Nantucket Landfill... did you know?
Methane is formed in landfills by a process called anaerobic digestion. This is a multi-stage process by which the organic wastes that have been placed underground in the landfill are degraded through activity of bacteria (called methanogens) on organic carbon in the waste and in the absence of oxygen. Methane formation occurs when there is sufficient organic carbon in the waste to support a population of methanogens; and when the waste is placed far enough underground such that there is little or no oxygen to inhibit the reaction.
The Nantucket Landfill emits little or no methane for several reasons. Most importantly, waste placed in the Landfill has generally already been processed in the Composter. The Composter breaks down the organic carbon in incoming waste through a different aerobic biological decomposition process involving bacteria that flourish in the presence of oxygen. Thus, organic carbon in the residual waste from the Composter, which comprises the bulk of what is sent to the Landfill, has already undergone decomposition before it gets to the Landfill and there is little or no organic carbon available for methane formation after it gets there.
Additionally, most of the residuals sent to the landfill are comprised of plastics and other inert materials that do not decompose and do not form methane when placed in the Landfill. Also, the Nantucket Landfill is relatively shallow; that is, the areas involving waste placement are less than 40 feet deep. In such shallow areas, the waste is exposed to oxygen that inhibits methane formation from any residual organic carbon that might have avoided decomposition in the Composter or during subsequent processing and storage. The residuals are baled before being sent to the Landfill, which provides another opportunity to expose organic carbon in waste to oxygen and inhibit later methane formation. Finally, the space between the bales after placement in the Landfill provides another opportunity for exposure to oxygen.
As a result, the MassDEP does not require the Nantucket Landfill to install an active landfill gas collection system like the one at Crapo Hill Landfill in New Bedford. At the Nantucket Landfill, the collection system would not capture enough methane to maintain a flame. In fact, there was a small collection system installed in Cell 2A when it accepted C&D waste before 2006, but the flare was shut down due to lack of methane to sustain a flame.
As part of the Town’s environmental monitoring program, CDM Smith measures methane concentrations at various monitoring wells on a periodic basis. Very little methane has been found and the reports can be sourced HERE.
Arguably, the potential for methane emissions from the Landfill may have increased recently because sludge that had been processed in the composter is now being diverted for separate processing and landfill disposal due to fears of PFAS contamination. The sludge is mixed with yard waste and composted separately to meet the MassDEP requirement to reduce its moisture content to below 20 percent before being placed in the Landfill. While this process also greatly reduces the organic carbon content of the sludge prior to Landfill disposal, it is likely not as effective as composter processing for degrading organic carbon content. Methane quantities, if any, are likely to be small.
Finally, gasification has its advantages for processing of certain wastes, although the technology is still developmental and emerging as applied to solid waste components in the U.S. The Town is investigating opportunities for gasification on Nantucket and is pursuing several pilot programs in this area.