocking of 75% of UV. An SPF of 40 is a transmittance of 1/40 or a blocking of 97.5% of UV. So the difference between nothing and SPF 4 is much, much larger than between SPF 4 and SPF 40, even though the number is 10x larger.

    In terms of GWP, it may not be completely clear what the actual global warming impact of a certain amount of a substance is (first you have to know the impact of CO2), but it does tell you how to compare different gases with each other. IE, it tells you how much better off you are burning waste methane from a landfill (producing CO2) than letting it get into the atmosphere as methane or burning methane instead of coal in a power plant.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Landfill methane gas emissions
  1. Abiogenic Methane (Replies: 1)

  2. Methane mystery (Replies: 4)

  3. The methane mystery (Replies: 11)

Loading...