1

Date: October 18 , 20 1 2

To: Rachel Schmeltz , U.S. EPA

From: Jeff Coburn, RTI International

Subject: Review of Weekly Landfill Gas Volumetric Flow and Methane Conce ntrations

Purpose

This memorandum documents the review of the landfill gas volumetric flow and methane concentration data provided by industry to assess the uncert ainty of the methane recovery when using a monthly versus weekly monitoring frequency for metha ne concentrations.

Background

On October 30, 2009, the EPA promulgated mandatory GHG reporti ng requirements for a variety of sources including municipal solid waste (MSW) l andfills as 40 CFR Part 98 Subpart HH. For MSW landfills that have landfill gas collec tion, Subpart HH requires continuous monitoring of landfill gas flow and either continuous moni toring (if continuous monitoring equipment is present) or weekly measurement of landfill gas methane concentration. Industry representatives have commented that the weekly measurem ent requirement is burdensome, particularly for closed landfills, and that monthly monitoring of methane concentration would provide an adequately accurate estimate of the methane rec overy. To assist the EPA in their review of this issue, industry representatives provided thei r weekly flow and methane concentration measurements for the 2011 reporting year (R Y2011). The data were provided on a landfill-specific basis with in indication of the lan dfill as either active or closed.

Conclusion

Shifting from weekly to monthly measurement frequency is pr ojected to increase the uncertainty of the methane recovery quantities for a single landfi ll from 8% to 10.5% with no significant bias. The uncertainty in the nationwide total is project ed to increase from ±0.4% to ±0.5%.

Analysis

Data were provided in two separate spreadsheets: one spre adsheet containing data for Waste Management landfills and one spreadsheet containing data fo r other (non-Waste Management) landfills. The data were initially reviewed to confirm th at the methane recovery values were calculated correctly. The volumetric flows were provi ded in units of standard cubic feet (scf) and methane concentrations were provided in units of volu me percent. Based on the data provided, no verification could be performed on the convers ion from actual cubic feet to scf (if flows were not auto-compensated). The flow rates and c oncentrations were assumed to be all on the same basis (either both on a wet basis or both on a dry basis, so no moisture correction factor was required) when determining volume of methane recovere d for each landfill.2

All calculations of methane recovery that were revi ewed for the Waste Management spreadsheet
were verified to be correct. The calculations in the non-Waste Management spreadsheet appeared to use the average annual methane concentration times the cumulative flow rather than determining the methane recovery per measurement period an d summing these values (or determining a flow-weighted average methane concentration , as done by Waste Management). Therefore, weekly and monthly methane recovery calcula tions were performed and then summed to determine the annual methane recovery following the met hodology required in Subpart HH. These annual recovery values were used for subsequent analy sis (rather than the annual recoveries contained in the non-Waste Management spreads heet as supplied to the EPA).

Five annual recovery estimates were developed for each of the 395 landfills in the data set: one based on the weekly measurement frequency for the entir e year and four based on a monthly measurement frequency (one using the first weekly concent ration measurement for each month, another using the second weekly concentration measureme nt for each month, etc.). The five annual recovery estimates for all landfills (Waste Ma nagement and non-Waste Management landfills) were compiled in a single spreadsheet for fur ther analysis. The four annual methane recovery estimates that were based on monthly methane concentration data were each separately compared to the annual methane recovery estimate determined using the weekly methane concentration data as currently required by Subpart HH. Distributional analyses were conducted for all active landfills, all closed/inactive landfill s, and all landfills (regardless of active status) in
the combined data set.

The variability of the weekly flow and concentration measurements were also analyzed. For each landfill, the relative standard deviation (the sta ndard deviation divided by the arithmetic mean) of the weekly flow values and the weekly concen tration values were determined. The calculated relative standard deviations of the weekly me asured values were used as a measure of the variability (normal fluctuations) in these paramete rs. The uncertainty associated with that variability was then evaluated using a Monte Carlo simul ation using 12 (for monthly) versus 52 (for weekly) measurement intervals. The overall unce rtainty with the calculated annual methane recovery was estimated using a simple sum of squares appro ach considering the accuracy of the flow measurement, the accuracy of the concentration m easurement, and the variability of the methane composition data.

Results

Data were available for 395 landfills; 296 of these were c lassified as active and 99 were classified as closed/inactive. The percent difference in the annual methane recovered calculated using monthly versus weekly methane concentrations appea rs to be normally distributed with no significant bias (i.e., central tendency of the percent difference values across different landfills was essentially zero). Figure 1 provides a distribution plot using “Week 1” concentration values for the monthly concentration assessment for active l andfills and Figure 2 provides the same distribution plot for closed landfills. The closed lan dfills appeared to have a slightly broader distribution, but this could be due to the smaller sample size.3

Figure 1. Distributional analysis of the percent difference i n the annual methane recovery using

monthly versus weekly methane concentration measuremen ts for active landfills.

