D. Format of Standards 1. Thermal Emissions EPA proposed, and is finalizing standards for HAP metals and chlorine ( the HAPs amenable to hazardous waste feed control) emitted by energy recovery units ( cement kilns, lightweight aggregate kilns, and liquid fuel boilers) expressed in terms of pounds of HAP attributable to the hazardous waste fuel per million british thermal units ( BTUs) of hazardous waste fired. 69 FR at 21219 20. EPA received many comments on this issue to which we respond below and in the Response to Comment Document. Some initial discussion of the issue is appropriate, however. a. Expressing Standards in Terms of a Normalizing Parameter is Reasonable. First, using a thermal emissions form of a standard is an example of expressing standards in terms of a normalizing parameter. EPA routinely normalizes emission standards either by expressing them as stack HAP concentrations or by expressing the standards in units of allowable mass emissions per amount of production or raw material processed. Emission concentration based standards normalize the size of each source by accounting for volumetric gas flowrate, which is directly tied to the amount of raw material each source processes ( and subsequently the amount of product that is produced). Metal and particulate matter emission standards for commercial and industrial solid waste incinerators are expressed in emission concentration format. See § 60.2105. The particulate matter standard for Portland cement kilns is expressed as mass of allowable emissions per mass of raw material processed. See § 63.1342. The particulate matter, mercury, and hydrogen chloride standards for nonhazardous waste industrial boilers are expressed as pounds of allowable emissions per million British thermal units ( BTUs). § See 63.7500. Technology based standards typically normalize emissions because such a format assures equal levels of control across sources per amount of raw material that is processed, and allows EPA to equally assess source categories that comprise units that differ in size. By normalizing the emissions standard we better ensure the same percentage of emission reduction per unit of raw material processed by each source. 101 See Weyerhaeuser v. Costle, 590 F. 2d 1011, 1059 ( D. C. Cir. 1978) ( technology based standards are typically expressed in terms of volume of pollutants emitted per volume of some type of unit of production). There is no legal bar to this approach since the statute does not directly address the question of whether a source emitting 100 units of HAP per unit of production but 100 units of HAP overall is a better performer ( or, for new sources, better controlled) than a source emitting 10 units of HAP per unit of production but emitting 101 units overall. 102 One commenter appeared to suggest that we should assess performance on mass feedrates and mass emission rates, without normalizing. Such an approach would yield nonsensical results because the best performing sources would more likely be the smallest sources in the source category ( smaller sources generally have lower mass emission rates because they process less hazardous waste). This would likely yield emission standards that would not be achievable by the larger sources that more likely are better controlled sources based on a HAP removal efficiency basis. 103 Normalization by unit of production is another way of expressing unit size, so that normalizing on this basis is a reasonable alternative to subcategorization on a plant size by plant size basis. See section 112 ( d) ( 1) ( size is an enumerated basis for subcategorizing). b. Using Hazardous Waste Thermal Input as the Normalizing Parameter is Permissible and Reasonable. Normalization of standards based on thermal input is analogous. For energy recovery units ( in this rule, kilns and most liquid fuel boilers), normalizing on the basis of thermal input uses a key feed input as the normalizing parameter, allowing comparison of units with different inputs rather than separately evaluating these units by size and type ( see section 112 ( d) ( 1)). Again, this approach is legally permissible. The statute does not answer the question of which source is better performing, the source emitting 100 pounds of HAP per million BTUs hazardous waste but 100 pounds of HAP overall or the source emitting 10 pounds of HAP per million BTUs hazardous waste but emitting 101 pounds overall. The approach also is reasonable. First, as with other standards expressed in normalized terms, by normalizing the emissions standard we ensure the same percentage of emission reduction per unit of raw material processed by each source, thus allowing meaningful comparison among sources. For example, emission concentration based standards normalize the size of each source by accounting for volumetric gas flowrate, which is directly tied to the amount of raw material each source processes ( and subsequently to the 101 A more familiar example of normalization is the Earned Run Average ( ERA), which normalizes a baseball pitchers' earned runs on the basis of nine innings pitched in order to make comparisons among pitchers possible. 102 Or, put another way, the statute does not directly address the question of whether a small source that emits 10 units of HAP is better than a much larger source with better back end control ( but feeding the same raw material at a higher mass feedrates) that emits 100 units of HAP. 103 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 6.0. amount of product that is produced), and assures equal levels of control per amount of product. Normalization on the basis of HAP amount in hazardous waste per BTU level in the hazardous waste similarly assures equal levels of control across sources per amount of raw material that is processed. Here, the raw material is the hazardous waste fuel, expressed as units of energy. It is reasonable to regard a hazardous waste fuel as a raw material to an energy recovery device. Indeed, fuels are the only input to boilers, so fuels are necessarily such units' sole raw material. 104, 105 Hazardous waste burning cement kilns and lightweight aggregate kilns produce a product in addition to recovered energy and so process other raw materials. However, the reason these units use hazardous waste as inputs is typically to recover usable energy from the wastes. Hence, the hazardous waste fuel is reasonably viewed as a raw material to these devices. In this regard, we note that our choice of normalizing parameter essentially says that best performers with respect to hazardous waste fuel burned in energy recovery units are those using the lowest HAP feedrate ( for metals and chlorine) per amount of energy recovered. 106 This approach accords well with the requirement in section 112 ( d) ( 2) that EPA take energy considerations into account in developing MACT, and also that the Agency consider front end means of control such as input substitution ( section 112 ( d) ( 2) ( A)). In addition, our choice furthers the RCRA goal of encouraging properly conducted recycling and reuse ( RCRA section 1003 ( b) ( 6)), which is of relevance here in that Congress directed EPA to consider the RCRA emission controls for hazardous waste combustion units in developing MACT standards for these units, and to ensure " to the maximum extent possible, and consistent with [ section 112 ]" that section 112 standards are " consistent" with the RCRA scheme. CAA section 112 ( n) ( 7). 107 Conversely, emission concentration based standards, the methodology that otherwise would be used to calculate emission concentration based standards, may result in standards that are biased against sources that recover more energy from hazardous waste. This may discourage sources from recovering energy from hazardous waste because such standards do not normalize each source's allowable emissions based on the amount of hazardous waste it processes for energy recovery purposes. See 69 FR at 21219 and responses below. Second, use of this normalizing parameter makes it much more likely that hazardous waste feed controls will be utilized by these devices as an aspect of emissions control. See 104 EPA thus has expressed the MACT standards for particulate matter, mercury, and hydrogen chloride standards for nonhazardous waste industrial boilers as pounds of allowable emissions per million BTUs. § See 63.7500. This normalization considers the total heat input into the combustion device. Normalizing by total heat input would not be appropriate for hazardous waste combustors for metals and chlorine because this would implicitly account for, and in turn require the use of, feed control of HAP in non hazardous waste fuels. This is inappropriate for the reasons discussed in Section III. B of this Part. 105 We distinguish ( i. e., subcategorize) liquid fuel boilers that process hazardous waste with heating values less than 10,000 BTU/ lb from those processing hazardous wastes with heating content greater than 10,000 BTU/ lb. Although boilers that process hazardous waste with heating values less than 10,000 BTU/ lb are still considered to be energy recovery units, we conclude a thermal emissions normalization approach for these sources is not appropriate. See Part Four, Section VI. D. 106 As explained earlier, the ultimate ranking of best performers then further evaluates system removal efficiency, best performers then being defined in terms of the combination of hazardous waste thermal feed and system removal efficiency. See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards", September 2005, Section 7.3. 107 EPA would adopt the thermal format for the standards, however, whether or not the approach furthered RCRA objectives. section 112 ( d) ( 2) ( A) ( use of measures reducing the volume of pollutants emitted through " substitution of materials"); CKRC, 255 F. 3d at 865 ( EPA to consider means of control in addition to back end pollution control technology when establishing MACT floors). As explained in our discussion of the SRE/ Feed methodology, the MACT floor level for metals and chlorine reflects the best combination of hazardous waste feedrate, and total HAP removal efficiency. See section III. B. However, if standards for energy recovery units are expressed in terms of mass of HAP per volume of stack gas, then it would be relatively easy for these energy recovery devices to achieve a standard, without decreasing concentrations of HAP in their hazardous waste fuels, by diluting the HAP contribution of hazardous waste with emissions from fossil fuel. A thermal emissions format prevents this type of dilution from happening because it ignores additions of stack gases attributable to burning fossil fuels. Weyerhaeuser, 590 F. 2d at 1059 ( use of production of a unit as a normalizing parameter serves " the commendable purpose" of preventing plants from achieving emission limitations via dilution). For example, assume there are two identical energy recovery units with identical back end control devices ( that reflect the performance of the average of the best performing sources). Source A fulfills 25% of its energy demand from the combustion of hazardous waste; source B fulfills 50% of its energy demand from the combustion of hazardous waste. Also assume that the hazardous waste for these two sources have equivalent energy contents. If these sources were required to comply with an emission concentration based standard ( e. g., ug/ dscm), source A would be allowed to feed hazardous waste containing twice the metal content ( on a mass concentration basis, e. g., ppm), and would be allowed to emit metal HAP at the same mass emission rate relative to source B. This is because this source is effectively diluting its emissions with the emissions that are being generated by the fossil fuels. 108 A thermal emissions standard format does not allow sources to dilute their emissions with the emissions from fossil fuel inputs because it directly regulates the emissions and feeds associated with the hazardous waste fuel. Under a thermal emissions format both sources would be required to feed hazardous waste with the same thermal feed concentrations ( on a lb HAP per million BTU hazardous waste basis), and source A would be required to process hazardous waste with an equivalent concentration of metal HAP ( on a mass basis) and also be required to emit half as much metal HAP ( on a mass emission rate basis) relative to source B, because source A is processing half as much hazardous waste fuel, thus vindicating the hazardous waste feed control aspect of the standard ( see also note below regarding the likelihood of sources using hazardous waste feed control). Further, the thermal feed concentration with which these sources must comply reflects the feed control of the average performance of the best performing sources ( on a mass of HAP per million BTU basis). Such a requirement assures that these sources are processing the cleanest hazardous waste fuels to recover energy and are reducing HAP emissions to MACT levels. We note that it would not be appropriate to express the emission standards for incinerators, hydrochloric acid production furnaces, and solid fuel boilers in terms of thermal emissions. As just explained, the choice of a normalizing parameter is fitted to the nature of the device to which it is applied in order to allow the most meaningful comparisons between devices of like type. We therefore conclude that a thermal emissions format ( i. e., normalizing parameter) for incinerators is not appropriate because the primary function of incinerators is to thermally treat hazardous waste ( as opposed to recovering energy from the 108 This example assumes there are no HAP emissions attributable to the fossil fuels. hazardous waste). See 67 FR at 17362 ( April 19, 1996). Our database indicates that most incinerators processed hazardous waste during their emissions tests that had, on average, heating values below 10,000 BTU/ lb. 109 We have emission test hazardous waste heating value information for 62 incinerators in our database. Of these 62 sources, 40 sources processed hazardous waste with an average heating value of less than 10,000 BTU/ lb. The other 22 sources processed hazardous waste with heating values greater than 10,000 BTU/ lb in at least one test condition, although we note that 14 of these 22 sources also processed hazardous waste in different test conditions with heating values lower than 10,000 BTU/ lb. 110 We assessed whether we should subcategorize incinerators, similar to how we subcategorize liquid fuel boilers, based on the BTU content of the hazardous waste. Incinerators do recover energy from processing high BTU wastes. Some incinerators are equipped with waste heat boilers, and high BTU hazardous waste can displace fossil fuels that otherwise would have to be burned to thermally treat low BTU wastestreams. However, such energy recovery is considered to be a secondary product because their primary function is to thermally treat hazardous waste. A thermal emissions normalization approach for incinerators that combust hazardous wastes with heating values greater than 10,000 BTU/ lb would therefore not be appropriate because the normalized parameter would not be tied to the primary production output that results from the processing of hazardous waste ( i. e., treated hazardous waste). In confirmation, no commenters suggested that we apply a thermal emissions format to incinerators. We also conclude that a thermal emission format is inappropriate for hydrochloric acid production furnaces. These devices recover chlorine, an essential raw material in the process, from hazardous waste. The classic normalizing parameter of amount of product ( HCl) produced is therefore the obvious normalizing parameter for these sources. It is true that some hydrochloric acid production furnaces recover energy from high BTU hazardous wastes. See 56 FR at 7141/ 1 and 7141 42 ( Feb. 21, 1991). Some sources are equipped with waste heat boilers, and high BTU wastes help sustain the combustion process, which is necessary to liberate the chlorine from the wastestreams prior to recovering the chlorine in the scrubbing systems. Again, energy recovery is not the primary function of these types of sources. 