C. Analysis of Variance Procedures to Assess Subcategorization We use analysis of variance ( ANOVA) to determine whether subcategories of sources have significantly different emissions. For two subsets of emissions, the variance of the data between the two subsets is compared to the variance within the subsets. The ratio of these two variances is called the F statistic. The larger the F statistic the more likely the underlying data distributions are different. To make a decision regarding the difference between the two subsets, we compare this calculated F statistic to an F value associated with a particular confidence level. One commenter has raised several concerns with our use of the ANOVA procedure in the selection of incinerator subcategories. Comment: The ANOVA procedure is based upon the assumption that the underlying distribution of both data sets has a normal shape. For incinerator emissions data this assumption is not valid. A log probability plot shows that particulate emission data is better described by a lognormal distribution. Prior to conducting the ANOVA procedure, the data should be log transformed. Response: We use probability plots, Skewness Coefficients, and Correlation Coefficient/ Shapiro Wilks testing to evaluate whether it is more appropriate to analyze emissions data for ANOVA and floor calculations assuming the data represent a normal or lognormal distribution. We believe it is reasonable to assume the data represent a normal distribution for several reasons. The purpose of the ANOVA subcategorization analysis is to determine if there is a significant difference in emission levels between potential subcategories to warrant establishing separate floors for the subcategories. Although in some cases it may appear that a data set in its entirety may be better represented by a lognormal distribution, the high emissions data causing the right hand skew will be truncated when we identify the best performing sources   those with the lowest emissions   to calculate floors. This moves the appearance of a skewed distribution toward one that is more symmetric and thus, more representative of a normal distribution. In addition, our analyses showed: ( 1) the probability plots do not suggest that either assumed distribution is significantly or consistently better; ( 2) the data set arithmetic averages tend to be in the neighborhood of the medians, indicating the data sets are not significantly skewed and more closely normal than lognormal; and ( 3) in some cases, neither assumed distribution could be statistically rejected. 1 Comment: Some of the data sets used for comparison have very few members. This means that the within group variance for a small data set would have to be very low for the two groups to be judged as separate. Response: We agree, but note that as the sample sizes change, the critical values are also changing depending on the degrees of freedom. Comment: Only emissions data were considered in the ANOVA tests. Feed rate and removal efficiency should have been considered as well. Response: Differences between subcategories in feedrates or system removal efficiency are irrelevant if there is no significant difference in emissions between the subcategories. The purpose of considering subcategorization is to determine if there are 1 USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 8.2. 2 design, operation, or maintenance differences between subcategories that could affect the type or concentration of HAP emissions and thus sources' ability to achieve the floor absent subcategorization. Consequently, it is appropriate to consider emissions only when evaluating subcategorization. Comment: The confidence level used by EPA for the F statistic in all cases was 95 percent. If the calculated F statistic were equal to this 95 percent confidence value, it would mean that there is only a 5 percent chance that data for the two subsets were drawn from the same parent distribution. A less stringent ( lower) confidence level would be more appropriate for this analysis. The commenter evaluated particulate emissions for specialty incinerators ( i. e., munitions, chemical weapons and mixed waste incinerators) and non specialty incinerators ( all others). The commenter log transformed the data and determined that there was only a 30 percent chance that the two data sets could come from the same parent distribution. This result, together with the vastly different operating characteristics for the two types of incinerators, argues for their being treated as separate categories, according to the commenter. Response: A confidence level of 95% assigns a probability of 0.95 of accepting the hypothesis when there is no difference between subcategories and hence a probability of 0.05 of rejecting a true hypothesis. This reduces the probability to 5% of rejecting a true hypothesis. A less stringent confidence level would increase the chances of rejecting a true hypothesis. The farther apart the averages of the two potential subcategories are, the more likely they are to be statistically different and the more likely you are to be wrong if you hypothesize that they are not different. A 95% confidence level is most often used for ANOVA because it is generally believed that being wrong one time out of 20 is an acceptable risk for purposes of ANOVA. In addition, statisticians are comfortable with a 95% confidence level because, in a normal distribution, 95% of the data fall within 2 ( actually 1.96) standard deviations of the mean. Other confidence levels could be used for ANOVA   99% or 90% if there is a good reason to deviate from the general default of 95%. A 99% confidence level is the second most commonly used confidence level and is generally used when it is very important that you be sure that you are right ( i. e., where you can only accept the risk of being wrong 1 time out of 100) before you classify the populations ( in this case subcategories) as different. Occasionally, but much less frequently, confidence levels of 90% or less are used. But, we note that these situations are so infrequent that some statistics books provide tables for the ANOVA F statistic only at the 95% and 99% confidence levels. For these reasons, we believe that the 95% confidence level is an appropriate level among those we could have reasonably selected.

VI. Emission Standards A. Incinerators Comment: A commenter states that EPA's subcategorization ( and assignment of differing dioxin/ furan standards as a result) between incinerators with wet or no air pollution control device and incinerators equipped with dry air pollution control devices or waste heat boilers is unlawful because incinerators equipped with a given type of 3 pollution control equipment are not different " classes," " types," or " sizes" of source. The commenter implies that EPA justifies this subcategorization by stating that these sources have different emission characteristics, which is no less unlawful and arbitrary than subcategorizing based on the pollution control devices they use. Response: We agree that it would not be appropriate to subcategorize source categories based on a given air pollution control technique. See 69 FR at 403 ( Jan. 4, 2004). As stated at proposal, we do not subcategorize incinerators with respect to dioxin/ furans based on the type of air pollution control device used. 69 FR at 21214. For example, with respect to dioxin/ furans, it would not be appropriate subcategorize based on whether a source is using: 1) good combustion practices; 2) a carbon bed; 3) an activated carbon injection system; or 4) temperature control at the inlet to its dry air pollution control device. These devices and practices are what control dioxin/ furan emissions. Today's final rule does not subcategorize based on these control devices and practices. Instead, our subcategorization approach recognizes the potential of some emission control equipment to create pollutant emissions that subsequently must be addressed. 2 Dioxin/ furans are unique in that these pollutants are not typically present in the process inputs, but rather are formed in the combustor or in post combustion equipment. The primary cause of dioxin/ furan emissions from incinerators not equipped with waste heat boilers is post combustion formation by surface catalyzed reactions that occur within the dry air pollution system. 3 This is evidenced by the statistically significant higher dioxin furan emissions for incinerators with dry air pollution control systems compared to those without dry systems. Incinerators with dry air pollution systems are designed to effectively control metal and particulate matter emissions through use of baghouses, electrostatic precipitators, etc. Incinerators that are designed in this manner have the potential for elevated dioxin/ furan emissions because dry air pollution control systems provide locations where surface catalyzed reactions can occur ( e. g., on particles on fabric filter bags or electrostatic precipitator plates). Thus, for purposes of dioxin/ furan formation and control, incinerators equipped with dry air pollution systems are in fact different " types" of incinerators because of their unique pollutant generation characteristics. On the other hand, incinerators with wet air pollution control systems are generally designed to effectively reduce total chlorine emissions ( with the use of wet scrubbers) and metals and particulate matter emissions. There generally is a tradeoff, however, in that these types of incinerators may not be as efficient in reducing particulate matter and metal emissions compared to incinerators that are equipped with baghouses and dry electrostatic precipitators. These types of incinerators generally do not have the potential to have elevated dioxin/ furan emissions because they do not provide locations 2 Although we subcategorize between incinerators with wet or no air pollution control device and incinerators equipped with dry air pollution control devices or waste heat boilers for the floor analysis, the calculated dioxin furan floors for both subcategories for existing sources were determined to be less stringent than the current interim standard. Subsequently, the final rule emission limitations for both subcategories are, for the most part, identical, and equivalent to the interim standard. See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 10.1, for further discussion. 3 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume IV: Selection of MACT Standards," September 2005, Section 3, for further discussion. 4 where surface catalyzed reactions can occur. For purposes of dioxin/ furan emission formation and control, sources with wet air pollution control systems are thus likewise different types of incinerators. 4 Subcategorizing dry air pollution systems and wet air pollution control systems for purposes of establishing a dioxin/ furan standard is no different than subcategorizing incinerators equipped with waste heat boilers. The waste heat boiler is the origin of the dioxin/ furan that is generated. These incinerators are designed to efficiently recover heat from the flue gas to produce useful energy. A result of this type of incinerator design, however, is that it also provides a location where surface catalyzed reactions can occur ( i. e., the boiler tubes), potentially resulting in elevated dioxin/ furan formation ( and emissions if not properly controlled). An alternative approach that does not subcategorize these sources, but rather identifies best performing sources as those sources with the lowest emissions irrespective of whether they have a wet or dry air pollution control device, would yield floors that would not be achievable unless all the sources, including the best performers, adopted beyond the floor technology. The calculated dioxin/ furan floor for existing incinerators and liquid fuel boilers using such an approach would be 0.008 and 0.009 ng TEQ/ dscm, respectively. 5 All of the best performing sources for these calculated floors had either wet air pollution systems or no air pollution control systems. The floor technology used by these sources is good combustion practices. As a result, these floor levels would not be replicable by these best performing sources nor duplicable by other sources through use of the same good combustion practices because of the uncertainties associated with dioxin/ furan generation mechanisms and rates that can vary both within sources and across sources, potentially leading to significant variability in emission levels. 6 Sources equipped with wet or no air pollution systems would thus likely be required to install carbon systems to comply with these standards, a technology used by only four incinerators ( none of which were best performers in the above discussed floor analysis). Such an outcome should be viewed as a beyond the floor technology and therefore assessed pursuant to the factors enumerated in section 112 ( d) ( 2). Furthermore, it is unclear, and perhaps doubtful, that these floors would be achievable by these sources even if they were to install beyond the floor controls such as activated carbon systems because no sources using activated carbon are currently achieving those floor levels. We therefore conclude that it is appropriate, and necessary, to subcategorize these types of incinerators for purposes of calculating dioxin/ furan floor standards.

