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Date: June 7 , 20 1 3 To: Rachel Schmeltz , U.S. EPA From: Jeff Coburn, RTIInternational Subject: Uncertainty

of Monthl y Landfill Gas Methane Concentration Measurements **Purpose** This memorandum document s the uncertainty of the methane recovery when using a monthly monitoring frequency with or without a minimum intervening period between “monthly” measurement events . **Background** On October 30 , 20 09 , the EPA pro mulgate d mandatory GHG reporting requirements for a variety of sources including municipal solid wa ste (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 weekl y measurement of landfill gas methane concentration . Industry representatives have commented that the weekly measurement requirement is burdensome, particularl y for closed landfills, and that monthly monitoring of methane concentration would provide an ad equatel y accurate estimate of the methane recovery . To assist the EPA in their review of this issue, industry representatives provided their weekl y flow and methane concentration measurements for the 2011 reporting year (RY2011). The data were provided o n a landfill - specific basis with an indication of the landfill as either active or closed. The EPA evaluated the weekl y measurement data, as documented in the memorandum “Review of Weekly

L andfill Gas Volumetric Flow and Methane Concentrations” from Jeff Coburn, RTI International, to Rachel Schmeltz, U.S. EPA, dated October 18, 2012 (see Docket Item No. EPA HQ - OAR - 2012 - 0934 - 0014). On April 2, 2013, the EPA proposed to allow monthly monitoring (rather than weekl y monitoring) for methane concentrations at landfills with gas collection systems based on the results of the October 2012 data anal ys es. Specifically , the proposed amendments required that the methane concentration be determined “...at least once each calendar month; if only

one measurement is made each calendar month, there must be at least fourteen days between measurements.” Industry representatives commented on the proposed rule stating that the 14 day interval was not necessary because: (1) the data showed little variability

in methane concentrations; (2) it “ significantl y and unnecessarily complicates scheduling ;” and (3) “it may not be possible to take a monthly reading at least fourteen day s apart due to the operating schedule of the device .” This memorandum evaluates the data originally pro vided by

the industry

representatives in light of these comments to determine if an intervening period between monthly samples is justified because it serves the intended purpose of limiting the uncertainty of the methane recovery .2

**Assertion 1: The d**  **ata s**  **how l** **ittle v**  **ariability in m**  **ethane c**  **oncentrations** The previous anal ys is (as documented in the October 18, 2012, memorandum) used “Week 1” concentrations for each month and the total monthly flow to develop one estimate of methane recovery . A second estimat e of methane recovery was developed using all “Week 2” concentration measurement. A third and fourth estimate of methane recovery was also developed using all “Week 3” and “Week 4” concentration measurements, respectivel y. In all of these cases, the reco very

estimates were made using concentration measurement data that were approximately

28 days apart and had to be at least 22 day s apart. If the rule did not include an intervening interval between monthly samples, it is possible that the landfill owner o r operator could determine the methane concentration just before midnight on January 31, wait a few minutes, and determine the methane concentration just after midnight on February 1 . This would qualify as monthly

samples as the samples are taken in different calendar months, even though they are minutes apart. In order to evaluate the impact of removing the intervening interval as suggested be commenters, the original data was again evaluated using a single methane concentration determination at the end of Month 1 for the cumulative flow for both Months 1 and 2, the methane concentration determination at the end of Month 3 for the cumulative flow for both Months 3 and 4, respectivel y. We note that subpart HH currentl y requires weekl y samples to be at le ast 3 day s apart, therefore using Week 4 for Month 1 and Week 1 for Month 2 would still indicate a minimum intervening period, so this approach was not used to evaluate the impact of removing the intervening period altogether. The annual methane recovery w as determined for each landfill based on “no intervening period” and compared to the annual methane recovery determined based on the weekly concentration measurements and flow rates. Figure 1 shows the distribution of the percent error in the monthl y meas urements as determined using the “no intervening period” values. Figure 2 shows the same distributional analysis determined previousl y using all “Week 1” methane concentration measurements. It is quite clear from a comparison of these figures that, while there is no significant bias (graphs still distributed around 0 error), there is a significantly broader range of errors. In the true monthly sampling paradigm (4 week intervals), there is only one landfill at the tails (outside ± 6.25 percent error). Fo r the “no intervening period” analysis, there are approximately

10 landfills at each tail. Under the true monthly sampling paradigm , the extreme values were - 6.3 percent and +6.5 percent; under the “no intervening period” anal ys is, the extreme values were - 21 percent and +13.2 percent. These results strongl y indicate that more accurate methane recovery values will be achieved if there is an intervening period between monthly samples. In October 18, 2012, RTI also performed a Monte Carlo anal ys is to projec t the impacts of the variability

in methane concentration measurements on the accuracy of the calculated methane recovery value when going from weekl y s ampling (n= 52 ) to monthly sampling (n=12). In order to project the uncertainty of monthl y sampling wi th no intervening period , the Monte Carlo anal ys is was repeated with n=6 ( assuming the no intervening period is taken to its extreme). The 95 th percentile confidence interval for the annual average methane concentration based on 52, 12, and 6 measurements i s ± 3.8 % , ± 7.8 % , and ± 11 % , respectivel y .3 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0

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- 6 . 2 5 %

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- 5 . 7 5 t o - 5 . 2 5 %

- 5 . 2 5 t o - 4 . 7 5 %

- 4 . 7 5 t o - 4 . 2 5 %

- 4 . 2 5 t o - 3 . 7 5 %

- 3 . 7 5 t o - 3 . 2 5 %

- 3 . 2 5 t o - 2 . 7 5 %

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- 0 . 7 5 t o - 0 . 2 5 %

- 0 . 2 5 t o 0 . 2 5 % 0 . 2 5 t o 0 . 7 5 % 0 . 7 5 t o 1 . 2 5 % 1 . 2 5 t o 1 . 7 5 % 1 . 7 5 t o 2 . 2 5 % 2 . 2 5 t o 2 . 7 5 % 2 . 7 5 t o 3 . 2 5 % 3 . 2 5 t o 3 . 7 5 % 3 . 7 5 t o 4 . 2 5 % 4 . 2 5 t o 4 . 7 5 % 4 . 7 5 t o 5 . 2 5 % 5 . 2 5 t o 5 . 7 5 % 5 . 7 5 t o 6 . 2 5 % > = 6 . 2 5 % **Figure 1.** Distributional anal ys is of the percent difference in the annual methane recovery using monthly methane concentration measurements with no intervening period .