111 Hydrochloric acid production furnace hazardous waste heating values range from 1,100 to11,000 BTU/ lb ( the median energy content for these sources is slightly above 6000 BTU/ lb). The range of hazardous waste heating contents from these sources is much lower 109 As discussed later, the heating values of hazardous wastes processed at cement kiln and lightweight aggregate kilns are primarily 10,000 BTU/ lb or greater. 110 These data are based on a compilation of heating contents for every incinerator test condition in the database where the source reported such heating content, and include both the most recent test conditions as well as older test conditions. Incinerator test condition heating values range from a low of 790 to a high of 19,800 BTU/ lb, with a median value of 7800 BTU/ lb. 111 EPA notes that when first adopting RCRA air emission standards for hydrochloric acid recovery furnaces ( then called ` halogen acid furnaces'), EPA indicated that those furnaces designed as boilers would be subject to the emission standards for boilers. 56 FR at 7040. This determination did not have regulatory consequence, since all hydrochloric acid production furnaces were subject to the same emission standards whether they were classified as boilers or as industrial furnaces. Thus, EPA was not concluding that some hydrochloric acid furnaces existed for the primary purpose of recovering energy in the 1991 rulemaking. 56 FR at 7139 ("[ Hydrochloric acid recovery furnaces] are typically modified firetube boilers that process secondary waste streams containing 20 to 70 per cent chlorine or bromine to produce a halogen acid product by scrubbing acid from the combustion gases"). than the ranges for cement kilns, lightweight aggregate kilns, and liquid fuel boilers, supporting the premise that energy recovery is of secondary importance. In addition, and critically, the hazardous waste that is processed in these units contains high concentrations of chlorine, confirming that the wastes serve as feedstock for hydrochloric acid production, even if the wastes also have energy value. 112 No commenters suggested that we apply a thermal emissions format to hydrochloric acid production furnaces. We consider the processing of hazardous waste in solid fuel boilers to be more reflective of energy recovery ( relative to incinerators and hydrochloric acid production furnaces) because these sources directly recover the heat that is released from the combustion of the waste streams. However, as stated at proposal, not all these sources are processing hazardous wastes for energy recovery. 69 FR at 21220. These boilers are generally not commercial units, and so tend to burn whatever hazardous wastes are generated at the facility where they are located. Heating values for this source category range from 1,300 to 10,500 BTU/ lb, with a median value of 8000 BTU/ lb. We therefore conclude that thermal emission standards for these sources are not appropriate because most of these sources are processing hazardous waste with energy content lower than 10,000 BTU/ lb. As discussed in section VI. D, we conclude that 10,000 BTU/ lb is an appropriate level that distinguishes whether thermal emission standards or mass emission concentration based standards are appropriate. We also note that no commenters suggested that we apply a thermal emissions format to solid fuel boilers. Comment: Commenters state that thermal emission standards are inappropriate because sources burning hazardous waste with a higher energy content or higher percent hazardous waste firing rate ( i. e., one that fulfills a greater percentage of its total energy demand from the hazardous waste) would be allowed to emit more HAP. Response: Part of this comment would apply regardless of what normalizing parameter is used. Technology based standards ( including MACT standards) are almost always expressed in terms of some type of normalizing parameter, i. e., " X" amount of HAP may be emitted per unit of normalizing parameter. This allows a meaningful comparison between units of different size and production capacity. A consequence is that the overall mass of HAP emissions varies, but the rate of control remains constant per the normalizing unit. As explained in the introduction to this section, this approach is both routine and permissible. Cement kilns, lightweight aggregate kilns, and liquid fuel boilers combust hazardous waste to recover valuable energy. Recovering energy is an integral part of their production process. As discussed at proposal, emission concentration based standards ( and the methodology that otherwise would be used to calculate emission concentration based standards) may result in standards that are biased against sources that recover more energy from hazardous waste. 69 FR at 21219. This may discourage sources from recovering energy from hazardous waste because such standards do not normalize each source's allowable emissions based on the amount of hazardous waste it processes for energy recovery purposes. A source that fulfills 100 percent of its energy demand from hazardous 112 Hazardous waste chlorine feedrates that are included in our database ( expressed as MTECs) range from a low of 46,000,000 ug/ dscm to a high of 294,000,000 ug/ dscm. On a mass chlorine percentage basis, these wastes range from 17% to 82%, noting that these percentages did not include the chlorine that was also spiked during the emissions tests). See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards", September 2005, Section 15. waste would be required to limit its mass HAP emissions to the same levels as an identical source that satisfies, for example, only 10 percent of its energy demand from hazardous waste and 90% from coal. This would inappropriately discourage the safe recovery of energy from hazardous waste, and could in turn result in greater consumption of valuable fossil fuels that otherwise would be consumed. Sources which fulfill a greater percentage of their energy demand from hazardous waste ( either by processing hazardous wastes that are higher in energy content, or by simply processing more hazardous waste) will be allowed to emit more HAP ( on a mass emission rate basis) than an identical source that satisfies less of its total energy demand from hazardous waste. This is appropriate because: 1) the source fulfilling a greater percentage of its energy demand from hazardous waste is processing more raw material than the other source ( the raw material being the energy content of the waste); and 2) the source fulfilling a lower percentage of its energy demand requirements from hazardous waste would not be allowed to dilute its emissions with nonhazardous waste fuels, and we would thus assure that all sources implement hazardous waste feed control to levels consistent with MACT. 113 This was illustrated in the example provided in the introduction to this comment response section. Similarly, two sources that combust hazardous waste with the same energy content and the same metal concentrations ( on both a thermal concentration and mass based concentration basis), but at different hazardous waste firing rates, would be required to achieve identical back end control device operating efficiencies to comply with a thermal emissions based standard. Holding these factors constant, thermal emission standards require sources to achieve identical percent reductions of the HAP that is processed within the combustor via removal with an air pollution control device. A thermal emission standard format is thus equally stringent for these sources on a percent HAP removal basis, irrespective of the amount of hazardous waste it processes for energy recovery, and better assures that sources burning smaller amounts of hazardous waste ( from an energy recovery perspective) are also controlling emissions as well as the average of the best performing sources. Sources processing higher energy content hazardous wastes would be allowed to feed hazardous wastes with higher metal and chlorine mass based concentrations relative to other sources combusting lower energy content wastes. To illustrate this, assume there are two sources ( named C and D) with identical back end control systems and identical mass feedrates of hazardous waste. Also assume the hazardous waste of source C has twice the energy content as compared to the hazardous waste processed by source D. A thermal emission standard will allow Source C to feed a hazardous waste that has twice the metals concentration ( as measured on a mass basis) as compared to source D, even though both sources would be required to comply with equivalent thermal feed rates limitations. Notably, however: 1) source C is displacing ( i. e., not using) twice as much valuable fossil fuel as the source with the lower energy content hazardous waste, and is feeding twice as much raw material   the raw material being energy content contained in the hazardous waste; 2) source C cannot exceed the feed control levels ( expressed on a lbs of HAP per million BTU basis)

113 Although the rule does not require use of feed control ( or any particular means of control to achieve a standard), the rule assures that all sources' emissions will reflect the emissions of the sources with the best hazardous waste federates expressed in terms of amount of HAP per BTU of hazardous waste. Because this format eliminates consideration of stack gas attributable to fossil fuel emissions, and thus eliminates the dilutive effect of these emissions, the likelihood that sources will in fact use hazardous waste feed control as part of their control strategy is great. that was achieved by the average of the best performing sources ( assuming its back end control efficiency is equivalent to the average performance demonstrated by the best performing sources); and 3) source D is required to have lower mass concentrations of metals in its hazardous waste because it is firing poorer quality hazardous waste fuel ( from an energy recovery perspective) and because it is feeding less of the same raw material ( measured by energy content). Thus, the thermal emissions format appropriately encourages and promotes the processing of clean, high energy content hazardous waste fuels ( consistent with evaluating hazardous waste feed control as an aspect of MACT, and not just relying on control solely through use of back end technology), and does so equally for all sources because it normalizes the allowable emissions based on the amount of energy each source recovers from the hazardous waste. Put another way, source C in the above example is controlling HAP emissions to the same extent as the average of the best performing sources per every BTU of hazardous waste fuel it processes ( as is source D). We note that this is a hypothetical example. In practice the average energy content of hazardous waste processed at cement kilns does not vary significantly across sources. Cement kilns burn hazardous wastes with relatively consistent energy contents because that is what their production process necessitates. This is supported by our database and by comments received from the Cement Kiln Recycling Coalition. 114 Heating values of hazardous wastes processed at cement kilns during compliance tests ( information which is included in our database) range from 10,300 to 17,600 BTU/ lb, with a median value of 12,400 BTU/ lb. We note that these are snapshot representations of hazardous waste heating content from these sources that originate from compliance tests. We also have long term average hazardous waste heating measurements from cement kilns indicating that the heating content of the hazardous wastes on average range from 9,900 to 12,200 BTU/ lb, with a median value of 11, 500 BTU/ lb. We thus conclude that the commenter's concern regarding sources being allowed to emit more HAP if they process hazardous waste with higher energy content is overstated for these sources. Energy content of hazardous wastes processed in liquid fuel boilers and lightweight aggregate kilns varies more than energy content of hazardous wastes processed by cement kilns, and sources with higher energy content wastes would be allowed to emit more metals than identical sources burning identical volumes of lower energy content wastes ( although the degree of control is identical per BTU of hazardous waste fuel processed). 115 Again, these are hypothetical examples. Each energy recovery unit will have an upper bound on the amount of energy it can process from the hazardous waste. Sources that process higher energy content hazardous wastes would not necessarily feed the same volume of hazardous waste as compared to sources processing lower energy content hazardous wastes because they cannot exceed the thermal capacity of their combustion unit. Under a thermal emission 114 See comment submitted by the Cement Kiln Recycling Coalition, USEPA, " Comment Response Document to the Proposed HWC MACT Standards, Volume 1: MACT Standards," September 2005, Section 3.3. Also see USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 23. 