August 2009, Vol. 21, No.8

Letters

To Fill or Not To Fill?

In “Another Reason Not to Landfill” [April], the authors found that landfilling of biosolids would emit more carbon dioxide equivalent than composting — 2.5 times more than existing compost operations and 3.4 times more than the upgraded compost option. We disagree on their findings and would like to point out that estimating emissions is a complicated process requiring robust data and site-specific assumptions.

The authors stated that fugitive methane and nitrous oxide emissions from compost operations were estimated by taking averages of several literature values. In 2008, Brown et al. (“Greenhouse gas balance for composting operations,” Journal of Environmental Quality [July], pp. 1396–1410) suggested values of 0.7 g of nitrous oxide per kg of biosolids (Czepiel et al. [1996], “Measurement of N2O from composted organic wastes,” Environmental Science Technology [July], pp. 2519–2525) and 1.9 kg methane per Mg (Hao et al. [2004], “Carbon, nitrogen balances, and greenhouse gas emissions during cattle feedlot manure composting,” Journal of Environmental Quality [January], pp. 37–44) for potential nitrous oxide and methane emissions from composting operations. Using these values, methane and nitrous oxide emissions would have been 89.4 and 424 Mg/yr of carbon dioxide equivalent, respectively, which are higher than the authors’ analysis of 18.7 and 169.8 Mg/yr of carbon dioxide equivalent.

Furthermore, it appears that fugitive methane from landfilling operations is the largest factor affecting the outcome of the authors’ analysis. According to the authors, the study used a landfill-related methane emission rate of 175 Mg/yr, which is the average value of five data sources, ranging from 27 Mg/yr methane derived from a California bioreactor landfill to 579 Mg/yr methane, a value obtained based on U.S. Environmental Protection Agency (EPA) modeling for methane generation from landfilled food discards. It is well known that the EPA’s landfill gas generation modeling tends to overly estimate methane generation of landfilled waste. Brown and Leonard ([2004], “Biosolids and global warming: evaluating the management impacts,” BioCycle [August] pp. 54–60) suggested a methane generation capacity of the biosolids equivalent to 250 ml methane per g of biosolids, which translates to an equivalent of 8828 ft3 methane per Mg of biosolids and is similar to value found in our own study (LACSD [2007], “Methane generation potential of landfilled biosolids”). Using Brown and Leonard’s value of 8828 ft3 methane per Mg of biosolids, and a conservative landfill gas collection efficiency of 75% (EPA’s default value), landfill-related methane emissions should be 91.7 Mg methane/yr (or 2108 Mg carbon dioxide equivalent/yr). This revised landfill-related methane emissions value of 91.7 Mg methane/yr (or 2108 Mg carbon dioxide equivalent/yr) is much smaller than the 175 Mg methane/year (or 4018 Mg carbon dioxide equivalent/yr) value used in the analysis presented in the article.

Applying the more representative values discussed above, the life-cycle assessment study presented in the article would lead to a conclusion that landfilled biosolids actually generate net greenhouse gas (GHG) impacts (1844 Mg/yr of carbon dioxide equivalent) that are comparable to composting options (1854 Mg/yr of carbon dioxide equivalent for current composting, 1419 Mg/yr of carbon dioxide equivalent for upgraded composting).

This analysis signifies the importance of a rigorous scientifically sound, case-specific life-cycle assessment when evaluating potential management options for biosolids. For example, if a potential landfill has a highly efficient landfill gas collection system, it could be that this is a superior option with respect to GHG emissions.

Stephen R. Maguin, Chief Engineer and General Manager
Dung Kong, Senior Engineer
Solid Waste Management Department Los Angeles County Sanitation Districts


The author responds:

I agree with some of your points — especially the statement that this kind of analysis is complicated and requires site-specific data — to the extent it is available. I feel we are all on the steep part of the learning curve on this topic. And there are many field measurements needed to confirm models and more closely bracket emissions (especially of methane and nitrous oxide) from various sources and processes.

The analysis for Merrimack included mostly site-specific data, short of actual air sampling for methane and nitrous oxide emissions. (The full project report is available at www.nebiosolids.org.) Because of the known high level of uncertainties in some of the assumptions, we tried to calculate the most significant emissions — e.g., methane — using several different approaches. Then we averaged the results. We also stressed several times in the article the high level of uncertainty in this kind of analysis.

That said, I stand by the overall conclusions, which even your analysis supports: Landfill disposal of Merrimack biosolids would require significantly less energy, but would emit more net emissions than upgraded composting (both of these future options involve new centrifuge dewatering).

I respectfully suggest that the EPA’s estimate of the landfill gas collection efficiency of 75% is not applicable to a highly putrescible material such as biosolids. When biosolids are placed in a traditional landfill cell, they do not wait to decompose until a year or more later when methane capture might begin, and they can quickly become anaerobic. I doubt that it will ever be possible to control or capture significant proportions of methane emissions from dewatered biosolids disposed in traditional format landfills. It may be that bioreactor landfill technology with injection of liquid biosolids could improve on the rate of methane capture from landfilled biosolids. But if you want to capture methane from biosolids, a better technology is anaerobic digestion, which is far more controllable. Despite this, we used a capture rate of 79% or 80% in most of our calculations for methane emissions from landfill disposal. At the same time, we attributed pretty much every emission we could think of to the composting operation.

Most importantly, the default factors from Brown et al. (2008) for composting operations that you cited do not necessarily apply. By looking at the particular situation of one biosolids program, rather than making general estimates (which is what Brown et al. is about), we can be more accurate in our analysis. The Merrimack composting operation involves enclosed, in-vessel initial composting, with air treated through a biofilter; there will be negligible emissions of methane — and likely low if any nitrous oxide emissions — from this process. The emissions of these two gases that we attributed to the composting operations were conservatively high values from curing only.

The important take-home points that I hope this article conveyed — and which are clearly supported by other literature (including the body of work of Brown) — are:

  • Total energy use in biosolids management programs does not predict the level of net greenhouse gas emissions.
  • The greenhouse gases that biosolids managers need to focus on now are methane and nitrous oxide (because of their high global warming potentials).
  • Landfill disposal of biosolids by current common processes inevitably creates significant fugitive methane emissions.
  • Putting to use the nutrients and organic matter in properly managed biosolids applied to soils provides numerous, documented benefits, including, in many cases, lower net GHG emissions. They provide carbon sequestration in soils and replacement of manufactured fertilizers with a locally recycled product.

Ned Beecher
North East Biosolids and Residuals Association Tamworth, N.H.

 

 

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