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United States Court of Appeals

FOR THE DISTRICT OF COLUMBIA CIRCUIT

Argued November 13, 2003 Decided February 24, 2004

No. 01-1053

NORTHEAST MARYLAND WASTE DISPOSAL AUTHORITY,

PETITIONER

v.

ENVIRONMENTAL PROTECTION AGENCY,

RESPONDENT

Consolidated with

01-1054, 01-1055, 02-1280, 02-1299, 03-1093

On Petitions for Review of an Order of the

Environmental Protection Agency

Timothy R. Henderson argued the cause for petitioners

Northeast Maryland Waste Disposal Authority, et al. With

him on the briefs was Warren K. Rich.

 Bills of costs must be filed within 14 days after entry of judgment.

The court looks with disfavor upon motions to file bills of costs out

of time.
2

James S. Pew argued the cause and filed the briefs for

petitioners Sierra Club and New York Public Interest Research Group.

H. Michael Semler and Stephen E. Crowley, Attorneys,

U.S. Department of Justice, argued the cause and filed the

brief for respondents.

Before: SENTELLE, HENDERSON and GARLAND, Circuit

Judges.

Opinion for the court Per Curiam.

Per Curiam: This action challenges the Emission Guidelines for Existing Small Municipal Waste Combustion Units,

65 Fed. Reg. 76,378 (Dec. 6, 2000), and the New Source

Performance Standards for New Small Municipal Waste

Combustion Units, 65 Fed. Reg. 76,350 (Dec. 6, 2000), promulgated by the United States Environmental Protection

Agency (EPA, Agency) pursuant to § 129 of the Clean Air

Act (CAA), 42 U.S.C. § 7429.1

 The petitioners include three

members of the municipal waste combustor industry (Industry Petitioners): Northeast Maryland Waste Disposal Authority (Northeast Maryland), which operates four municipal

waste combustor (MWC) units in Harford County, Maryland;2

Dutchess County Resource Recovery Agency, which operates

1 Under the regulatory scheme, for existing units EPA promulgates ‘‘emission guidelines’’ which ‘‘do not directly regulate any

MWC units, but TTT require States to develop plans to limit air

emissions from existing small MWC units.’’ 65 Fed. Reg. at 76,379.

For new units, by contrast, EPA promulgates ‘‘new source performance standards’’ with which MWC operators must directly comply.

See id. Both the new source performance standards and the

emission guidelines for existing sources are commonly referred to

as ‘‘standards.’’

2 Waste Energy Partners (WEP), a one-time owner of the Harford County, Maryland facility, originally filed this action. On July

1, 2002, however, Northeast Maryland acquired WEP’s ownership

interest in the Harford facility. Accordingly, on August 14, 2002,

we added Northeast Maryland as a party to WEP’s petition in No.

01–1053, and on November 4, 2003, we granted WEP’s motion to

withdraw from these proceedings.
3

two MWC units at a facility in Poughkeepsie, New York and

Islip Resource Recovery Agency, which operates two MWC

units at a facility in Islip, New York. The petitioners also

include two environmental organizations: the New York Public Interest Research Group (NYPIRG) and the Sierra Club

(collectively identified as Sierra Club). For the reasons set

out below, we grant the petitions in part and deny the

petitions in part.

I.

The challenged rulemaking is now in its third decade. In

1987 EPA issued an advance notice of a rulemaking to

regulate pollutants produced by MWC emissions pursuant to

§ 111 of the CAA, 42 U.S.C. § 7411, which requires EPA to

develop emission standards generally for each category of

pollutant EPA determines ‘‘causes, or contributes significantly to, air pollution which may reasonably be anticipated to

endanger public health or welfare,’’ 42 U.S.C. § 7411(b)(1)(A).