4 A similar analogy applies to incinerators that are not equipped with air pollution systems. These incinerators are not designed to control emissions of metals, chlorine, and particulate matter ( perhaps because emission levels are low due to low HAP feed levels). Similar to incinerator types with wet systems, this design does not provide the locations for surface catalyzed reactions to occur, which leads us to conclude that these are different types of incinerator with respect to dioxin/ furan control. 5 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 20 and Appendix C, tables labeled " E INC all DF" and " E LFB all DF". 6 Dioxin/ furan formation mechanisms are complex. Sources equipped with wet or no air pollution control systems cannot rely on good combustion practices alone to achieve these floor levels because they cannot " dial in" to a specific emission level, as is the case with typical back end control systems that control particulate matter and metals, for example. See Part Four, Section IV. B. 5 B. Cement Kilns 1. Hg Standard Comment: Several commenters recommend that EPA use a commenter submitted dataset, which includes three years of data documenting day to day levels of mercury in hazardous waste fuels fired to all hazardous waste burning cement kilns, to identify a MACT floor for existing and new cement kilns. Several commenters state that existing cement kilns should have the option to comply with either of the following mercury standards: ( 1) a hazardous waste feed concentration limit, expressed in ppmw, based on an evaluation of the five best performing sources within the commenter submitted dataset ( documenting day to day levels of mercury in the hazardous waste over a three year period); or ( 2) a hazardous waste maximum theoretical emissions concentration ( MTEC), expressed in units of ug/ dscm, developed by projecting emissions of the best performing sources assuming mercury concentrations in the hazardous waste were at the source's 99th percentile level in the commenter submitted dataset. To identify the best performing sources, the commenter suggests selecting the five sources with the lowest median mercury concentrations in the dataset. For existing sources, the commenters' evaluation yields a hazardous waste feed concentration limit of 3.3 ppmw and a stack concentration emission limit of 150 ug/ dscm ( rounded to two significant figures and considering mercury contributions only from the hazardous waste). For new cement kilns, the commenters recommend a mercury standard in the format of a hazardous waste feed concentration limit only, expressed in ppmw, based on the single source with the lowest 99th percentile level of mercury in hazardous waste. The commenters recommend a mercury standard of 1.9 ppmw for new sources. Response: We agree with commenters that the commenter submitted dataset documenting the day to day levels of mercury in hazardous waste fuels fired to all hazardous waste burning cement kilns is the best available data to identify floor levels for existing and new cement kilns. See discussion in Part Four, Section I. D. However, we disagree with the commenters' suggested format of the mercury standard for existing sources. Establishing the mercury standard as the commenters' suggest ( i. e., 3.3 ppmw in the hazardous waste feed or 150 ug/ dscm as a hazardous waste MTEC) fails to consider the interim mercury standards. As discussed in Part Four, Section III. E, there can be no backsliding from the levels of performance established in the interim standards. While not every source feeding hazardous waste with a maximum mercury concentration of 3.3 ppmw would exceed the interim standard, most sources using more than 50 percent hazardous waste as fuel ( i. e., replacing at least half its fossil fuel with hazardous waste) would exceed the interim standard, emitting mercury higher than the levels allowed under § § 63.1204( a)( 2) and 63.1206( b)( 15) of the interim standards. 7 The hazardous waste MTEC of 150 ug/ dscm calculated by the commenters is also higher than the level currently allowed under § 63.1206( b)( 15) of the interim standards. Since sources cannot backslide from the levels of the interim standards, if we were to accept the commenters' floor analysis results as presented ( which we are not), then we would " cap" each calculated standard ( i. e., 3.3 ppmw hazardous waste feed concentration and 150 ug/ dscm in stack emissions) at the interim standard level. This would result in a mercury standard for existing sources of 3.3 ppmw hazardous waste feed and a hazardous waste feed 7 USEPA, " Technical Support Document for HWC MACT Standards, Volume III: Selection of MACT Standards," Section 23.4, September 2005. 6 MTEC of 120 ug/ dscm or 120 ug/ dscm as a stack gas concentration limit. We note this is similar to the mercury standard adopted today: a hazardous waste feed concentration limit of 3.0 ppmw and a hazardous waste feed MTEC of 120 ug/ dscm or 120 ug/ dscm as a stack gas concentration limit. For an explanation of why we derived a level of 3.0 ppmw from the data, see Section 7.5.3 of Volume III of the Technical Support Document. The commenters' suggested new source mercury standard of 1.9 ppmw in the hazardous waste has the same deficiency. New sources with a hazardous waste fuel replacement rate of approximately 75% could emit mercury at levels higher than currently allowed under the interim standards. After capping the calculated standard at the interim standard level, we would identify the mercury standard for new sources as a hazardous waste concentration limit of 1.9 ppmw in the hazardous waste and a hazardous waste feed MTEC of 120 ug/ dscm or 120 ug/ dscm as a stack gas concentration limit. For reasons discussed in Section 7.5.3 of Volume III of the Technical Support Document, this is indeed the mercury standard we are promulgating for new cement kilns. The commenters also suggest that the best performing sources should be identified as those with the lowest three year median concentration of mercury in hazardous waste. Although this approach would be permissible, we conclude that it is more appropriate to identify the best performers ( or single best performer for new sources) by selecting those with the lowest 99th percentile upper level mercury concentrations. ( This is not a statistically determined upper prediction limit; there is sufficient data for an arithmetically calculated 99th percentile to reliably reflect sources' performance.) We believe that this approach best accounts for the variability experienced by best performing sources over time. A detailed discussion of the MACT floor analysis for existing and new cement kilns is presented in Section 7.5.3 of Volume III of the Technical Support Document. In summary, the mercury standard for existing cement kilns is 3.0 ppmw in the hazardous waste feed and 120 ug/ dscm as a hazardous waste maximum theoretical emission concentration feed limit or 120 ug/ dscm as a stack gas concentration limit. For new sources the mercury standard is 1.9 ppmw in the hazardous waste feed and 120 ug/ dscm as a hazardous waste maximum theoretical emission concentration feed limit or 120 ug/ dscm as a stack gas concentration limit. 8 Comment: Two commenters oppose EPA's proposed approach to base compliance with the mercury standard on averaged annual emissions. The commenters state an annual average would allow mercury emissions to exceed the interim standard because a source could burn high concentrations of mercury waste over a short period and still comply with an annual limit by burning low concentration wastes at other times. These commenters support the concept of a 12 hour rolling average feedrate limit ( i. e., the current requirement under the interim standards) in conjunction with an emission standard no less stringent than the interim standard. Response: We agree with these comments. Cement kilns must establish a 12 hour rolling average feedrate limit of mercury to comply with these standards. The mercury standards for cement kilns are " capped" at the interim standard level to prevent backsliding from the current level of performance. This is accomplished by expressing 8 Please note that we do not regard this standard as a work practice standard under section 112( h)( 1) of the Act, because part of the standard includes an emission limit which is measured at the stack. EPA believes the special requirements of section 112( h)( 1) apply when a work practice is the exclusive standard. 7 the standard as a limit on the mercury concentration in the hazardous waste ( with the rolling average) and either an emission concentration limit or hazardous waste maximum theoretical emission concentration feed limit. See § 63.1209( l)( 1)( iii). 2. Total Chlorine Comment: One commenter states that the proposed MACT floor approach is inconsistent with the statutory definition of MACT because EPA's selection of a routinely achievable system removal efficiency ( SRE) was arbitrary and not representative of the best performing sources. Instead, the commenter suggests EPA identify a MACT SRE based on the five sources with the best SREs and apply that SRE to the MACT chlorine feed level. Later, in supplemental comments, the same commenter suggests two alternative approaches to identify a floor level. One approach applies a ranking methodology based on emissions and chlorine feed, and the second suggested approach applies a triple ranking method based on emissions, feed, and chlorine SRE. Other commenters, however, supported EPA's proposed approach. Response: We are adopting the same approach we proposed at 69 FR at 21259. As we explained, this is a variant of the SRE/ Feed approach, the variant involving the degree of system removal efficiency achieved by the best performing sources. In summary, to determine the floor level we first identify the best performing sources according to their hazardous waste chlorine feedrate. The best performing sources are those that have the lowest maximum theoretical emissions concentration ( MTEC), considering variability. We then apply an SRE of 90 percent ( the specific point in contention) to the best performing sources' total MTEC ( i. e., thus evaluating removal of total chlorine across the entire system, including chlorine contributions to emissions from all feedstreams such as raw materials and fossil fuels) to identify the MACT floor, which is expressed as a stack gas emissions concentration in parts per million by volume. This approach defines the MACT floor as an emission level that the best performing sources could achieve if the source limits the feedrate of chlorine in the hazardous waste to the MACT level ( i. e., the level achieved by the average of the best performing five sources) while also achieving an SRE that accounts for the inherent variability in raw material alkalinity and ( to a lesser degree) cement kiln dust recycle rates, and production requirements. 69 FR at 21259. Under this approach, we are evaluating hazardous waste feed control as we do for other sources. One commenter objects to our determination that an SRE of 90 percent is representative of the best performing sources because we have not established a MACT SRE   the average SRE achieved by the best performing sources. There is no doubt that the cement manufacturing process is capable of capturing significant quantities of chlorine when favorable conditions exist within the kiln system. Our usual approach of establishing an SRE by ranking the most efficient SREs taken from individual compliance tests, however, would result in a standard that would not be achievable because it may not be duplicable by the best performers or certainly would not be replicable by others, given that it is a function of various highly variable parameters, especially levels of alkali metals ( e. g., sodium and potassium) and volatile compounds ( e. g., chlorine and sulfur) in the raw materials. Alkalis and volatiles vary at a given best performer facility ( in fact, at all facilities) as different strata are mined in the quarry, and across facilities due to different sources of raw materials. Raw material substitution is 8 infeasible and counter to the objective of producing quality product ( i. e., a product with low alkali content). Cement kilns thus are not able to design or operate to achieve a specific SRE at the high ( most efficient) end of the range of test conditions. This is demonstrated by our calculations of system removal efficiency data, which is essentially a collection of performance " snapshots." See SRE data summarized in Table 1 at the end of this response; see also Mossville, 370 F. 3d at 1242 ( maximum emission variability associated with raw material variability needs to be accounted for in MACT floor determination since the standard must be met at all times under all operating conditions). The performance data of the " apparent" best performers   upwards of 99 percent  identified by the commenter are simply a snapshot in the possible range of performance and are not replicable in the future due to factors which are uncontrollable by the source, as just explained. In confirmation, cement kilns achieving this level of removal in one test proved incapable of replicating their own result in other tests even though individual sources each have their own proprietary source of raw materials. See results in table for Giant ( SC), Essroc ( IN), Holcim ( MO), Giant ( PA), and LaFarge ( KS) all of whom would violate a 99+ percent standard based on their own operating results.

Table 1   Summary of System Removal Efficiency Data for Wet Process Cement Kilns9 Facility No. Runs in Data Base Low SRE Run (%) High SRE Run (%) Average SRE of All Runs (%) LaFarge ( OH) 3 99.1 99.4 99.3 Giant ( SC) 24 95.5 99.8 99.0 Essroc ( IN) 13 97.3 99.9 98.7 Holcim ( MO) 6 96.4 99.9 98.4 LaFarge ( KS) 12 95.7 99.3 98.1 Giant ( PA) 17 87.7 99.4 97.1 Continental ( MO) 3 95.7 97.0 96.5 Ash Grove ( AR) 37 85.1 98.8 95.1 Texas Industries ( TX) 6 88.8 97.0 93.6 Holcim ( MS) 9 76.5 99.2 90.0 However, the data indicate that SRE is reasonably quantifiable to a point. Based on our data base of system removal efficiency information from 130 test conditions where total chlorine was evaluated, we conclude that a system removal efficiency of 90 percent is a reasonable estimate of MACT SRE. 10 We also reject the commenter's three suggested alternative approaches to identify a MACT SRE to apply to the MACT feed level. The commenter's methods all suffer a common flaw: they fail to recognize and take into account the limitations of the total 9 See Section 3.6 of Volume II ( Specific MACT Standards) of Comment Response Document, September 2005. 10 As discussed a number of times earlier, we are not basing any standards on feed control of HAP in raw material and fossil fuel input. We instead are controlling HAP attributable to those inputs by means of end of stack emission standards which reflect removal of HAP by some type of control device. This approach is consistent with the discussion above, since we are not basing the cement kiln chlorine standard on control of any raw material input, but rather on some type of back end removal efficiency. 9 chlorine SRE data. For example, as just demonstrated, available data show that considering the SRE data associated with the most recent compliance test as a ranking factor will result in unachievable standards due to the varying effectiveness of chlorine capture ( which impacts emissions) depending on the raw material mix characteristics. Considering only the most recent compliance test data as suggested yields results that are unachievable because the best performer's SRE data are likely biased high ( e. g., sources that happen to test under favorable conditions are likely to be identified as best performers), which would not be replicable by even that source on a day to day basis. 3. Semivolatile and Low Volatile Metals Comment: Commenters oppose EPA's proposed approach to treat each kiln as a separate and unique source in the SRE/ Feed MACT floor analysis for cement kilns. 11 Commenters state that the approach is an improper way to perform a statistical analysis and reduces the variability in emissions that otherwise would be observed in a MACT pool of five unique sources. Variability is reduced because co located kilns at the same plant share many of the factors that comprise front end and back end controls. As a result, the calculated MACT floors for SVMs and LVMs for cement kilns are too stringent. The commenters' recommended solution ( in instances where co located kilns are among the top five performers) is to use only the data from the best performing colocated kiln, exclude any lesser performing kilns at the plant site, and then include the next best performing non co located kiln in the MACT pool. Implementing their recommendation, the commenters state that the MACT floor for SVMs increases from 4.0 x 10 4 to 7.4 x 10 4 lbs/ MMBtu and the floor for LVMs increases from 1.4 x 10 5 to 1.8 x 10 5 lbs/ MMBtu. Another commenter generally supports EPA's approach noting that the variability factor applied to the emissions data already accounts for variability. Response: We consider sources that are not identical as unique sources and emissions data and information from unique sources are considered separate sources in the floor analyses. An example of an " identical" source in our data base is compliance test data from a similar on site combustion unit used in place of a compliance test for another unit ( i. e., emissions testing of an identical unit was not conducted). These sources and their associated data are called " data in lieu of" sources in our data based on the RCRA provisions under § 266.103( c)( 3)( i). We acknowledge that co located sources may in fact share certain similar operation features ( e. g., use of raw material from the same quarry, use of the same coal and hazardous waste burn tank to fire the kilns); however, given that the co located sources ( except those designated as data in lieu of) are not designed identically, and given their hazardous waste feed control levels were not identical during testing, we conclude we must consider each source as a unique source in the floor analyses. 12 Comment: Commenter states that EPA's proposed standards for new cement kilns are unachievable due to problems with its accounting for variability, in part because EPA did not consider geographic differences when assessing feed control levels. The concentrations of hazardous constituents in the waste in a particular region are likely to be different than in the waste from another geographical region due to types of industrial 11 It is common for cement manufacturing plants to operate multiple cement kilns at the same plant. 12 Nonetheless, we analyzed the SVM and LVM floors for cement kilns as suggested by the commenter. Results of the analysis are presented in " Technical Support Document for HWC MACT Standards, Volume III: Selection of MACT Standards," Section 8.8, September 2005. 10 sectors located within each region. Sources cannot reasonably arrange for transportation of lower HAP wastes generated across the country and cannot treat the hazardous waste to remove or reduce HAP concentrations. The commenter cites several court decisions that support their assertions. Commenter believes that while this represents a problem for developing both the new and existing source floors, it is a greater predicament for the new source floor because this floor level is based on test data for only one source. Response: We are not obligated to account for varying hazardous waste feed control levels occurring because of differing HAP generation rates in different locations ( for commercial sources), or because different production process types generate higher or lower levels HAP concentration wastes. Hazardous waste feed control is a legitimate control technology. The commenter seems to suggest that we should subcategorize low feeding sources and high feeding sources based on their hazardous waste feed control level. This would inappropriately subcategorize sources based on differing levels of controls, which we do not do. See 69 FR at 403 ( January 5, 2004). Nonetheless, as previously discussed, the SRE/ Feed methodology lessens the impact of feed control variations across commercial units because it results in fewer situations where best performing back end controlled sources ( from a particulate matter emissions perspective) cannot achieve the semivolatile and low volatile metal design levels and floors. For new source standards, the single best performing cement kiln sources for semivolatile metals and low volatile metals were not the lowest hazardous waste feed controlled source ( both floors were based on sources with the fourth best, i. e., lowest, hazardous waste feed control level). We therefore do not believe these sources are atypically low hazardous waste feeders relative to the other best performing sources in the existing source MACT pools.

