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OAR Box 1787 

Prepped by Ollie Stewart 

Document Number: 

74) ll-C-32 

Docket Number: 

A-90-37 

Printed 11/4/2008 10:22:19 AM Sheet 74 of 147 
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

Office of Air Quality Planning and Standards 

Research Triangle Park, North Carolina 27711 

Pt- 70-37 

MAY i9ag 

Mr. John Paul 

Supervisor, Regional Air Pollution 

Control Agency 

451 W. Third Street 

P.O. Box 972 

Dayton, Ohio 45422 

Dear Mr. Paul: 

}2 A .\.i i ')•> 

As we agreed, I am providing you with a draft copy for 

review by the STAPPA/ALAPCO NSR Subcommittee of a summary of 

"top-down" BACT guidance. The draft is also being concurrently 

reviewed by the EPA Regional Offices. Please have the 

Subcommittee submit their comments by June 19, 1989. 

We are currently working on developing more detailed 

"top-down" BACT guidance material. However, in response to some 

of the broader questions and issues raised by State and local 

agencies and permit applicants regarding the implementation of 

"top-down" BACT, we have prepared the attached summary document. 

The purpose of the document is to act as a general guide through 

the "top-down" process until the more detailed guidance becomes 

available. 

If you have any questions about the draft document, please 

contact me at FTS 629-5621 or Mr. David Solomon of the New Source 

Review Section at (FTS) 629-5375. 

Sincere] 

Air 

John caacagni 

/ Director 

(lity Management Division 

Enclosure 

cc: G. Emison (w/enclosure) 

W. Becker (w/enclosure) 

Air Division Director, Regions I-X (w/enclosure) 
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"TOP-DOWN" BEST AVAILABLE CONTROL TECHNOLOGY: 

A SUMMARY 

Environmental Protection Agency 

Office of Air Quality Planning and Standards 

Air Management Division 

Noncriteria Pollutants Program Branch 

New Source Review Section 

May 25, 1989 
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TOP-DOWN BACT: A SUMMARY 

I. BACKGROUND 

On December 1, 1987, the Environmental Protection Agency's 

(EPA) Assistant Administrator for Air and Radiation issued a 

memorandum that established certain program initiatives designed 

to improve the effectiveness of the new source review programs. 

Among these was the "top-down" methodology for determining best 

available control technology (BACT). The Office of Air Quality 

Planning and Standards is responsible for implementing many of 

these initiatives, including the development of specific guidance 

on the "top-down" approach to determining BACT. Recently, this 

office has received numerous inquiries from the EPA Regional 

Offices, State and local agencies and permit applicants 

requesting guidance on the implementation of "top-down" BACT. 

The purpose of this overview is to provide a general summary of 

the "top-down" methodology in order to assist permitting 

authorities and prevention of significant deterioration (PSD) 

applicants in conducting BACT analyses. The EPA is also 

currently working on additional guidance material. 

In calling for the use of the "top-down" approach in BACT 

determinations, EPA has not changed the BACT requirement for any 

specific source or category of source. The definition of BACT as 

contained in the Clean Air Act, the PSD regulations and the State 

implementation plans of individual States has not changed. 

Rather, the "top-down" approach is a change in the methodology of 

the BACT selection process. There are two key criteria that must 

be satisfied in any BACT analysis under the Clean Air Act. 

First, the permit applicant must consider the most stringent 

control technologies available. Second, if the applicant 
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proposes less stringent controls, it must demonstrate, using 

objective data, that the most stringent controls are not 

achievable due to source-specific energy, environmental, or 

economic impacts, and the permitting authority must exercise its 

informed judgment before accepting that determination. The EPA's 

experience was that prior BACT methodologies were deficient in 

fulfilling the core BACT requirements. The EPA expects that the 

top-down approach will be more effective in assuring that BACT 

analyses comply with these requirements. 

II. THE "TOP DOWN" PROCESS 

The first step in a "top-down" analysis is to identify, for 

the emissions unit in question, all "available" control options. 