See Assessment of Municipal Waste Combustor Emissions

Under the Clean Air Act, 52 Fed. Reg. 25,399, 25,399 (July 7,

1987). In 1989 EPA issued proposed emission regulations

imposing limits on the MWC emission levels for specific

pollutants, based on the level of emissions achievable with the

best pollution control technology, but did not prescribe specific control technologies to be used to achieve the limits. See

Standards of Performance for New Stationary Sources; Municipal Waste Combustors, 54 Fed. Reg. 52,251 (Dec. 20,

1989).

In 1990 the Congress enacted CAA § 129, 42 U.S.C.

§ 7429, which expressly requires EPA to establish specific

standards for each ‘‘solid waste incineration unit.’’3

 The

standards must ‘‘reflect the maximum degree of reduction in

emissions of air pollutants listed under section (a)(4) that

[EPA], taking into consideration the cost of achieving such

3 The statute defines a ‘‘solid waste incineration unit’’ as ‘‘a

distinct operating unit of any facility which combusts any solid

waste material from commercial or industrial establishments or the

general public.’’ 42 U.S.C. § 7429(g)(1).
4

emission reduction, and any non-air quality health and environmental impacts and energy requirements, determines is

achievable for new or existing units in each category.’’ Id.

§ 7429(a)(2).4

 These standards are known as ‘‘maximum

achievable control technology’’ or ‘‘MACT’’ standards. The

statute limits EPA’s discretion to determine the stringency of

MACT standards. MACT standards must be at least as

stringent as the MACT floor set for each pollutant. The

MACT floor for new units is defined as ‘‘the emissions control

TTT achieved in practice by the best controlled similar unit.’’

Id. The MACT floor for existing units is defined as ‘‘the

average emissions limitation achieved by the best performing

12 percent of units in the category.’’ Id. The statute mandates two ‘‘categories’’ within both existing and new units

(defined in terms of combustion capacity), with different

deadlines for promulgating standards, id. § 7429(a)(1)(B)-(C),

and further provides that EPA ‘‘may distinguish among

classes, types, TTT and sizes of units within a category in

establishing [MACT] standards,’’ id. § 7429(a)(2).

In 1994 EPA proposed new standards governing MWC

units pursuant to § 129. See Standards of Performance for

New Stationary Sources: Municipal Waste Combustors, 59

Fed. Reg. 48,198 (Sept. 20, 1994). The Agency proposed

distinct sets of standards for new and for existing sources, as

the statute contemplates, and broke down both source types

4 Subsection (a)(4) of § 129 provides:

The performance standards promulgated under TTT this section

and applicable to solid waste incineration units shall specify

numerical emission limitations for the following substances or

mixtures: particulate matter (total and fine), opacity (as appropriate), sulfur dioxide, hydrogen chloride, oxides of nitrogen,

carbon monoxide, lead, cadmium, mercury, and dioxins and

dibenzofurans. The Administrator may promulgate numerical

emissions limitations or provide for the monitoring of postcombustion concentrations of surrogate substances, parameters or

periods of residence time in excess of stated temperatures with

respect to pollutants other than those listed in this paragraph.

42 U.S.C. § 7429(a)(4).
5

into two categories based on the aggregate plant capacity for

municipal solid waste (MSW), that is, based on the sum of the

maximum amount of waste each MWC unit located at a

particular site is designed to combust daily. Thus, within

both existing and new source types, EPA created a large unit

category — consisting of units located at plants with an

aggregate MSW capacity greater than 250 tons per day

(tpd) — and a small unit category — consisting of units

located at plants with an aggregate MSW capacity of 250 tpd

or less (but greater than 35 tpd).

In 1995 EPA issued its final standards, which generally

tracked the proposed ones. See Standards of Performance

for New Stationary Sources and Emission Guidelines for

Existing Sources: Municipal Waste Combustors, 60 Fed.