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0

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- 6 . 2 5 %

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- 5 . 7 5 t o - 5 . 2 5 %

- 5 . 2 5 t o - 4 . 7 5 %

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- 3 . 7 5 t o - 3 . 2 5 %

- 3 . 2 5 t o - 2 . 7 5 %

- 2 . 7 5 t o - 2 . 2 5 %

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- 0 . 2 5 t o 0 . 2 5 % 0 . 2 5 t o 0 . 7 5 % 0 . 7 5 t o 1 . 2 5 % 1 . 2 5 t o 1 . 7 5 % 1 . 7 5 t o 2 . 2 5 % 2 . 2 5 t o 2 . 7 5 % 2 . 7 5 t o 3 . 2 5 % 3 . 2 5 t o 3 . 7 5 % 3 . 7 5 t o 4 . 2 5 % 4 . 2 5 t o 4 . 7 5 % 4 . 7 5 t o 5 . 2 5 % 5 . 2 5 t o 5 . 7 5 % 5 . 7 5 t o 6 . 2 5 % > = 6 . 2 5 % **Figure 2**  **.** Distributional anal ys is of the percent difference in the annual methane recovery using monthly concentration measurements one month apart .4

**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 ± 1 0.5 % when going from a weekly to a true monthly

monitoring frequency . I f there is no intervening period between monthly samples , the uncertainty

increases to ± 13% . Thus, the inclusion of an intervening interval in the monthly sampling requirement helps to limit the increase in the uncertainty associated with the methane recovery values when switching from weekl y to monthly

monitoring. **Table 1. Estimate of the Overall Uncertainty for Methane Recovery for a Single Landfill**  **. Pa**  **rameter Uncertainty**  **Square of Uncertainty**

**N=52 N=12**  **N=6 N=52**  **N=12 N=6**  Accuracy of flow meter ±5% ±5% ±5% 25 25 25 Accuracy of methane co ncentration ±5% ±5% ±5% 25 25 25 Variability

of methane concentration ±3.8% ±7.8% ± 11 % 14.44 60.84 121 Sum of Squares 64.44 110.84 171 Overall Uncertainty (square root of Sum of Squares) ± 8.0% ± 10.5% ± 13.0% **Assertion 2: The intervening period “**  **significantly and unnec essarily complicates**  **scheduling ”**  No data were presented by industry commenters as to how the intervening period “significantl y” complicates scheduling. If the sampling is planned for approximately the 15 th of every month, the subsequent month’s sampling can be conducted any time during the following month. Thus, if the landfill owner schedules sampling to be conducted near the 15 th of each month, the landfill owner of operator has significant latitude to reschedule a certain month’s sampling event if an issue arises with the recovery system and the s ys tem cannot be sampled on the 15 th . Therefore, it is not clear how this requirement “significantl y” complicates scheduling. Furthermore, the weekl y sampling requirement currentl y in subpart HH requires a 3 day interv al between weekl y sampling . As such , monthly sampling with a 14 day

intervening period is much less burdensome or complicated than the existing weekl y measurement requirement with a 3 day intervening period . As for “unnece ssarily” complicating the rule, the “no intervening analysis” presented under Assertion 1 provides strong eviden ce that the intervening interval is necessary to limit measurement error and uncertainty associated with monthly sampling .5

**Assertion 3: The i ntervening period may make it impossible to collect samples**  **for some**  **months at some landfills**  **.** The majority

of landfills operate their recovery s ystem continuously . From the weekl y monitoring data submitted from the industry , onl y 22 of 395 landfills had landfill gas flow for less than 52 weeks per year and onl y 12 of 395 landfills had landfill gas flow for less than 5 0 weeks per year . If a landfill’s gas collection sy stem is down for only one or two weeks a year, the 14 - day

intervening period will not hin der the collection of monthly samples . On the other extreme, i f a landfill has no recovered gas flow for an entire month, then that landfill would have to use missing data procedures regardless of whether or not there is an intervening period between monthl y samples (in fact, no concentration measurement is need since the cumulative flow for the month is zero) . I f the landfill owner or operator generall y plans to sample earl y each month or near the 15 th of the month, the next month’s sample can be taken any time during the month. The only

time the lack of an intervening period would allow a landfill owner or operator to collect a monthly

gas sample is when there is only gas flow during the last week of a month and during the first week of the next month. The weekl y flow periods for the 1 2 landfills that have landfill gas flow for less than 5 0 weeks per year were evaluated to determine how often the 14 day intervening period would prevent a landfill owner or operator from collecting a monthly sample that th ey

could have otherwise collected if there were no intervening period between monthly samples. We could not identify one month for any

landfill within the entire data set where missing data provisions would have to be used under the monthl y sampling with a 14 - day intervening period but not under the monthly sampling with no intervening period. Based on the industry ’s weekl y sampling data and this anal ys is , we conclude that the 14 - day interval between monthly sampling events will not increase the prevalence at which facilities must rely on missing data procedures.