BACT analysis is one of the most technically demanding tasks in air quality permitting. Every Prevention of Significant Deterioration permit requires it for each pollutant that triggers PSD review. The analysis has to be documented, defensible, and consistent with how comparable sources have been permitted elsewhere. Getting it wrong does not just delay the permit. It can result in more stringent conditions than necessary or a challenged permit that reopens months of work.
The EPA top-down methodology is the standard approach. It has five steps, each building on the last. This article walks through each step with enough detail to apply it correctly.
What triggers BACT analysis
BACT applies when a new major stationary source or a major modification to an existing source triggers PSD review. That happens when the project's net emissions increase exceeds the significance threshold for a regulated pollutant. For NOx and VOC, that threshold is 40 tons per year in most areas. For CO it is 100 tons per year. For PM2.5 it is 10 tons per year.
Once triggered, BACT must be determined for each pollutant that exceeds the significance threshold. A project that triggers PSD for NOx and CO needs separate BACT analyses for each pollutant. The analyses are independent but draw from a common body of regulatory precedent.
BACT vs MACT vs LAER: BACT applies to PSD permits in attainment areas. LAER (Lowest Achievable Emission Rate) is a stricter standard that applies in nonattainment areas under nonattainment NSR. MACT (Maximum Achievable Control Technology) applies to hazardous air pollutants under Section 112 of the Clean Air Act. They are related concepts but distinct requirements with different analytical frameworks.
The five steps of top-down BACT
Identify all available control technologies
The first step is a comprehensive survey of all control technologies that could apply to the emission unit in question. This means every technology that has been applied to similar sources anywhere, regardless of whether it has been permitted in your state or at a comparable scale. The EPA Regional Clearinghouse (RBLC) database is the primary source. Review all BACT determinations for similar source categories in the past five years. Also review EPA guidance documents, vendor literature, and engineering studies. The goal is a complete list, not a short one. Technologies eliminated for cost or feasibility reasons in step one will be rejected in later steps with documented justification.
Eliminate technically infeasible technologies
Review each technology identified in step one and determine whether it is technically feasible for the specific emission unit being permitted. Technical infeasibility must be grounded in specific engineering reasons, not preference or cost. A technology is technically infeasible if it cannot be physically applied to the source, if it would interfere with the operation or safety of the process, or if it has never been demonstrated in practice on a similar source type. Document the basis for each elimination. Vague or unsupported claims of infeasibility will not survive agency review.
Rank remaining technologies by control effectiveness
The technologies that survive step two are ranked from most stringent to least stringent. Stringency is measured by the emission rate achievable with the technology applied, expressed in the same units as the permit limit will be written. For a combustion unit, that might be pounds of NOx per MMBtu. For a coating operation, it might be pounds of VOC per gallon of coating applied. The most effective technology sits at the top. The applicant proposes BACT starting from the top and works down only if the top-ranked technology is eliminated in step four.
Evaluate economic, energy, and environmental impacts
For each technology starting from the most stringent, evaluate whether the cost is disproportionate, whether energy impacts are significant, and whether secondary environmental impacts outweigh the air quality benefit. Cost effectiveness is typically expressed as dollars per ton of pollutant reduced. EPA guidance provides benchmarks, but there is no universal threshold. Cost effectiveness is evaluated in the context of similar determinations and the specific project economics. A cost that was acceptable for a 500 MMBtu/hr unit at a large industrial facility may be disproportionate for a 15 MMBtu/hr unit at a smaller facility. Document the analysis with actual cost estimates, not approximations.
Select BACT
The most stringent technology that survives the step four evaluation is BACT. The permit limit is written at the emission rate achievable with that technology, applied to the specific unit under consideration. The BACT limit must be expressed in a form that is enforceable and verifiable through monitoring. If monitoring of the BACT parameter directly is not practicable, surrogate parameters that demonstrate compliance must be defined and included in the permit conditions.
Where applicants make mistakes
Starting the list too narrow. Omitting technologies from step one because they seem obviously infeasible or uneconomical shortcuts the process and produces a vulnerable analysis. Every technology that could conceivably apply should appear in step one. The documentation of why it was eliminated is what makes the analysis defensible, not the decision to omit it from the start.
Generic cost effectiveness claims. Stating that a technology is too expensive without specific cost documentation does not meet the standard for elimination in step four. The analysis must include capital cost estimates, annualized cost estimates, and a calculated cost effectiveness ratio in dollars per ton of reduction achieved. Using published vendor quotes or engineering cost estimates, not rules of thumb.
Ignoring recent RBLC comparables. Agency reviewers check the RBLC database. If a technology was accepted as BACT for a similar source in another state within the past three years at a more stringent level than what the applicant is proposing, that will be flagged. The analysis needs to address recent precedent directly, either by proposing the same level or by explaining why site-specific factors justify a different determination.
Not connecting the limit to the technology. The BACT limit has to be achievable with the selected technology operating as designed. Proposing a technology and then writing a limit that the technology cannot reliably achieve creates a compliance problem before the facility even opens.
How AI assists with BACT analysis
AirComply retrieves current RBLC determinations for comparable source categories, applies top-down methodology framework to the specific source characteristics, identifies comparable permit limits from recent agency decisions, and produces permit-ready language for the proposed BACT limit and monitoring conditions. The analysis still requires a licensed professional to review, validate, and sign. What AirComply eliminates is the hours spent manually searching the RBLC database and assembling the documentation framework from scratch.
Run BACT analysis faster
AirComply retrieves current RBLC comparables and applies top-down methodology to your specific source.
Request demo accessThe bottom line
BACT top-down analysis is not complicated in concept. The steps are clear. The challenge is doing it rigorously: comprehensive identification of technologies, specific documentation of eliminations, real cost estimates, and a proposed limit that is both defensible against current precedent and achievable by the selected technology.
Permits that get challenged on BACT usually fail because of incomplete step-one lists, unsupported elimination rationales in step four, or proposed limits that are not consistent with recent comparable determinations. The remedy for all three is thoroughness in the analysis and documentation, not creative argumentation after the fact.