Reg. 65,387 (Dec. 19, 1995) (1995 Rule). Two MWC facility

operators petitioned this court to review the 1995 Rule,

asserting that EPA violated § 129’s unambiguous language

when it defined large and small units based on the aggregate

MSW combustion capacity of the plant at which a MWC unit

is located rather than on the combustion capacity of the

individual MWC unit itself. We agreed with the petitioners

and vacated the standards, holding that ‘‘the EPA’s use of

aggregate plant MSW capacity rather than unit MSW capacity in the 1995 standards to create categories of MWC units

for MACT purposes violates the plain meaning of section 129

and exceeds the EPA’s statutory authority.’’ Davis County

Solid Waste Mgmt. v. EPA, 101 F.3d 1395, 1411 (D.C. Cir.

1996). Subsequently, on EPA’s motion for rehearing, the

court modified the remedy to vacate only the small unit

standards because it concluded ‘‘the Davis opinion will not

meaningfully alter the [new source performance standards] or

the emission guidelines applicable to [existing] large units and

that vacating the large unit standards will have a significant

deleterious effect.’’ Davis County Solid Waste Mgmt. v.

EPA, 108 F.3d 1454, 1460 (D.C. Cir. 1997) (rehearing).

In August 1999 EPA proposed new standards for the

category of small MWC units, which it defined as units ‘‘with

a combustion design capacity of 35 to 250 tons per day.’’
6

Emission Guidelines for Existing Stationary Sources: Small

Municipal Waste Combustion Units, 64 Fed. Reg. 47,234,

47,236 (Aug. 30, 1999).5

 Existing small units were further

divided into three subcategories according to type and aggregate plant capacity: Class A, consisting of ‘‘nonrefractorytype small MWC units located at plants with an aggregate

plant capacity greater than 250 tons per day of MSW’’; Class

B, consisting of ‘‘refractory-type small MWC units located at

plants with an aggregate plant capacity greater than 250 tons

per day of MSW’’;6

 and Class C, consisting of all ‘‘small

MWC units located at plants with an aggregate plant capacity

less than or equal to 250 tons per day of MSW.’’ Id. New

small units were divided into only two subcategories, strictly

by aggregate plant capacity: Class I, consisting of small units

located at plants with aggregate plant capacities greater than

250 tons of MSW per day, and Class II, consisting of small

units located at plants with aggregate plant capacities less

than or equal to 250 tons of MSW per day. New Source

Performance Standards for New Small Municipal Waste

Combustion Units, 64 Fed. Reg. 47,276, 47,279 (Aug. 30,

1999).

Following comment and hearing, in December 2000 EPA

issued its final standards, which established subcategories by

aggregate plant capacity alone both for existing units, 65 Fed.

Reg. 76,378, and for new units, 65 Fed. Reg. 76,350, (collectively, the 2000 Rule). For both existing and new units, Class

I consists of small MWC units located at plants with aggregate plant capacities greater than 250 tons of MSW per day,

5 EPA has not yet promulgated standards to regulate units with a

design capacity of 35 tpd or less, although a consent decree entered

in Sierra Club v. Whitman, No. 01–1537 (D.D.C. filed July 16,

2001), requires it to do so by November 30, 2005. Because such

units are not regulated, hereinafter our references (regarding both

the proposed and the final rule) to units with a combustion capacity

of equal to or less than 250 tpd refer to MWC units with MSW

capacity between 35 tpd and 250 tpd.

6 A ‘‘refractory type’’ MWC unit is one ‘‘that has no energy

recovery (such as through a waterwall) in the furnace of the

municipal waste combustion unit.’’ 64 Fed. Reg. at 47,262.
7

while Class II comprises small MWC units located at plants

with aggregate plant capacities equal to or less than 250 tons

of MSW per day. 65 Fed. Reg. at 76,379 (existing small

units); 65 Fed. Reg. at 76,351 (new small units). Within each

subcategory EPA calculated a MACT floor for each pollutant

and set a standard at or beyond the floor.

