AREA A/B ENGINEERING REPORT - Waste Management

1.4.2 Permanent Storage of Biogenic Carbon in Landfills (Sequestration)
Geosyntec Consultants
Carbon sequestration is the permanent removal of biogenic carbon (i.e., carbon of
recent plant origin rather than the fossil carbon found in coal, natural gas, or oil) from
the atmosphere. The major biodegradable (i.e., biogenic carbon) components of
MSW are cellulose and hemicelluloses (C&H), which are complex carbohydrates that
form the main structural components of cells in all green plants. C&H are thus the most common
organic compounds on Earth. However, although C&H will decompose anaerobically to methane
and CO2, the complete decomposition of C&H within a landfill is not expected. In addition, many
common components of the waste mass are wood-based and contain lignin. Lignin is highly
recalcitrant to anaerobic biodegradation under landfill conditions, and will not undergo any
significant decomposition. 4
Given these conditions, “carbon sequestration” as applied to landfills refers to carbon that is of
plant origin (including wood, paper, cardboard, food waste, and green yard waste) that does
not degrade after disposal, but rather is permanently stored in the landfill in a stable form that
cannot be emitted as a greenhouse gas (GHG) such as methane or CO2. As discussed further in
Section 7.2, the degree of biodegradation that may be achieved in a landfill, combined with the
carbon that is sequestered and permanently stored in the landfill, are important factors in
understanding the potential for landfills to emit greenhouse gases to the atmosphere.
1.4.3 Development of a Leachate Biofilter in Bottom-Most Waste Layers
Researchers have noted that leachate strength (in terms of organic indicators,
primarily the measured biological oxygen demand or BOD, and chemical oxygen
demand or COD) from the upper waste layers of a landfill is invariably higher than
that in leachate collected from lower waste layers. Independent studies in the U.S.,
Japan, and China have demonstrated the capacity of lower layers of MSW to accelerate
improvement in leachate quality. This suggests that the bottom-most layers of the waste are well
decomposed due to moist conditions and the presence of efficient biodegradation in this
environment. These degraded waste layers act as a biofilter, with an attenuating capacity for
consuming degradable organics in leachate. Moreover, a landfill does not have to be operated
under conditions of enhanced degradation to realize the benefits of a basal biofilter layer
because moisture will tend to accumulate in the bottommost waste layers of landfills, except at the
very driest sites. This is of immense value in effectively evaluating long-term leachate conditions
because it allows organic indicator parameters such as BOD and COD to be used as a primary
measure of overall leachate quality. Existing research demonstrates that if an improving trend in
the concentration of such indicators in leachate can be demonstrated and leachate continues to be
4 Carbon sequestration in landfills is discussed by Barlaz (1998 and 2006), Barlaz, et al (2007), and is recognized in
two seminal reports: (i) USEPA (2006) “Solid Waste Management and Greenhouse Gases: A Life-Cycle Assessment of
Emissions and Sinks (3rd Edition),” and (ii) Intergovernmental Panel on Climate Change (IPCC, 2006) “Guidelines for
National Greenhouse Gas Inventories; the National Greenhouse Gas Inventories Programme.” A number of state GHG
inventories have been conducted, such as the California Greenhouse Gas Inventory (developed by the California Air
Resources Board in response to Assembly Bill 32, 2006), and include landfills as sink for sequestered carbon (for
additional information see www.arb.ca.gov/cc/inventory/inventory.htm).
MD10186.doc 24 29 March 2009