Available control options are those air pollution control 

technologies with a practical potential for application to the 

emissions unit and the regulated pollutant under evaluation. The 

control alternatives should include not only existing controls 

for the source category in question, but also (through technology 

transfer) controls applied to similar source categories and gas 

streams, and innovative control technologies. Technologies 

required under lowest achievable emission rate (LAER) 

determinations are available for BACT purposes and must also be 

included as control alternatives. 

Later in the BACT analysis, one or more of the options may 

be eliminated from consideration because they are determined to 

be technically infeasible or have unacceptable energy, economic, 

and environmental impacts on a case-by-case basis. However, at 

the outset, applicants should identify (but not, at this time, 

conduct a detailed study of) all control options with potential 

application to the emissions unit under review. 
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In the second step, the technical feasibility of the control 

options identified in step one is evaluated with respect to the 

source-specific factors at the emissions unit. In general, a 

demonstration of technical infeasibility should be clearly 

documented and should show, based on physical, chemical, and 

engineering principles, that technical difficulties would 

preclude the successful use of the control option on the 

emissions unit under review. Technically infeasible control 

options are then eliminated from further consideration in the 

BACT analysis. For example, in cases where the level of control 

in a permit has not yet been achieved in practice (e.g., a source 

committed to a permitted level but the project was cancelled or 

the permitting agency has found that the permit limitations are 

unachievable by the permitted source), and supporting 

documentation showing why such limits are not technically 

feasible is provided, the option may be eliminated from further 

consideration. 

In step three, all remaining control alternatives not 

eliminated in step two are ranked in order of control 

effectiveness, with the most effective control alternative at the 

top. The list should present the array of control alternatives 

and include the following: 

- control efficiencies (percent pollutant removed); 

- expected emission rate (tons per year); 

- expected emissions reduction (tons per year); 
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- economic impacts (e.g.,; cost per ton, incremental and 

total costs); 

- environmental impacts (includes any significant or unusual 

other media impacts (e.g., water or solid waste), and, at a 

minimum, the relative ability of each control alternative to 

control emissions of toxic or hazardous air contaminants); 

- energy impacts. 

A list should be prepared for each pollutant and for each 

emissions unit (or grouping of similar units) subject to a BACT 

analysis. 

At this point the analysis validates the suitability of the 

top control option in the listing for selection as BACT, or 

provides clear justification that the top candidate is 

inappropriate as BACT. If the applicant accepts the top 

alternative in the listing as BACT from an economic (and energy) 

standpoint, the applicant proceeds to consider whether impacts of 

unregulated air pollutants or impacts in other media would 

justify selection of an alternative control option. If there are 

no outstanding issues regarding collateral environmental impacts, 

the analysis is ended and the results accepted as BACT. In the 

event that the top candidate is shown to be inappropriate, due to 

energy, environmental, or economic impacts, the rationale for 

this finding needs to be fully and carefully documented for the 

public record. Then the next most stringent alternative in the 

listing becomes the new control candidate and is similarly 

evaluated. This process continues until the technology under 

consideration cannot be eliminated by any source-specific 
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environmental, energy, or economic impacts which demonstrate that 

alternative to be inappropriate as BACT. 

The core of a BACT analysis, after identification of 

available control options, is the consideration of energy, 

environmental, and economic impacts to select the final level of 

control. The applicant is responsible for presenting an 

objective evaluation of each impact. Consequently, both 

beneficial and adverse impacts should be discussed and, where 

possible, quantified. In general, the BACT analysis should focus 

on the direct impact of the control alternative. The following 

discussion, under each of the three headings of energy, 

environmental, and economic impacts, briefly touches on a number 

of factors which may be considered in the respective impacts 

analyses. These factors provide general guidelines only and are 

not intended as an exclusive list of considerations for BACT. 