On February 2, 2001 Waste Energy Partners, together

with other parties to the administrative proceeding, petitioned

EPA for reconsideration, and all Industry Petitioners filed

petitions for review of the final standards with the court. On

February 5, 2001 NYPIRG filed a petition for administrative

reconsideration, and on February 6, 2001 Sierra Club filed a

petition for judicial review of the standards. EPA denied

WEP’s petition for reconsideration on August 7, 2002, J.A.

2317, and denied NYPIRG’s petition on August 14, 2002, J.A.

2319.

II.

Under § 307(d)(9) of the CAA, the court reviews EPA

action as follows:

In the case of review of any action of the Administrator

to which this subsection applies, the court may reverse

any such action found to be —

(A) arbitrary, capricious, an abuse of discretion, or

otherwise not in accordance with law;

(B) contrary to constitutional right, power, privilege,

or immunity;

(C) in excess of statutory jurisdiction, authority, or

limitations, or short of statutory right; or

(D) without observance of procedure required by law,

if (i) such failure to observe such procedure is arbitrary or capricious, (ii) the requirement of paragraph

(7)(B) has been met, and (iii) the condition of the last

sentence of paragraph (8) is met.7

7 Paragraph 7(B) provides: ‘‘Only an objection to a rule or

procedure which was raised with reasonable specificity during the

period for public comment (including any public hearing) may be
8

42 U.S.C. § 7607(d)(9). We apply this standard of review

seriatim to each of the petitioners’ challenges to EPA’s

standards.

A. Industry Challenges

We begin with the challenges raised by Industry Petitioners. Each of these petitioners owns and operates small

MWC units — i.e., units with capacities equal to or less than

250 tpd. Because each of these small MWC units is located

at a plant with multiple units, the aggregate capacity of which

exceeds 250 tpd, each is classified as a Class I facility under

the 2000 Rule. Industry Petitioners challenge the emission

limits set by the 2000 Rule for existing units on both substantive and procedural grounds. We consider those challenges

below.

1. Substantive Challenges

Industry Petitioners raise two substantive challenges to the

2000 Rule. First, they contend that § 129(a)(2) of the Clean

Air Act requires EPA to establish one MACT floor for all

existing units within the small unit category, and that the

Agency therefore exceeded its statutory authority by establishing different MACT floors for subcategories of units (i.e.,

Class I and Class II units). Second, Industry Petitioners

argue that, even if EPA may subcategorize when setting

MACT floors, the Act does not permit it to do so on the basis

of aggregate plant capacity. The consequence of this unlawful subcategorization, they protest, is that their Class I units

are subjected to more stringent standards than they would be

raised during judicial review.’’ 42 U.S.C. § 7607(d)(7)(B). The

final sentence of paragraph 8 provides: ‘‘In reviewing alleged

procedural errors, the court may invalidate the rule only if the

errors were so serious and related to matters of such central

relevance to the rule that there is a substantial likelihood that the

rule would have been significantly changed if such errors had not

been made.’’ Id. § 7607(d)(8).
9

if MACT floors were instead calculated on a category-wide

basis.8

a. Subcategorization

Both Industry Petitioners and EPA contend that our opinion in Davis County Solid Waste Management v. EPA, 101

F.3d 1395 (D.C. Cir. 1996), governs the question of subcategorization authority — although each side draws a different

lesson from that case. Accordingly, we begin with a brief

recap of Davis.

As noted above, EPA promulgated an earlier round of

standards to regulate municipal waste combustion in 1995.