115 The hazardous waste heating values of liquid fuel boilers range from 2200 to 21000 BTU/ lb, with a median value of 14,800. Heating values of lightweight aggregate kilns range from 4900 to 16,900 TU/ lb, with a median value of 14,800. We note that the low end heating value for lightweight aggregate kilns reflects one source and is not typical of heating values used by the other commercial lightweight aggregate kiln facilities, and are similar to the heating values of cement kilns. standard format, the mass emission rates that would be allowed for identical sources that fulfill 100 percent of their energy demand from hazardous waste and that have differing hazardous waste energy contents would be identical. Although the source with the higher energy content hazardous waste would have a higher allowable mass based hazardous waste feed concentration, this source would have to process less hazardous waste ( on a mass basis) to remain within its thermal capacity. This helps to ensure that its mass HAP emission rate is similar to other sources that process lower energy content hazardous waste. One commenter's apparent concern with thermal emissions seems to center on an assertion that sources will intentionally blend nonhazardous, high heating value wastes or fuels with low energy, high metal bearing hazardous wastes in order to increase the energy content of these metal bearing wastes so that they will be subject to higher allowable emissions via thermal emission standards. We specifically address that comment later as it relates to commercial energy recovery units ( lightweight aggregate kilns and cement kilns). We note here, however, that we do not consider that comment to be of practical concern for liquid fuel boilers because they do not engage in commercial fuel blending practices. Comment: A commenter states that EPA's assessment of thermal emissions to identify the relevant best sources is inappropriate because thermal emissions are not emission levels, but rather a ratio of emissions to the heat content in a source's hazardous waste. Response: This comment challenges the basic idea of normalization, since the comment would be the same regardless of the normalizing parameter being used. Thermal emissions are emission levels that are normalized to account for the amount of energy ( i. e., raw material) these sources recover by processing hazardous waste. Similarly, a mass emission concentration ( i. e., ug/ dscm) is a ratio of the emissions to the volume of combustion gas that is generated, which normalize emissions to account for differences in the size of the combustion units ( as well as differences in production capacity). This rulemaking assesses performance and expresses emission standards in both of these formats; both formats normalize the emissions so that we may better assess emission control efficiencies equally across sources based on the percent of HAP in the feed ( whether thermal feed or feed normalized based on combustor size) 116 that is controlled or removed from the stack gas prior to being emitted into the atmosphere. As discussed above, technology based standards have historically assessed performance after normalizing emissions based on the amount of raw material processed by the given industry sector. Thermal emissions normalize each source's emissions based on the amount of raw material ( hazardous waste fuel) it processes, and are therefore appropriate to assess and identify the relevant best performers. Finally, as previously explained, this approach is consistent with both the language of section 112 ( d) ( 2) and ( 3), and the purpose of these provisions. Comment: A commenter states that EPA's assessment of thermal emissions to identify the relevant best sources is inappropriate because it ignores HAP emissions attributable to the nonhazardous fuel and raw material. Response: Thermal emission standards do not directly control HAP emissions attributable to the fossil fuels and raw material, in the sense that we did not assess feed control of fossil fuels or raw materials. However, this issue is not related to our choice to 116 For emission concentration based standards we normalize hazardous waste feed control levels by calculating what we call maximum theoretical emission concentrations, which are equivalent to the HAP mass feed rate divided by gas flow rate. use thermal content of hazardous waste as a normalizing parameter. Rather, the issue is whether feed control of fossil fuels and raw materials is a feasible means of control at all. We have determined that it is not, and that only back end control ( expressed as system removal efficiency) is feasible. Moreover, today's rule controls emissions from HAP in raw material and fossil fuels. All non mercury metal HAP emissions attributable to fossil fuels or raw material are effectively and efficiently controlled to the level of the average of the best performing sources with the surrogate particulate matter standard, as well as the system removal efficiency component of the SRE/ Feed methodology. Comment: EPA has failed to document sources' actual feedrates. Feedrates are presented either as MTECs ( where hazardous waste HAP feedrates are divided by gas flow rates) or as thermal feedrates, ( where feedrate is expressed as the mass of HAP per million BTUs of hazardous waste fired). This is impermissible, since it does not measure actual feed levels. Response: This comment essentially takes the position that it is legally impermissible to normalize standards, i. e., express standards on a common basis. EPA rejects this comment for the reasons stated in the introduction to this section. Comment: A commenter states that an increasing number of fuel blenders are producing fuels with a minimum heating content and maximum metals content in order to maximize revenues because high metal bearing wastes command a higher revenue on the commercial waste market. The commenter states that thermal emission standards are not appropriate because they are based on the implicit assumption that energy recovery entails metals feed. Response: Contrary to what the commenter suggests, the thermal emissions format will more likely discourage the alleged practice of fuel blenders producing fuels with a minimum heat content and maximum metals content because the standard limits the allowable metal emissions based on the amount of energy contained in the hazardous waste. Thus, a source with a lower energy waste would have to ensure that the mass concentration of metals is also lower to comply with the thermal emission formatted standard. The source would consequently emit less metals ( on a mass basis) because of the lower metal mass concentration in the waste fuel. Thermal emission standards reflect the reality that the hazardous waste fuels that are currently processed safely and efficiently in energy recovery units to displace valuable fossil fuel do in fact contain metal HAP. From a feed control perspective, the thermal emissions format appropriately requires sources to process high energy content hazardous waste fuels that reflect the thermal feed control levels achieved by the average of the best performing sources, and does so equally for all sources because it normalizes the allowable emissions based on the amount of energy each source recovers from the hazardous waste. Comment: A commenter states that EPA should be concerned that fuel blenders and kilns will use the thermal emission standard format to increase the allowable metals feedrates for their units. The commenter claims that sources could inappropriately convert nonhazardous waste fuel to hazardous waste fuel by simply putting coal in a bunker in which hazardous waste was once stored, or mixing nonhazardous waste fuel oil with hazardous waste. The commenter states that a facility with a low hazardous waste firing rate, and relatively low allowable emissions can become a facility with a high hazardous waste percent firing rate, with higher allowable emissions, simply by ` creative' use of the hazardous waste mixture rule. The commenter suggests that EPA clearly state that the hazardous waste thermal emission standards apply only to the hazardous waste portion of the fuel blend mixture. The commenter further suggests that EPA require fuel blenders to report the amount of nonhazardous waste fuel that is contained in the fuel blend, and that cement kilns use this to determine allowable metal feed rates based on the original hazardous waste energy content. Response: We do not believe hazardous waste combustors will engage in the practice of redesignating their fossil fuels, i. e., coal, as hazardous wastes with creative use of the mixture rule in order to increase their allowable metal HAP emission rate. That would require large quantities of coal to be newly classified as hazardous waste. The coal, and the unit where the coal is stored, would subsequently become subject to all applicable subtitle C requirements, which include storage and closure/ post closure requirements. We believe this disincentive will discourage this hypothetical practice. Moreover, as previously discussed, today's rule does not allow cement kiln or lightweight aggregate kiln emissions to exceed the interim standards. The fact that we are issuing emission standards for some pollutants in the thermal emissions standard format will not encourage fuel blenders to send more metals to these commercial energy recovery sources because their allowable emission concentrations are, by definition, either equivalent to or more stringent than the current limitations with which they are complying. Thus, even if the fuel blenders and energy recovery units engaged in this practice, they could not emit more metals than they are currently allowed to emit. We therefore conclude that it is not necessary to promulgate complicated regulatory provisions that would increase the reporting and recordkeeping requirements of fuel blenders and energy recovery units in order to address a hypothetical scenario that likely would never occur. Finally, we note that combustion of certain high HAP metal content wastes is already prohibited under RCRA rules. See 40 CFR § 268.3. Such wastes remain prohibited from combustion even if they are mixed with fossil fuel so that the mixture has a higher energy content. U. S. v. Marine Shale Processors, 81 F. 3d 1361, 1366 ( 5th Cir. 1996) ( an unrecyclable hazardous waste is not recycled when it is mixed with a usable non waste and the mixture is processed). Thus, the dilution prohibition in § 268.3 serves as a further guard against the commenter's concern. Comment: A commenter states that the thermal emissions format may be problematic because it is based on a flawed assumption that metal HAP from the cement kiln raw material and hazardous waste partition in equal proportions to the total stack gas emissions. The commenter believes that metal retention in the raw materials is higher than the hazardous waste, suggesting that thermal emission standards allow an arbitrary increase in allowable hazardous waste metals emissions. The commenter suggests that EPA require that compliance demonstrations be conducted only under conditions where the metals content in the hazardous waste is significantly higher than the metal content in the raw material to minimize this bias. Response: The commenter has not provided any emissions data to support this claim, nor does the EPA know of data available that reaches this conclusion. We do not believe there is a significant difference in the partitioning rates of these metals in a cement kiln. 117 117 We reference comments submitted by the cement kiln recycling coalition that address this very point. See USEPA, " Comment Response Document to the Proposed HWC MACT Standards, Volume 1: MACT Standards," September 2005, Section 3.3. We have evaluated these comments and find them persuasive on this issue. Even if there is a difference, this would not result in an arbitrary increase of allowable hazardous waste metals emissions. The thermal emission standards were calculated using thermal emissions data that are based on each source's compliance test. These tests were conducted at hazardous waste feed control levels that represented the upper bound of feed control levels these sources see on a day to day basis. To accomplish this, sources spiked metals into the hazardous waste prior to combusting the wastes. The amount of metals that were contained in the hazardous waste streams, after accounting for these spiked metals, far exceeded the metal levels that were contained in the raw material. Thus the differences in partitioning, if any, would likely be overshadowed by the fact that the majority of the metals were contained in the hazardous waste. Notably, any partitioning bias that that may be present would also have been present during these compliance tests. As a result, this potential bias would be built into the emission standard and thus would not result in an arbitrary increase in allowable hazardous waste metals emissions because these sources will again demonstrate compliance under testing conditions similar to those used to generate the data used to calculate the MACT floors. We conclude that it is not necessary to provide additional prescriptive regulatory language that would require sources to demonstrate system removal efficiencies under testing conditions that exhibit a high ratio of hazardous waste metal content to raw material metal content because the regulations implicitly require sources to demonstrate hazardous waste metal feed control levels that represent the upper range of their allowable feed control levels. 118 Comment: A commenter states that compliance with standards expressed in a thermal emissions format is problematic because the measurement of energy content of hazardous waste fuel blends is subject to significant variability due to the nature of the test. The commenter also claims that heating value measurements of waste streams that are mixtures of solids and liquids tend be biased high, which would inappropriately give these sources higher allowable metal emission limitation. Response: There are standard ASTM procedures that reliably measure the energy content of the hazardous waste. Any parameter that is measured for compliance purposes is subject to method imprecision and variability. We do not believe that hazardous waste energy content measurements result in imprecision and variability above and beyond the measurement methods that are currently used to assure compliance with emission concentration based standards. The commenter did not provide evidence that supports the claim that energy content measurement and/ or sampling methods consistently result in a positive bias. If a bias were consistently present for these types of wastes, then one would expect it to be also reflected in the measured data for which we based the emission standards, which would fully address the commenter's concern. Nonetheless, we note that all hazardous waste sampling and analysis procedures must be prescribed in each source's feedstream analysis plan, which can be reviewed by the permitting authority upon request. These feedstream analysis plans must ensure that sampling and analysis procedures are unbiased, precise, and that the results are representative of the feedstream. See § 63.1208( b)( 8). More information on obtaining a 118 Although today's final rule allows sources to extrapolate their allowable hazardous waste feed control levels to levels that are higher than the level demonstrated in the comprehensive performance test, sources must still spike metals into the hazardous waste during the test in order to assure that the system removal efficiency used for the extrapolation procedure is reliable and accurate. representative samples can be found in EPA's SW 846 publication. 