A. Energy Impacts 

Applicants should examine the energy requirements of the 

control technology and determine whether the use of that 

technology results in any significant or unusual energy penalties 

or benefits. A source may, for example, benefit from the 

combustion of a concentrated gas stream rich in volatile organic 

compounds; on the other hand, more often extra fuel or 

electricity is required to power a control device or incinerate a 

dilute gas stream. If such penalties or benefits exist, they 

should be quantified. Certain types of control technologies have 

inherent energy penalties associated with their use. While these 

penalties should be quantified, so long as they are within the 

normal range for the technology in question, such penalties 

should not, in general, be considered adequate justification for 

nonuse of that technology. 
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Energy impacts should consider only direct energy 

consumption and not indirect energy impacts. For example, the 

applicant could estimate the direct energy impacts of the control 

alternative in units of energy consumption at the source ( e.g., 

Btu, kWh, barrels of oil, tons of coal). The energy requirements 

of the control options should be shown in terms of total and 

incremental (units of energy per ton of reduction) energy costs. 

The energy impact analysis may also address the concern over 

the use of locally scarce fuels. The designation of a scarce 

fuel may vary from region to region, but in general a scarce fuel 

is one which is in short supply locally and can be better used 

for alternative purposes, or one which may not be reasonably 

available to the source either at the present time or in the near 

future. 

B. Environmental Impacts 

The environmental impacts analysis is not to be confused 

with the air quality impact analysis, which is conducted to 

demonstrate that the source (using the level of control 

eventually selected as BACT) will not cause or contribute to a 

violation of any applicable national ambient air quality standard 

or PSD increment. The environmental impacts analysis, in 

contrast, concentrates on impacts other than impacts on air 

quality (i.e., ambient concentrations) due to emissions of the 

regulated pollutant in question, such as solid or hazardous waste 

generation or discharges of polluted water from a control device, 

visibility impacts, or emissions of unregulated pollutants. 
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The applicant should identify any significant or unusual 

environmental impacts associated with a control alternative that 

have the potential to affect the selection or rejection of a 

control alternative. Some control technologies may have 

potentially significant secondary environmental impacts. 

Scrubber effluent, for example, may affect water quality and land 

use, and, similarly, technologies using cooling towers may affect 

visibility, other examples of secondary environmental impacts 

could include hazardous waste discharges, such as spent catalysts 

or contaminated carbon. Generally, these types of environmental 

concerns become important when sensitive site-specific receptors 

exist or when the incremental emissions reduction potential of 

the top control is only marginally greater than the next most 

effective option. However, the fact that a control device 

creates liquid and solid waste that must be disposed of does not 

necessarily argue against selection of that technology as BACT, 

particularly if the control device has been applied to similar 

facilities elsewhere and the solid or liquid waste problem under 

review is not significantly greater than in those other 

applications. On the other hand, where the applicant can show 

that unusual circumstances at the proposed facility create 

greater problems than experienced elsewhere, this may provide a 

basis for the rejection of the top control alternative as BACT. 

The generation or reduction of toxic and hazardous 

emissions, including compounds not regulated under the Clean Air 

Act, are considered as part of the environmental impacts 

analysis. On June 23, 1986, the EPA Administrator remanded the 

PSD permit decision for the North County Resource Recovery 

project in California to EPA Region XI for reconsideration. At 

issue in the remand was whether appropriate consideration had 

been given, within the BACT determination, to the control of air 
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pollutants not subject to regulation under the Clean Air Act. 

The remand strongly affirmed that the permitting authority should 

take into account the ability of a given control alternative for 

regulated pollutants to reduce emissions of unregulated 

pollutants in making BACT decisions. Consequently, the ability 

of a given control alternative to control toxic or hazardous air 

contaminants must be considered in the BACT analysis and, as 

appropriate, raay affect the outcome of the analysis. 

C. Economic Impacts 

In the economic impact analysis, primary consideration 

should be given to quantifying the cost of control (e.g., total 

cost, dollars per ton of pollutant removed, incremental costs per 

ton of pollutant removed) and not the economic situation of the 

individual source. For control alternatives that have been 

effectively employed in the same source category, the economic 

impact of such alternatives on the particular source under review 

should be not nearly as pertinent to the BACT decision making 

process as the total and incremental cost effectiveness of the 

alternative. Thus, where a control technology has been 

successfully applied to similar sources in a source category, an 

applicant should concentrate on documenting significant cost 

differences, if any, between those other sources and the 

particular source under review. 

