Carbon Sequestration in Landfills: Documentation from Field Samples
Investigators: North Carolina State University
Cellulose and hemicellulose are the major biodegradable components of municipal solid waste (MSW) in landfills and their decomposition to methane (CH4) and carbon dioxide (CO2) is well documented (Barlaz, 2006). The extent of biodegradation is important because CH4 is a greenhouse gas with an infrared activity 21 times that of CO2 for a 100 year time horizon. When captured, this CH4 can be flared, in which case it is converted to CO2, or converted to energy in the form of steam or electricity. When CH4 is converted to energy, it offsets the utilization of fossil fuels and the resulting emissions. A recent estimate indicates that there are 1150 landfill gas-to-energy projects worldwide (Willumsen, 2003).
A number of factors limit the decomposition of cellulose and hemicellulose in landfills, including environmental conditions such as moisture, pH, and temperature, as well as the availability of these substrates to microorganisms. Thus, while significant decomposition of these materials occurs, the complete conversion of cellulose and hemicellulose buried in landfills would not be expected. This is significant because the organic carbon that does not biodegrade is sequestered.
Carbon sequestration is the permanent removal of carbon from the atmosphere. As applied to landfills, carbon sequestration refers to the fact that some of the carbon that is of plant origin (biogenic C), including wood, paper, food waste, and green waste, is not degraded after disposal. Rather it is permanently stored (or sequestered) in the landfill. Landfills have significant value as carbon sinks for the following reasons:
– Lignin does not biodegrade under the anaerobic conditions that dominate landfills, thus any lignin buried will remain sequestered.
– Less than 100% of the cellulose and hemicellulose that is buried in a landfill will biodegrade. The amount of cellulose and hemicellulose that will biodegrade is specific to each waste component. For example, the cellulose in grass clippings biodegrades to a greater extent than cellulose in newsprint.
The objective of this proposal is to quantify the amount of carbon sequestration that is occurring in landfills using methods that can be expected to satisfy any carbon accounting program in which sequestration is considered. At this time, the methods to account for carbon sequestration are evolving and it is premature to focus on any one agency (IPCC, EPA, CCX, California Air Resources Board). This research proposes two activities:
1. Research to demonstrate a method for the documentation of carbon sequestration at landfills. This research was originally described in a research proposal to EREF in 2008. I am attaching that proposal as Appendix A. Funds are requested to support a graduate student who will be writing a manuscript on this topic.
2. Research to implement a new analytical method for lignin. The reason that a new method is needed is because materials such as plastics, synthetic textiles and rubber all interfere with the current method, leading to artificially high lignin contents in refuse samples. This is problematic because the ratio of cellulose plus hemicellulose to lignin is used extensively to monitor solids biodegradation in landfills. Additional detail is provided below. Funds are requested to partially support the purchase of an HPLC system for the lignin analysis and to support a graduate student to implement the analytical method.
The alkaline CuO oxidation method for lignin analysis will be implemented to quantify lignin in refuse. In this method, lignin oxidation breakdown products are analyzed by HPLC (or GC-MS) (GoÃ±i and Montgomery 2000). Successful application of this method will result in an improved understanding of waste composition and the components of waste that decompose. This method also would overcome errors associated with plastics and rubber in waste, which are lumped with lignin in the conventional analytical sulfuric acid method used to determine lignin content. As a result, the lignin content measured using the conventional method can be artificially high, and varies depending on the plastics, rubber, and textiles in a waste sample.
In the alkaline CuO oxidation method, lignin is hydrolyzed into its monomeric constituents including vanillic acid, vanillin, acetovanillone, syringic acid, syringealdehyde, acetosyringone, p-coumaric acid and ferulic acid. These constituents are then related back to the total lignin content in a sample. This method will also make it possible to explore the extent to which these constituents can be used to characterize waste composition. For example, cinnamyl phenols which are represented by coumaric and ferulic acids, are only associated with non-woody lignin as would be present in grass, and possibly leaves and food waste. The lignin in softwoods (gymnosperms) is dominated by vanillyl phenols, while the lignin in hardwoods (angiosperms) is dominated by syringyl phenols. This may be significant, as in ongoing research at NCSU has shown significant differences in the anaerobic biodegradability of hardwoods and softwoods. After ~400 days of monitoring, 1% and 5.5% of the organic carbon has been mineralized in a softwood (spruce) and hardwood (red oak) sample, respectively. Given the heterogeneous nature of solid waste, molecular markers such as the composition of phenolic monomers in lignin have the potential to provide previously unavailable information on the extent of decomposition of specific components, or classes of components, in waste.
De la Cruz, F. B., Chanton, J. P., & Barlaz, M. A. (2013). Measurement of carbon storage in landfills from the biogenic carbon content of excavated waste samples. Waste Management, 33(10), 2001-2005. doi:http://dx.doi.org/10.1016/j.wasman.2012.12.012