119 These procedures involve acquiring several sub samples that provide integration over the breadth, depth and surface area of the waste container and obtaining replicate samples ( see Ch. 13.3.1 of SW 846). Comment: A commenter states that BTU measurements can be reported as either a higher heating value or a lower heating value, and suggests that EPA require sources to use the lower heating value calculation when determining allowable hazardous waste feed control levels. The commenter seems to imply that use of higher heating values will inappropriately result in higher allowable metal feed rates for fuel blends that contain aqueous waste. Response: The BTU data in our database that we use to calculate the emission standards reflect higher heating values. It is standard practice in the incineration/ combustion industry to report the gross heat of combustion ( or higher heating value). We conclude that sources should use the higher heating value rather than the lower heating value for all compliance determinations because these are method based emission standards. Fuel blends that contain aqueous wastes will not be inappropriately rewarded with higher allowable feed rates because any fuel mixture that contain aqueous mixtures will have lower reported heating values, irrespective of whether they are reported as higher heating values or lower heating values. 120 E. Standards Can Be No Less Stringent Than the Interim Standards Comment: Several commenters oppose EPA's position in the proposed rule that the replacement standards can be promulgated at a level no less stringent than the interim standards for incinerators, cement kilns, and lightweight aggregate kilns. In instances where the calculated replacement standard is less stringent than the interim standard, the commenters oppose EPA's position of " capping" the replacement standard at the level of the interim standard to prevent backsliding from those levels. Instead, commenters recommend that EPA calculate and finalize the existing and new source floor levels without regard to the interim standards. One commenter also notes that the interim standards are simply a placeholder without the necessary statutory basis to qualify as emission limitations for purposes of establishing MACT floors. Another commenter, however, supports EPA's position to prevent backsliding to levels less stringent than the interim standards. Response: We maintain that the replacement standards can be no less stringent than existing standards, including the interim standards under § § 63.1203 1205, for incinerators, cement kilns, and lightweight aggregate kilns. These standards were promulgated on February 13, 2002, and sources were required to comply with them no later than September 30, 2003, unless granted a one year extension ( see § 63.1206( a)). Thus, all hazardous waste combustors are currently complying with the interim standards. The comment that the standards lack some type of requisite statutory pedigree misses the central point of our interpretation of the statute: motivation for achieving a standard ( be it regulatory compulsion, statutory requirement, or some other reason) is irrelevant in determining levels of MACT 119 SW 846, " Test Methods for Evaluating Solid Waste, Physical/ Chemical Methods 120 The difference between the higher heating value and lower heating value of an aqueous waste is insignificant relative to the difference in heating value between an aqueous waste and an organic liquid waste fuel. floors. National Lime v. EPA, 233 F. 3d at 640. What matters is the level of performance, not what motivated that level. As a result, the replacement standards promulgated today ensure that sources will emit HAP at levels no higher than levels achieved under current regulations. We do this in this rule, when necessary, by either capping a calculated floor level by the interim standard ( when both the calculated floor level and interim standard are expressed in the same format of the standard) or by adopting dual standards in cases where formats of the standard vary ( so that comparison of stringency cannot be uniformly determined ( as for cement kilns and lightweight aggregate kilns, as explained in the preceding section above and in the following response). In this case, the sources are subject to both the replacement and interim standards. Comment: One commenter states that some proposed standards expressed in a thermal emissions format would allow some sources to emit semivolatile metals at levels higher than the interim standard. The commenter states that EPA reached incorrect conclusions when making relative stringency comparisons between standards expressed in a thermal emissions and mass concentrations format because, in part, EPA assumed an average F factor ( e. g., semivolatile metals for cement kilns). 121 In addition, the commenter notes that the actual relationship between standards expressed in terms of thermal emissions and mass concentrations is complex and depends on a number of factors. As a result, the commenter urges EPA to adopt dual standards ( i. e., promulgate the MACT standard as both the standard expressed in a thermal emissions format and also the interim standard expressed in a mass concentration format) to prevent backsliding. Response: Even though a source may operate in compliance with a standard expressed in a thermal emission format, a source may or may not also be in compliance with the corresponding mass concentration interim standard ( e. g., the semi and low volatile metal emission standards for cement and lightweight aggregate kilns of § § 63.1204 and 63.1205, respectively). As reflected in the comment, making a judgment as to whether a replacement standard is more stringent than the interim standard for the HAP is not always a straightforward calculation. As we discussed in the proposed rule122 and echoed by the commenter, comparing standards in the thermal emissions format to those in a mass concentration format involves assumptions that vary on a site specific basis and can vary over time, including the hazardous waste fuel replacement rate, contributions to emissions from nonhazardous waste inputs such as raw materials and nonhazardous waste fuels such as coal, how close to the standard a source elects to comply, the system removal efficiency demonstrated during testing, and the type and composition, including heating value, of fuels burned. To ensure that sources operating under standards expressed in a thermal emissions format will not emit HAP metals at levels higher than currently achieved under the interim standards, we adopt a dual standard to prevent emissions increasing to levels higher than the interim standards. The dual standard structure includes both the standard expressed in a thermal emissions format and the interim standard, which is expressed in a mass concentration format. We apply this concept to several standards including semivolatile 121 An F factor is an estimate of the amount of combustion gas volume that is generated per fuel heat input for a given type of fuel, expressed in units, for example, cubic feet of combustion gas per million British thermal units ( Btu) of fuel burned. In the proposal, EPA used F factors to convert the emission standards expressed on a thermal basis to mass concentrations in order to make a judgment as to the relative stringency of the proposed MACT standards relative to the interim standards. 122 For example, see 69 FR at 21255 258, 267 271. metals, low volatile metals, and mercury123 for cement kilns and semivolatile metals and low volatile metals for lightweight aggregate kilns. This approach ensures that sources are not emitting HAP metals above the levels of the interim standards because we cannot reliably determine that emissions under a standard expressed in a thermal emissions format would not exceed the interim standard for all sources in the category. See § § 63.1220( a)( 2)( a)( 4), and ( b)( 2)( b)( 4) and 63.1221( a)( 3)( a)( 4) and ( b)( 3)( b)( 4). We evaluated the relative stringency of the standards expressed in the thermal emissions format compared to the interim standards for the entire source category in order to determine if the dual standard scheme could be avoided. We determined that we could not. For some HAP groups we found that many sources in the category would have the potential to exceed the interim standards for that HAP. 124 In this case, we considered simply " capping" the standard expressed in the thermal emission format by the interim standard ( i. e., the promulgated standard would only be expressed in a mass concentration format). However, we conclude that this approach would not be appropriate because the standard expressed in a thermal emission format would likely be more stringent than the mass concentration for some sources, and the statute requires that MACT floors reflect this superior level of performance. In other cases we found that the standards expressed in the thermal emissions format would not likely exceed the interim standards by the majority of sources operating under typical conditions. 125 While our analysis ( based on information in our data base) shows in these cases that the emission standard expressed in a thermal emission format would not likely result in an exceedance of the interim standard, this conclusion may not be true because the assumptions may not be valid for a particular source or site specific factors may change in future operations. For example, HAP metal emissions could increase over time due to increases in HAP contributions from raw materials or alternative raw materials. Given this potential, we adopt dual standards for the HAP metal standards in order to ensure that standards expressed in a thermal emissions format will not exceed emission levels achieved under the interim standards. 126 Comment: Several commenters state that the interim standards do not reflect the average performance of the best sources, and so cannot be the basis for floor levels. Response: In those few situations where we have established floor levels at the level of the interim standards, we have done so as the best means of estimating performance of the best performing sources. Based on the available data to us, the average of the best 123 Although the mercury standard promulgated for cement kilns is not expressed using a thermal emission format basis, the same concept applies because the mercury standard is a hazardous waste feed concentration standard, which is a different format than the interim standard. 124 An example for each category is semivolatile metals thermal emissions standard for existing cement and lightweight aggregate kilns. See USEPA, " Final Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," Section 23.1, September 2005. 125 An example is the emission standards for low volatile metals for existing and new cement kilns and new lightweight aggregate kilns. See USEPA, " Final Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," Section 23.1, September 2005. 126 In response to a comment regarding the implementation of dual standards, we note the promulgation of a new provision allowing sources to petition the Administrator to waive the HAP metal feedrate operating parameter limits for either the emissions standards expressed in a thermal emissions format ( or the mercury feed concentration standard for cement kilns) or the interim standards based on documentation that the feedrate operating parameter limit is not needed to ensure compliance with the relevant standard on a continuous basis. See new § 63.1209( g)( 1)( iv) and Comment Response Document, Volume I, Section 3.5. performing sources exceeds the level of the interim standards in a few instances. Under these circumstances, the binding regulatory limit becomes the best means available to us to estimate performance. See Mossville, 370 F. 3d at 1241 42 ( accepting regulatory level as a floor standard where sources' measured performance is not a valid means of determining floor levels, and where such data contains results as high as those regulatory levels).

F. How Can EPA's Approach to Assessing Variability and its Ranking Methodologies be Reasonable when they Result in Standards Higher than the Interim Standards?

A commenter argued that EPA's floor methodologies, in particular its consideration of variability beyond that demonstrated in single test conditions, the SRE/ feed and Air Pollution Control Device methodologies, must be arbitrary because in a few instances projected standards using these approaches were higher than the current interim standards, a level every source ( not just the best performers) are achieving. Commenters also noted that one of the new source standards calculated under these approaches was higher than an existing source standard, another arbitrary result. EPA believes that these seeming anomalies ( which are infrequent) result from the database used to calculate performance and standards, rather than from the approaches to assessing variability or the two questioned floor methodologies. The data base is from test results which preceded EPA's adoption of the interim standards. Thus, the level of performance required by the later rule is not necessarily reflected in pre rule test data. In confirmation, some of the standards computed using straight emission approaches also are higher than the interim standards. Other anomalies arise simply due to scarcity of data ( floor levels for certain HAP emitted by lightweight aggregate kilns especially, where there are only nine sources total). In these situations there is a greater likelihood that one or more of the best performing sources will have relatively high emissions because we are required to use data from five sources to comprise the MACT pool whenever we have data from fewer than 30 sources, and a small amount of data can skew the result. See § 112( d)( 3)( B). 127 For example, many of the calculated new source chlorine floors were slightly higher than the calculated existing source standards because we assumed all sources with measured emissions below 20 ppmv were in fact emitting at 20 ppmv ( see part four, section I. C). We generally are unable to differentiate a single best performing source among these best performers because many/ all of the best performing sources emissions are adjusted to the same emission level. The calculated new source floor can be slightly higher than the existing source floor because the variability factor that is applied to the single best performing source is based on only one test condition ( with three emission test runs). This results in a higher level of uncertainty relative to the existing source standard, which is based on a compilation of emissions data from several sources that have essentially the same projected emissions as a result of the method bias correction factor. The variability factor that is applied to the emissions of the single best performing source is therefore higher than the variability factor for the existing source floor because there are fewer degrees of freedom in the statistical analysis. 128 Likewise, many of the calculated solid fuel boiler new source standards were 127 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 19, for further discussion.

