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MEMORANDUM 

To: Amy Hambrick, U.S. EPA, Sector Policies and Programs Division/Natural 

Resources and Commerce Group 

From: Eastern Research Group, Inc. 

Date: January 2011 

Subject: Revised Estimation of Baseline Emissions from Existing Sewage Sludge 

Incineration Units 

1.0 INTRODUCTION 

The U.S. Environmental Protection Agency (EPA), under section 129 of the Clean Air 

Act (CAA), is required to regulate emissions of nine pollutants from sewage sludge incineration 

(SSI) units: hydrogen chloride (HCl), carbon monoxide (CO), lead (Pb), cadmium (Cd), mercury 

(Hg), particulate matter (PM), total mass basis dioxins/furans (TMB PCDD/PCDF) and toxic 

equivalency basis dioxin/furans(TEQ PCDD/PCDF), nitrogen oxides (NOx), and sulfur dioxide 

(SO2). In order to assess the effects of regulatory requirements on these pollutants, it is first 

necessary to determine their emissions at the current level of control at each SSI unit. These 

emissions are referred to as the baseline emissions.

This memorandum describes the development of baseline emissions estimates for existing 

sources in the SSI source category. Section 2.0 discusses the sources of data used in the 

development of the baseline emission estimates, Section 3.0 presents the methodology used to 

estimate baseline emissions, and Section 4.0 summarizes the results of this analysis. Tables 1-1 

and 1-2 summarize the baseline emissions estimated for the nine section 129 pollutants regulated 

by SSI emission guidelines for existing sources in two subcategories, multiple hearth (MH) 

incinerators, and fluidized bed (FB) incinerators. Table 1-1 presents ‘actual’ emissions, based on 

dry sludge feed rates reported by facilities and additional extrapolated data. Table 1-2 presents 

‘potential’ emissions, based on known dry sludge design capacities and additional extrapolated 

data. Unit design capacities and average feed rates are discussed in detail in the database revisions 

memorandum.1

Table 1-1. Summary of Baseline Emissions for Existing SSI Units (tons per year) – 

Based on Actual Dry Sludge Usage 

Subcategory Cd Pb Hg HCl SO2 NOx CO PM 

PCDD/PCDF 

(mass) 

PCDD/PCDF 

(TEQ) 

FB 0.0015 0.0056 0.039 1.59 42.6 316 73 24.9 0.0000007 0.00000006 

MH 0.91 2.37 0.855 26.6 660 2,060 8,430 308 0.0000056 0.00000038 

Total 0.91 2.37 0.894 28.2 702 2,380 8,500 333 0.0000063 0.00000044 
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Table 1-2. Summary of Baseline Emissions for Existing SSI Units (tons per year) – 

Based on Dry Sludge Capacity 

Subcategory Cd Pb Hg HCl SO2 NOx CO PM 

PCDD/PCDF 

(mass) 

PCDD/PCDF 

(TEQ) 

FB 0.0022 0.0077 0.058 2.17 56.7 476 101 37.2 0.0000010 0.00000008 

MH 1.15 3.07 1.15 41.2 1,020 2,800 11,475 408 0.0000076 0.00000051 

Total 1.15 3.07 1.21 43.3 1,080 3,280 11,580 445 0.0000086 0.00000059 

2.0 DATA SOURCES 

Emissions information on SSI units was collected from two sources. An information 

collection request (ICR) survey was sent to nine owners of SSI’s units, who provided 

characterization information (costs, controls, operating information) for their units.2 The survey 

respondents also provided emissions test data from 16 SSI units. Some of the tests were 

conducted in response to the ICR; other test reports were for tests conducted within five years 

prior to the ICR request. A second source of emissions information was test reports collected 

from State environmental agencies’ public databases for nine SSI units (all MH units). The 

emission tests in these reports were conducted between 2000 and 2009. The emissions 

information and ICR responses are further discussed in the memorandum, “Facility, Unit, and 

Emissions Test Database for the Sewage Sludge Incineration Source Category”.3

Baseline emissions were calculated for 204 SSI units, of which 144 have the MH 

combustor design and 60 have the FB combustor design. As indicated above, emissions 

information was gathered on 26 SSI units (20 MH and 6 FB units). The information in the 

emissions test reports were then applied to the other SSI units based on their characteristics and 

controls. The revised inventory of SSI units is discussed in the database revisions memorandum.1 

