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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: Post-Proposal SSI Database Revisions and Data Gap Filling Methodology 

1.0 INTRODUCTION 

This memorandum summarizes the data revisions made to the Sewage Sludge 

Incinerators (SSI) unit inventory and emissions database after the rule’s proposal. Details 

about the database and its initial development can be found in the Inventory Database 

memorandum1 and the emissions database memorandum.2 The data discussed in this 

memorandum reflects the data used in preparation of the final standards. Section 2.0 

describes the changes made to the basic inventory data based on comments received after 

proposal. Section 3.0 describes the revised data gap filling methodology used for 

parameters such as unit capacities, feed rates, flow rates, and hours of operation. Section 

4.0 describes additional adjustments made to the extrapolated data on a unit-specific 

basis. 

2.0 INVENTORY REVISIONS 

Comments containing facility- or unit-specific data were reviewed and compared 

to the original inventory data. Where applicable, new data or data corrections were 

incorporated. Table 2-1 documents the changes made to the inventory data, as well as any 

additional supplemental data that could be used in subsequent cost or emissions analyses. 

The inventory revisions resulted in the removal of 19 multiple hearth (MH) units 

and 1 fluidized bed (FB) unit from the inventory, as well as the addition of 6 new 

fluidized bed units. The new inventory contains 144 MH units and 60 FB units. 

The dry sludge capacities for 13 units were added or revised based on comments. 

In many cases, commenters provided the number of dry tons or wet tons incinerated per 

year. If operational hours were also provided, these values were used to estimate an 

average feed rate in dry tons per hour. Similarly, if an average feed rate and the annual 

tonnage incinerated were provided, these values were used to estimate operational hours. 

Other similar calculations were made as appropriate, to fill in as many gaps as possible, 

based on the comment data. Operational hours were updated for more than 50 units, 

average feed rates were updated for 38 units, and average flow rates were updated for 24 

units. Additionally, service area populations were updated for about 40 units. 
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Baseline emissions and/or concentrations for several units were provided in the 

comments. These data could not be used to revise the emissions database because 

background documentation (test reports) were not provided to verify the information or to 

convert the results into units used in the analyses. 

Table 2-2 presents a clean version of all of the ‘known’ data based on the original 

inventory, ICR responses, and data received during the comment period. Red text 

indicates revisions made based on data received during the comment period. The data in 

this table was used, as described in the next section, to fill data gaps for all units with 

limited or no data. 

3.0 FILLING DATA GAPS BASED ON REVISED DATA 

Parameter values for units with no data were extrapolated based on the data 

shown in Table 2-2. The final values are presented in Table 3-1. The methodology used 

for each parameter is described below. 

A. Sludge Capacity (dry tons per hour) 

Known dry sludge capacities were summed by owning entity, and these total 

capacities were plotted against the owner’s service area population. The linear regression 

yielded an R2 value of 0.66, and it was considered sufficient to use this linear relationship 

to extrapolate unknown capacity values based on an owner’s population. Calculated 

capacities per owner, and subsequently per unit, are presented in Table 3-2. 

B. Hours of Operation 

The annual hours of operation for some facilities were provided during the 

comment period. However, for facilities with multiple units, commenters did not always 

provide information on whether or not all units operate at the same time. This 

information is important in determining an average number of hours per year each

individual unit operates. For instance, PAAlleghenyCounty reported in their ICR survey 

that they incinerate 8,760 hr/year (or full time). However, they also specify that only one 

unit operates at a time. Therefore, each of the facility’s units is assigned an average of 

4,380 hours per year. This methodology was applied to all multi-unit facilities for which 

it was specified that not all units operate simultaneously. For multi-unit facilities for 

which it was specified that all units operate at the same time, the total facility operating 

hours were applied to each unit, rather than divided among the units. For 2-unit facilities 

for which this information was not specified, it was assumed that only one unit operates 

at a time, as this appears to be the more common practice. Table 3-3 lists the hours 

reported for each facility or unit, interpreted as described above. 

These ‘known’ hours were then averaged based on the number of units per 

facility. As previously noted, 2-unit facilities were treated as two groups: those for which 

both units run simultaneously, and those for which only one operates at a time. Averages 
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are presented in Table 3-3. These averages were applied to fill the gaps for units with 

unknown operating hours. 

C. Annual Sludge Incinerated (dry tons per year) 

Data gaps for the amount of dry tons per year (dtpy) incinerated at a given unit 

were filled as follows: 

1. Calculations based on other ‘known’ parameters: 

- If wet tons per year were provided, this value was multiplied by 0.28 (an 

average moisture content for dry sludge) to convert to dtpy. 

- As described in Section 2, if ‘known’ values for both average feed rate and 

operational hours were available, these were multiplied to get dtpy. 

These calculated values are presented in the ‘Calculated dry tons/yr (based on 

known info) column of Table 3-4A. 