C. Lightweight Aggregate Kilns 1. Mercury Standard Comment: One commenter, an operator of lightweight aggregate kilns subject to this rule, recommends that EPA establish the mercury standard for lightweight aggregate kilns at a hazardous waste feed concentration limit of 3.3 ppmw for existing sources and 1.9 ppmw for new sources, which is the same standard suggested in public comments by a trade organization representing hazardous waste burning cement kilns. The commenter notes that these mercury limits are appropriate for lightweight aggregate kilns because the commenter's two lightweight aggregate manufacturing facilities participate in the same hazardous waste fuel market as the majority of cement kilns. Moreover, the commenter maintains that its parent company also owns and operates two cement kilns and that its lightweight aggregate kilns receive hazardous waste from many of the same generators that provide hazardous waste fuel to the cement kilns. Consequently, the commenter states that the cement industry's data set of actual mercury feed concentrations in the hazardous waste best represents the full range of hazardous waste fuel concentrations that exist in the waste fuel market ( see also Part Four, Sections I. D and E). Response: We disagree with the commenter. Although the cement industry's set of mercury feed concentration data in the hazardous waste may represent the full range of concentrations for the cement kiln source category, we cannot conclude the same for lightweight aggregate kilns because the commenter states that the mercury dataset are 11 only applicable to its kilns. 13 Further, the commenter provides no specific information or data to support the conclusion that its suggested approach is justified for the other lightweight aggregate kiln facility. We also disagree with the commenter as to the appropriateness of establishing the mercury standard in the format of a hazardous waste feed concentration ( i. e., 3.3 ppmw for existing sources and 1.9 ppmw for new sources) for lightweight aggregate kilns. A hazardous waste feed concentration standard is improper for this source category because one lightweight aggregate kiln facility's sources ( although not the commenter's) controls mercury emissions using wet scrubbing. Thus, a hazardous waste feed concentration standard would inappropriately limit the mercury concentration in hazardous waste for sources that use control equipment capable of capturing mercury. A source with control equipment should not be restricted to a hazardous waste feed concentration standard that is based on sources that can only control mercury emissions through limiting the amount of mercury in the hazardous waste. In any case, as explained earlier in our discussion of cement kiln mercury standard, we believe that it is preferable to establish an emission standard to assure that the actual amount of mercury emitted by these sources is controlled by means of a numerical standard for stack emissions. Comment: One commenter agrees that a source may not be able to achieve the mercury standard due to raw material contributions that might cause an exceedance of the emission standard in spite of a source using properly designed and operated MACT floor control technologies, including controlling the levels of metals in the hazardous waste. The commenter opposes the proposed alternative standard of 42 ug/ dscm, which is expressed as a hazardous waste maximum theoretical emissions concentration. Instead, the commenter suggests that EPA maintain the alternative standard options of § § 63.1206( b)( 15) or 63.1206( b)( 9). Response: We agree with the commenter that the mercury standard should address the concern of raw material contributions causing an exceedance of the emission standard. We also agree that the proposed alternative standard of a hazardous waste maximum theoretical emissions concentration of 42 ug/ dscm is an improper standard because the underlying data are unrepresentative. See discussion in Part Four, Section I. E. We note that the mercury standard promulgated today is 120 ug/ dscm as a stack gas concentration limit or 120 ug/ dscm as a hazardous waste maximum theoretical emission concentration feed limit. The alternative mercury standard sought by the commenter under § 63.1206( b)( 15) is a limit of 120 ug/ dscm as a hazardous waste maximum theoretical emission concentration, which is included in the mercury standard promulgated today. This should address the commenter's concern. Comment: One commenter supports a mercury standard with short term compliance limits ( e. g., 12 hour rolling average feedrate limits) as opposed to the annual limit proposed. Response: For reasons discussed in Part Four, Section I. E, we are using a different mercury dataset than at proposal. We solicited comment on a floor approach 13 We note that the commenter submitted dataset is not amenable for use in establishing standards expressed in a thermal emission format because sufficient information on the characteristics of the hazardous waste ( e. g., heating value of hazardous waste) were not provided. 12 using these data in a notice14 sent directly to certain commenters. We are adopting that approach today. The monitoring requirements of the mercury standard for lightweight aggregate kilns includes short term averaging periods ( i. e., not to exceed a 12 hour rolling average), as recommended by the commenter. 2. Total Chlorine Standard Comment: One commenter supports excluding from the floor analysis all lightweight aggregate kiln sources that lack air pollution control devices for chlorine, such as scrubbing technology. The floor analysis should simply exclude sources without back end controls according to the commenter. Response: We disagree. For the final rule, we are using the SRE/ Feed MACT floor approach which defines best performers as those sources with the best combined front end hazardous waste feed control and back end air pollution control efficiency. The commenter's suggestion would exclude emissions data from two of the three facilities in this source category even though valid emissions data from these sources are available ( and therefore ordinarily to be used, see CKRC, 255 F. 3d at 867), and these sources achieved the best front end hazardous waste feed control in the category. We note that the best feedrate controlled sources have hazardous waste thermal feed levels that are approximately one fifth the level of the source's with back end controls. These data describe the level of performance of sources in the category and must be evaluated in the MACT floor analysis. We also note that even if we were to implement the commenter's suggestion, the MACT floor results would not change for existing and new lightweight aggregate kilns because the total chlorine emissions data of the source with back end air pollution controls ( after considering variability) are higher than the standards promulgated today. Thus, the commenter's suggestion also would result in a standard that would be capped by the interim standard. 3. Beyond the Floor Standards Comment: One commenter opposes EPA's proposed decision to promulgate a beyond the floor standard for dioxin/ furans for existing and new lightweight aggregate kilns based on performance of activated carbon injection. Response: For the final rule, we conclude that a beyond the floor standard for lightweight aggregate kilns is not warranted. The Clean Air Act requires us to consider costs and non air quality impacts and energy requirements when considering more stringent requirements than the MACT floor. In the proposed rule, we estimated that the incremental annualized compliance costs for lightweight aggregate kilns to achieve the beyond the floor standard would be approximately $ 1.8 million and would provide an incremental reduction in dioxin/ furan emissions of 1.9 grams TEQ per year ( see 69 FR at 21262). At proposal we judged costs of approximately $ 950,000 per additional gram of dioxin/ furan TEQ removed as justified, and, therefore, we proposed a beyond the floor standard. Since proposal, we made several changes to the dioxin/ furan data base as the result of public comments. One implication of these changes is a lower national emissions estimate for dioxin/ furans for lightweight aggregate kilns. We now estimate an incremental reduction in dioxin/ furan emissions of 1.06 grams TEQ per year with costs ranging between $ 1.6 and $ 2.2 million per additional gram of dioxin/ furan TEQ removed. Based on these costs and consideration of the non air quality impacts and energy requirements ( including more waste generated in the form of spent activated 14 See docket item OAR 2004 0022 0370. 13 carbon, and more energy consumed), we conclude that a beyond the floor standard for existing and new lightweight aggregate kilns is no longer justified. For an explanation of the beyond the floor analysis, see Section 12.1.2 of Volume III of the Technical Support Document. We note that EPA also retains its authority under RCRA section 3005( c) ( the so called omnibus permitting authority) by which permit writers can adopt more stringent emission standards in RCRA permits if they determine that today's standards are not protective of human health and the environment.

