General Provisions: MACT Determinations - The MACT Floor Finding

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The MACT Floor Finding

This section will explain four approaches for determining a MACT floor. If the emissions information is available, the first three methods should be considered before the permitting authority concludes that a MACT floor cannot be determined. The three emissions-based methods include using:

1. Existing state and local air toxic control regulations;
2. Control efficiency ratings; or
3. Emission reduction ratios.

A fourth method, the technology approach, can be used when insufficient emissions data are available to determine an average emissions limitation.

The first method compares air pollution regulations in different states. The second method is applicable when the control technologies under consideration can be assigned an efficiency rating for HAP emission reductions. This is most likely to occur with add-on control devices. The third method can be used for add-on control devices, work practices, recycling, reuse or pollution prevention strategies. Depending on the format of available information, a hybrid of the three approaches may be necessary. The fourth method involves determining which technology is being used by the best performing sources in the category and then determining the emissions limit that the technology is capable of achieving in practice on a continuous basis.

Calculation of the MACT Floor

The Clean Air Act (as amended in 1990) instructs the US EPA to set emission standards for new sources based on the emissions control achieved in practice by the best controlled similar source and to set emission standards for existing sources based on an average emission limitation achieved by the best performing 12 percent of existing sources or best performing five sources in the source category or subcategory for categories with fewer than 30 sources.

The word "average" can have several different meanings, including arithmetic mean, median, and mode. As stated previously, the EPA published a Federal Register notice describing these methods of determining the average as well as other common sense considerations at 59 FR 29196 et.seq., June 6, 1994. A copy of this notice is contained in Appendix B of on Guidelines for MACT Determinations under Section 112(j) Requirements[PDF]

The following examples illustrate the average as represented by the mean, median, and mode.

Example 1

Under some circumstances the arithmetic mean results in a number that may not correspond to the application of a specific control technology. If there is a large discrepancy between the amount of emission reductions that can be achieved by available control options, other factors should be considered to determine the MACT floor. This is illustrated with the following example:

Example 2

An arithmetic mean is computed for the best performing 12 percent of storage tanks. There are 10 sources among the best performing 12 percent of storage tanks. Two tanks are controlled at 99 percent, and the remaining 8 tanks are not controlled. The emissions limitations considered in the floor calculation are:

If there is a large discrepancy between the amount of emission reductions that can be achieved by available control options, the median should be used in lieu of the arithmetic mean to determine the average emission limitation equal to the MACT floor. A median is the value that falls in the middle of a set of numbers when those numbers are arranged in an increasing order of magnitude; in other words, there will be an equal number of values above and below the median. If the middle falls between two values, the median is equal to the arithmetic mean of those two numbers. This situation will occur when there is an even number of values in the set of numbers. In this example, the median would be 0-percent reduction, and this could be selected as the MACT floor.

However, if there is a large discrepancy between the control technologies used to establish a median such that no technology could realistically obtain a reduction close to the median, the mode should be used to calculate the MACT floor. A mode is the most frequent occurrence among a set of data. In Example 1, there are two modes, 95-percent and 88-percent emission reductions. In Example 2, the mode would be equal to 0-percent emission reduction. When there is more than one mode in the data set, the MACT floor should be based on the least degree of emission control. However, the existence of more than one mode may be an indicator that the MACT should be established at a level of control more stringent than the MACT floor.

The mode may also be used as a method to compute an average emission limitation if the emissions data for a source category or subcategory are not numerically based. This situation could occur if sources were regulated by several different equipment or work practice standards. Unless a specific level of emission reduction can be associated with each different standard or unless the standards can be ranked in some order of increasing level of control, an arithmetic mean and median cannot be calculated. A mode could be used if one of the control options is used more frequently by the best performing 12 percent of existing sources. This is illustrated in the Example 3 below:

Example 3

• There are 44 tanks in the source category or subcategory.
• Five sources are among the best performing 12 percent of existing sources. These five tanks are subject to the following regulations in the source category or subcategory:

3 of the 5 must be covered and vented to a carbon canister;
2 of the 5 must use a fixed roof.

The mode would be to cover and vent the tank to a carbon canister.