General Provisions: MACT Determinations - MACT Emission Limitation and Permit Conditions

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The purpose of this section is to provide information on some procedures that may be used to determine MACT emission limitations. This information is for guidance only and does not impose legally binding requirements for either the permitting authority or an owner or operator.

MACT Emission Limitation

The MACT emission limitation established by the permitting authority is based on the level of emission reductions that can be obtained by the affected source when MACT is applied and properly operated and maintained. The MACT emission limitation should be based on an overall reduction of all HAP emissions. The MACT emission limitation may need to account for differing kinds of equipment within the affected source and may include emission averaging provisions to allow such equipment to achieve MACT in the most cost-effective manner possible. The permitting authority may establish a MACT emission limitation for an individual HAP when the risk to human health and the environment warrants such an emission limitation. If it is not practicable to establish a specific numerical or efficiency limitation, then a specific design, process, or control technology should be designated as the MACT emission limitation. For example, a floating roof with a primary and secondary seal on a storage vessel or an equipment leak detection and repair practice could be determined as MACT.

Determining the expected emission reductions from an add-on control may require some engineering judgment. In some instances, the add-on control may achieve different levels of reduction efficiency even when it is applied to the same type of emission unit. Lower efficiency ratings may be due to different operational parameters or poor maintenance practices. The MACT emission limitation should be based on the level of control that the technology is likely to obtain for all emission units operating under good operational and maintenance practices.

When control efficiencies are used to establish a MACT floor, the MACT emission limitation could be expressed as this efficiency. In other words, all sources could be required to reduce emissions by some percent (i.e., 90-percent reduction). Additional terms and conditions would be necessary to make this practically enforceable, but such an emission limitation may be appropriate when all emission units are operated relatively homogeneously within the source category or subcategory.

The uncontrolled emission level for an emission unit is the maximum amount of HAP that could be emitted from the emission unit using current design specifications at full capacity utilization in the absence of controls. It could be computed using a variety of different formats, i.e. tons/yr, lbs/hr, lbs/ton, etc. The following sources of information may be acceptable:

1. Engineering calculation using material balance or emission factors;
2. Actual emission data from similar emission units;
3. Average annual hourly emission rate multiplied by hours of operation;
4. Emission limits and test data from EPA documents, including background information documents;
5. State emission inventory questionnaires for comparable sources;
6. Federal or State enforceable permit limits; or,
7. For equipment leaks use, "Protocols for Equipment Leak Emission Estimates," EPA-453/R-93-026.

The selection of the uncontrolled emission level will likely require some engineering judgment on the part of the permitting authority. Typical throughputs, flow rates, concentrations, etc. should be used to estimate a uncontrolled emission level that can be applied to the source category or subcategory. The definition of a control technology includes the use of pollution prevention and source reduction strategies. The permitting authority should take into consideration the use of such control measures when computing the uncontrolled emission level for an emission unit.

Alternative Ways to Comply

Once the permitting authority determines the MACT emission limitation, the applicant will determine a control strategy that allows the affected source to meet MACT. In many cases, this will be through the application of the MACT technologies. However, in some cases, the emission unit at the major source may already be controlled to some extent with an existing control technology. The owner or operator could demonstrate that using additional control strategies in combination with existing controls will allow the emission unit to achieve the required emission reductions. For instance, an emission unit may currently be controlled with a baghouse. The MACT emission limitation for the emission unit may be based on use of an electrostatic precipitator. The emission unit may be able to meet the MACT emission limitation by installing a series of baghouses in lieu of the electrostatic precipitator.

Applicable Monitoring, Reporting and Recordkeeping, and Compliance

The permitting authority should identify monitoring parameters in consultation with the applicant to assure compliance with the MACT emission limitation. However, the permitting authority is ultimately responsible for these monitoring parameters, as well as reporting and recordkeeping requirements at permit issuance.