D. Liquid Fuel Boilers 1. Mercury Standard Not Achievable when Burning Legacy Mixed Waste Comment: One commenter states that the proposed liquid fuel boiler mercury standard is not achievable by a commercial boiler, DSSI ( Diversified Scientific Services, Inc.) that burns mercury bearing low level radioactive waste that is also a hazardous waste ( so called ` mixed waste') that was generated years ago ( so called, legacy waste). The waste is an organic liquid containing high concentrations of mercury. The boiler is equipped with a wet scrubber which provides good mercury control   93%, system removal efficiency according to the commenter. The commenter states that the proposed liquid fuel boiler mercury standard is not achievable using feedrate control and/ or additional back end control. Waste minimization is not an option because the waste has already been generated. Further, available national treatment capacity for mercury bearing, low level radioactive organic hazardous waste is very limited. The only other hazardous waste combustion facility authorized to treat such waste is the Department of Energy incinerator at Oak Ridge, Tennessee. Waste treatment volumes at that facility are restricted by the mercury feed rate limitation for the incinerator. In addition, the feedrate of the waste cannot be practicably reduced because of the large back log of waste that must be treated. The commenter suggests that their boiler be subject to the incinerator mercury standard because the mixed waste has far higher concentrations of mercury than wastes burned by other boilers and, as a consequence, the boiler is more incinerator like with respect to the feedrate of mercury. Response. We agree with the commenter's suggestion. The final rule subjects this commercial liquid fuel boiler to the mercury standard for incinerators. We are classifying this source as a separate type of source for purposes of the mercury standard, because the type of mercury containing waste it processes is dramatically different from that processed by other liquid fuel boilers, effectively making this a different type of source for purposes of a mercury standard15. The source thus feeds mercury at concentrations exceeding that of any boiler but at concentrations within the range processed by hazardous waste incinerators. The maximum test condition average MTEC16 for mercury for the remaining liquid fuel boilers is 20 ug/ dscm. All the liquid fuel boiler mercury data represent " normal" data, i. e., data that were not spiked. ( The lack of spiked data in the liquid fuel boiler data base, in and of itself, indicates that these sources do not process mercury bearing waste and do not need the operational flexibility 15 See CAA section 112 ( d) ( 1)), authorizing EPA to distinguish among different " types... of sources within a category or subcategory" in developing MACT standards. 16 Maximum theoretical emission concentration is the feedrate normalized by gas flowrate assuming zero system removal efficiency. 14 gained by spiking to account for occasional higher concentration mercury wastes.) DSSI's 2002 mercury test condition average MTEC was spiked to 3500 ug/ dscm. In other words, DSSI needs the operational flexibility to feed 175 times more mercury than any other liquid fuel boiler. Incinerators, on the other hand, had mercury MTECs that ranged to 110,000 ug/ dscm in 2002. In fact, DSSI's mercury feed rate is the eighth highest of the 40 incinerators, including DSSI, for which we have 2002 mercury feed rate data. DSSI's process feed is thus within the upper range of mercury feed found at incinerators. We believe it is well within the broad discretion accorded us in section 112( d)( 1) to subcategorize among " types" and " classes" of sources within a category. See also Weyerhaeuser v. Costle, 590 F. 2d at 254, n. 70 ( D. C. Cir. 1978) ( similar raw waste characteristics justify common classification) and Chemical Manufacturers Ass'n v. EPA, 870 F. 2d 177, 253 54 and n. 340 ( 5th Cir. 1989) ( same). We note that this boiler will be subject to the liquid fuel boiler standards for all HAP other than mercury ( the only HAP where the issue of appropriate classification arises). Not surprisingly, given the disparity in waste concentration levels, the DSSI boiler, even though equipped with back end control comparable to best performing commercial incinerators, achieves mercury emission levels less than an order of magnitude higher than the other hazardous waste burning liquid fuel boilers, few of which use back end control that is effective for mercury. 17 This emission disparity likewise indicates that DSSI is treating a different type of waste than other liquid fuel boilers. The nature of the mercury bearing waste further confirms that it is of a different type than that processed by other hazardous waste burning liquid fuel boilers. The waste is a remediation waste, a type of waste burned routinely by commercial hazardous waste incinerators but almost never by a liquid fuel boiler. Moreover, the waste is a legacy, mixed waste generated decades ago in support of the United States' strategic nuclear arsenal. It is not amenable to the types of control all other liquid fuel boilers use to reduce mercury emissions   some type of feed control or other minimization technique. We investigated whether any waste minimization options are feasible for this waste, and find that they are not. Normally, waste minimization is accomplished by one of three means: eliminating the use of mercury in the process to prevent it from being in the waste; pretreating the waste before burning to remove the mercury; or sending it to another facility better suited to handle the waste. Changing the production process to eliminate or reduce the mercury content of the waste is not an option because this waste has already been generated. Pretreatment is already practiced to the maximum extent feasible by settling out and separating the heavier mercury from the liquid components after thermal desorbtion. The remaining organic liquid that is burned by the mixed waste boiler contains concentrations of mercury ( in organo mercury and other organic soluble forms) that are orders of magnitude higher than burned by other liquid fuel boilers. Much of the waste cannot be feasibly pretreated to remove mercury because this legacy, mixed waste comes from many highly diverse sources. It is not practical or feasible to investigate how to remove the mercury from wastes of such varied and unique origins.

17 USEPA, " Technical Support Document for HWC MACT Standards, Volume I: Description of Source Categories," September 2004, Section 2.4.4. 15 Only one other facility could potentially treat this mixed waste, DOE's incinerator at Oak Ridge, Tennessee, whose permit allows the incinerator to manage mixed waste. However, waste treatment volumes for mercury bearing wastes at that facility are restricted by the mercury feed rate limitation in the incinerator's permit. The DOE incinerator alone cannot assure national capacity for mercury bearing, low level radioactive organic hazardous waste. In addition, the back end emission controls of the mixed waste boiler are superior to those used by most incinerators, including the Oak Ridge incinerator. This boiler uses a highly effective wet scrubbing system the principal MACT floor back end control for mercury used by incinerators that achieves over 93% system removal efficiency. This is superior control compared to most incinerators, including the one at Oak Ridge which achieves 75 to 85% removal. 18 Thus, this mixed waste boiler is reasonably classified a different type of source with respect to mercury waste than other hazardous waste burning liquid fuel boilers, based on the nature of the waste burned and confirmed by the source's mercury emissions. We note that, although the final rule subjects only the DSSI mixed waste boiler to the incinerator mercury standard, we would conclude that any other liquid fuel boiler with the same fact pattern ( i. e., that met the same criteria as the DSSI boiler as discussed above) should also be subject to the incinerator mercury standard rather than the liquid fuel boiler mercury standard. Comment. One commenter states that EPA's standards for all sources must reflect the actual emission levels achieved by the relevant best sources. If EPA wishes to subject the boiler source and incinerators to the same emission standards, however, it is entirely within the Agency's power to do so. Response. We agree. There is no functional difference between this boiler and incinerators with respect to mercury feed rate and the type of waste processed ( incinerators often treat remediation wastes). Therefore, the most relevant sources for the purposes of clarification in this case are incinerators, not liquid fuel boilers. Accordingly, we have classified DSSI as an incinerator for purposes of a mercury standard ( i. e., made it subject to the mercury standard for incinerators), and have included the DSSI mercury data with the incinerator data when assessing mercury standards for incinerators. Comment. In something of a contradiction, the same commenter argues that the mixed waste boiler source ( DSSI) does not claim that it cannot meet the relevant mercury standard for liquid fuel boilers, but only that it cannot do so " using either feedrate control or MACT floor back end emission control." Floors must reflect the emission levels that the relevant best sources actually achieve, not what is achievable through the use of a chosen emission control technology. It is flatly unlawful and essentially contemptuous of court for EPA even to entertain the source's argument that the source should be subject to a less stringent emission standard based on the levels they believe would be achievable through the use of one chosen control technology. The commenter also states that the source acknowledges that it could achieve a better emission level, and apparently meet the relevant standards, by using activated carbon. Their argument that doing so would generate large quantities of spent 18 For more explanation concerning mixed waste sources, limitations on the concentrations of mercury fed to these sources, and the system removal efficiency achieved, see USEPA, " Technical Support Document for HWC MACT Standards, Volume III: Selection of Standards," September 2005, Section 8.7. 16 radioactive carbon does not support its attempt to avoid Clean Air Act requirements; the alternative to the source accumulating large quantities of radioactive carbon is releasing large quantities of radioactive and toxic pollution into the environment. Response. DSSI cannot meet the liquid boiler mercury standard because it burns a unique waste that resembles wastes processed by hazardous waste incinerators ( in terms of mercury concentration and provenance) and is unlike any mercury containing waste burned by the remaining liquid fuel boilers. See the earlier discussion showing that DSSI needs the operational flexibility to feed 175 times more mercury than any other liquid fuel boiler, but that DSSI's process feed is within the upper range of mercury feed found at incinerators. We agree that DSSI is processing different types of mercury bearing wastes than those combusted by all other liquid fuel boilers. We believe that establishing a different mercury standard for DSSI is warranted, as it would for any source with demonstrably unique, unalterable feedstock which is more difficult to treat than that processed by other sources otherwise in the same category. How DSSI chooses to comply with the incinerator mercury standard ( for example, whether it must use some other type of emissions control technology) is not germane to this decision. We note that today's mercury standard for incinerators will force this source to lower its mercury emissions, since it is unlikely that it can meet today's 120 ug/ dscm standard at all times without some changes in operations. Comment. The source argues that waste minimization is not feasible for legacy mixed waste that has already been generated. It is not possible to travel back in time and unmake mixed legacy waste that already has been created. That obvious fact, however, lends no support to their argument that it should be allowed to burn mixed legacy waste with less stringent emission standards, according to one commenter. Response. As discussed above, the mercury standard for liquid fuel boilers is not achievable for this source because it is a different type and class of boiler, based on the type of mercury containing hazardous waste it processes. Because this boiler has mercury feed rates that resemble those of incinerators not liquid fuel boilers and waste minimization is not possible, subjecting the boiler to the mercury incinerator standard is a reasonable means of sub categorization pursuant to the discretionary authority provided us by section 112( d)( 1) of the Clean Air Act. Comment. The commenter states that it is entirely possible to dispose of mixed legacy waste without burning it. Specifically, currently available technologies such as chemical oxidation and precipitation can be used to treat mixed legacy waste without burning it and without releasing mercury into the air. Therefore, mixed legacy waste should not be burned at all; it should be disposed of safely through the application of one of these more advanced technologies. Response. First, these wastes must be treated before they can be land disposed. RCRA sections 3004( d), ( g)( 5), and ( m). They also must meet a standard of 0.025 mg/ l measured by the Toxicity Characteristic Leaching Procedure before land disposal is permissible. 40 CFR section 268.40 ( standard for " all other nonwastewaters that exhibit the characteristic of toxicity for mercury"). 19 EPA's technical judgment is that it would 19 Although the legacy waste that DSSI is burning is nominally classified as a nonwastewater due to its high organic content, it is in fact a liquid matrix, meaning that the treatment standard of 0.025 ug/ l is effectively a total standard. 