See 60 Fed. Reg. 65,387. Unlike the 2000 Rule, which applies

only to the category of small (250 tpd or less) MWC units and

which subcategorizes that category based on aggregate plant

capacity, the 1995 Rule categorized units based on aggregate

plant capacity. As a consequence, the 1995 Rule grouped a

number of small MWC units with individual capacities of less

than 250 tpd into the same category as large units with

individual capacities greater than 250 tpd, because those

small units were located at facilities with aggregate capacities

greater than 250 tpd. Id. In Davis, we found the 1995 Rule

unlawful, concluding that the Clean Air Act created two

8 This is so, Industry Petitioners contend, because Class I units

have more efficient pollution control systems than the smaller Class

II units. By creating a subcategory of better-performing units, and

calculating the MACT floor on that basis, the resulting standard is

more stringent than it would be if EPA grouped all MWCs in the

small MWC unit category together and calculated one MACT floor

for all of them. This is especially true, petitioners continue, because if EPA were to establish one MACT floor for all small MWC

units, the Agency would have to include in its calculation ‘‘very

small’’ units — those with capacities of less than 35 tpd — a group

that is currently unregulated. See supra note 5. On the other

hand, were EPA to adopt Industry Petitioners’ view, Class II units

as well as below-35 tpd units would be subject to more stringent

controls than they are under the 2000 Rule. Respondent’s Br. at

25–26. Indeed, according to EPA, a MACT floor applicable to all

small MWCs would be stricter than most Class II units and below35 tpd units could feasibly achieve. Id. at 26 & n.29.
10

separate ‘‘categories of MWC units based on unit capacity,

units with unit MSW capacities above 250 tons/day and units

with unit MSW capacities of 250 tons/day or less.’’ 101 F.3d

at 1410. In support, we relied on the fact that § 129(a)(1)

imposed ‘‘different dates by which the standards for large and

small MWC units must be promulgated,’’ and that it therefore

‘‘separately define[d] these two types of MWC units.’’ Id. at

1403.9

Although the only question in Davis was the lawfulness of

including both large and small units within the same category, the opinion contains dicta upon which each side has seized

regarding the question of subcategorization. EPA focuses on

the Davis court’s suggestion — repeated four times in the

opinion — that the Agency may ‘‘exercise[ ] its discretion to

distinguish among units within a category and create[ ] subcategories of small units, for which it can then calculate MACT

floors and standards separately.’’ Id. at 1408; see also id. at

1404–05, 1405 n.11, 1409 n.12, 1411. EPA reads this statement as advising that, while the Agency is obligated to

categorize MWCs based on unit capacity, it remains free to

subcategorize the small unit category based on other factors.

See Respondent’s Br. at 27; see also 64 Fed. Reg. at 47,237

(quoting passage from Davis and concluding that the court’s

decision allows EPA to exercise its discretion to set MACT

floors based on subcategories of small units). In opposition,

Industry Petitioners maintain — ‘‘[w]ith all due respect’’ to

the Davis court — that the above-quoted material ‘‘is directly

at odds with’’ other sentences in the same opinion. Reply Br.

at 5. In particular, petitioners rely on Davis’ statement that,

‘‘in order to promulgate emissions standards, the EPA must

first calculate the MACT floors, and the EPA cannot calculate

the MACT floors until it has studied the emissions levels of

all units in the relevant category,’’ as barring EPA from

9 Under 42 U.S.C. § 7429(a)(1), EPA was to promulgate standards for MWC units with capacities of more than 250 tpd by

November 15, 1991, but did not have to promulgate standards for

MWC units with capacities of 250 tpd or less until November 15,

1992.
11

calculating MACT floors based on anything other than a

category-wide basis. Davis, 101 F.3d at 1404.

Given that Davis — which did not involve subcategorization

within a category at all, but rather an attempt by the Agency

to collapse two statutory categories into one — contains no

holding on the subcategorization question at issue here, we

see little to be gained by striving to reconcile its dicta.

Instead, we look directly to the relevant statutory language in

order to determine whether EPA’s action was authorized.

