Methane Generation from a Biodegradable Polymer

Once again we have a case of a press release overstating a research result. And that is rather sad, as the original research is rather interesting (at least at one level, as I shall explain).

As we all are well aware now, the creation and demand for biodegradable polymers continues to grow every year. While this is considered a good outcome by environmentalists and the general public, it is well known that most biodegradable material ends up in landfills, a buried tomb where the biodegradation that was originally desired for the product (conversion to CO₂ and H₂O) doesn't really occur since a landfill is generally lacking in O2. Instead of the complete oxidation reactions, incomplete reduction reactions occur, leading largely to the formation of CH₄ (methane), potent greenhouse gas, far more potent than CO₂.

There is a new report in Environmental Science and Technology (open access!) this month that look at this issue from two perspectives – that of the national overview of the landfills located in the US and how much methane that can/could/will generate, and then also looking at the degradation of specific components in the landfills. This was the part of the report that I thought was especially interesting. The big unknown is the generation rate for a landfill, not only in the present, but also in the future. Enter a Monte Carlo analysis to determine the sensitivity of the authors results to the assumptions. I was pretty impressed with the work that the authors put into trying to capture this and ensure that their results are meaningful.

From the small-picture perspective, the authors determine the degradation kinetics for 4 (and only 4) input materials, these being food waste, newsprint, office paper and a biodegradable polymer poly(3-hydroxybutyrate-co-3-hydroxyoctanoate) (PHBO) and found that the faster a material degrades, the more likely it is to produce methane, a hihgly potent greenhouse gas.

PHBO? Why this polymer, one that as far as I know isn't commercially available? (The article doesn't even state the source of the material that they tested.)

This is more than just a strictly academic questions, since PLA (polylactic acid) is actually being produced and consumed on a somewhat volumetric basis? And wouldn't the results be significantly different if PLA were being studied instead of PHBO? There certainly is reason to suspect similar results since PHBO and PLA are both polyesters, but PLA certainly has more oxygen per repeat unit than the PHBO does. Some data would be really helpful.

But more importantly, the researchers never overstep the bounds of their research. They a quite clear in their conclusions that they are only talking about 1 biodegradable polymer, not all biodegradable polymers.

Unfortunately, that is not what was being stated in the PR blurb and what is being discussed around the internet. To make a blanket generalization that ALL biodegradable polymers will produce so much methane as to be overall unfavorable is not a correct statement to make based on the evidence here. But to know that, you have to read the report and not just a the hype. (Quite a challenge, huh?)

Lastly, what is also not stated anywhere is that while CO₂ is most certainly a powerful greenhouse gas, it has a short lifetime in the atmosphere compared to CO₂. That only has merit if the amount of CH₄ is decreasing over time - i.e., that no new material is being generated.