17 be very difficult to meet this standard by any means other than combustion. Moreover, as an organic liquid, the waste is readily amenable to treatment by combustion. In addition, combustion is a legal form of treatment for the waste. EPA did not propose to change or otherwise reconsider these treatment standards in this rulemaking, and is not doing so here. We note, however, that 40 CFR 268.42 and 268.44 provide means by which generators and treatment facilities can petition the Agency to seek different treatment standards from those specified by rule, and set out requirements for evaluating such petitions. We note further that, because this waste is radioactive, exceptional precautions need to be taken in its handling. The nonthermal treatment alternatives mentioned by the commenter ignore the potential for radiation exposure if nonthermal treatment is used. Concerns ( some of which are mentioned in DSSI's comment) include: nonthermal treatment would ( or could) increase worker exposure; desire to reduce handling of radioactive materials in general; need to avoid contaminating equipment that subsequently requires decontamination or handling as radioactive material; minimizing the generation of additional radioactive waste residues; reducing the amount of analysis of radioactive materials, which causes potential exposure, generation of radioactive wastes and equipment; wastes are varied and often of small volumes, which makes it difficult to develop routine procedures. Nonthermal treatment alternatives are also not currently available to DOE to manage the diversity and volume of DOE mixed waste. It is thus our belief that the commenter has not fully explored the implications of its position, especially with regard to radiation exposure. If the commenter wishes to pursue this issue, EPA believes the appropriate context is through the Land Disposal Restriction mechanisms described above. Comment. The commenter states that the source argues that feedrate control is not " practical." There appears to be no record evidence indicating what would make feedrate control impractical and why any such obstacle could not be overcome. Response. Feedrate control to the extent necessary to achieve the liquid fuel boiler standards is not practical for reasons just discussed. This source is one of two available sources that is authorized to treat mixed waste, and the other source is not likely to have the ability to burn mercury bearing organic waste in the future due to permit limitations and size constraints. Comment. The commenter states that mixed legacy waste should not be burned at all. If there are truly no other facilities that are currently permitted to dispose of mixed legacy waste, such waste should be stored until a facility that can treat such waste safely e. g., through chemical oxidation can be permitted. Response. The commenter's suggestion is beyond the scope of today's rulemaking. The suggestion is also illegal, since RCRA prohibits the storage of hazardous waste for extended periods. See RCRA section 3004( j); and Edison Electric Inst. v. EPA, 996 F. 2d 326, 335 37 ( DC Cir. 1993) ( illegal under RCRA section 3004( j) to store hazardous waste pending development of a treatment technology). EPA also notes that it retains authority under RCRA section 3005( c) ( the so called omnibus permitting authority) by which permit writers can adopt more stringent emission standards in RCRA permits if they determine that today's standards are not protective of human health and the environment. 18 2. Different Mercury, Semivolatile Metals, Chromium, and Total Chlorine Standards for Liquid Fuel Boilers Depending on the Heating Value of the Hazardous Waste Burned. Comment. Several commenters state that liquid fuel boilers should have an alternative concentration based standard in addition to the thermal emission based standard. Liquid fuel boilers are typically " captive" units that burn waste fuels generated from on site or nearby manufacturing operations, rather than accepting wastes from a wide variety of other sources. Because they have captive fuel sources, operators generally do not have fuel blending capabilities. Liquid fuel boilers " burn what they have," and as such have very limited operational flexibility. EPA should not penalize boilers that have the same mass concentrations of metals or chlorine in their waste compared to other boilers, but which wastes have a lower heating value than wastes burned by other boilers. ( The " penalty" is that emissions limits that are normalized by the heating value of the hazardous waste require that less volume of lower heating value waste can be burned compared to higher heating value fuel.) This problem is made worse by the limited data base for liquid fuel boilers, the lack of historical data to verify that these standards are achievable over time, and having most or all of the measured emissions below detection limits. In addition, most of the mercury and semivolatile metal data EPA has in the data base were obtained during normal operations and while the source demonstrated compliance with RCRA's chromium standard   the other metals data were available only because stack method Method 29 reports data for all RCRA metals, even ones that are not at issue for the compliance test. ( Sources generally elected to comply with the BIF Tier I metals emissions levels, but Tier III for chromium. Thus, the Method 29 test for chromium will give emissions results for all the metals   even those not subjected to stack testing   not just chromium.) Response. As explained earlier in Part Four, Section V. A., EPA has selected normalizing parameters that best fit the input to the combustion device. A thermal normalizing parameter ( i. e., expressing the standards in terms of amount of HAP contributed by hazardous waste per thermal content of hazardous waste) is appropriate where hazardous waste is being used in energy recovery devices as a fuel, since the waste serves as a type of fuel. Using a thermal normalizing parameter in such instances avoids the necessity of subcategorizing based on unit size. The commenters raise the other side of the same issue. As the commenters point out, some liquid fuel boilers burn lower Btu hazardous waste because that is the waste available to them, and those with waste that has a low heating value are, in their words, " penalized," compared to those with a high( er) heating value. Also, since these are not commercial combustion units, they normally lack the opportunity to blend wastes of different heating values to result in as fired high heating value fuels. If boiler standards are normalized by hazardous waste heating value, sources with lower heating value waste must either reduce the mass concentration of HAP or increase the waste fuel heating value ( or increase the system removal efficiency) compared to sources with wastes having the same mass concentration of HAP but higher heating value. Moreover, the thermal normalizing parameter is not well suited for a hazardous waste that is not burned entirely for its fuel value. In cases where the lower heating value waste is burned, the boiler is serving   at least in part   as a treatment device for the lower heating value hazardous waste. When this occurs, the better normalizing parameter is the unit's gas flow ( a different means of accounting for sources of different size), 19 where the standard is expressed as amount of HAP per volume of gas flow ( the same normalizing parameter used for most of the other standards promulgated in today's final rule.) The commenters requested that liquid fuel boilers be able to select the applicable standard ( i. e., to choose between normalizing parameters) and further requested that we assess the performance of these units ( for the purpose of establishing concentration based MACT floor levels) by using the same MACT pool of best performing sources expressed on a thermal emissions basis. Neither of these suggestions is appropriate. Choice of normalizing parameter is not a matter of election, but rather reflects an objective determination of what parameter is reasonably related to the activity conducted by the source. Moreover, the commenter's suggestion to use thermal emissions to measure best performance for a concentrationbased standard does not make sense. It arbitrarily assumes that the best performers with respect to low and high heating value wastes are identical. Instead, we have established two subcategories among the liquid fuel boilers: those burning high and those burning low heating value hazardous waste. The normalizing parameter for sources burning lower energy hazardous waste is that used for the other hazardous waste treatment devices, gas flow rate, so that the standard is expressed as concentration of HAP per volume of gas flow ( a concentration based form of the standard.) The normalizing parameter for sources burning higher energy content hazardous waste is the thermal parameter used for energy recovery devices, such as cement kilns and lightweight aggregate kilns. For the purposes of calculating MACT floors, the best performers are then drawn from those liquid fuel boilers burning lower energy hazardous waste for the lower heating value subcategory, and from those liquid fuel boilers burning higher energy hazardous waste for the higher heating value subcategory20. ( See Section 23.2 of Volume III of the Technical Support Document for more information.) Moreover, liquid fuel boilers are not irrevocably placed in one or the other of these subcategories. Rather, the source is subject to the standard for one or the other of these subcategories based on the as fired heating value of the hazardous waste it burns at a given time. Thus, when the source is burning for energy recovery, then the thermal emissions based standard would apply. When the source is burning at least in part for thermal destruction, then the concentration based standard would apply. This approach is similar to how we have addressed the issue of normalization in other rules where single sources switch back and forth among inputs which are sufficiently different to warrant separate classification. 21 20 We also agree that liquid fuel boilers present several unique circumstances, namely: they are often unable to blend fuel and have limited operational flexibility as a result; our data base on these sources' performance is relatively small; much of our mercury and semivolatile metals data is at or near detection limits; and much of the mercury and semivolatile metals data was obtained for other purposes, namely from risk burns or as a result of Method 29 testing to demonstrate compliance with a RCRA chromium standard. While not immediately important to the topic at hand   namely that not all liquid fuel boilers burn for energy recovery   they are secondary issues that we need to closely consider to make sure we do not estimate what the best performing 12% of sources are achieving in an unreasonable manner. 21 See NESHAP for Stationary Combustion Turbines, 40 CFR section 63.6175 ( definitions of " diffusion flame gas fired stationary combustion turbine", " diffusion flame oil fired stationary combustion 20 We next considered what an appropriate as fired heating value would be for each liquid fuel boiler subcategory. Although we have used 5000 Btu/ lb ( the heating value of lowest grade fuels such as scrap wood) in past RCRA actions as a presumptive measure of when hazardous waste is burned for destruction ( see, e. g. 48 FR 11159 ( March 16, 1983)), we do not think that measure is appropriate here. We used the 5,000 Btu/ lb level to delineate burning for destruction from burning for energy recovery at a time when that determination meant the difference between regulation and nonregulation. See 50 FR 49166 167 ( Nov. 29, 1985). This is a different issue from choosing the most reasonable normalizing parameter for regulated units ( i. e., units which will be subject to a standard in either case). Instead, we are adopting a value of 10,000 Btu/ lb as the threshold for subcategorization. This is approximately the heating value of commercial liquid fossil fuels. 63 FR 33782, 33788 ( June 19, 1998) It is also typical of current hazardous waste burned for energy recovery. Id. Moreover, EPA has used this value in its comparable fuel specification as a means of differentiating fuels from waste. See id. and Table 1 to 40 CFR section 261.38, showing that EPA normalizes all constituent concentrations to a 10,000 Btu/ lb level in its specification for differentiating fuels from wastes. We next examined the waste fuel being burned at cement kilns and lightweight aggregate kilns, which burn hazardous waste fuels to drive the process chemistry to produce products22, to cross check whether 10,000 Btu/ lb is a reasonable demarcation value for subcategorizing. 10,000 Btu/ lb is the minimum heating value found in burn tank and test report data we have for cement kilns and lightweight aggregate kilns23. We believe the cement kiln and light weight aggregate kiln data confirm that this is an appropriate cutpoint, since these sources are energy recovery devices that blend hazardous wastes into a consistent, high heating value fuel for energy recovery in their manufacturing process. We then separated the liquid fuel boiler emissions data we had into two groups, sources burning hazardous waste fuel with less than 10,000 Btu/ lb and all other liquid fuel boilers, and performed separate MACT floor analyses. ( See Sections 13.4, 13.6, 13.7, 13.8, and 22 of Volume III of the Technical Support Document.) We calculated concentration based MACT standards for these sources from their respective mercury, semivolatile metals, chromium, and total chlorine data. Liquid fuel boilers will need to determine which of the two subcategories the source belongs in at any point in time. Thus, you must determine the as fired heating value of each batch of hazardous waste fired so that you know the heating value of the hazardous waste fired at all times. 24 If the as fired heating value of hazardous wastes turbine", " lean pre mix gas fired stationary combustion turbine" and " lean premix oil fired stationary combustion turbine"). 22 The Norlite light weight aggregate kiln was not included in this analysis because they claim they are not burning for energy recovery. The waste Norlite burns is 4,860 Btu/ lb or lower. This is indicative of a source burning solely for thermal treatment of the waste and not, at least in part, for energy recovery. See 40 CFR 266.100( d)( 2)( ii). 23 The cement kiln burn tank data and test report data shows the minimum heating values of 9,900 and 10,000 Btu/ lb, respectively, for the hazardous waste. The minimum lightweight aggregate kiln heating values for hazardous waste was 10,000 Btu/ lb, excluding the Norlite source. 24 If you burn hazardous waste in more than one firing nozzle, you must determine the massweighted average heating value of the as fired hazardous waste across all firing nozzles. 21 varies above and below the cutpoint ( i. e., 10,000 Btu/ lb) at times, you are subject to the thermal emissions standards when the heating value is not less than 10,000 Btu/ lb and the mass concentration standards when the heating value is less than 10,000 Btu/ lb. To avoid the administrative burden of frequently switching applicable operating requirements between the subcategories, you may elect to comply with the more stringent operating requirements that ensure compliance with the standards for both subcategories. Comment. EPA's attempt to give actual performance two different meanings within a single floor approach is unlawful, unexplained, internally inconsistent, and arbitrary. If EPA believes that mass based emissions constitute sources' actual performance, the best performing sources must be those with the best mass based emissions   not thermal emissions. Response. As just explained, we agree with this comment, and have developed MACT floors independently for the two subcategories of liquid fuel boilers. Thus, we have defined two separate MACT pools based on the thermal input of the waste fuel and derived two separate and consistent MACT standards for sources when they burn solely for energy recovery, and when they do not. We also note that a source cannot " pick and choose" the less stringent of the two standards and comply with those. The source must be in compliance with the set of standards that apply. 