Tuesday, June 7, 2011

Is biodegradability a beneficial attribute for discarded solid waste?



A paper that I co-wrote with my advisor Dr. Morton Barlaz entitled "Is Biodegradability a Desirable Attribute for Discarded Solid Waste? Perspectives from a National Landfill Greenhouse Gas Inventory Model" has been making the news lately. The official press release can be found here. From what I've seen, a variety of people have taken the results to support a number of pre-conceived notions. But, I'll address that in a future post. I want this post to focus on what was done and what the results show.

Greenhouse gas flows in a landfill.

The foundation of this study is a life-cycle assessment (LCA) of the greenhouse gas (GHG) emissions from discarding waste in both national average landfills and state-of-the-art landfills. A state-of-the-are landfill is one where the landfill collects the generated methane and beneficially uses it. This LCA consisted of adding up all of the GHG emissions associated with every step of landfilling. The fundamental equation for determining the total GHG emissions from landfilling is shown below and is essentially a restatement of the image above (Sorry the equation is in picture form. I need to learn how to do equations in html.).


The units in each term of the equation are kg. Fossil fuels are used in the construction, operations, final cover, leachate management, and long term monitoring phases. These only amount to about 7 kg CO2 equivalents* (CO2e), though and aren't very important when compared to the other terms.

Source EPA
Fugitive methane is very important and pretty interesting from a modeling standpoint. But, I want to cover landfill gas modeling in a separate post. Basically, different materials decay at different rates, and landfills that collect gas tend to increase their collection efficiency over time. Collected methane is assumed to be destroyed with near 100% efficiency. It is also assumed that 10% of the methane passing through the soil will be oxidized to CO2, which is similar to combusting it, but is done by aerobic microbes. This 10% level is pretty conservative though, other studies have put the number at anywhere between 20-50% (Chanton et al., 2011). So, the methane that isn't collected or oxidized is emitted to the atmosphere.

The next major component is the electricity offsets. The EPA estimates that 69% of waste is in landfills that collect methane. About half of this waste is in landfills that beneficially use the methane. The others just flare the methane, which converts it to CO2. For this study we assumed all of the beneficially used methane was used for electricity generation. The electricity offsets are then due to the avoided emissions from the electricity that subsequently was not produced at either coal or natural gas plants. We only considered coal and natural gas because they are the two sources that can cost effectively adjust to changes in electricity demand. The marginal costs of nuclear and hydro are just too low for it to make any sense to reduce their output due to the availability of another source. So, we then assumed that our offset electricity was composed of 72% coal and 28% natural gas because this represents the adjusted national split. This leads to an offset of 1.02 kg CO2e per kWh.

The final component is the landfill carbon balance is carbon storage. Typical materials in solid waste do not fully degrade under anaerobic conditions (i.e. without oxygen). Since most of the typical materials that degrade at all are biogenic (i.e. plant or animal based) the carbon in those materials was recently removed from the atmosphere as plants were growing (e.g., trees for paper and food crops). This carbon is then considered sequestered in the landfill and will remain there as long as the landfill is undisturbed. On geological time scales this material could form peat or perhaps lignite. The amount of carbon stored for each material was measured based on previous laboratory studies (Staley and Barlaz, 2009; Federle et al., 2002).

So, what happens when you put all of these components together? The main results are shown in Figure 1 of the paper (reproduced below with modifications).
Greenhouse gas emissions from materials disposed in national average and state-of-the-art landfills. (Units and colors have changed for readability).
As I stated above, a state-of-the-art landfill is one where gas is collected and beneficially used. What these results really show that has been ignored by a lot of the articles that have reported on this study is that there are huge benefits to be had from collecting and beneficially using landfill gas. Disposing of mixed MSW in a state-of-the-art landfill  is actually carbon negative, but it is currently slightly positive using our current national average landfill infrastructure. Actually, all of the materials except for food waste have negative carbon emissions in a state-of-the-art landfill. This is fairly low hanging fruit for greenhouse gas mitigation efforts. So, when reporters have asked what consumers should do, the first thing that comes to mind is to ensure that their local landfill collects the generated methane and beneficially uses it if at all possible.

