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Date: October 18, 2012

To: Rachel Schmeltz, U.S. EPA

From: Jeff Coburn, RTI International 

Subject: Review of Weekly Landfill Gas Volumetric Flow and Methane Concentrations

Purpose 

This memorandum documents the review of the landfill gas volumetric flow and methane 

concentration data provided by industry to assess the uncertainty of the methane recovery when 

using a monthly versus weekly monitoring frequency for methane concentrations. 

Background 

On October 30, 2009, the EPA promulgated mandatory GHG reporting requirements for a 

variety of sources including municipal solid waste (MSW) landfills as 40 CFR Part 98 Subpart 

HH. For MSW landfills that have landfill gas collection, Subpart HH requires continuous 

monitoring of landfill gas flow and either continuous monitoring (if continuous monitoring 

equipment is present) or weekly measurement of landfill gas methane concentration. Industry 

representatives have commented that the weekly measurement requirement is burdensome, 

particularly for closed landfills, and that monthly monitoring of methane concentration would 

provide an adequately accurate estimate of the methane recovery. To assist the EPA in their 

review of this issue, industry representatives provided their weekly flow and methane 

concentration measurements for the 2011 reporting year (RY2011). The data were provided on a 

landfill-specific basis with in indication of the landfill as either active or closed. 

Conclusion 

Shifting from weekly to monthly measurement frequency is projected to increase the uncertainty 

of the methane recovery quantities for a single landfill from 8% to 10.5% with no significant 

bias. The uncertainty in the nationwide total is projected to increase from ±0.4% to ±0.5%. 

Analysis 

Data were provided in two separate spreadsheets: one spreadsheet containing data for Waste 

Management landfills and one spreadsheet containing data for other (non-Waste Management) 

landfills. The data were initially reviewed to confirm that the methane recovery values were 

calculated correctly. The volumetric flows were provided in units of standard cubic feet (scf) 

and methane concentrations were provided in units of volume percent. Based on the data 

provided, no verification could be performed on the conversion from actual cubic feet to scf (if 

flows were not auto-compensated). The flow rates and concentrations 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 recovered for each landfill. 
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All calculations of methane recovery that were reviewed 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 and summing these values (or 

determining a flow-weighted average methane concentration, as done by Waste Management). 

Therefore, weekly and monthly methane recovery calculations were performed and then summed 

to determine the annual methane recovery following the methodology required in Subpart HH. 

These annual recovery values were used for subsequent analysis (rather than the annual 

recoveries contained in the non-Waste Management spreadsheet 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 entire year and four based on a monthly 

measurement frequency (one using the first weekly concentration measurement for each month, 

another using the second weekly concentration measurement for each month, etc.). The five 

annual recovery estimates for all landfills (Waste Management and non-Waste Management 

landfills) were compiled in a single spreadsheet for further 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 landfills, 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 standard deviation divided by the arithmetic 

mean) of the weekly flow values and the weekly concentration values were determined. The 

calculated relative standard deviations of the weekly measured values were used as a measure of 

the variability (normal fluctuations) in these parameters. The uncertainty associated with that 

variability was then evaluated using a Monte Carlo simulation using 12 (for monthly) versus 52 

(for weekly) measurement intervals. The overall uncertainty with the calculated annual methane 

recovery was estimated using a simple sum of squares approach considering the accuracy of the 

flow measurement, the accuracy of the concentration measurement, and the variability of the 

methane composition data. 

Results 

Data were available for 395 landfills; 296 of these were classified as active and 99 were 

classified as closed/inactive. The percent difference in the annual methane recovered calculated 

using monthly versus weekly methane concentrations appears 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 landfills and Figure 2 provides the same 

distribution plot for closed landfills. The closed landfills appeared to have a slightly broader 

distribution, but this could be due to the smaller sample size. 
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Figure 1. Distributional analysis of the percent difference in the annual methane recovery using 

monthly versus weekly methane concentration measurements for active landfills. 

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

monthly versus weekly methane concentration measurements for closed landfills. 
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A bias may occur if high flow events are more likely associated with high methane 

concentrations (or, alternatively, low methane concentrations). If the flow and methane 

concentrations are independent variables, then no bias would 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 Week 4 concentrations was 0.27%). 

When determining the annual quantity of methane recovered across all 395 landfills, the values 

determined using monthly concentration measurements varied from -0.11 to 0.25% of the 

aggregate methane recovery determined using the weekly measurement data; the average 

monthly measurement total was 0.07% greater than the weekly aggregate total. The 95th

percentile confidence interval for the percent difference between monthly and weekly calculated 

methane recovery totals was -3.2 to 4.0% for active landfills, -4.5 to 4.6% for closed landfills, 

and -3.5 to 4.5% for the combined dataset. While these confidence 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 often 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 frequencies. 

The impact of using a monthly rather than weekly monitoring frequency was also evaluated 

based on the added uncertainty of the calculated annual methane 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 are each ±5%; this assumption is based on 

the requirement in 40 CFR Part 98 Subpart A for flow monitors to be accurate within 5%. The 

relative standard deviations of weekly flow and methane concentration data were used to assess 

the variability in these parameters for an individual landfill. The 95th percentile flow variability 

was approximately ±70% and the 95th percentile methane concentration variability was ±27%. 

The high flow variability is not a concern because continuous flow measurements are required. 

That is, the continuous flow monitor will capture the variability 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 uncertainty in the average methane concentration 

determined using 12 versus 52 measurements. The 95th percentile confidence interval for the 

annual average methane concentration with 52 measurements (with a single measurement 

variability of 27%) was ±3.8%; the 95th percentile confidence interval for the annual average 

methane concentration with 12 measurements (with a single measurement variability of 27%) 

was ±7.8%. These results suggest that switching from weekly to monthly measurements will 

increase the uncertainty by ±4%, which agrees well with the 95th percentile confidence intervals 

for the percent difference calculations. 

Table 1 summarizes the sum of squares approach for determining the overall uncertainty 

associated with the methane recovery values for a single landfill. As seen in Table 1, the overall 

uncertainty of the methane recovery values for a single reporter is expected to increase from 

±8% to ±10.5% when going from a weekly to a monthly monitoring frequency. Note: the 

uncertainty in the cumulative nationwide methane recovery total is much less than the 

uncertainty for an individual landfill because the positive and negative estimates tend to cancel 

out. The uncertainty in a cumulative total is the uncertainty of a single measurement value 
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divided by the square root of the number of values in the summation. 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 cumulative methane recovery for these landfills 

in projected to increase from ±0.4% to ±0.5%. 

Table 1. Estimate of the Overall Uncertainty for Methane Recovery 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%