3.0 METHODOLOGY FOR ESTIMATING BASELINE EMISSIONS 

All the emissions information collected were for stack tests conducted following all the 

control devices, i.e., controlled emissions at the baseline level of control. For units where 

emissions information was gathered, the average concentration of the individual test runs was 

used to calculate their baseline emissions. If multiple tests were conducted for a unit, then the 

average was calculated as the average of all the test runs. Baseline emissions on an annual basis 

were calculated from the concentration reported in the test information corrected to 7 percent 

oxygen (either parts per million volume dry (ppmvd), milligrams per dry standard cubic meter 

(mg/dscm), or nanograms per dry standard cubic meter (ng/dscm), the flue gas flow rate of the 

emission stream corrected to 7 percent oxygen in dry standard cubic feet per minute (dscfm), and 

the hours of operation of the unit. Attachment A shows the calculations used for converting 

concentration values to emission rates. 
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For the remaining units where emissions information was not available, baseline emissions 

were calculated using average concentrations, flow rates, and hours of operation. The 

development and use of these parameters is discussed in this section. 

3.1 Assignment of Concentrations

In order to calculate baseline emissions for all units, an average concentration was 

calculated from the known information and assigned to the remaining units without data. Slightly 

different approaches were taken for each subcategory, as described below. Table 3-1 presents the 

assumed baseline concentrations for each pollutant for each unit. 

MH Units: 

First, the uncontrolled average concentration for each unit with test data was calculated 

using the following equation: 

Uncontrolled Concentration = (Controlled Concentration) ÷ [1- (% control efficiency/100)] 

The control efficiencies used in the calculation are presented in Table 3-2. The efficiencies 

were based on assumptions used in previous EPA regulations, particularly the industrial, 

commercial, and institutional boiler NESHAP, and incorporate engineering judgment based on 

information provided by EPA testing personnel, internet web searches, and EPA technical 

documents and fact sheets on control devices.4-6 For this analysis, whenever there were multiple 

control devices affecting the same pollutant, the highest reduction efficiency for all the controls 

was used. For example, if a unit had a venturi scrubber and a fabric filter, the control efficiency of 

the fabric filter for cadmium and lead control was assigned to the combination because fabric 

filters are more efficient in controlling these pollutants. While some additional control may be 

achieved from multiple controls in series, most of the controls currently used at SSI units do not 

generally overlap in their effectiveness for most pollutants. The assumption also provides the 

most conservative estimate of performance. Although some units use thermal oxidizers or 

afterburners and achieve lower CO emissions levels, reduction efficiency was not assigned to them 

for CO because data were not available to determine a percent reduction value. For these units, 

the baseline level of CO emissions will be overstated. FGR has been used on combustion devices 

to reduce NOx emissions. However, the amount of NOx reduced varies widely, ranging from 20 

percent to 80 percent, and site-specific factors often affect the performance. Emissions test data 

collected by EPA showed that one unit providing emission test data operates a MH unit with 

FGR. However, its emission levels are similar to units without FGR. So no conclusion could be 

made on FGR performance. For CO, NOx, and PCDD/PCDF, the average of emissions from all 5 

tested MH units were applied, because emissions of these pollutants are influenced more by the 

combustion mechanism of MH units rather than add-on control device technologies used at MH 

units. 

Once all concentration data were converted to uncontrolled levels, all the uncontrolled 

data points for a pollutant in a subcategory were averaged to develop average uncontrolled 

concentration levels. The average uncontrolled levels developed for each pollutant were then 

used to estimate emissions after application of existing controls (referred to as baseline emissions) 
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for each SSI unit without emissions data by applying the pollutant control efficiencies (from Table 

3-2) for the control devices currently in use (as identified in the SSI inventory database). The 

following equation was used: 

Baseline Concentration = (Uncontrolled Concentration) x [1-(% control efficiency/100)] 

FB Units: 

Four of the six FB units with emissions data have wet electrostatic precipitators in 

addition to venturi and impingement tray scrubbers. For Cd, Pb, Hg, and PM, emissions from 

these units were averaged, and that average was applied directly to other units having WESPs. 

For other units not having WESPs as part of their controls, the Cd, Pb, Hg, and PM emissions 

values from the FB with the most basic controls (venturi scrubbers and impingement tray 

scrubbers) were considered indicative of the emission levels achieved with these scrubbers, and 

were assigned to units with these controls. 

For CO, NOx, and PCDD/PCDF, the average of emissions from all tested FB units were 

applied, because emissions of these pollutants are influenced more by the combustion mechanism 

of FB units rather than add-on control device technologies used at FB units. 

For HCl and SO2, the same back-calculation method used for the MH subcategory was 

applied to fill data gaps. 