2. These ‘known’ values were summed by unit owner and plotted against the owner 

service area population. This was done separately for 1-unit entities and 2-unit 

entities, as shown in Table 3-4B. The equation for the resulting linear regression was 

then used to extrapolate data, based on population and number of units per entity. For 

entities owning more than 2 units, the ratio of dtpy to owner service area population 

for known data was determined and multiplied by the populations of entities with no 

data. Specific calculations are included in Table 3-4C. In some cases for entities 

owning multiple units, the units don’t have identical capacities or operational hours. 

To account for these differences in these cases, annual dry tons per year were 

adjusted based on the fraction of total entity annual operating hours contributed by 

each unit. 

D. Average Feed Rate (dry tons per hour) 

Once all gaps were filled for annual operating hours and annual dry tons 

incinerated, these values were used to fill in the remaining gaps for the average dry tons 

per hour. These calculations can be found in Table 3-5. 

E. Average Flow Rate (dry standard cubic feet per minute) 

Several different types of flow rates were required for different types of analyses. 

For cost analyses, flow rates needed to represent a ‘potential’ flow rate based on sludge 

capacity in order to estimate control costs. These maximum flow rates were represented 

by the set of flow rates extrapolated based on unit design capacities, as described below. 

For emission reductions and other analyses, another set of flow rates that would represent 

values based on ‘actual’ sludge usage was needed. These ‘average’ flow rates were 

extrapolated based on average feed rates. Additionally, for emissions analyses, flow rates 

needed to be converted to 7% oxygen. 
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Known flow rates, based on ICR survey responses, submitted test data, and 

additional data provided during the comment period, were plotted against known dry 

sludge capacities. The resulting linear regression was used to extrapolate flow rate values, 

using the known and extrapolated capacity data, for the remaining units with no flow rate 

data. These flow rates were used in costing algorithms. Calculations are shown in Table 

3-5, in the ‘Final Flow Rate (dscfm) Based on Capacity’ column. The same equation was 

then applied to the actual and extrapolated feed rates in order to determine flow rates 

based on actual sludge usage. Calculations can be found in the ‘Final Flow Rate (dscfm) 

Based on Avg. Feed Rate’ column in Table 3-6. 

Before applying these two sets of flow rates to emissions calculations, they 

needed to be converted to 7% oxygen, in order to be compatible with emissions 

concentration data. For units having emissions data, flow rates were converted directly 

based on reported O2 values. For units without emissions data, average O2 values were 

calculated for each subcategory, based on emissions test data, and those averages were 

applied to each set of final flow rates. The average O2 value for FB units was 10.1 

percent and the average for MH units was 10.5 percent. These calculations are shown in 

the last two columns of Table 3-6. For the emissions analyses, the 7% O2-converted flow 

rates based on capacity were used to determine ‘potential’ emissions, and those based on 

average feed rates were used to determine ‘actual’ emissions. Further details on how 

these emissions were calculated can be found in the baseline emissions memo3 and the 

cost and emission reduction memo.4

4.0 FINAL ADJUSTMENTS TO EXTRAPOLATED DATA 

Because operational hours per year, dry tons per year, and dry tons per hour are 

all related, and because by definition the unit capacity must be greater than or equal to the 

sludge feed rate, several discrepancies became apparent when all extrapolated data were 

compared. Where appropriate, on a unit-specific basis, extrapolated data were revised to 

resolve these discrepancies. For instance, as an artifact of the methodologies used to fill 

data gaps, the City of New Orleans (East Bank) was assigned a design capacity of 2.94 

dtph, but an average feed rate of 3.02 dtph. The unit capacity was therefore revised to be 

3.02. In cases where this happened but the unit capacity was a ‘known’ value (rather than 

extrapolated), extrapolated values for the other parameters were adjusted to bring the 

average feed rate down to the capacity value. In many cases, an average utilization factor 

(see Utilization Factor column in Table 2-2) could be applied to revise the feed rate. 

Table 4-1 presents the refinements that were made. Table 4-1A highlights in red 

the units for which refinements are needed based on the initial extrapolated data. Table 4-

1B documents final parameter adjustments and, where appropriate, includes comments to 

describe what was done. These changes were incorporated in the flow rate calculations as 

well as in the final gaps filled table (Table 3-1). 
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5.0 REFERENCES 

1. “Development of the Inventory Database for the Sewage Sludge Incinerator Source 

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

Environmental Protection Agency. June, 2010. 

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. “Revised Estimation of Baseline Emissions From Existing Sewage Sludge 

Incineration Units” Memorandum from Eastern Research Group, Inc. to Amy 

Hambrick, U.S. Environmental Protection Agency. January, 2011. 

4. “Revised Cost and Emissions Reduction of Complying with the MACT Floor for 

Existing SSI Units” Memorandum from Eastern Research Group, Inc. to Amy 

Hambrick, U.S. Environmental Protection Agency. January 2011.