Date: November 30, 2009 

Subject: Revised Sulfur Dioxide MACT Floor, Data Variability Analysis, and Emission 

Limit for New Large HMIWI 

EPA Contract No. EP-D-06-118; Work Assignment No. 3-09; SPPD No. 02/30 

RTI Project No. 0210426.003.009 

From: Thomas Holloway 

To: Ketan Patel 

OAQPS/SPPD/NRCG (E143-03) 

U.S. Environmental Protection Agency 

Research Triangle Park, NC 27711 

I. Background

On October 6, 2009, the U.S. Environmental Protection Agency (EPA) promulgated its 

response to the remand of the new source performance standards (NSPS) and emission guidelines 

(EG) for hospital/medical/infectious waste incinerators (HMIWI) by the U.S. Court of Appeals for 

the District of Columbia Circuit (the Court) and satisfied the Clean Air Act (CAA) Section 

129(a)(5) requirement to conduct a review of the standards every 5 years. The promulgated rule 

revised the NSPS and EG emission limits for all nine of the CAA Section 129 pollutants--—

particulate matter, lead, cadmium, mercury, chlorinated dibenzo-p-dioxins/dibenzofurans, carbon 

monoxide, nitrogen oxides (NOX), hydrogen chloride, and sulfur dioxide (SO2). Emission limits 

were revised for all three HMIWI size subcategories (large, medium, and small) for the NSPS and 

all four HMIWI size subcategories (large, medium, small, and small rural) for the EG. 

Following promulgation of the revised emission limits, an industry representative informed 

EPA of an error in the published NSPS emission limit for NOX for new large HMIWI, which did 

not appear to reflect EPA’s described analytical process for adopting the revised standards.1

 On 

review, EPA staff determined that the published revised NOX NSPS for new large HMIWI indeed 

did not reflect EPA’s intent in the final rule. Prior to promulgation, the NOX emission limit for 

new large HMIWI was tentatively estimated to be 150 parts per million by volume, dry basis 

(ppmvd), based on test data for the best-performing unit and taking into account the variability in 

the test data. This tentative NOX emission limit for new large HMIWI would be higher than the 

corresponding NOX emission limit for existing large HMIWI (140 ppmvd). In order to prevent a 

situation where a new source would have a less stringent emission limit than an existing source, 

EPA decided to assign the NOX emission limit for existing large sources as the NOX emission 

limit for new large sources. However, a NOX NSPS limit of 130 ppmvd was erroneously 

published, which did not correspond to EPA’s description of its standard-setting process. EPA is 

proposing to amend the HMIWI NSPS to include the correct NOX NSPS limit of 140 ppmvd for 

new large HMIWI, which matches the final NOX EG limit and reflects EPA’s intent in the October 

6, 2009 final rule. No further analysis is necessary, and this issue is not discussed any further in 

this memorandum.
After receiving industry’s notice about the erroneous NOX emission limit, we reviewed the 

other NSPS and EG emission limits published in the October 6, 2009 notice to look for any similar 

errors, and we determined that the published, revised SO2 NSPS for new large HMIWI also did 

not reflect EPA’s intent in the final rule. The purpose of this memorandum is to present the 

methodology and results of the revised NSPS emission limit determination for SO2. 

II. SO2 Emission Limit-New Large Sources

For the large HMIWI size subcategory, the SO2 emissions estimate associated with the best 

controlled similar unit is 0.462 ppmvd. In our analysis for the October 6, 2009 final rule, we 

indicated that the SO2 test run data for the best performer were normally distributed.2

 Based on 

that conclusion, we estimated a 99 percent upper limit of 1.59 ppmvd for SO2 and published an 

SO2 emission limit of 1.6 ppmvd for new large HMIWI.2 However, a closer examination of the 

SO2 emissions data for the best performer indicates that the data are actually lognormally 

distributed. (Note: In the emission limit determinations conducted for the October 6, 2009 final 

rule, we erroneously referred to the 99 percent estimated values as “upper confidence limits” or 

“UCLs.” In this memorandum, we are using the more accurate term “upper limits.”) 

