Q361. I am required to provide the values of Equations HH-5 to HH-8. How are the waste depth data and surface area data used?

A361. The rule requires determination of both surface area and waste depth for each area type specified in Table HH-3.  The surface areas by area type are used to determine the area-weighted collection efficiency (CE) of the landfill gas collection system.  However, methane generation is expected to be proportional to waste quantities, so the surface area weighting used to determine the CE of the landfill gas collection system may overestimate or underestimate the actual CE if the different areas have significantly different waste depths.

The waste depths by area type data, although not used directly in the emissions calculations, are being collected for policy analysis purposes. EPA plans to use the waste depth data reported to develop separate “waste quantity-weighted” CEs and back-calculated methane generation and emissions quantities (i.e., Equations HH-7 and HH-8, respectively).  These values will then be compared with the surface area weighted average CEs.  If significantly different results between the two methods of calculating CE exist, EPA may elect to revise the calculation methodology of the average CE for a landfill.  As the surface area data were deferred from reporting until reporting year 2013, we were not able to perform the analysis prior to issuing the technical corrections to the rule in 2013.

Please note that the area-weighted calculation method for the gas collection system was selected because we had data to suggest that CE varied as a function of cover type and because it was the easiest means to account for different CEs based on cover types.