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MEMORANDUM 

To: David Putney, U.S. Environmental Protection Agency (EPA), OAQPS 

From: David Green and Matt Hakos, RTI International 

Date: November 13, 2014 

Subject: Development of Emissions Standards to Address Risks for the Primary Aluminum 

Production Source Category Pursuant to Section 112(f) of the Clean Air Act 

In developing the risk assessment for chronic exposures, we used the estimated annual average 

ambient air concentration of each of the hazardous air pollutants (HAP) emitted by each source for 

which we have reliable emissions data in the source category. The air concentrations at each nearby 

census block centroid were used as a surrogate for the chronic inhalation exposure concentration for all 

the people who reside in that census block. 

We then estimated the maximum individual risk (MIR) for each facility as the cancer risk 

associated with a continuous lifetime (24 hours per day, 7 days per week, and 52 weeks per year for a 

70-year period) exposure to the maximum concentration at the centroid of an inhabited census block. 

Individual cancer risks were estimated by multiplying the estimated lifetime exposure to the ambient 

concentration of each of the HAP (in micrograms per cubic meter) by its unit risk estimate (URE), 

which is an upper bound estimate of an individual’s probability of contracting cancer over a lifetime of 

exposure to a concentration of 1 microgram of the pollutant per cubic meter of air. For residual risk 

assessments, we generally use URE values from the EPA’s Integrated Risk Information System (IRIS). 

For carcinogenic pollutants without EPA IRIS values, we look to other reputable sources of cancer 

dose-response values, often using California EPA (CalEPA) URE values, where available. 

More details regarding the risk assessment methods and risk results are available in the Federal 

Register (FR) Notice and in the document titled Residual Risk Assessment for the Primary Aluminum 

Production Source Category in Support of the 2014 Supplemental Proposal (Risk Assessment), which 

is available in the docket for this action (Docket ID No. EPA-HQ-OAR-2011-0797). 

As explained in the supplemental proposal FR Notice and the Risk Assessment, when 

considering the risks associated with MACT-allowable emissions, we estimate that the MIR could be 

up to 300-in-1 million if the Columbia Falls Aluminum Company facility were to resume its primary 

aluminum operations with three vertical stud Soderberg two (VSS2) potlines. This estimated risk 

results mainly from estimated allowable emissions of polycyclic organic matter (POM), nickel and 

arsenic. Based on the model outputs, we estimate that 59% of this MIR risk is due to POM, 22% is due 

to arsenic, and 16% is due to nickel. The MIRs for all other facilities in the source category were less 

than 100-in-1 million. 

In order to ensure the risks associated with Soderberg facilities are acceptable, we evaluated the 

potential to reduce MACT-allowable VSS2 potline emissions for the primary HAP driving the cancer 

risks (i.e., POM, arsenic and nickel) to levels that would reduce the cancer risks to 100-in-1 million or 

less (i.e., the presumptive ceiling of acceptable risk, based on the Benzene NESHAP, 54 FR 38044, 

September 14, 1989). 
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Regarding POM, the current NESHAP includes an emissions limit for POM of 5.7 pounds per 

ton of aluminum produced (lb/ton) for VSS2 potlines. As noted above, the one facility driving the 

allowable risks has been idle for 5 years and has not operated at capacity since 2000. Also, the VSS2 

potline technology is less efficient than the newer, lower emitting prebake technologies. All indications 

are that this facility will not reopen. However, based on available data from the most recent years that 

it was operating, we estimate that if this one VSS2 facility did reopen and if it installed wet roof top 

scrubbers that it could achieve a POM emissions limit of 1.9 lb/ton (0.85 Kg/Mg) of aluminum 

produced, which would be a significant reduction in allowable POM emissions from this facility. This 

limit is 3 times lower than the current limit for POM from VSS2 potlines. Furthermore, given that 

there would be variability in emissions during normal operations, in order for this facility to comply

with a limit of 1.9 lb/ton at all times, it would need to have average POM emissions considerably lower 

than 1.9 lb/ton. Therefore, under the authority of CAA section 112(f)(2), the EPA is proposing a POM 

emission limit for VSS2 potlines of 1.9 lb/ton (0.85 Kg/Mg) of aluminum. This would reduce the 

estimated cancer risks due to allowable POM by at least a factor of 3. 

As mentioned above and in the supplemental proposal FR Notice, the one remaining Soderberg 

plant has been idle for about 5 years and we believe it is highly unlikely that the facility will reopen, 

due to its less efficient aluminum production method. However, if this facility does resume operations, 

we estimate that the capital costs for the roof top wet scrubbers would be about $30 million and that 

annualized costs would be about $8 million. 

