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.
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).|
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 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.
*kg CO2e = 1 x kg fossil CO2 + 25 x kg methane - (44/12) kg C stored
- 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
- Chanton, J.; Abichou, T.; Langford, C.; Spokas, K.; Hater, G.; Goldsmith, D.; Barlaz, M. A. Observations on the methane oxidation capacity of landﬁll soils. Waste Manage. 2011, 31, 914 925.
- 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.
- 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.