The main result that has been reported is an analysis of the effects of decay rate on greenhouse gas emissions in a national average landfill. Essentially, we exercised the model with 4 different theoretical biodegradable materials, and plotted the greenhouse gas emissions from disposing of those materials in a national average landfill as we vary the decay rate (i.e., how quickly the material releases its methane). The results of this analysis are shown below.
Greenhouse gas emissions from hypothetical biodegradable materials versus decay rate.  (Units and colors have changed for readability).
It is the results in this figure that have been widely reported for two reasons. Firstly, it shows that the more degradable a material is, the greater the CO2 emissions are from it. Basically, this means that degrading and generating methane is worse than just storing the carbon in the landfill. It also shows that a slower decay rate also leads to decreased greenhouse gas emissions. The best material is the one that does not degrade at all. It should be pointed out that this is for a national average landfill. The results would be somewhat different for a state-of-the-art landfill, but it can be easily shown that degradation of a carbohydrate or any degradable material with a hydrogen:oxygen atom ratio of 2:1 always leads to increased greenhouse gas emissions versus its non-degradable counterpart with standard electricity conversion efficiencies. Essentially, even with 100% gas collection the electricity offset is not large enough to offset the carbon storage.

It is this analysis that has led to headlines about biodegradable materials being bad for the environment. I would say it shows that there are negative global warming impacts associated with disposing of  biodegradable materials in landfills. One would need to study the entire life-cycle of the material to know if it was better or worse than the alternatives. One should also look at other environmental factors (i.e., resource conservation, biodiversity impacts, etc.) before making a final judgement. What this study does suggest is that the best first step is to ensure we are aggressively collecting methane from landfills. Increasing composting infrastructure could also be beneficial, and the development of non-degradable materials from biogenic sources may also be beneficial and worth further study. In the end, one must always take a systemic approach to analyzing complex problems.

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*kg CO2e = 1 x kg fossil CO2 + 25 x kg methane - (44/12) kg C stored

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Cited Literature

  1. Levis, J. W., Barlaz, M. A. “Is Biodegradability a Desirable Attribute for Discarded Solid Waste? Perspectives from a National Landfill Greenhouse Gas Inventory Model” Environ Sci Technol, 2011, doi: 10.1021/es200721s
  2. Chanton, J.; Abichou, T.; Langford, C.; Spokas, K.; Hater, G.; Goldsmith, D.; Barlaz, M. A. Observations on the methane oxidation capacity of landÔ¨Āll soils. Waste Manage. 2011, 31, 914 925.
  3. Staley, B. F.; Barlaz, M. A. Composition of municipal solid waste in the united states and implications for carbon sequestration and methane yield. J. Environ. Eng. -ASCE 2009, 135, 901–909.
  4. Federle, T. W.; Barlaz, M. A.; Pettigrew, C. A.; Kerr, K. M.; Kemper, J. J.; Nuck, B. A.; Schechtman, L. A. Anaerobic biodegradation of aliphatic polyesters: Poly(3-hydroxybutyrate-co-3-hydroxyoctanoate) and poly(epsilon-caprolactone). Biomacromolecules 2002, 3, 813–822.

3 comments:

  1. Dear Jim,

    Thank you very much for this "crystal clear" explanation on your study. I am a material chemistry Phd but some details of your study were quite hard to understand for a non-specialist like me.

    I have one question: Why did you use PHBO for your study ? I think I have a quite good knowledge of the bioplastic market and I've nerver heard anything about this material ... Why don't you use a commercial available one as PLA for example ?

    Thank you very much for your answer,

    Best Regards,

    Antoine

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  2. Sorry, for the delay in my response. I have been traveling.
    We would've loved to have directly modeled PLA in the study, unfortunately the necessary anaerobic degradation data is not publicly available. Full decay rate and methane potential tests are expensive and were not part of this study. We did address the benefits of biodegradability generally in the sensitivity analysis, though. It clearly shows that a decreased rate and extent of anaerobic degradability will lead to reduced GHG emissions, with completely recalcitrant materials leading to the least possible emissions. I hope that answered your question.

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  3. Interesting to say the least. Articles that came out on the public forum are exactly opposite of what you and we are trying to achieve. Awareness that collecting methane gas in current disposal methods create a carbon negative outcome.

    Jack Roberts

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