That language is contained in § 129(a)(2) of the Clean Air

Act, which, with sentence numbers inserted for ease of subsequent discussion, states as follows:

Emissions standard

[1] Standards applicable to solid waste incineration

units promulgated under TTT this section shall reflect the

maximum degree of reduction in emissions of [listed air

pollutants] that the Administrator, taking into consideration the cost of achieving such emission reduction, and

any non-air quality health and environmental impacts

and energy requirements, determines is achievable for

new or existing units in each category. [2] The Administrator may distinguish among classes, types (including

mass-burn, refuse-derived fuel, modular and other types

of units), and sizes of units within a category in establishing such standards. [3] The degree of reduction in

emissions that is deemed achievable for new units in a

category shall not be less stringent than the emissions

control that is achieved in practice by the best controlled

similar unit, as determined by the Administrator. [4]

Emissions standards for existing units in a category may

be less stringent than standards for new units in the

same category but shall not be less stringent than the

average emissions limitation achieved by the best performing 12 percent of units in the categoryTTTT

42 U.S.C. § 7429(a)(2). In brief summary: § 129(a)(2)’s first

sentence directs EPA to set overall emission standards that

(inter alia) reflect the maximum degree of achievable emissions reduction (‘‘beyond-the-floor’’ MACT levels); the second
12

sentence grants EPA discretion to distinguish among units

within a category in establishing emission standards; the

third sentence instructs EPA that emission standards for new

units must ‘‘not be less stringent than’’ a specified level (the

new-unit MACT ‘‘floor’’); and the fourth sentence instructs

the Agency that emission standards for existing units must

‘‘not be less stringent than’’ a (different) specified level (the

existing-unit MACT ‘‘floor’’).

Chevron U.S.A. Inc. v. Natural Resources Defense Council,

Inc., 467 U.S. 837 (1984), governs our review of Industry

Petitioners’ claim that the 2000 Rule conflicts with

§ 129(a)(2). As the Supreme Court has recently explained,

under Chevron ‘‘we must decide (1) whether the statute

unambiguously forbids the Agency’s interpretation, and, if

not, (2) whether the interpretation, for other reasons, exceeds

the bounds of the permissible.’’ Barnhart v. Walton, 535

U.S. 212, 218 (2002). Industry Petitioners contend that the

2000 Rule cannot survive the first step of the Chevron

inquiry. We disagree, concluding that the Rule survives both

steps of Chevron because the statutory language is ambiguous, and the Agency’s interpretation is reasonable. See

Barnhart v. Thomas, 124 S. Ct. 376, 380 (2003).

In support of its Chevron argument, Industry Petitioners

focus on § 129(a)(2)’s fourth sentence. That sentence states

that emissions limitations for existing units ‘‘shall not be less

stringent than the average emissions limitation achieved by

the best performing 12 percent of units in the category.’’ 42

U.S.C. § 7429(a)(2) (emphasis added). By its plain terms,

petitioners insist, this provision requires EPA to calculate one

MACT floor for all existing units in the small MWC category.

The problem with this argument is that it reads the fourth

sentence of § 129(a)(2) in isolation, as if it were the only

sentence in the section rather than the final sentence of four.

As the Supreme Court has instructed, ‘‘the words of a statute

must be read in their context and with a view to their place in

the overall statutory scheme.’’ Davis v. Michigan Dep’t of

Treasury, 489 U.S. 803, 809 (1989); see National R.R. Passenger Corp. v. Boston & Maine Corp., 503 U.S. 407, 417
13

(1992). And when we follow that instruction and look — as

EPA does — at the second sentence of § 129(a)(2), we find

express authorization for the Agency to ‘‘distinguish among

classes, types TTT, and sizes of units within a category in

establishing such standards.’’ 42 U.S.C. § 7429(a)(2) (emphasis added).

Petitioners contend that EPA cannot rely on the second

sentence of § 129(a)(2) for authorization because, in their

view, that sentence permits subcategorization only after

MACT floors are calculated. That is so, they argue, because

the second sentence refers to subcategorization in the setting

of ‘‘such standards,’’ which petitioners read as pertaining only

to the beyond-the-floor levels described in the bulk of the

section’s first sentence. On that reading, subcategorization is

permissible in setting beyond-the-floor levels, but not in

setting the floors themselves.