3. Alternative Particulate Matter Standard for Liquid Fuel Boilers Comment: A commenter requested that EPA establish standards that allow boilers the option to comply with either a concentration based particulate matter standard or thermal emissions based particulate matter standard. Response: We determined that it is appropriate to express the particulate matter emission standard as a concentration based standard consistently across source categories and not to give boilers the option to comply with a thermal emissions based particulate matter standard. As discussed in Part Four, Section III. D as well as the preceding section, metal and chlorine concentration based emission standards can be biased against sources that process more hazardous waste ( from an energy demand perspective), in part because the SRE/ Feed methodology assesses feed control of each source when identifying the best performing sources; the ranking procedure thus favors sources with lower percentage hazardous waste firing rates ( keeping all other assessment factors equal). The thermal emission standard format eliminates this firing rate bias, which amounts to a limitation on the amount of raw material ( hazardous waste fuel to an energy recovery device) that may be processed, when identifying best performing sources. The methodology we use to identify best performing sources for particulate matter emissions is not affected by the firing rate bias in the manner that metal and chlorine emissions are. This is primarily because we define best performing sources as those with the best back end air pollution control technology; feed control is not assessed ( specifically ash feed control) for raw materials, fossil fuel, or unenumerated HAP metal in the hazardous waste. The hazardous waste firing rate bias is therefore not present when we identify the best performing particulate matter sources because a source's hazardous waste firing rate is not a direct factor in the ranking procedure. We also note that four of the nine best performing liquid fuel boilers for particulate matter are equipped with fabric filters. Particulate matter emissions from sources equipped with fabric filters are not significantly affected by ash inlet loading. 22 This is not true for metals and chlorine, given metal and chlorine emissions from fabric filters tend to increase at increased feed rates. See Volume III of the Technical Support Document, Sections 5.3 and 7.4. We conclude that the hazardous waste firing rate issue is not a concern for these sources given their particulate matter emissions would not be significantly affected by increased hazardous waste firing rates. 4. Long term, Annual Averaging Is Impermissible Comment: Standards expressed as long term limits are legally impermissible because those levels, by definition, would sometimes be greater than the average emission levels achieved by the best performing sources. Compliance also must be measured on a continuous basis, under section 302( k) of the Act. Thus, floor levels ( and standards) for mercury expressed as long term limits are illegal. Response: The commenter maintains that the statutory command in section 112( d)( 3)( A) to base floor standards for existing sources on " the average emission limitation achieved by the best performing 12 percent of ... existing sources" precludes establishing standards expressed as long term averages because certain daily values could be higher. We do not accept this position. The statute does not state what type of " average" performance EPA must assess. Long term, i. e., annual, averaging of performance is quite evidently a type of average, and so is permissible under the statutory text. Moreover, it is reasonable to establish standards on this basis ( the standards being the average of the best performing sources, expressed as a long term average), where sufficient data exist. Indeed, since the principal health concern posed by the emitted HAP is from chronic exposure ( i. e. cumulative exposure over time), long term standards ( which reduce the long term distribution of emitted HAP) arguably would be preferable in addressing the chief risks posed by these sources' emissions. We establish standards with long term averaging limits whenever we use normal data to estimate long term performance. We do this in the few instances where there are insufficient data ( whether normal data or compliance test data) to estimate each source's short term emission levels ( e. g., mercury and semivolatile metal standards for liquid fuel boilers). 25 One or two snapshot data based on normal operations are not likely to reflect a source's short term operating levels in part because feed control levels can vary over time. 26 See Mossville, 370 F. 3d at 1242 ( varying feed rates lead to different emission levels, and this variability must be encompassed within the floor standard because the standard must be met at all times). As a result, snapshot normal emissions, when averaged together, better reflect a source's long term average emissions. An emission standard based on normal data that is averaged together, but expressed as a short term limit, would not be achievable by the best performing sources because it would not 25 Two emission standards in this rulemaking are based on normal data but are expressed as short term limits ( the mercury standards for lightweight aggregate and cement kilns). However, in these instances we had enough normal data to reasonably estimate each source's maximum emissions, thus allowing us to express the standard as a short term limit. See USEPA, " Technical Support Document for HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Sections 11.2 and 12.2. 26 This is not the case for floors that are based on compliance tests because sources spiked their hazardous wastes to account for variability in hazardous waste feedrate. See Part Four, Section III. C above. Normal data, however, are a snapshot of what occurred on that day and are not likely to be representative over the long term, especially for mercury and semivolatile metals for liquid fuel boilers, where these limited data were almost entirely below the analytic detection limit. 23 adequately account for their emissions variability. See National Wildlife Federation v. EPA, 286 F. 3d at 572 73 ("[ c] ontinuous operation at or near the daily maximum would in fact result in discharges that exceed the long term average. Likewise, setting monthly limitations at the 99th percentile would not insure that the long term average is met"). Long term limits better account for this variability because such limits allow sources to average their varying feed control levels over time while still assuring average emissions over this period are below the levels demonstrated by the best performing sources. Indeed, under the commenter's approach where no averaging of intra source data would be allowed, sources would not be in compliance with the standards during the performance tests themselves. The tests consist of the average of three data runs, so half of the emissions weighted data points would be impermissibly higher than the average during the test used to derive today's emission standards. EPA also does not see that section 302( f) of the Act, cited by the commenter, supports its position. That provision indicates that the emission standards EPA establishes must limit the quantity, rate, or concentration of air pollutants on a continuous basis. A standard expressed as a long term average does so by constraining the overall distribution of emissions to meet a long term average. Also, long term limits result in emission standards that are lower than those that otherwise would be implemented on a short term basis. The short term limit would have to reflect the best performing sources' short term emissions variability ( i. e., the maximum amount of variability a source could experience during a single test period). National Wildlife Federation, 286 F. 3d at 571 73. Comment. Other commenters argued the opposite point, that ERA has no data to show that an annual average is achievable, and EPA should establish a longer averaging period. Response. We believe that all sources can achieve the mercury and semivolatile metals standards for liquid fuel boilers on an annual basis using some combination of MACT controls, i. e., feed control, back end control, or some combination of both. We agree that we have a small data set for these standards, but also believe that it is intuitive that a liquid fuel boiler can meet these standards on an annual basis, because one year is sufficiently more than any seasonal ( i. e., several month long) production of certain items that may not be represented by the tests we have. This informs us that an average of less than a year may not be achievable. It does not inform us that averaging of more than a year is required, since variations that occur with a year are averaged together. An annual average is sufficient for a source to determine whether an individual waste stream impacts negatively on the compliance of the liquid fuel boiler and take measures to address the issue. 5. Gas Fuel Boilers Comment. How can a boiler burning only gaseous waste also be burning hazardous waste? Uncontained gases are not considered hazardous waste under RCRA. Why are boilers that burn only gasses part of the liquid fuel boiler subcategory? Response. We agree with the commenter that boilers that burn gasses are unlikely to burn hazardous wastes. However, gas fuel hazardous waste boilers have existed in the past27, and we believe we need to define a MACT standard for them. Therefore, we 27 For example, sources 2014 and 2015 owned by Environmental Purification Industries in Toledo, Ohio, were considered hazardous waste boilers at the time the Phase II data base was noticed in the June 24 included gas fuel boilers in the liquid fuel boiler subcategory for reasons cited in the proposed rule. See 69 FR at 21216.

E. General 1. Alternative to the Particulate Matter Standards Comment: Commenters state that some incinerators are currently complying with the alternative to the particulate matter standard provision pursuant to the interim standards. See § 63.1206( b)( 14). The eligibility and operating requirements for the alternative to the particulate matter standard in the Interim Standards are different than the proposed alternative to the particulate matter standard in the replacement rule. Specifically, the proposed alternative to the particulate matter standard would no longer require sources to demonstrate a 90% system removal efficiency or a minimum hazardous waste metal feed control level to be eligible for the alternative. Commenters request that EPA clarify in the final rule that the proposed alternative to the particulate matter standard supersedes the requirements in the Interim Standards. Response: We are finalizing the alternative to the particulate matter standard for incinerators as proposed, with the exception that the alternative metal emission limitations have been revised as a result of database changes since proposal. See § 1219( e) and part three, section II. A. We considered superseding the interim standard alternative to the particulate matter standard requirements ( 63.1206( b)( 14)) immediately ( upon promulgation) by replacing it with the revised alternative standard provisions finalized in today's rule. Although the eligibility requirements for the alternative to the particulate matter standard finalized today are less stringent than the interim standard requirements, the metal emission limitations that are also required by the alternative finalized today are by definition equivalent to or more stringent than the metal limitations in the interim standard alternative. We therefore cannot completely supersede the interim standard provisions immediately ( upon promulgation) because sources have three years to comply with more stringent standards. We are instead revising the interim standard provisions of § 63.1206( b)( 14) to only reflect the revised alternative standard eligibility criteria ( specifically, we have removed the requirements to achieve a given system removal efficiency and hazardous waste metal HAP feed control level). 28 These eligibility criteria revisions become effective immediately with respect to the interim standards because they are less stringent than the current requirements. Sources should modify existing Notifications of Compliance and permit requirements as necessary prior to implementing these revised procedures. Comment: One commenter is opposed to the alternative to the particulate matter standard because it ignores the health effects/ benefits that are attributable to particulate matter. Response: Particulate matter is not defined as a hazardous air pollutant pursuant the NESHAP program. See CAA 112( b)( 1). We control particulate matter as a surrogate for metal HAP. See part four, section IV. A. As a result, a particulate matter standard is 27, 2000, despite the fact that these boilers burned only gasses. These boilers have since stopped burning hazardous waste. 28 Sources can only use § 63.1206( b)( 14) for purposes of complying with the interim standards. After the compliance date for today's rule, incinerators electing to comply with the alternative to the particulate matter standard must comply with the provisions found in § 63.1219( e). 25 not necessary in instances where metal HAP emission standards can alternatively and effectively control the nonmercury metal HAP that is intended be controlled with the surrogate particulate matter standard. The alternative to the particulate matter standard in the final rule accomplishes this. We acknowledge that particulate matter emission reductions result in health benefits. That in itself does not give EPA the authority under § 112( d)( 2) to directly regulate particulate matter, however. 2. Assessing Risk as Part of Consideration of Nonair Environmental Impacts Comment: Commenter states that EPA has inappropriately failed to consider emissions of persistent bioaccumulative pollutants in its beyond the floor analysis despite EPA's acknowledgment that these HAPs have non air quality health and environmental impacts. Response: EPA has taken the consistent position that considerations of risk from air emissions have no place when setting MACT standards, but rather are to be considered as part of the residual risk determination and standard setting process made under section 112 ( f) of the statute. EPA thus interprets the requirement in section 112 ( d) ( 2) that we consider " non air quality health and environmental impacts" as applying to the by product outputs from utilization of the pollution control technology, such as additional amount of waste generated, and water discharged. 29 EPA's interpretation was upheld as reasonable in Sierra Club v. EPA, 353 F. 3d 976, 990 ( D. C. Cir. 2004) ( Roberts, J.).