128 For a single test condition the t factor used in variability factor calculation has n 1 degrees of freedom where n is the number of runs for that condition. For the MACT floor calculation the t factor has X N degrees slightly higher than the calculated existing source standards because, as discussed above, there are fewer degrees of freedom when assessing the variability from a single best performing source. The solid fuel boiler " anomalies" also occur using a straight emissions methodology. See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September, 2005, Section 19, for further discussion that summarizes and explains these so called anomalies.

IV. Use of Surrogates A. Particulate Matter as Surrogate for Metal HAP Comment: A commenter states that EPA's use of particulate matter as a surrogate for nonenumerated metals is unlawful and arbitrary and capricious because although particulate matter emissions may provide some indication of how good a source's end of stack control of such metals is, it does not indicate what its actual metal emission levels are. 129 The commenter states that emissions of these metals can vary based on metal feed rate without having any appreciable effect on particulate matter emission levels. Thus a particulate matter standard does not necessarily ensure that metal emissions are reduced to the metal emission levels achieved by the relevant best performing sources. To support this assertion, the commenter states that EPA is on record saying " low particulate matter emissions do not necessarily guarantee low metal HAP emissions, especially in instances where the hazardous waste feeds are highly concentrated with metal HAP." 69 Fed. Reg. at 21221. Response: The final rule uses a particulate matter standard as a surrogate to control: 1) emissions of nonenumerated metals that are attributable to all feedstreams ( both hazardous waste and remaining inputs); and 2) all nonmercury metal HAP emissions ( both enumerated and nonenumerated metal HAP) from the nonhazardous waste process feeds at cement kilns, lightweight aggregate kilns, and liquid fuel boilers ( e. g., emissions attributable to coal and raw material at a cement kiln, and emissions attributable to fuel oil for liquid fuel boilers). Incinerators, liquid and solid fuel boilers may elect to comply with an alternative to the particulate matter standard that would limit emissions of all the semivolatile metal HAPs and low volatile metal HAPs. See § 63.1219( e). The particulate matter standard is a necessary, effective, and appropriate surrogate to control nonmercury metal HAPs. The record demonstrates overwhelmingly that when a hazardous waste combustor emits particulate matter, it also emits nonmercury HAP metals as part of that particulate matter, and that when particulate matter is removed from of freedom where X is the total number of runs from all sources in the MACT pool and N is the number of sources in the pool. See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September, 2005, Section 7.1 for more information on the floor calculation procedure. 129 " Enumerated" metals are those HAP metals directly controlled with an emission limit, i. e., lead, cadmium, chromium, arsenic and beryllium. The remaining nonmercury metal HAP ( i. e., antimony, cobalt, manganese, nickel, and selenium) are called " nonenumerated" metal HAP ( note that arsenic and berrylium are nonenumerated metals for liquid fuel boilers because the low volatile metal emission standard applies only to chrome). emissions the nonmercury HAP metals are removed with it. 130 Nonmercury metal HAP emissions are therefore reduced whenever particulate matter emissions are reduced. The particulate matter standard thus is an effective and appropriate surrogate that assures sources are controlling these metal HAP with an appropriate back end control technology. National Lime v. EPA, 233 F. 3d at 639. The nonenumerated metal HAP are no different than other semivolatile or low volatile metals in that they also will be effectively controlled with a back end particulate matter air pollution control device. We also considered the possibility of developing a standard for nonenumerated HAP metals instead of a PM standard ( i. e., regulating these metals directly, rather than through use of a surrogate). We conclude for several reasons, however, that issuing emission standards for these nonenumerated metals in lieu of a particulate matter standard would not adequately control nonmercury metal HAPs to levels achieved by the relevant best performing sources. We generally lack sufficient compliance test emissions data for the noneneumerated metals to assess the relevant best performing sources, because, as discussed below, most of these metals were not directly regulated pursuant to RCRA air emission standards. 131 Although we have more emissions data for these metals that are based on ( so called) normal operations, we still lack sufficient emissions data to establish nonenumerated metal standards for all the source categories. Use of normal data may also be problematic because of the concern raised by the cement kiln and lightweight aggregate kiln stakeholders that our normal metals emissions data obtained from compliance tests are not representative of the range of actual emissions at their sources. Cement kiln and lightweight aggregate kiln stakeholders submitted long term hazardous waste mercury feed control data that support their assertion. Although these stakeholders did not submit longterm normal hazardous waste feed control data for the nonenumerated metals, we can still see that use of the normal nonenumerated metal snapshot emissions in our database to determine MACT floors could raise similar concerns with respect to whether the normal data in fact represents average emissions at these sources, and their level of performance. Use of particulate matter emissions data to assess the relevant best performers for nonenumerated metal HAP is therefore more appropriate for two reasons. Compliance test data better account for emissions variability and avoid the normal emissions bias discussed above. We also have much more particulate matter emissions data from more sources, which better allows us to evaluate the true range of emissions from all the sources within the source category and to assess and identify the relevant top performing 12 percent of the sources. It would be inappropriate to assess total stack gas emissions of nonenumerated metals for cement kiln and lightweight aggregate kilns when determining the relevant best performers because these emissions would, in part, reflect the metal feed levels in these 130 This statement is equally true for any emitting source, not just hazardous waste combustors. It is well established that semivolatile and low volatile metals exist in solid particulate form at typical air pollution control device operating temperatures. This is supported by 1) known operating temperature ranges of air pollution control devices used by hazardous waste combustors; 2) known metal volatility equilibrium relationships; and 3) extensive technical literature. See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 3.1.

131 At best, we may have enough compliance test data for antimony and selenium to adequately assess relevant best performers for only incinerators and lightweight aggregate kilns. sources' nonhazardous waste process feedstreams. This is not appropriate because nonhazardous process feedstream control is not a feasible means of control. See part four, section III. B. 1. A potential solution to this problem would be to identify the relevant best performers by assessing each source's hazardous waste thermal emissions for these nonenumerated metals ( given that hazardous waste thermal emissions exclude by definition emissions attributable to inputs other than hazardous waste, i. e. raw materials and fossil fuels). This, however, would be problematic because, aside from the data limitation issues, the majority of the nonenumerated metals data reflect normal emissions which often do not contain the highest feed rates used by the source. As a result, we cannot assess performance on a thermal emissions basis because of the uncertainty associated with system removal efficiencies at such low metal feedrates. Furthermore, even if we could issue hazardous waste thermal emissions standards for these metals, a particulate matter emission standard would still be necessary to control nonmercury metal HAP emissions from the nonhazardous waste process feedstreams. Emission standards for these nonenumerated metals could require sources to implement hazardous waste feed control ( for these metals) to comply with the standard. 132 We are less assured that these sources were implementing hazardous waste feed control for these nonenumerated metals at the time they conducted the emissions tests ( which serve as the basis for floor calculations) because most of these metals were never directly regulated pursuant to the RCRA emission standards. 133 This means that sources tended to optimize ( or at least concentrate their efforts on) control of the metals that are regulated. Although these metals were being controlled with each source's back end control device, sources may not have been controlling these metal feedrates because they probably were not subject to specific feedrate limitations ( feed control of the enumerated metal HAP does not ensure feed control of these nonenumerated metal HAP). Furthermore, simultaneous feed control of all these metals, when combined with enumerated semivolatile and low volatile metals, may not be possible because the best performing sources for all these metals may collectively represent a hazardous waste feedstream that does not exist in practice ( from a combined metal concentration perspective) because there likely would be different best performers for each of the metal HAP or metal HAP groups. 134 We thus conclude that back end control as measured and assessed by each source's particulate matter emissions is 132 Sources that otherwise would be equipped with what is considered to be a MACT back end control devices ( i. e., a control device achieving the final rule particulate matter standard) may not be able to achieve these metal emissions standards due to varying metal feed levels ( both within sources and across sources). Such an outcome may require a source to limit the amount of metal that is fed into the combustion unit to achieve the standard. 133 Antimony is the only nonenumerated metal that is directly regulated pursuant to the boilers and industrial furnace regulations. See § 266.106. 134 We generally cannot combine these nonenumerated metals into the associated semivoltile or low volatile metal volatility groupings promulgated in this final rule for purposes of establishing " grouped" emission standards because we cannot mix compliance test data with normal emissions data when calculating floors ( the majority of the standards included in this final rule are based on compliance test data, and the majority of the data we have for nonenumerated metals being normal). Furthermore, if we were to separately group the normal nonenumerated metal emission data into their associated semivolatile or low volatile metal group, we may encounter data limitation issues because each source would need to have measured each of the nonenumerated metals in that associated metal volatility group in order for us to conclude that the emission data adequately represents the sources combined emissions of semivolatile or low volatile metals. the appropriate floor technology to assess when identifying the relevant best performers for nonenumerated HAP metals and estimating these sources' level of performance. Comment: A commenter states that EPA's rationale for use of particulate matter as a surrogate for nonenumerated metals is flawed because EPA has provided no data in the proposal to justify its hypothesis that particulate matter is an appropriate surrogate for nonenumerated metal HAP. The commenter also states that the proposed emission standards for particulate matter for existing sources discriminate against boilers and process heaters that burn clean ( i. e., little or very low concentrations of HAP metals) hazardous waste fuels. The commenter suggests that if there are sufficient data, EPA should consider developing an alternative emission standard for total HAP metals for new and existing liquid fuel boilers, as was done for the Subpart DDDDD National Emission Standards for Hazardous Air Pollutants for Industrial/ Commercial/ Institutional Boilers and Process Heaters. Response: As previously discussed in this section, particulate matter reflects emissions of nonmercury metal HAPs because these compounds comprise a percentage of the particulate matter ( provided these metals are fed into the combustion unit). The technologies that have been developed and implemented to control particulate matter also control nonmercury metal HAP. Since non mercury metal HAP is a component of particulate matter, we can use particulate matter as a surrogate for these metals. Further justification for the use of particulate matter as a surrogate to control metal HAP is included in the technical support document. 135 We conclude that we do not have enough nonenumerated metal emissions data to calculate alternative total metal emission floors for liquid fuel boilers. The most problematic of these metals are manganese and cobalt, where we have emission data from only three sources. We have much more compliance test particulate matter emissions data from liquid fuel boilers, and thus conclude that the particulate matter standard best reflects the emission levels achieved by the relevant best performers. Similar to the above discussion, calculating an alternative total metal emissions floor raises questions regarding the method used to calculate such floors. Hazardous waste combustor metal emissions have traditionally been regulated in volatility groupings because the volatility of the metal affects the efficiency of back end control ( i. e., semivolatile metals are more difficult to control than low volatile metals because they volatilize in the combustor and then condense as small particulates prior to or in the emission control device). When identifying the best performing sources, we previously have, in general, only evaluated sources that have metal emissions information for every metal in the volatility grouping. This approach could prove to be problematic since it is not likely many sources will have emissions data for all the metals. Although we could not calculate alternative total metal emission floor standards based on the available emissions data we have, we agree with the commenters' view that sources that burn hazardous waste fuels with low levels of nonenumerated metals should be allowed to comply with a metals standard rather than the particulate matter standard. We proposed an alternative to the particulate matter standard ( see 69 FR at 21331) for incinerators, liquid, and solid fuel boilers that was a simplified version of the alternative particulate matter standard that is currently in effect for incinerators pursuant to the interim standards ( see § 63.1206( b)( 14)). We received no adverse comment and are promulgating 135 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 3.1. this alternative as proposed. The alternative metal standards apply to both enumerated and nonenumerated metal HAP, excluding mercury. For purposes of these alternative requirements, each nonenumerated metal is classified as either a semivolatile or a low volatile metal and subsequently grouped with the associated semivolatile and low volatile enumerated metals. The semivolatile and low volatile metals standards under this alternative are the same as those that apply to other liquid fuel boilers, but the standard would apply to all metal HAP, not just those enumerated in the generic low volatile metal and semivolatile metal standards. See § § § 63.1216( e), 63.1217( e) and 63.1219( e).