But Industry Petitioners’ reading is not the only reasonable

way to read § 129(a)(2). It is, of course, possible that the

second sentence’s use of the phrase ‘‘such standards’’ refers

to the first sentence as a whole. But another valid reading is

that ‘‘such standards’’ refers merely to the opening phrase of

the first sentence: ‘‘standards applicable to solid waste incineration promulgated under TTT this section.’’ And that

phrase can be read as encompassing both the beyond-thefloor requirements of the balance of the first sentence and the

floor requirements of the third and fourth sentences. Read

in this fashion, the second sentence authorizes the Agency to

distinguish among units ‘‘within a category’’ during all stages

of the MACT standard-setting process described in the section.

The order of the sentences in § 129(a)(2) further supports

EPA’s view that the second sentence does not only authorize

subcategorization after MACT floors are established. Indeed, the second sentence, which expressly permits subcategorization, precedes the two sentences that mandate the

establishment of MACT floors. And it is certainly reasonable

to conclude that a statutory provision that authorizes an

agency to take a particular action contemplates that such
14

action will be taken before — rather than after — another

action that is not even mentioned until a subsequent provision. Thus, if one reads the sentences of § 129(a)(2) in order,

the second sentence appears to contemplate that EPA may

first distinguish among units in a category, and then apply

the resulting subcategories when setting MACT floors. See

Holloway v. United States, 526 U.S. 1, 6 (1999) (‘‘In interpreting the statute at issue, ‘[w]e consider not only the bare

meaning’ of the critical word or phrase ‘but also its placement

and purpose in the statutory scheme.’ ’’ (quoting Bailey v.

United States, 516 U.S. 137, 145 (1995))).

Finally, still further support for EPA’s view is provided by

close attention to § 129(a)(2)’s third sentence, which immediately follows the authorization to subcategorize and directs

the Agency to establish MACT floors for new units. That

sentence states that ‘‘for new units in a category,’’ the MACT

floor ‘‘shall not be less stringent than the emissions control

that is achieved in practice by the best controlled similar

unit.’’ 42 U.S.C. § 7429(a)(2) (emphasis added). The word

‘‘similar’’ may reasonably be read as referring to a unit that is

in the same subcategory. Indeed, to find otherwise would

work the disfavored result of giving the word ‘‘similar’’ no

effect. See TRW Inc. v. Andrews, 534 U.S. 19, 31 (2001);

Duncan v. Walker, 533 U.S. 167, 174 (2001). And, if EPA can

group ‘‘similar’’ units together in setting the MACT floor for

new units, then Industry Petitioners’ central contention —

that the (second) subcategorization sentence must be limited

to beyond-the-floor calculations — cannot be sustained.

In sum, we conclude that § 129(a)(2) is at least ambiguous

on the question of whether EPA may subcategorize the small

unit category when establishing MACT floors for MWCs, and

that the Agency’s construction of the section as permitting

such subcategorization is permissible.10

10 In a footnote, Industry Petitioners argue that an altogether

different provision of the Clean Air Act shows that Congress knew

how to explicitly grant EPA discretion to set MACT floors for

subcategories when it wanted to do so. Petitioners’ Br. at 13 n.6.
15

b. Subcategorization by Aggregate Plant Size

We now turn to Industry Petitioners’ second substantive

claim, that even if EPA may subcategorize when setting

MACT floors, the Clean Air Act does not permit it to do so on

the basis of aggregate plant capacity. This inquiry centers

on § 129(a)(2)’s second sentence, which provides: ‘‘The Administrator may distinguish among classes, types (including

mass-burn, refuse-derived fuel, modular and other types of

units), and sizes of units within a category in establishing

such standards.’’ 42 U.S.C. § 7429(a)(2). According to Industry Petitioners, aggregate plant capacity is not encompassed within any of the sentence’s three permissible grounds

of distinction: class, type and size of unit.