Where there are a sufficient number of datapoints for the best performer, we have used the 

skewness of the data to determine the distribution of the data.2

 Because normal distributions 

typically have a skewness of zero, we have concluded that those datasets with a skewness less than 

0.5 are normally distributed, while those with a skewness of 0.5 or greater are lognormally 

distributed. Closer examination of the SO2 emissions data for the best performer shows the 

skewness of the data to be 0.54, which indicates the data are actually lognormally distributed. The 

skewness of the data was calculated by applying the Microsoft Excel SKEW function to the SO2

test run data for the best performer. (See Appendix A at the end of this memorandum for the test 

run data.) 

We calculated the 99 percent upper limit for SO2 using the following Microsoft Excel 

equation, based on a lognormal distribution of the SO2 test run data for the best performer: 

Lognormal distribution: 99% upper limit = EXP{[AVERAGE(Natural Log Values of Test Runs for Best 

Performer)] + [STDEV(Natural Log Values of Test Runs for Best Performer) x TINV(2 x probability, n -1 

degrees of freedom)]}, for a one-tailed t-value (with 2 x probability), probability of 0.01, and sample size of 

18 

Using this equation, we estimated the 99 percent upper limit for SO2 for new large HMIWI 

to be 8.04 ppmvd. (See Table 1 below.) As with the other emission limits, we determined the SO2

emission limit by rounding up the upper limit to two significant figures, in accordance with 

standard engineering practices. We estimate the SO2 NSPS emission limit should be 8.1 ppmvd, if 

our standard-setting process is to be correctly followed. EPA is proposing to amend the HMIWI 

NSPS to include the correct SO2 limit of 8.1 ppmvd for new large HMIWI, which reflects EPA’s 

intent in the October 6, 2009 final rule. 

2
3

Table 1. Summary of SO2 MACT Floor Results 

for New Large HMIWI 

Parameters SO2, ppmvd 

No. of sources = 1 

Top performer = 0.462 

Skewness = 0.54 

Kurtosis = -1.57 

Distribution = Lognormal 

No. of test runs = 18 

99% upper limit of top performer (test runs) = 8.04 

Limit (based on 99% upper limit) = 8.1 

III. References

1. E-mail from Lindsey W. Kroos, ALL4 Inc., to Ketan Patel, EPA. October 27, 2009. 

“Question Around Final HMIWI NSPS Emission Limit.” 

2. Memorandum from Thomas Holloway, RTI, to Katen Patel, EPA. July 2, 2009. Revised 

MACT Floors, Data Variability Analysis, and Emission Limits for Existing and New HMIWI.
Appendix A 

New Sources / Current Large Subcategory 

MACT Floor Unit and Test Runs
Table A-1. SO2 MACT Floor Unit – New Sources / Current Large Subcategory

FACID UNITID Facility name Unit no. City 

State 

abbr Category

120 120--2 Waste Management Resource Recovery 

and Recycling Center 

Unit 2 Anahuac TX L 

Note: L = Large HMIWI; DIFF = Dry sorbent injection/fabric filter.
Table A-2. SO2 MACT Floor Test Runs – New Sources / Current Large Subcategory 

FACID UNITID Facility name 

Unit 

no. City 

State 

abbr 

Category APCD code SO2 test date Param

eter 

120 120--2 Waste Management Resource 

Recovery and Recycling Center 

Unit 

2 

Anahuac TX L DIFF 9/20/02 Run 1 

Run 2 Run 3 

10/15/02-10/16/02 Run 1 

Run 2 Run 3 

11/10/04 Run 1 

Run 2 Run 3 

12/5/05-12/8/05 Run 1 

Run 2 Run 3 

12/4/06-12/9/06 Run 2 

Run 3 Run 4 

12/3/07-12/5/07 Run 1 

Run 2 Run 3 

Notes: L = Large HMIWI; DIFF = Dry sorbent injection/fabric filter.