B. Carbon Monoxide/ Hydrocarbons and DRE as Surrogates for Dioxin/ Furan Comment: One commenter states that the dioxin/ furan floors for new and existing solid fuel boilers is unlawful and arbitrary and capricious. EPA established the floor for dioxin/ furan for these sources as compliance with the carbon monoxide or hydrocarbon standard and the destruction and removal efficiency ( DRE) standard. The commenter states that EPA has not shown that carbon monoxide or hydrocarbon emissions correlate to dioxin/ furan emissions, and, accordingly, has not shown that the carbon monoxide or hydrocarbon standard, together with the DRE standard, are valid surrogates. This commenter also states that it is inappropriate for EPA to use carbon monoxide or hydrocarbons and DRE as surrogates to establish dioxin/ furan floors for liquid fuel boilers with wet or no air pollution control devices and for hydrochloric acid production furnaces. The commenter believes EPA inappropriately justifies these surrogates by claiming that a numerical dioxin/ furan floor would not be replicable by the best sources or duplicable by the others. The commenter states that EPA has no discretion to avoid setting floors for a HAP just because it believes that HAP is not controlled with a technology. Rather, EPA must set floors reflecting the relevant best sources' actual performance. Such floors necessarily will be duplicable by the relevant best sources themselves. That they cannot be replicated by other sources is irrelevant according to the commenter. In addition, the commenter states that EPA does not claim or demonstrate that the carbon monoxide and hydrocarbon floors for solid fuel boilers reflect the average emission levels achieved by the relevant best sources. Finally, the commenter also notes that EPA appears to argue that its carbon monoxide or hydrocarbon standard and DRE standard could be viewed as work practice standards under section 112( h) which allows EPA to establish work practice standards in lieu of emission standards only if it is not be feasible to set the former. Because EPA has made no such demonstration, setting work practice standards to control dioxin/ furan emissions from boilers would be unlawful according to the commenter. Response: The commenter raises four issues: ( 1) are the carbon monoxide/ hydrocarbon standard and the DRE standard adequate surrogate floors to control dioxin/ furan; ( 2) floors for existing sources must be established as the average emission limitation achieved by the best performing sources irrespective of whether the limitation is duplicable by the best performing sources or replicable by other sources; ( 3) EPA has not explained how the carbon monoxide and hydrocarbon floors reflect the average emission limitation achieved by the relevant best sources; and ( 4) EPA cannot establish work practice standards for dioxin/ furan under section 112( h) because it has not demonstrated that setting an emission standard is infeasible under section 112( h)( 1). Carbon Monoxide and Hydrocarbons Are Adequate Surrogates to Control Dioxin/ Furan when Other Controls Are Not Effective or Achievable. Carbon monoxide and hydrocarbons ( coupled with the DRE standard) are the best available surrogates to control dioxin/ furan emissions when a numerical floor would not be achievable and when other indirect controls, such as control of the gas temperature at the inlet of a dry particulate matter control device to 400F, are not applicable or effective. 136 As we explained at proposal, operating under good combustion conditions to minimize emissions of organic compounds such as polychlorinated biphenyls, benzene, and phenol that can be precursors to dioxin/ furan formation is an important requisite to control dioxin/ furan emissions. 137 See 69 FR at 21274. Minimizing dioxin/ furan precursors by operating under good combustion practices plays a part in controlling dioxin/ furan emissions, and that role is substantially enhanced when there are no other dominant factors that relate to dioxin/ furan formation and emission ( e. g., operating a dry particulate matter control device at temperatures above 400F). Carbon monoxide and hydrocarbons are widely accepted indicators of combustion conditions. The current RCRA regulations for boilers and hydrochloric acid production furnaces use emissions limits on carbon monoxide and hydrocarbons to control emissions of toxic organic compounds. See 56 FR 7150 ( February 21, 1991) documenting the relationship between carbon monoxide, combustion efficiency, and emissions of organic compounds. In addition, carbon monoxide and hydrocarbons are used by many CAA standards for combustion sources to control emissions of organic HAP, including: MACT standards for hazardous waste burning incinerators, hazardous waste burning cement kilns, hazardous waste burning lightweight aggregate kilns, Portland cement plants, and industrial boilers; and section 129 standards for commercial and industrial waste incinerators, municipal waste combustors, and medical waste incinerators. Finally, hydrocarbon emissions are an indicator of organic hazardous air pollutants because hydrocarbons are a direct measure of organic compounds. Commenters on our proposed MACT standards for hazardous waste incinerators, cement kilns, and lightweight aggregate kilns stated that EPA's own surrogate evaluation138 did not demonstrate a relationship between carbon monoxide or hydrocarbons and organic HAP at the carbon monoxide and hydrocarbon levels evaluated. See 64 FR at 52847 ( September 30, 1999). Several commenters on that proposed rule noted that this should not have been a surprise given that the carbon monoxide and hydrocarbon emissions data evaluated were generally from hazardous waste combustors operating under good combustion conditions ( and thus, relatively low carbon monoxide and hydrocarbon levels). Under these conditions, emissions of HAP were generally low, which made the 136 As discussed in Part Two, Section V, we view the carbon monoxide, hydrocarbon, and destruction removal efficiency standards as unaffected by the Court's vacature of the September 1999 challenged regulations for incinerators, cement kilns, and lightweight aggregate kilns. We are therefore not repromulgating and reopening consideration of these standards in today's final rule for these source categories. 137 Operating under good combustion conditions also helps minimize soot formation on boiler tubes. Research has shown that operating under conditions that can form soot followed by operating under good combustion conditions can lead to dioxin/ furan formation. See Section 2.4 of Volume III of the Technical Support Document. 138 See Energy and Environmental Research Corporation, `` Surrogate Evaluation of Thermal Treatment Systems,'' Draft Report, October 17, 1994. demonstration of a relationship more difficult. These commenters noted that there may be a correlation between carbon monoxide and hydrocarbons and organic HAP, but it would be evident primarily when actual carbon monoxide and hydrocarbon levels are higher than the regulatory levels. We agreed with those commenters, and concluded that carbon monoxide and hydrocarbon levels higher than those we established as emission standards for hazardous waste burning incinerators, cement kilns, and lightweight aggregate kilns are indicative of poor combustion conditions and the potential for increased emissions organic HAP. We continue to believe that carbon monoxide and hydrocarbons are adequate surrogates for organic HAP which may be precursors for dioxin/ furan formation and note that the commenter did not explain why our technical analysis is problematic. Emissions that Are Not Replicable or Duplicable Are Not Being " Achieved". The commenter believes that floors must be established as the average emission limitation of the best performing sources irrespective of whether they are replicable by the best performing sources or duplicable by other sources. To the contrary, emission levels that are not replicable by the best performing sources are not being " achieved" by those sources and cannot be used to establish the floor. For solid fuel boilers, we explained at proposal why dioxin/ furan emissions are not replicable by the best performing sources ( or duplicable by other sources): there is no dominant, controllable means that sources are using that can control dioxin/ furan emissions to a particular level. See 69 FR at 21274 75. We explained that data and information lead us to conclude that rapid quench of post combustion gas temperatures to below 400 ° F   the control technique that is the basis for the MACT standards for dioxin/ furan for hazardous waste burning incinerators, and cement and lightweight aggregate kilns   is not the dominant dioxin/ furan control mechanism for coal fired boilers. We believe that sulfur contributed by the coal fuel is a dominant control mechanism by inhibiting formation of dioxin/ furan. Nonetheless, we do not know what minimum level of sulfur provides significant control. Moreover, sulfur in coal causes emissions of sulfur oxides, a criteria pollutant, and particulate sulfates. For this reason, as well as reasons stated at 69 FR 21275, we are not specifying a level of sulfur in coal for these sources as a means of dioxin/ furan control. The same rationale applies to liquid fuel boilers with no air pollution controls or wet air pollution control systems and to hydrochloric acid production furnaces there is no dominant, controllable means that sources are using that can control dioxin/ furan emissions to a particular emission level. 139 Thus, best performer dioxin/ furan emissions are not replicable by the best performing sources ( or duplicable by other sources). For these sources, the predominant dioxin/ furan formation mechanism for other source categories   operating a fabric filter or electrostatic precipitator above 400F   is not a factor. Given that these sources are not using controllable means to control dioxin/ furan to a particular emission level, there is no assurance that the best performers can achieve in the future the emission level reported in the compliance test in our data base. Put another way, the test data do not reflect these sources' variability, and the variability is largely unquantifiable given the uncertainties regarding control mechanisms plus the environmental counter productiveness of encouraging use of higher sulfur coal. Hence, that reported emission level is not being " achieved" for the purpose of establishing a floor.

