Gas production, composition and emission at a modern disposal site receiving waste with a low-organic content

AV Miljø is a modern waste disposal site receiving non-combustible waste with a low-organic content. The objective of the current project was to determine the gas generation, composition, emission, and oxidation in top covers on selected waste cells as well as the total methane (CH4) emission from the disposal site.

The investigations focused particularly on three waste disposal cells containing shredder waste (cell 1.5.1), mixed industrial waste (cell 2.2.2), and mixed combustible waste (cell 1.3). Laboratory waste incubation experiments as well as gas modeling showed that significant gas generation was occurring in all three cells.

Field analysis showed that the gas generated in the cell with mixed combustible waste consisted of mainly CH4 (70%) and carbon dioxide (CO2) (29%) whereas the gas generated within the shredder waste, primarily consisted of CH4 (27%) and nitrogen (N2) (71%), containing no CO2. The results indicated that the gas composition in the shredder waste was governed by chemical reactions as well as microbial reactions.

CH4 mass balances from three individual waste cells showed that a significant part (between 15% and 67%) of the CH4 generated in cell 1.3 and 2.2.2 was emitted through leachate collection wells, as a result of the relatively impermeable covers in place at these two cells preventing vertical migration of the gas.

At cell 1.5.1, which is un-covered, the CH4 emission through the leachate system was low due to the high gas permeability of the shredder waste. Instead the gas was emitted through the waste resulting in some hotspot observations on the shredder surface with higher emission rates. The remaining gas that was not emitted through surfaces or the leachate collection system could potentially be oxidized as the measured oxidation capacity exceeded the potential emission rate.

The whole CH4 emission from the disposal site was found to be 820±202kgCH4d−1. The total emission rate through the leachate collection system at AV Miljø was found to be 211kgCH4d−1. This showed that approximately ¼ of the emitted gas was emitted through the leachate collections system making the leachate collection system an important source controlling the overall gas migration from the site.

The emission pathway for the remaining part of the gas was more uncertain, but emission from open cells where waste is being disposed of or being excavated for incineration, or from horizontal leachate drainage pipes placed in permeable gravel layers in the bottom of empty cells was likely.