Normally the submittal of very detailed and comprehensive 

project cost data is not necessary. However, where initial 

control cost projections on the part of the applicant appear 

excessive or unreasonable (in light of recent cost data) more 

detailed and comprehensive cost data may be necessary to document 

the applicant's projections. 
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It is important to keep in mind that BACT is a primarily 

technology-based standard. However, unusual circumstances may 

greatly affect the cost of controls in a specific application and 

should be documented. An example of an unusual circumstance 

might be the unavailability in an arid region of the large 

amounts of water needed for a scrubbing system. Shipping water 

from a distant location might add unreasonable costs to the 

alternative, thereby justifying its rejection on economic 

grounds. Where unusual factors exist that result in 

cost/economic impacts beyond the range normally incurred by other 

sources in that category, the technology can be rejected provided 

the applicant has adequately identified the circumstances, 

including the cost or other analyses, that show what is 

significantly different about the proposed source. 

Where the cost of a control alternative for the specific 

source being reviewed is within the range of normal costs for 

that control alternative, the alternative may also be eligible 

for rejection in limited circumstances. This may occur, for 

example, where the control alternative has not been required as 

BACT (or its application as BACT has been extremely limited), and 

there is a clear demarcation between recent BACT control costs in 

that source category and control costs for sources in the 

category which have been driven by other constraining factors. 

To justify rejection of an alternative on these grounds, the 

applicant must demonstrate to the satisfaction of the permitting 

agency that costs of pollutant removal (e.g., dollars per total 

ton removed and in some instances incremental ton removed) for 

the control are disproportionately high when compared to the cost 

of control for the pollutant in recent, acceptable BACT 

determinations. Specifically, the applicant must document that 

the cost to the applicant of the control alternative is 

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significantly beyond the range of recent costs normally 

associated with BACT for the type of facility (or BACT control 

costs in general) for the pollutant. This type of analysis 

should essentially demonstrate that a technically and 

economically feasible control option is nevertheless, by virtue 

of the magnitude of its associated costs and limited application, 

unreasonable or otherwise not "achievable" as BACT in the 

particular case. Total cost and cost effectiveness numbers 

should be factored into this analysis. However, such economic 

information must be coupled with a comprehensive demonstration, 

based on the other objective factors described in this document, 

that the technology is inappropriate in the specific 

circumstance. 

Also, the BACT analysis should not focus on only one element 

of the economic analysis, as the results may be misleading. In 

particular, undue focus on incremental cost effectiveness can 

give an impression that the cost of a control alternative is 

unreasonably high when in fact the total cost effectiveness is 

well within the normal range. For another example, there may be 

a case where the capital cost of a control option appears 

excessive when presented as a percent of the total project cost. 

However, if in this case a large emissions reduction is 

projected, low or reasonable cost effectiveness numbers may 

validate the option as an appropriate BACT alternative 

irrespective of the high capital costs. 

D. Other Considerations 

Examples of other possible circumstances, not necessarily 

economic in nature, which might affect a BACT analysis are: 

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1) When exceeding otherwise appropriate costs by a moderate 

amount would result in a substantial additional emissions 

reduction. 

2) When a control technology would achieve controls of more 

than one pollutant (including toxic pollutants). 

3) Where the proposed BACT level would cause a violation of 

an applicable air quality standard or increment. A permit 

cannot be issued to a source that would cause or contribute 

to such a violation. 

4) When there are legal constraints outside of the Clean Air 

Act requiring the application of a more stringent technology 

than one which otherwise would have been determined to be 

; BACT. 

In all cases, a complete BACT analysis must consider 

environmental and energy, as well as economic impacts in the BACT 

analysis. 

III. SUMMARY 

In summary, all available control technologies are ranked, 

and the most stringent alternative is considered initially in the 

BACT analysis. However, when supported by a complete and 

objective review, technologies that can be demonstrated to be 

infeasible, unreasonable, or otherwise not achievable considering 

source-specific energy, economic, environmental, or technological 

reasons can be set aside. 

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