VII. Health Based Compliance Alternative for Total Chlorine A. Authority for Health Based Compliance Alternatives Comment: One commenter states there is no established health threshold for either HCl or chlorine. Response: Although EPA has not developed a formal evaluation of the potential for HCl or chlorine carcinogenicity ( e. g., for IRIS), the evaluation by the International Agency for Research on Cancer stated that there was inadequate evidence for carcinogenicity in humans or experimental animals and thus concluded that HCl and chlorine are not classifiable as to their carcinogenicity to humans ( Group 3 in their categorization method). Therefore, for the purposes of this rule, we have evaluated HCl and chlorine only with regard to non cancer effects. In the absence of specific scientific evidence to the contrary, it has been our policy to classify non carcinogenic effects as threshold effects. RfC development is the default approach for threshold ( or nonlinear) effects. Comment: One commenter states that the proposal is an inappropriate forum for bringing forward such a significant change in the way that MACT standards are established under Section 112( d) of the Clean Air Act. A precedent setting change of the magnitude that EPA has raised should be discussed openly and carefully with all affected parties, rather than being buried in several individual proposed standards. Response: Including health based compliance alternatives for hazardous waste combustors does not mean that EPA will automatically provide such alternatives for other 29 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume V: Emission Estimates and Engineering Costs," September 2005, Section 6, for a discussion of the non air impact that were assessed for this final rule. 26 source categories. Rather, as has been the case throughout the MACT rule development process, EPA will undertake in each individual rule to determine whether it is appropriate to exercise its discretion to use its authority under CAA section 112( d)( 4) in developing applicable emission standards. Stakeholders for those affected rules will have ample opportunity to comment on the Agency's proposals. Comment: One commenter states that the proposed approach is contrary to the intent of the CAA which explicitly calls for a general reduction in HAP emissions from all major sources nationwide through the establishment of MACT standards based on technology, rather than risk, as a first step. Response: For pollutants for which a health threshold has been established, CAA section 112( d)( 4) allows the Administrator to consider such threshold level, with an ample margin of safety, to establish emission standards. Comment: One commenter states that the proposed approach would take the national air toxics program back to the time consuming NESHAP process that existed prior to the Clean Air Act Amendments of 1990. Response: We disagree that allowing a health based compliance alternative in the final rule will alter the MACT program or affect the schedule for promulgation of the remaining MACT standards. Today's rule is the last MACT rule to be promulgated, and the health based compliance alternative did not delay promulgation of the rule. Comment: A commenter is concerned that the proposal would remove the benefit of the " level playing field" that would result from the proper implementation of technology based MACT standards. Response: Providing health based compliance alternatives in the final rule for sources that can meet them will assure the application of a uniform set of requirements across the nation. The final rule and its criteria for demonstrating eligibility for the health based compliance alternatives apply uniformly to all hazardous waste combustors except hydrochloric acid production furnaces. The final rule establishes two baseline levels of emission reduction for total chlorine, one based on a traditional MACT analysis and the other based on EPA's evaluation of the health threat posed by emissions of HCl and chlorine. All hazardous waste combustor facilities must meet one of these baseline levels, and all facilities have the same opportunity to demonstrate that they can meet the alternative health based emission standards. We also note that additional uniformity is provided by limiting the health based compliance alternatives for incinerators, cement kilns, and lightweight aggregate kilns to the emission levels allowed by the Interim Standards. Comment: Several commenters state that site specific emission limits are inappropriate under section 112( d)( 4) because they are not emission standards. One commenter asserts that the Agency's position that the limits are based on uniform procedures is flawed because the process allows " any scientifically accepted, peerreviewed risk assessment methodology for your site specific compliance demonstration." This is not a " uniform" procedure, according to the commenter. There are a host of variables that influence the results of an accepted methodology. The commenter reasons that, without some standardization of those variables, there is no uniform or standard analysis. Each permitting authority could establish its view of appropriate variables; there would be no national consistency. 27 Several other commenters assert that EPA has the authority to establish an exposure based emission limit for total chlorine. One commenter notes that one issue that often arises when considering risk based standards is whether EPA has authority under section 112 to establish an exposure based emission limit. The commenter states that the concern seems to be that some stakeholders construe the Act's statutory provisions as requiring uniform emission limitations at all facilities, rather than emissions that are measured at places away from the source and that vary from facility to facility. The commenter does not see any legal impediment to establishing exposure based limits. The commenter notes that, first, under section 112, EPA has authority to establish " emission standards." Emission standards are defined to be a requirement established by the State or the Administrator which limits the quantity, rate or concentration of emissions of air pollutants on a continuous basis . . . to assure continuous emission reduction, and any design, equipment, work practice or operational standard promulgated under this chapter. EPA's alternate risk based emission standard will limit the quantity, rate or concentration of the emissions. The commenter states that there is no requirement in the definition that specifies where the emission standard is to be measured, nor is there such a requirement anywhere in the statute. Second, the commenter notes that EPA's proposed exposure based limit will result in facilities establishing operating parameter limitations, or OPLs. These OPLs qualify as emission limitations because they are " operational standards" being promulgated under section 112, according to the commenter. They will be measured at the facility, not at the point of exposure. Finally, the commenter reasons that the limitations EPA is establishing are uniform. They uniformly protect the individual most exposed to emission levels no higher than a hazard index of 1.0. Consequently, the commenter believes that there is nothing in the statute that prevents the Agency from promulgating exposure based emission standards. Response: We agree with the commenters who believe the Agency has the authority to establish health based compliance alternatives under a national exposure standard. In particular, we agree with the commenter that the health based compliance alternatives are national standards since they provide a uniform and national measure of risk control, and also that the health based compliance alternatives are " emission standards" because they limit the quantity, rate or concentration of total chlorine emissions. Section 112( d)( 4) authorizes EPA to bypass the mandate in section 112( d)( 3) in appropriate circumstances. Those circumstances are present for hazardous waste combustors other than hydrochloric acid production furnaces. Section 112( d)( 4) provides EPA with authority, at its discretion, to develop health based compliance alternatives for HAP `` for which a health threshold has been established,'' provided that the standard reflects the health threshold `` with an ample margin of safety.'' Both the plain language of section 112( d)( 4) and the legislative history indicate that EPA has the discretion under section 112( d)( 4) to develop health based compliance alternatives for some source categories emitting threshold pollutants, and that those standards may be less stringent than the corresponding MACT standard ( including floor standards) would be. 30 EPA's use of such standards is not limited to situations where 30 See also Legislative History at 876 ( section 112 ( d) ( 4) standard may be less stringent than MACT). 28 every source in the category or subcategory can comply with them. As with technologybased standards, a particular source's ability to comply with a health based standard will depend on its individual circumstances, as will what it must do to achieve compliance. In developing health based compliance alternatives under section 112( d)( 4), EPA seeks to ensure that the concentration of the particular HAP to which an individual exposed at the upper end of the exposure distribution is exposed does not exceed the health threshold. The upper end of the exposure distribution is calculated using the `` high end exposure estimate,'' defined as `` a plausible estimate of individual exposure for those persons at the upper end of the exposure distribution, conceptually above the 90th percentile, but not higher than the individual in the population who has the highest exposure'' ( EPA Exposure Assessment Guidelines, 57 FR 22888, May 29, 1992). Assuring protection to persons at the upper end of the exposure distribution is consistent with the `` ample margin of safety'' requirement in section 112( d)( 4). We agree with the view of several commenters that section 112( d)( 4) is appropriate for establishing health based compliance alternatives for total chlorine for hazardous waste combustors other than hydrochloric acid production furnaces. Therefore, we have established such compliance alternatives for affected sources in those categories. Affected sources which believe that they can demonstrate compliance with the health based compliance alternatives may choose to comply with those compliance alternatives in lieu of the otherwise applicable MACT based standard. Comment: One commenter states that the risk assessments would not provide an ample margin of safety because background exposures are not taken into account. There is no accounting for other chlorine compounds from other sources at the facility, or from other neighboring facilities. The commenter believes that there is no evidence in the section 112( f) residual risk assessments produced thus far that emissions from collocated sources will actually be pursued by EPA. The commenter also notes that the Urban Air Toxics program cannot be relied upon to address ambient background. This program, required under section 112( k), was to be completed by 1999. However, the strategy has not been finalized and the small amount of activity in this area is focused on voluntary emission reductions rather than federal requirements. Finally, the commenter notes that control of criteria pollutants via State Implementation Plans to achieve compliance with the NAAQS is problematic. For particulate matter ( PM) and ozone, new NAAQS were set in 1997 and seven years later the nonattainment designations are still being determined. The designation process will be followed by a 3 year period to prepare State Implementation Plans and several more years to carry out those plans. In the meantime, there will be high levels of PM and ozone in the air near many hazardous waste combustors in New Jersey which will exacerbate exposures to chlorine and hydrogen chloride. Response: Total chlorine missions from collocated hazardous waste combustors must be considered in establishing health based compliance alternatives under § 63.1215. Ambient levels of HCl or chlorine attributable to other on site sources, as well as off site sources, are not considered, however. As we indicated in the Residual Risk Report to Congress and in the recent residual risk rule for Coke Ovens, the Agency intends to consider facility wide HAP emissions as part of the ample margin of safety determination for CAA section 112( f) residual risk actions. 70 FR at 19996 998 ( April 15, 2005); see also, 54 Fed. Reg. at 38059 ( Sept. 14, 1989) ( benzene NESHAP). 29 Comment: Several commenters state that acute exposure guideline levels ( AEGLs) are once in a lifetime exposure levels. They assert that, because short term exposures at a Hazard Index greater than 1.0 may occur more than once in a lifetime, using AEGLs for the purpose of setting risk based short term limits for HCl and chlorine does not provide an " ample margin of safety." Response: To assess acute exposure, we proposed to use acute exposure guideline levels for 1 hour exposures ( AEGL 1) as health thresholds. We have investigated commenters' concerns, however, and conclude that AEGLs are not likely to be protective of human health because individuals may be subject to multiple acute exposures at a Hazard Index greater than 1.0 from hazardous waste combustors. Consequently, we use acute Reference Exposure Levels ( aRELs) rather than acute exposure guideline levels ( AEGLs) as acute exposure thresholds for the final rule. See also Part Two, Section IX. D above. Acute RELs are health thresholds below which there would be no adverse health effects while AEGL 1 values are health thresholds below which there may be mild adverse effects. Acute exposures are relevant ( in addition to chronic exposures) and the acute exposure hazard index of 1.0 could be exceeded multiple times over an individual's lifetime. Although we concluded at proposal that the chronic exposure Hazard Index would always be higher than the acute exposure Hazard Index, and thus would be the basis for the total chlorine emission rate limit, this conclusion relates to acute versus chronic exposure to a constant, maximum average emission rate of total chlorine from a hazardous waste combustor. See 69 FR at 21300. We explained that acute exposure must nonetheless be considered when establishing operating requirements to ensure that short term emissions do not result in an acute exposure Hazard Index of greater than 1.0. This is because total chlorine and chloride feedrates to a hazardous waste combustor ( e. g., commercial incinerator) can vary substantially over time. Although a source may remain in compliance with a feedrate limit with a long term averaging period ( e. g., 12 hour, monthly, or annual) based on the chronic Hazard Index, the source could feed chlorine during short periods of time that substantially exceed the long term feedrate limit. This could result potentially in emissions that exceed the one hour ( i. e., acute exposure) Hazard Index. Consequently, we discussed at proposal the need to establish both short term and long term total chlorine and chloride feedrate limits to ensure that neither the chronic exposure nor the acute exposure Hazard Index exceeds 1.0.31 We conclude that 1 hour Reference Exposure Levels ( aRELs) are a more appropriate health threshold metric than AEGL 1 values for hazardous waste combustors given that the acute Hazard Index limit of 1.0 may be exceeded multiple times over an individual's lifetime, albeit resulting from uncontrollable factors. The California Office of Health Hazard Assessment has developed acute health threshold levels that are intended to be protective for greater than once in a lifetime exposures. The acute exposure levels are called acute Reference Exposure Levels and are available at http:// www. oehha. ca. gov/ air/ acute_ rels/ acuterel. html.

31 Note that we conclude for the final rule that most sources are not likely to exceed the acute Hazard Index because they will establish a 12 hour rolling average chlorine feedrate limit and their chlorine feedrates are not likely to vary substantially over that averaging period. Thus, we believe that most sources will not be required to establish an hourly rolling average chlorine feedrate limit. The owner/ operator must determine whether the hourly rolling average chloride feedrate limit can be waived under § 63.1215( d). 30 The 1 hour REL values for hydrogen chloride and chlorine are 2.1 mg/ m3 and 0.21 mg/ m3, respectively. The AEGL 1 values for hydrogen chloride and chlorine are 2.7 mg/ m3 and 1.4 mg/ m3, respectively. Although there is little difference between the 1 hour REL and AEGL 1 values for hydrogen chloride, the 1 hour REL for chlorine is substantially lower than the AEGL 1 value. In summary, we believe that aRELs are a more appropriate health threshold metric than AEGL 1 values for establishing health based compliance alternatives for hazardous waste combustors because aRELs are " no adverse effect" threshold levels that are intended to be protective for multiple exposures. Comment: One commenter states that the health based compliance alternative is unlawful because the proposal does not address ecological risks that may result from uncontrolled HAP emissions, including risks posed to those areas where few people currently live, but sensitive habitats exist. Response: An ecological assessment is normally required under CAA section 112( d)( 4) to assess the presence or absence of " adverse environmental effects" as that term is defined in CAA section 112( a)( 7). To identify potential multimedia and/ or environmental concerns, EPA has identified HAP with significant potential to persist in the environment and to bioaccumulate. This list does not include hydrogen chloride or chlorine. We also note that health based total chlorine emission limits for incinerators, cement kilns, and lightweight aggregate kilns cannot be higher than the current Interim Standards. See § 63.1215( b)( 7). Thus, the ecological risk from total chlorine emissions from these sources will not be increased under the health based limits. In addition, we note that only 2 of 12 solid fuel boilers have total chlorine emissions higher than 180 ppmv, and only 1 liquid fuel boiler has emissions higher than 170 ppmv. Thus, boilers generally have low total chlorine emissions which would minimize ecological risk. Consequently, we do not believe that emissions of hydrogen chloride or chlorine from hazardous waste boilers will pose a significant risk to the environment, and facilities attempting to comply with the health based alternatives for these HAP are not required to perform an ecological assessment.