139 We note that the same rationale also applies to incinerators with wet or no air pollution control equipment and that are not equipped with a waste heat boiler. Finally, we note that beyond the floor controls such as activated carbon can control dioxin/ furan to a particular emission level. If a source were to install activated carbon, it could achieve the level demonstrated in a compliance test, after adjusting the level to account for emissions variability to ensure the measurement was replicable. The commenter argues that such a result is mandatory under the straight emissions approach ( the only way the commenter believes best performers can be determined). Doing so, however, would amount to a surreptitious beyond the floor standard ( forcing adoption of a control technology not used by any existing source), without considering the beyond the floor factors set out in section 112( d)( 2). In fact, we considered beyond the floor standards based on use of activated carbon for these sources   solid fuel boilers, liquid fuel boilers with wet or no emission control device, and hydrochloric acid production furnaces but rejected them for reasons of cost. The cost effectiveness ranged from $ 2.5 million to $ 4.9 million per gram TEQ of dioxin/ furan removed. In contrast, the cost effectiveness of the beyond the floor standard we promulgate for liquid fuel boilers equipped with dry emission control devices is $ 0.63 million per gram TEQ of dioxin/ furan removed. 140 Consequently, we are not promulgating a beyond the floor standard for dioxin/ furan for these sources, and do not believe we should adopt such a standard under the guise of determining floor levels. The Carbon Monoxide and Hydrocarbon Floors Are Appropriate MACT Floors. We explained at proposal why the carbon monoxide standard of 100 ppmv and the hydrocarbon standard of 10 ppmv are appropriate floors. See 69 FR at 21282. The floor level for carbon monoxide of 100 ppmv is a currently enforceable Federal standard. Although some sources are achieving carbon monoxide levels below 100 ppmv, it is not appropriate to establish a lower floor level because carbon monoxide is a conservative surrogate for organic HAP. Organic HAP emissions may or may not be substantial at carbon monoxide levels greater than 100 ppmv, and are extremely low when sources operate under the good combustion conditions required to achieve carbon monoxide levels in the range of zero to 100 ppmv. 141 ( See also the discussion below regarding the progression of hydrocarbon oxidation to carbon dioxide and water). As such, lowering the carbon monoxide floor below 100 ppmv may not provide significant reductions in organic HAP emissions. Moreover, it would be inappropriate to establish the floor blindly using a mathematical approach   the average emissions for the best performing sources because the best performing sources may not be able to replicate their emission levels ( and other sources may not be able to duplicate those emission levels) using the exact types of good combustion practices they used during the compliance test documented in our data base. This is because there are myriad factors that affect combustion efficiency and, subsequently, carbon monoxide emissions. Extremely low 140 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Sections 12, 13, and 15. 141 We note, however, that this general principle may not always apply. There are data that indicate that even though carbon monoxide levels are below 100 ppmv, hydrocarbon levels may not always be below 10 ppmv. See 64 FR at 52851 and Part Four, Section IV B. and C. of this preamble. An example of how this might occur, although not a likely practical scenario, is if combustion is quenched before substantial carbon monoxide can be generated, leaving unburned hydrocarbons in the stack gas. Because of this potential ( although unlikely) concern, the rule requires sources that elect to monitor carbon monoxide rather than hydrocarbons to conduct a one time test to document that hydrocarbons are below 10 ppmv and to establish operating limits on parameters that affect combustion conditions ( i. e., the same operating parameters that we use for compliance assurance with the DRE standard). See § 63.1206( b)( 6). carbon monoxide emissions cannot be assured by controlling only one or two operating parameters. We proposed a floor level for hydrocarbons of 10 ppmv even though the currently enforceable standard for boilers and hydrochloric acid production furnaces is 20 ppmv because: ( 1) although very few sources elect to comply with the RCRA standard for hydrocarbons rather than the standard for carbon monoxide, those that comply with the hydrocarbon standard have hydrocarbon levels well below 10 ppmv; and ( 2) reducing hydrocarbon emissions within the range of 20 ppmv to 10 ppmv may reduce emissions of organic HAP. Although all sources are likely to be achieving hydrocarbon levels below 10 ppmv, it is not appropriate to establish a lower floor level because hydrocarbons are a surrogate for organic HAP. Although total hydrocarbons would be reduced at a floor level below 10 ppmv, we do not know whether organic HAP would be reduced substantially. As combustion conditions improve and hydrocarbon levels decrease, the larger and easier to combust compounds are oxidized to form smaller compounds that are, in turn, oxidized to form carbon monoxide and water. As combustion continues, carbon monoxide is then oxidized to form carbon dioxide and water. Because carbon monoxide is a difficult todestroy refractory compound ( i. e., oxidation of carbon monoxide to carbon dioxide is the slowest and last step in the oxidation of hydrocarbons), it is a conservative surrogate for destruction of hydrocarbons, including organic HAP, as discussed above. As oxidation progresses and hydrocarbon levels decrease, the larger, heavier compounds are destroyed to form smaller, lighter compounds until ideally all hydrocarbons are oxidized to carbon monoxide ( and then carbon dioxide) and water. Consequently, the relationship between total hydrocarbons and organic HAP becomes weaker as total hydrocarbon levels decrease to form compounds that are not organic HAP, such as methane and acetylene. 142 Moreover, as discussed above for carbon monoxide, it would be inappropriate to establish the floor blindly using a mathematical approach   the average emissions for the best performing sources because the best performing sources may not be able to replicate their emission levels ( and other sources may not be able to duplicate those emission levels) using the exact types of good combustion practices they used during the compliance test documented in our data base. This is because there are myriad factors that affect combustion efficiency and, subsequently, hydrocarbon ( and carbon monoxide) emissions. Extremely low hydrocarbon emissions cannot be assured by controlling only one or two operating parameters. The Standards for CO and HC Are Not Work Practice Standards.. The floor standards for CO or HC for boilers and hydrochloric acid production furnaces are quantified emission limits. The standards consequently are not work practice standards ( even though they represent levels showing good combustion control). CAA section 302( k). EPA's reference to section 112( h)( 1) at proposal ( 69 FR at 21275) was consequently erroneous.

142 USEPA, Technical Support Document for HWC MACT Standards, Volume III: Selection of MACT Standards and Technologies, July 1999, Section 12.1.2. C. Use of Carbon Monoxide and Total Hydrocarbons as Surrogate for Non Dioxin Organic HAP143 Comment: A commenter states that neither the total hydrocarbon nor carbon monoxide standard alone provides adequate surrogate control for organic HAP. Accordingly, EPA must include standards for both. Hazardous waste combustors could have total hydrocarbon levels below the standard during the carbon monoxide compliance tests, but higher total hydrocarbon levels at other times during normal operation because there are many variables that can affect total hydrocarbon emissions, and these will not all be represented during the carbon monoxide compliance test. The commenter states that EPA is on record stating that carbon monoxide limits alone may not by itself minimize organic emissions because products of incomplete combustion can result from small pockets within the combustion zone where adequate time, temperature, turbulence and oxygen have not been provided to completely oxidize these organics. The commenter also states that EPA is on record stating that total hydrocarbon levels can exceed good combustion condition levels when carbon monoxide levels are below 100 ppmv. Response: The final rule requires compliance with destruction and removal efficiency and carbon monoxide or hydrocarbon standards as surrogates to control nondioxin organic HAP emissions144 from liquid fuel boilers, solid fuel boilers, and hydrochloric acid production furnaces. These are effective and reliable surrogates to control organic HAP. We conclude that simultaneous measurement of both total hydrocarbons and carbon monoxide with continuous emission monitors is not necessary because each serves as a reliable surrogate to control organic HAP emissions. The commenter has cited EPA preamble language that was included in the April 19, 1996 proposed rule for hazardous waste incinerators, cement kilns, and lightweight aggregate kilns. In that rule we proposed to require compliance with both the total hydrocarbon standard and the carbon monoxide standard. We requested comment on whether these requirements were redundant, and we later requested comment on whether we should allow sources to comply with either the carbon monoxide standard or the total hydrocarbon standard. We clarified, however, that allowing sources to comply with the carbon monoxide standard would be contingent on the source demonstrating compliance with the hydrocarbon standard during the compliance test. We believed this was necessary because we had limited data that showed a source could have total hydrocarbon levels exceeding 10 ppmv even though their carbon monoxide emission levels were below 100 ppmv. EPA subsequently promulgated this approach in the September 1999 Final Rule. 62 FR 52829. Today's rule adopts the same approach for liquid and solid fuel boilers and hydrochloric acid production furnaces. We again conclude that it is not necessary to require sources to verify compliance with both of these standards on a continuous basis with two separate continuous emission monitors, given the redundancy of these measurement techniques. Total hydrocarbon emission measurements are a more direct 143 As discussed in part two, section V, we view carbon monoxide, hydrocarbon, and destruction removal efficiency standards as unaffected by the Court's vacature of the September 1999 challenged regulations for incinerators, cement kilns, and lightweight aggregate kilns. We are therefore not re promulgating and did not reconsider these standards in today's final rule for these source categories. 144 As discussed in the previous section, these standards are also used as surrogates to control dioxin/ furans for hydrochloric acid production furnaces, solid fuel fired boilers, and liquid fuel fired boilers that are not equipped with dry air pollution control devices. indicator of organic HAP emissions than carbon monoxide. Hence, continuous compliance with this standard always assures that organic HAP are well controlled. Carbon monoxide is a conservative indicator of combustion efficiency because it is a product of incomplete combustion and because it is a refractory compound that is more thermally stable than hydrocarbons. The hydrocarbon products of incomplete combustion that are simultaneously formed during incomplete, or inefficient, combustion conditions can be subsequently oxidized later in the combustion process. In such instances carbon monoxide will likely still be prevalent in the exhaust gas even though the products of incomplete combustion were later oxidized. The conservative nature of carbon monoxide as an indicator of good combustion practices is supported by our data. At carbon monoxide levels less than 100 ppmv, our data indicates that there is no apparent relationship between carbon monoxide and hydrocarbons ( other than that hydrocarbon levels are generally below 10 ppm when carbon monoxide levels are below 100 ppm). For example, a source with a carbon monoxide level of 1 ppm is no more likely to have lower measured hydrocarbons than a source achieving a carbon monoxide emission level of 100 ppm. 145 We consider the few instances where the data showed total hydrocarbon levels above 10 ppmv while carbon monoxide levels are below 100 ppmv to be anomalies. Even so, we have accounted for this by requiring compliance with the hydrocarbon standard during the compliance test if a source elects to comply with the carbon monoxide standard. See § § § 63.1216( a)( 5)( i), 1217( a)( 5)( i), and 1218( a)( 5)( i). We disagree with the commenter's assertion that the total hydrocarbon compliance demonstration during the compliance test is insufficient. Sources are required to establish numerous operating requirements based on operating levels that were demonstrated during the test, including minimum operating temperature, maximum feed rates, minimum combustion zone residence time, and operating requirements on the hazardous waste firing system that control liquid waste atomization efficiency. Sources must comply with these operating requirements on a continuous basis. Compliance with these requirements, in addition to the requirements to comply with the carbon monoxide and destruction and removal standards, adequately assure sources are controlling organic HAP emissions to MACT levels. Comment: A commenter states that EPA's proposed use of surrogates for organic HAP do not ensure that each of the organic HAP ( e. g., polychlorinated biphenyls and polyaromatic hydrocarbons) are reduced to the level of the HAP emitted by the relevant best performing sources. EPA has not shown the necessary correlation between either the total hydrocarbon or carbon monoxide standards and organic HAP, and neither is a reasonable surrogate according to the commenter. Response: Carbon monoxide and total hydrocarbon monitoring are widely used and accepted indicators of combustion efficiency, and hence control organic HAP, which are destroyed by combustion. 146 Sources that are achieving carbon monoxide of emission levels of 100 ppm or a hydrocarbon emission levels of 10 ppm are known to be operating 145 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 3.2 and USEPA, " Final Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards and Technologies," July 1999, Section 5.1. 146 This is why almost all of the RCRA Land Disposal Restiction treatment standards for organic waste, which standards are for the most part established at an analytic detection level for the organic HAP in question plus a variability factor, are based on the performance of combustion technology. See 40 CFR Part 268.40 43. pursuant to good combustion practices. This is supported by an extensive data analysis we used to support identical standards for incinerators, cement kilns, and lightweight kilns which were promulgated in the September 1999 Final Rule. We are applying the same rationale to support these standards for boilers and hydrochloric acid production furnaces. Today's rule requires continuous compliance with either a carbon monoxide and hydrocarbon standard, in combination with a destruction and removal efficiency standard, as surrogates to control organic HAP. We conclude that sources which comply with these standards are operating under efficient combustion conditions, assuring non dioxin organic HAP are being oxidized, thus limiting emissions to levels reflecting MACT. Efficient combustion of hazardous waste minimizes emissions of organic HAP that are fed to the combustion chamber as well as emissions attributable to products of incomplete combustion that may form within the combustion chamber or post combustion. We are not capable of issuing emission standards for each organic HAP because of data limitations and because such emission standards may not be replicable by individual sources or duplicable by the other best performing sources because of the complex nature of combustion and post combustion formation of products of incomplete combustion.

