Q779. I have a site that started the reporting year with extraction wells installed in only one third of the landfill. During the second half of the year, additional wells were installed and brought on line to cover the entire landfill.  How do I calculate the collection efficiency (CE) for this situation?

A779. Determine CE based on the landfill gas collection system coverage as it existed during the first part of the year (CE_start) A.  In the instance where a single gas collection system that is operated all year long, but was effectively expanded at some point during the reporting year, we recommend that you determine an area-weighted average collection efficiency (CE) for the landfill gas collection system (LFGCS) as follows:

1. Determine CE again for the landfill gas collection system as it existed at the end of the year (CE_end). 

2. Calculate a weighted average CE (CE_ave) for the year based on the number of months (or weeks or days) the LFGCS existed like that.  For example, if the landfill gas collection system was upgraded near the end of May, CE_ave = 5/12*CE_start + 7/12*CE_end.

This approach should allow Equations HH-7 and HH-8 to provide an accurate estimate of methane generation and emissions during the reporting year.  This same approach can be used if a gas collection system did not operate for a portion of the reporting year.