Regarding emissions of nickel and arsenic, as explained in the Revised Draft Development of 

the RTR Emissions Dataset for the Primary Aluminum Production Source Category, which is available 

in the docket for this action, we estimate that the Columbia Falls Aluminum Company facility (again, 

presuming the highly unlikely scenario wherein this facility resumes primary aluminum reduction 

operations) would emit about 4,420 pounds of nickel per year and 672 pounds of arsenic per year, 

assuming operation of 3 potlines at full capacity. We estimate that wet roof scrubbers would achieve a

50 percent reduction in secondary potline emissions of these metals. See CFAC BART Analysis in the 

docket (Docket No. EPA-HQ-OAR-2011-0797). Therefore, with the wet scrubbers installed and 

operating, we estimate annual average emissions would be about 2,210 pounds of nickel per year and 

336 pounds of arsenic per year. The subject Columbia Falls Aluminum Company facility is the only 

facility in the U.S. that utilizes the VSS2 potline technology. Therefore, to estimate emissions of nickel 

and arsenic from this facility and to account for variability in emissions during normal operation, we

analyzed data on emissions of nickel and arsenic from the Alcoa-Massena East facility. We chose to 

use emissions data from the Alcoa-Massena East facility because it is the only primary aluminum 

reduction facility in the U.S. (other than the Columbia Falls Aluminum Company facility) that utilizes

Soderberg potline technology. Specifically, the Alcoa-Massena East facility utilizes horizontal stud 

Soderberg (HSS) potline technology. Our analysis of the nickel and arsenic emissions data from the 

Alcoa-Massena East facility results in our estimate of nickel and arsenic emissions of no more than 

0.14 lb/ton and 0.012 lb/ton, respectively, from the Columbia Falls Aluminum Company facility. 

To ensure the primary HAP metals (i.e., arsenic and nickel), that are driving some of the 

allowable cancer risks (along with the POM), are limited to acceptable levels of emissions, we are 

proposing facility-wide total potline emissions limits for arsenic and nickel that reflect a 50 percent 

reduction in the estimated potline emissions of those metals, along with a consideration of the 
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variability in emissions. The EPA is proposing these limits pursuant to section 112(f)(2) in order to 

ensure risks will be acceptable from the Soderberg potline subcategory. 

As mentioned above, our analysis of nickel emissions from the Columbia Falls Aluminum 

Company facility indicates that, after accounting for normal variability, nickel emissions from this 

facility are no more than 0.14 lb/ton of aluminum produced. Therefore, after installation of wet roof 

scrubbers, we estimate nickel emissions of no more than 0.07 lb/ton of aluminum produced from this 

facility. Similarly, we estimate that the installation of wet roof scrubbers would reduce arsenic 

emissions from this facility from no more than 0.012 lb/ton of aluminum produced to 0.006 lb/ton of 

aluminum produced. Therefore, pursuant to section 112(f)(2), the EPA is proposing emissions limits of 

0.07 lb/ton of aluminum produced for nickel and 0.006 lb/ton of aluminum produced for arsenic for the 

Columbia Falls Aluminum Company facility. 

Given that these reductions of metals would be achieved using the same controls used for 

POM, there would be no additional cost of control, and there would be risk reductions associated with 

reduced HAP metal emissions. Assuming the wet scrubbers would achieve a 50 percent reduction in 

HAP metal emissions, and assuming the facility would run 3 potlines, which is the most potlines it 

operated since the year 2000, we estimate that the roof top wet scrubbers would limit emissions of 

nickel and arsenic from potlines to about 2,210 pounds per year of nickel and 336 pounds per year of 

arsenic, on a facility-wide basis. 

Regarding post-control risks, we estimate that with a POM emission limit of 1.9 lbs/ton of 

aluminum produced for the VSS2 potline subcategory (that is 3 times lower than the current POM 

emission limit for that potline subcategory) and with nickel and arsenic emission limits that reflect a 50 

percent reduction in potential emissions of those metals, that the post control risks would be 

approximately 100-in-1 million or less, if the plant did reopen. 

Compliance with these emission standards would be determined by conducting annual 

emissions tests including tests of production based nickel and arsenic emissions, and continuous 

compliance would be demonstrated by facilities conducting various types of parametric monitoring, as 

described in the supplemental proposal FR Notice. These nickel and arsenic emission limits, would 

also apply to all new Soderberg potlines at any facility. We estimate that these nickel and arsenic 

emission limits, in conjunction with the proposed POM emission limits will decrease the MIR for the 

Columbia Falls Aluminum Company facility to approximately 100-in-1 million or less.