V. Additional Issues Relating to Variability and Statistics Many commenters raised issues relating to emissions variability and statistics other than those discussed above in Section III. A: ( 1) variability dampening for data sets containing nondetects; ( 2) imputation of variability to address variability dampening for data sets containing nondetects; and ( 3) our analysis of variance procedures to identify subcategories. We present comments and responses on the remaining topics below.

A. Data Sets Containing Nondetects Comment: One commenter states that EPA's approach of assuming measurements that are below detection limits are present at the detection limit dampens the variability of the data set. Thus, the variability of ranking parameters is understated when ranking sources to identify the best performers and emissions variability is understated when calculating the floor. Response: We agree with the commenter. For the final rule, we use an approach to address nondetects whereby a value is assigned to each nondetect within its possible range such that the 99th percentile upper prediction limit for the data set ( i. e., test condition runs for each source) is maximized. Although this approach maximizes the deviation among runs containing nondetect measurements, the test condition average is lower because we no longer assume the nondetect analyte is present at the level of detection. See response to comments discussion below for more information on this statistical approach to address variability of nondetects. We use this measurement imputation approach to address variability of feedrate data sets containing nondetects for source ranking purposes and to address variability of emissions data sets containing nondetects when calculating floors. We do not apply the measurement implementation approach to system removal efficiency ( SRE) data sets where feedrates or emissions contain nondetects, however. Statistical imputation of nondetect SREs is complicated given that SRE is derived from feedrate and emissions data, both of which could contain nondetect measurements. 147 Our inability to apply the imputation approach to SREs is not a major concern, however, because system removal efficiency is used as a source ranking criterion only ( i. e., it is not used as the standard, except for hydrochloric acid production furnaces where there are no nondetect feedrate or emissions measurements), and there are few instances where system removal efficiencies are derived from nondetect feedrate or emissions data.

B. Using Statistical Imputation to Address Variability of Nondetect Values On February 4, 2005, EPA distributed by email to major commenters on the proposed rule a direct request for comments on a limited number of issues that were raised by the public comments on the proposed rule. The nondetect measurement imputation approach discussed above was one of the issues for which we requested comment. We discuss below the major comments on the approach. Comment: Most commenters state that they agree with either the concept or the approach in principle but cannot provide substantive comments. These commenters indicate they cannot provide substantive comments because they cannot determine the implications of using the approach given that we did not provide the resulting floor calculations. One commenter suggests that, before blindly applying this arbitrary estimate of a nondetect value, a reality check should be done to validate that this is reasonable by consulting what is published on the method variability, as well as by checking variability factors derived for other data in the database that are above the detection limit. Another commenter voiced significant concerns with the approach. The commenter states that EPA contradicts its assumption at proposal that all data that are reported as nondetect are present at the detection limits by now admitting that the true value is between zero and the level of detection. The commenter concludes that EPA now proposes to retreat from its assumption that undetected pollutants are always present at the detection limits not because that assumption is false but because it does not generate sufficiently lenient floors. The commenter believes that this underscores that EPA's statistical analysis approach cannot possibly give an accurate picture of any source's actual emission levels. Accordingly, it cannot possibly satisfy EPA's obligation to ensure that its floors reflect the average emission levels achieved by the relevant best performing sources. The commenter also states that EPA's imputation approach is independently flawed because it assumes again inaccurately that the value for a nondetect is always either the highest value or lowest value in the allowable range. In reality the undetected values will necessarily fall in a range between the highest and lowest, and thus yield less variability than EPA would assume. Response: We agree in theory with the commenter who suggests that the results of the imputation approach should be checked to see if it overstates variability for nondetect data by comparing the results of the imputation approach with the actual variability for detected measurements in the data set. We considered comparing the relative standard deviation derived from the imputation approach for data sets with nondetects, to the relative standard deviation for the data set using a regression analysis. Under the regression analysis approach, we considered relating the relative standard deviation of detected data sets to the 147 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005 Section 7.3. average measurement. We would determine this relationship for each standard for which we have nondetect data, and use the relationship to impute the standard deviation for a data set containing nondetects. 148 We could not perform this analysis, however, because: ( 1) we have very few detected measurements for the data sets for several standards and could not establish the relationship between relative standard deviation and emission concentration for those data sets; and ( 2) moreover, for many data sets where detected measurements would have been adequate to establish the relationship, it would have been problematic statistically to extrapolate the relationship to the very low values assigned to the nondetect measurements ( e. g., 100% of the detection limit; the value assigned by our statistical imputation approach). 149 This commenter also suggests that we check the resultant standard deviation after imputation by consulting what is published on the method variability. The commenter did not explain, however, how method variability relates to the variability of nondetect data. Moreover, we believe that the imputation approach is one approach we could have reasonably used to estimate variability of nondetect data. We first attempted to apply standard statistical techniques to address the nondetect issue. We investigated standard interval censoring techniques to calculate maximum likelihood estimates ( MLE) of the average and standard deviation that provide the best fit for a normal distribution for the data containing nondetect values, taking into account that each nondetect data point can be anywhere within its allowable interval. These techniques are not applicable, however, to data sets where all data are nondetects, as is the case for many of our data sets. In that situation, we approximated the mean as the average of the midpoints of the nondetect intervals, and the standard deviation as one half of the possible range of the data. After working with this MLE/ Approximation approach for some time and iteratively developing complicated algorithms to address problems as they arose, we concluded that we needed a simpler approach that could be applied to all data sets. Accordingly, we developed the statistical imputation approach discussed in Section IV. A above. For 22 separate floors, we compared the results of the approaches we considered for nondetects: ( 1) nondetects present at the detection limit ( i. e., full detection limit approach); ( 2) MLE; ( 3) MLE combined with an approximation approach ( i. e., MLE/ Approximation approach; and ( 4) statistical imputation. 150 The MLE approach was only applicable to 2 of the 22 floor data sets, and the numerical algorithm failed to converge on an answer for one of those. The MLE/ Approximation approach sometimes results in floors that are unrealistically high ( i. e., it calculated 5 of 22 floors that were higher than the statistical imputation approach, which always produces floors that are equal to or higher than assuming nondetects are present at the full detection limit), and sometimes fails to converge on an answer. Because of these limitations, we do not use either the MLE or MLE/ Approximation approach.

148 Note that, under this approach, we would continue to assume that the nondetect analyte is present at the detection limit. 149 Note that this was not the case where we use a regression analysis of relative standard deviation versus total chlorine measurements to impute a standard deviation for values below 20 ppmv that we corrected to 20 ppmv to address the low bias of Method 0050. In that situation, we have several total chlorine measurements very close to 20 ppmv. 150 See USEPA, " Technical Support Document for the HWC MACT Standards, Volume III: Selection of MACT Standards," September 2005, Section 5.4. We believe the statistical imputation approach is preferable to the full detection limit approach because it: ( 1) accounts for variability of data sets containing nondetects; ( 2) can be applied to all data sets containing nondetects; and ( 3) results in reasonable floor levels. In most cases, floors calculated using statistical imputation are close to those calculated by the full detection limit approach. The statistical imputation approach can produce substantially higher floors than the full detection limit approach, however, when a relatively high nondetect is reported because of a high detection limit. Nonetheless, the statistical imputation approach calculated floors that were 30% higher than the full detection limit approach for only 2 of the 22 floors. We reject the comment that our approach to handling nondetect data is a mere manipulation to raise the floor. The commenter observes that EPA appears to determine that its initial approach of assuming the worst case for nondetect data   that the data are present at the detection limit   did not produce floors that were high enough, and consequently applies another manipulation   statistical imputation of nondetect measurements   that assumes the nondetect data are present at lower levels but nonetheless generates floors that are even higher than before. Although the commenter is correct about the outcome of our handling of nondetect data   the floors are generally higher after statistically imputing nondetect measurements than if nondetects are simply assumed to be present at the detection limit   our rationale for handling nondetects is sound. At proposal, we assumed that nondetects are present at the detection limit. We do not know ( nor does anyone else) whether a nondetect value is actually present at 1% or 99% of the detection limit. We thought that assuming that all values were at the limit of detection would reasonably estimate the range of performance a source could experience for these nondetect measurements. This approach inherently maximizes the average emissions but minimizes emissions variability. Commenters on the proposed rule state that assuming nondetects are present at the detection limit dampens emissions variability   a consideration necessary to ensure that a source's performance over time is estimated reasonably. Mossville, 370 F. 3d at 1242 ( daily maximum variability must be accounted for in MACT standards [ including floors] which must be achieved continuously). See also CMA, 870 F. 2d at 232 ( EPA not even obligated to use data from plants that consistently reported nondetected values in calculating variability factors for best performing plants). We agree with these commenters, and are using the statistical imputation approach to address the concern. Relative to our proposed approach of assuming nondetect measurements are present at the detection limit, the statistical imputation approach reduces the average of the data set for a source while maximizing the deviation of the data set. These are competing and somewhat offsetting factors when calculating the floor for existing sources given that we use a modified 99th percentile upper prediction limit to calculate the floor   the floor is the average of the test condition averages for the best performers plus the pooled variance of their runs. See CMA, 870 F. 2d at 232 ( upholding approach to variability for datasets with nondetect values where various conservative assumptions in methodology offset less conservative assumptions). We further disagree with this commenter's view that the statistical imputation approach is independently flawed because it assumes that the value for a nondetect is always either the highest value or lowest value in the allowable range. The commenter states that, in reality, the undetected values will necessarily fall in a range between the highest and lowest, and thus yield less variability than EPA would assume. Although the commenter is correct that the true value of a nondetect measurement is likely to be in the range between the highest or lowest value possible rather than at either extreme, we do not know where the true value is within that range. To ensure that variability is adequately considered in establishing a floor, the statistical imputation approach, by design, maximizes the deviation by assuming the nondetect value is at one end of the range or the other, whichever results in a higher average for the data set.