B. Implementation of the Health Based Standards Comment: Several commenters are concerned that the health based compliance alternative will place an intensive resource demand on state and local agencies to review and approve facilities' eligibility demonstrations, and State and local agencies may not have adequate expertise to review and approve the demonstrations. One commenter states that permitting authorities do not have the expertise to review eligibility demonstrations that are based on procedures other than those included in EPA's Reference Library, as would be allowed. The commenter also states that, if the healthbased compliance alternative is promulgated, EPA should establish one standard method for the analyses so there is consistency nationwide. If EPA offers more than one method, EPA should do all of the risk assessment reviews, instead of passing the responsibility, without clear direction, to the permitting authorities, according to the commenter. Response: The health based compliance alternatives for total chlorine that EPA has adopted in the final rule should not impose significant resource burdens on states. 31 The required compliance demonstration methodology is structured in such a way as to avoid the need for states to have significant expertise in risk assessment methodology. We have considered the commenters' concerns in developing the criteria defining eligibility for these compliance alternatives, and the approach that is included in the final rule provides clear, flexible requirements and enforceable compliance parameters. The final rule provides two ways that a facility may demonstrate eligibility for complying with the health based compliance alternatives. First, look up tables allow facilities to determine, using a limited number of site specific input parameters, whether emissions from their sources might cause the Hazard Index limit to be exceeded. Second, if a facility cannot demonstrate eligibility using a look up table, a modeling approach can be followed. The final rule presents the criteria for performing this modeling. Only a portion of hazardous waste combustors will submit eligibility demonstrations for the health based compliance alternatives. Of these sources, several should be able to demonstrate eligibility based on simple analyses using the look up tables. However, some facilities will require more detailed modeling. The criteria for demonstrating eligibility for the compliance alternatives are clearly defined in the final rule. Moreover, under authority of RCRA section 3005( c)( 3), multi pathway risk assessments will typically have already been completed for many hazardous waste combustors to document that emissions of toxic compounds, including total chlorine, do not pose a hazard to human health and the environment. Thus, state permitting officials have already reviewed and approved detailed modeling studies for many hazardous waste combustors. The results of these studies could be applied to the eligibility demonstration required by this final rule. Because these requirements are clearly defined, and because any standards or requirements created under CAA section 112 are considered applicable requirements under 40 CFR part 70, the compliance alternatives would be incorporated into title V programs, and states would not have to overhaul existing permitting programs. Finally, with respect to the burden associated with ongoing assurance that facilities that opt to do so continue to comply with the health based compliance alternatives, the burden to states will be minimal. In accordance with the provisions of title V of the CAA and part 70 of 40 CFR ( collectively `` title V''), the owner or operator of any affected source opting to comply with the health based compliance alternatives is required to certify compliance with those standards every five years on the anniversary of the comprehensive performance test. In addition, if the facility has reason to know of changes over which the facility does not have control, and these changes could decrease the allowable HCl equivalent emission rate limit, the facility must submit a revised eligibility demonstration. Further, before changing key parameters that may impact an affected source's ability to continue to meet the health based emission standards, the source is required to evaluate its ability to continue to comply with the health based compliance alternatives and submit documentation to the permitting authority supporting continued eligibility for the compliance alternative. Thus, compliance requirements are largely self implementing and the burden on states will be minimal. Comment: One commenter suggests that the look up tables would have more utility if EPA developed tables for each source category to ensure the HCl equivalent emission rate limits reflected stack parameters representative of each source category. Similarly, another commenter notes that a look up table designed to be applicable to all 32 hazardous waste combustors is very conservative and will have limited utility. This commenter does not suggest that EPA develop look up tables for each class of hazardous waste combustors, however. Rather, the commenter suggests that since look up tables have already been developed for industrial boilers that do not burn hazardous waste32 hazardous waste combustors should be allowed to use those look up tables instead of the look up tables proposed for hazardous waste combustors. Response: We noted at proposal that the emission rates provided in the look up table for hazardous waste combustors are more stringent than those promulgated for solid fuel industrial boilers that do not burn hazardous waste. This is because the key parameters used by the SCREEN3 atmospheric dispersion model ( i. e., stack diameter, stack exit gas velocity, and stack exit gas temperature) to predict the normalized air concentrations that EPA used to establish HCl equivalent emission rates for solid fuel industrial boilers that do not burn hazardous waste are substantially different for hazardous waste combustors. Thus, the maximum HCl equivalent emission rates for hazardous waste combustors would generally be lower than those EPA established for solid fuel industrial boilers that do not burn hazardous waste. Nonetheless, we agree with the commenter's concerns that the look up tables would have more utility if they better reflected the range of stack properties representative of hazardous waste combustors. Accordingly, we examined the stack parameters for all hazardous waste burning sources in our data base ( except for hydrochloric acid production furnaces that are not eligible for the health based emission standards). After analyzing the relationships among the various stack parameters ( i. e., stack height, stack diameter, stack gas exhaust volume, and exit temperature), we concluded that the look up table should be modified to treat both stack diameter and stack height as independent variables rather than relying on stack height alone. We developed separate tables for short term ( i. e., 1 hour) HCl equivalent emissions limits to protect against acute health effects and long term ( i. e., annual) emission limits to protect against chronic effects from exposures to chlorine and hydrogen chloride. As discussed above, we used the acute Reference Exposure Level ( aREL) developed by Cal EPA as the benchmark for acute health effects. We used EPA's Reference Concentrations ( RfC) as the benchmark for chronic health effects from exposures occurring over a lifetime. Emission limits in the look up table are expressed in terms of HCl toxicity equivalent emission rates ( lbs/ hr). To convert your total chlorine emission rate ( lb/ hr) to an HCl equivalent emission rate, you must adjust your chlorine emission rate by a multiplicative factor representing the ratio of the HCl health risk benchmark to the chlorine health risk benchmark. For 1 hour average HCl equivalent emission rates, the ratio is the ratio of the aREL for HCl ( 2100 micrograms per cubic meter) to the aREL for chlorine ( 210 micrograms per cubic meter), or a factor of 10.33 For annual average emissions, the ratio is the ratio of the RfC for HCl ( 20 micrograms per cubic meter) to the RfC of chlorine ( 0.2 micrograms per cubic meter), or a factor of 100. See § 63.1215( b).

32 See Table 2 of Appendix A to Subpart DDDDD, Part 63. 33 We note that this factor of 10 ratio of the aRELs of HCl to chlorine is based on current aREL values and is subject to change. You must use current aREL ( and RfC) values when you conduct your eligibility demonstration. See § 63.1215( b)( 4 and 5). 33 We used the SCREEN3 air dispersion model to develop the emission limits in the look up tables. SCREEN3 is a screening model that estimates air concentrations under a wide variety of meteorological conditions in order to identify the meteorological conditions under which the highest ambient air concentrations are likely to occur and what the magnitude of the ambient air concentrations are likely to be. The SCREEN3 model implements the procedures in EPA's " Screening Procedures for Estimating the Air Quality Impact of Stationary Sources, Revised" ( EPA 454/ R 92 019, U. S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, October 1992). Included are options for estimating ambient air concentrations in simple elevated terrain and complex terrain. Simple elevated terrain refers to terrain elevations below stack top. We did not use the complex terrain option in the development of the look up tables because of the site specific nature of plume impacts in areas of complex terrain. Therefore, the look up tables cannot be used in areas of complex terrain ( which we define generally as terrain that rises above stack top). Sources located in complex terrain ( i. e., as a practical matter, sources other than those that are located in flat or simple elevated terrain as discussed below and thus cannot use the lookup tables) must use site specific modeling procedures to establish HCl equivalent emission rates. We looked at two generic terrain scenarios for purposes of the look up table. In one we assumed the terrain rises at a rate of 5 meters for every 100 meter run ( i. e., a slope of 5 percent) and that terrain is " chopped off" above stack top ( following the convention for such analyses in simple elevated terrain). In the other we assumed flat terrain. As can be seen from the tables in § 63.1215, the emission limits with flat terrain are significantly higher than those with simple elevated terrain. To reasonably ensure that the emission limits are not substantially over stated ( e. g., by a factor of 2), the simple elevated terrain table must be used whenever terrain rises to an elevation of one half ( 1/ 2) the stack height within a distance of 50 stack heights. For both the simple elevated terrain and flat terrain scenarios, we performed model runs for urban and rural dispersion conditions, with and without building downwash. We selected the highest ( ambient air concentration) values at each distance from among the four runs for each of the terrain scenarios. As can be seen from the tables in § 63.1215, the HCl equivalent emission rate limits range from 0.13 pounds per hour on an annual average ( for a 0.3 meter diameter stack that is 5 meters tall that lies within 30 meters of the property boundary) to 340 pounds per hour ( for a 4.0 meter diameter stack that is 100 meters tall that lies 5000 meters from the property boundary) when located in simple elevated terrain. In flat terrain, the range is from 0.37 to 1100 pounds per hour on an annual average. This contrasts with the look up table at proposal, where the comparable range was from 0.0612 pounds per hour ( for a 5 meter stack height at a distance of 30 meters) to a maximum of 18 pounds per hour ( for stack heights of 50 meters or greater, at distances of 500 meters or greater). If you have more than one hazardous waste combustor on site, the sum of the ratios for all combustors of the HCl equivalent emission rate to the HCl equivalent emission rate limit cannot exceed 1.0. See § 63.1215 ( c)( 3)( v). This will ensure that the Hazard Index of 1.0 is not exceeded considering emissions from all on site combustors. Comment: Several commenters state that facilities should be allowed to establish 34 an averaging period for the total chlorine and chloride feedrate limit that is shorter than an annual rolling average. Commenters are referring to the feedrate limit to ensure compliance with the annual average HCl equivalent emission rate limit. Commenters are concerned with the data handling issues that could arise from calculating, recording, and reporting an annual rolling average feedrate level that is updated hourly, and note that a shorter averaging period would make the limit more stringent. Response: We agree with commenters, and conclude, moreover, that a 12 hour averaging period rather than an annual averaging period will be imposed on the vast majority of sources as a practical matter. This is because sources must establish a limit on the feedrate of total chlorine and chloride to ensure compliance with the semivolatile metals emission standards. See § 63.1209( n). The feedrate limit for total chlorine and chloride is established under § 63.1209( n) as the average of the hourly rolling averages for each test run, and the averaging period is 12 hours. Thus, the averaging period for the feedrate limit for semivolatile metals   12 hour rolling average updated hourly   trumps the annual rolling average averaging period that would otherwise apply here. 34 Sources may also demonstrate compliance with the semivolatile metals standard by assuming all semivolatile metals in feedstreams are emitted. See § 63.1207( m)( 2). Sources that do not have emission control equipment, such as most liquid fuel boilers, are particularly likely to use this approach. Under this approach, there is no concern regarding increased volatility of metals as chlorine feedrates increase, and such sources are not subject to a feedrate limit for chlorine for compliance assurance with the semivolatile metal standard. These sources may establish an averaging period for the feedrate of total chlorine and chloride for compliance with the health based compliance alternative for total chlorine of not to exceed one year. 35 Comment: Several commenters offered suggestions on whether a short term feedrate limit was needed for total chlorine and chloride ( i. e., chlorine) as EPA suggested, and if EPA continues to consider it necessary, how the limit should be established. One commenter states that it is not necessary to set short term limits for chlorine feedrates. If EPA concludes that short term limits are necessary, however, the commenter recommended these options: ( 1) cap the feedrate at a level that is extrapolated up to the feedrate associated with Interim Standard for incinerators; ( 2) if the facility uses the site specific option to set emission limits, the dispersion models can easily be used to set a 1 hour ( or longer) limit; and ( 3) if the facility uses the look up table ( which at proposal provided only annual average HCl equivalent emission rate limits), a short term limit can be set based on a multiplier of the annual limit   10 times the annual limit as recommended by documents in EPA's Air Toxics Risk Assessment Reference Library.