Figure 2. Distributional analysis of the percent difference i n the annual methane recovery using

monthly versus weekly methane concentration measuremen ts for closed landfills.4

A bias may occur if high flow events are more likely ass ociated with high methane concentrations (or, alternatively, low methane concent rations). If the flow and methane concentrations are independent variables, then no bias w ould be expected. Considering central tendency values, the -0.25 to 0.25% difference category was always the most frequent category (i.e., the mode category) when considering the combined ( active and closed landfill) dataset; the central tendency value (median) was in this category for all but monthly values determined using Week 4 concentrations (median percent difference using We ek 4 concentrations was 0.27%). When determining the annual quantity of methane recovered a cross all 395 landfills, the values determined using monthly concentration measurements varied f rom -0.11 to 0.25% of the aggregate methane recovery determined using the weekly measur ement data; the average monthly measurement total was 0.07% greater than the weekl y aggregate total. The 95 th percentile confidence interval for the percent differe nce between monthly and weekly calculated methane recovery totals was -3.2 to 4.0% for active landf ills, -4.5 to 4.6% for closed landfills,
and -3.5 to 4.5% for the combined dataset. While these confi dence intervals may suggest a slightly high bias when using the monthly measurement data, particularly for open landfills, a very large sample set (on the order of thousands) is oft en needed to characterize the extreme values well. Based on the cumulative methane recovery totals, it appears there is no significant bias when using monthly rather than weekly measurement fr equencies.

The impact of using a monthly rather than weekly monitor ing frequency was also evaluated based on the added uncertainty of the calculated annual met hane recovery for an individual landfill. Two areas of uncertainty were considered: accuracy and variability. Accuracy is how closely the monitoring device measures the true value of a parameter. It is assumed that the accuracy of the flow and concentration measurements ar e each ±5%; this assumption is based on the requirement in 40 CFR Part 98 Subpart A for flow monito rs to be accurate within 5%. The relative standard deviations of weekly flow and methane c oncentration data were used to assess the variability in these parameters for an individual l andfill. The 95 th percentile flow variability was approximately ±70% and the 95 th percentile methane concentration variability was ±27%. The high flow variability is not a concern because conti nuous flow measurements are required. That is, the continuous flow monitor will capture the va riability in flow, which is why continuous flow monitoring was required. The variability in the methane concentration was used
in a Monte Carlo simulation to determine the uncertain ty in the average methane concentration determined using 12 versus 52 measurements. The 95 th percentile confidence interval for the annual average methane concentration with 52 measurements (with a single measurement variability of 27%) was ±3.8%; the 95 th percentile confidence interval for the annual average methane concentration with 12 measurements (with a sing le measurement variability of 27%) was ±7.8%. These results suggest that switching from weekl y to monthly measurements will increase the uncertainty by ±4%, which agrees well with t he 95 th percentile confidence intervals for the percent difference calculations.

Table 1 summarizes the sum of squares approach for determining th e overall uncertainty associated with the methane recovery values for a sin gle landfill. As seen in Table 1, the overall uncertainty of the methane recovery values for a singl e reporter is expected to increase from ±8% to ±10.5% when going from a weekly to a monthly monitor ing frequency. Note: the uncertainty in the cumulative nationwide methane recov ery total is much less than the uncertainty for an individual landfill because the posit ive and negative estimates tend to cancel out. The uncertainty in a cumulative total is the unce rtainty of a single measurement value5

divided by the square root of the number of values in the su mmation. Assuming the 395 landfills
in the dataset reflect the total number of facilities that would switch from weekly to monthly measurements, the projected uncertainty in the cumulati ve methane recovery for these landfills in projected to increase from ±0.4% to ±0.5%.

Table 1. Estimate of the Overall Uncertainty for Methane Rec overy for a Single Landfill.

Parameter Uncertainty Square of Uncertainty

Weekly Monthly Weekly Monthly
Accuracy of flow meter ±5.0% ±5.0% 25 25

Accuracy of methane concentration ±5.0% ±5.0% 25 25

Variability of methane concentration ±3.8% ±7.8% 14.4 60.8

Sum of Squares 64.4 110.8
Overall Uncertainty (square root of Sum of Squares) ±8.0% ±10.5%