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Federal Register / Vol. 86, No. 5 / Friday, January 8, 2021 / Proposed Rules
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well below 1 0.0007. This information, in addition to the conservative healthprotective assumptions built into the screening assessment, leads us to conclude that adverse effects from acute exposure to emissions of this HAP from this source category are not anticipated.
Maximum noncancer hazard due to ingestion exposures estimated using health-protective risk screening assumptions are below an HQ of 1 0.4
for the Tier 2 fisher scenario. The estimated ingestion cancer risk is zero since we did not identify any carcinogenic HAP emitted from the source category. Considering all of the health risk information and factors discussed above, as well as the uncertainties discussed in section III of this preamble, we propose that the risks posed by emissions from the Mercury Cell Chlor-Alkali Plant source category are acceptable.
2. Ample Margin of Safety Analysis As directed by CAA section 112f2, we conducted an analysis to determine whether the current emissions standards provide an ample margin of safety to protect public health. Under the ample margin of safety analysis, we evaluated the cost and feasibility of available control technologies and other measures including the controls, measures, and costs reviewed under the technology review that could be applied to this source category to further reduce the risks or potential risks due to emissions of HAP from the source category.
As described above, the only HAP
emitted from this source category posing any risks of potential concern is elemental mercury, with a maximum noncancer acute HQ of 2 based on the REL. Therefore, we considered potential options to reduce mercury emissions under the ample margin of safety analysis. The options we considered under the ample margin of safety analysis are the exact same control options described under the technology review section of this preamble see section IV.D below.
First, as described in greater detail under the technology review section, we evaluated the option of requiring a combination of implementing a cell room monitoring program and performing work practices as an approach to minimize mercury emissions. Under the technology review section, we determined that this option does constitutes a development in emissions control practices pursuant to CAA section 112d6 with very low costs, and, therefore, we are proposing these requirements under the technology review. However, since the
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one operating facility already conducts these two actions, we do not expect any actual reductions in emissions and, therefore, we would expect no actual reductions in risks. Since this option is not expected to result in any risk reductions, we are not proposing to adopt those requirements pursuant to CAA section 112f.
The other option we considered under the CAA section 112d6 technology review described in section IV.D of this preamble as well as under CAA
sections 112d2 and 3, as described in section IV.A of this preamble, is to require zero mercury emissions from existing sources, which is the requirement for new and reconstructed mercury cell chlor-alkali production sources. This option would eliminate process vent and fugitive mercury emissions as it would force the remaining facility to convert the operation to a non-mercury process or close the mercury cell operation. As described in more detail in sections IV.A and IV.D of this preamble, we estimate the capital cost of converting the one remaining mercury cell facility to membrane cells is just over $69
million. The estimated emissions of mercury would be reduced from 126
pounds to zero pounds per year.
Considering the costs of conversion annualized over a time period of 20
years, the annual costs are estimated to be approximately $2.8 million, which results in a cost effectiveness of approximately $22,000 per pound of mercury emissions eliminated. With regard to reductions in risks due to HAP
emissions as a result of this option, since this option would force conversion or closure of the remaining one mercury cell facility, the risks due to emissions of HAP for the source category would be zero, since there would be no facilities in the source category.
Nevertheless, after considering the options described above, since the risks due to mercury emissions are already low with a maximum acute noncancer HQ of less than or equal to 2 based upon the 1-hour REL and a maximum HQ of 0.0007 based on AEGL2 and ERPG2, and given the costs described above, and because of the substantial uncertainties in the emissions estimates and cost estimates, we are not proposing any additional standards for mercury under CAA section 112f.
In summary, considering the very low cancer risks MIR far less than 1-in-1
million and very low chronic noncancer risks HI of 0.05 to individuals exposed to HAP emitted from this source category, and after considering possible options for
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mercury as described above, we are proposing a determination that the existing standards provide an ample margin of safety to protect public health.
3. Adverse Environmental Effect Based on the results of the environmental risk screening analysis, we do not expect an adverse environmental effect, as defined by CAA
section 112a7, as a result of HAP
emissions from this source category, and we are proposing that it is not necessary to set a more stringent standard to prevent, taking into consideration costs, energy, safety, and other relevant factors, an adverse environmental effect.
D. What are the results and proposed decisions based on our technology review?
As noted above, 40 CFR part 63, subpart IIIII currently includes emission limitations for mercury emissions from process vents including emissions from end-box ventilation systems, hydrogen systems, and mercury recovery facilities and work practices for fugitive mercury emissions from the cell room.
We have identified a development for cell room fugitive mercury emissions.
With regard to fugitive mercury emissions from the cell room, the current rule at 40 CFR 63.8192a through f requires a suite of equipment standards and work practices. It also provides the option, in lieu of the work practices otherwise required under CAA
sections 63.8192a through d, to institute a cell room monitoring program to continuously monitor the mercury vapor concentration in the upper portion of each cell room. See 40
CFR 63.8192 introductory text, and 40
CFR 63.8192g. The single mercury cell facility still operating complies via this alternative. However, while not required to do so under the current regulation, the facility also performs all the work practices. Therefore, the EPA
determined that the combination of implementing a cell room monitoring program and performing work practices constitutes a development in emissions control practices. This combination was the proposed option in the June 11, 2008, action 73 FR 33258, and also included as a co-proposal in the March 14, 2011 76 FR 13852, action. Since the only facility in the source category is already implementing the monitoring program and performing these work practices, there would be no costs with the exception of additional recordkeeping and reporting costs or additional mercury emission reductions associated with implementing a standard that requires a combination of these practices.

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