3.2 Determining Flow Rates

The flue gas flow rate exiting the SSI units where data were not available was 

extrapolated using the linear regression relating known flue gas flow rates to the dry tons of 

sludge fired. Two feed rates were considered to show the range of potential emissions. The first 

was the average sludge feed rate based on information provided by sources. The second was the 

emissions based on the unit design capacity. We estimated the design capacity emissions because 

the amount of wastewater treated (and sludge produced) may vary significantly based on changes 

in population or sources of wastewater. Facilities have the potential to burn up to their units 

permitted capacity although they may not be doing so currently. Once flow rates were 

determined, they were adjusted to 7% O2, using average reported O2 values for each subcategory. 

More information about how unit capacity values, average feed rates, and average and maximum 

flow rates were developed can be found in the database revisions memorandum.1 

3.3 Development of Default Operating Hours

For some of the surveyed SSI units, the operating time was provided.2 Additionally, some 

operational hours were provided during the post-proposal comment period. Known operational 

hours were averaged based on the number of units at each facility, and these averages were then 

applied to fill in data gaps for this parameter. For instance, the average number of hours reported 

for facilities having only 1 unit was 5,004 hrs/yr. This value was applied to all units operating at 
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facilities with just the one unit. Further details on how operational hours were assigned to units 

with no data can be found in the database revisions memorandum.1 

4.0 SUMMARY OF RESULTS 

Table 4-1 presents the ‘actual’ baseline concentrations and calculated emissions for each 

pollutant and at each SSI unit. Table 4-2 presents the ‘potential’ baseline concentrations and 

calculated emissions for each pollutant and at each SSI unit. Flue gas flow rates and operating 

hours assigned for each SSI unit are also listed in these tables. 

5.0 REFERENCES 

1. Post-Proposal SSI Database Revisions and Data Gap Filling Methodology. Memorandum 

from Eastern Research Group, Inc. to Amy Hambrick, U.S. Environmental Protection 

Agency. January 2011. 

2. Facility, Unit, and Emissions Test Database for the Sewage Sludge Incineration Source 

Category. Memorandum from Eastern Research Group, Inc. to Amy Hambrick, U.S. 

Environmental Protection Agency. June 2010. 

3. Inventory Database for the Sewage Sludge Incineration Source Category. Memorandum 

from Eastern Research Group to Amy Hambrick, U.S. Environmental Protection Agency. 

June 2010. 

4. Development of Baseline Emission Factors for Boilers and Process Heaters at 

Commercial, Industrial, and Institutional Facilities. Memorandum from Amanda Singleton 

and Graham Gibson, Eastern Research Group to Jim Eddinger, U.S. EPA. April 2010. 

5. Technology Transfer Network, Clean Air Technology Center. 

http://www.epa.gov/ttn/catc/products.html 

6. A Comparison of Fluid Bed and Multiple Hearth Biosolids Incineration. Ky Dangtran, 

John Mullen, and Dale Mayrose. Paper presented at the 14th Annual Residuals and Sludge 

Management Conference. February 27-March 1, 2000, Boston MA
Attachment A 

Conversion of Units 
A-1 

The following calculations were used to develop ton/year emission estimates: 

PM, Pb, Cd and Hg

Concentration “X” given in mg/dscm, flow rate “FR” in dscf/minute (dscfm), and annual hours 

“H” (hours/year): 

[X(mg/dscm) x FR(dscf/min) x 60(min/hr) x H(hr/year)] ÷ 

[35.3147(dscf/dscm) x 453,592(mg/lb) x 2,000(lb/ton)] = (ton/yr) 

PCDD/PCDF 

Concentration “X” given in ng/dscm, flow rate “FR” in dscf/minute (dscfm), and annual hours 

”H” (hours/year): 

[X(ng/dscm) x FR(dscf/min) x 60(min/hr) x H(hr/year)] ÷ 

[35.3147(dscf/dscm) x 1,000,000 (ng/mg) x 453,592(mg/lb) x 2,000(lb/ton)] = (ton/yr) 

HCl, NOx, SO2, CO 

Concentration “X” given in ppmvd, flow rate “FR” in dscf/minute (dscfm), annual hours “H” 

(hours/year), and molecular weight “MW” as follows: HCl = 36.45, NOx = 46, SO2 = 64.06, 

CO = 28.01: 

[X(ppmvd) x MW(lb/lbmol) x FR(dscf/min) x 60(min/hr) x H(hr/year)] ÷ 

[1,000,000 x 385.5(dscf/lbmol) x 2,000(lb/ton)] = (ton/yr) 
A-2 

Tables 3-1 through 4-2 

See BaselineEmissionsMemo_Table3-1to4-2.xlsx