Once again, Industry Petitioners insist that their position is

compelled by dicta in this court’s opinion in Davis. In

support, they point to a fragment of a sentence in a footnote

that states: ‘‘EPA cannot use location to override the MWC

unit categories established by Congress.’’ Davis, 101 F.3d at

1405 n.11. But even if the word ‘‘location’’ is correctly read

as a reference to a unit’s location at a plant of a specified

aggregate capacity, the complete footnote is at least equally

supportive of EPA’s position: that while location may not be

used to combine statutory categories, it may be used to

subcategorize within each category. The footnote states:

That provision is CAA § 112(d)(3), which governs standards for

emissions from major stationary sources, and provides:

Emission standards promulgated under this subsection for

existing sources TTT shall not be less stringent TTT than — (A)

the average emission limitation achieved by the best performing 12 percent of the existing sources TTT in the category or

subcategoryTTTT

42 U.S.C. § 7412(d)(3). But the fact that Congress had available a

clearer way of expressing what EPA believes the legislature said in

§ 129(a)(2) does not compel us to reject EPA’s interpretation.

Although § 112(d)(3)’s single-sentence formulation is clearer, we

cannot conclude that EPA was unreasonable in relying on two of

§ 129(a)(2)’s sentences to do the same work.
16

We emphasize that we do not hold that the EPA is

precluded from ever taking a unit’s location into account,

but simply that EPA cannot use location to override the

MWC unit categories established by Congress. Section

129(a)(2) gives the EPA broad discretion to differentiate

among units in a category, and there is nothing in the

text of section 129(a)(2) that would prevent the EPA

from subcategorizing within the two categories of MWC

units TTT on the basis of the units’ location, provided the

EPA indicated why such a subcategorization was appropriate.

Id. (emphasis added); see also id. at 1411.

As before, we need not spend time deconstructing Davis’

dicta, because the words of the statute must ultimately decide

the issue. See Ernst & Ernst v. Hochfelder, 425 U.S. 185, 201

(1976). As we have noted, § 129(a)(2) authorizes EPA to

‘‘distinguish among classes.’’ ‘‘Class’’ is an ambiguous term.

It is not defined in the Clean Air Act, and the dictionary

definition — ‘‘a group, set, or kind marked by common

attributes’’ — could hardly be more flexible. WEBSTER’S

THIRD NEW INTERNATIONAL DICTIONARY 416 (1976) (3rd meaning). There is certainly nothing about the term or its dictionary definition that precludes the use of aggregate plant

capacity as a factor for drawing distinctions among units.

Because the term ‘‘class’’ is ambiguous, we would now

ordinarily take Chevron’s second step and ask whether it was

reasonable for the Agency to construe that term as permitting subcategorization based on aggregate plant capacity.

But because Industry Petitioners regard this case as governed by step one of Chevron, their briefs do not dispute that,

assuming subcategorization is permitted at all, aggregate

capacity is a reasonable criterion. Petitioners’ Br. at 11, 16.

Instead, they contend that EPA failed to provide any explanation at all for subcategorizing on that basis. Because we

ultimately find that argument dispositive, we pretermit our

discussion of Chevron and proceed directly to that challenge.11

11 Industry Petitioners also contend, somewhat elliptically, that

the 2000 Rule is inconsistent with the statute because the emission
17

2. Procedural Challenges

In addition to their substantive challenges, Industry Petitioners level a number of procedural attacks against the 2000

Rule. Specifically, petitioners claim that in promulgating the

regulations, EPA: (1) failed to articulate a rationale for its

decision to subcategorize on the basis of aggregate plant

capacity; (2) failed to respond to significant comments; (3)

promulgated a rule that was not a logical outgrowth of the

rule the Agency originally proposed; (4) relied on latedocketed materials; and (5) wrongfully denied a request for a

new round of public comment.

The Clean Air Act limits the scope of our review of these

claims. In particular, we may not consider an objection to a

rule or procedure unless it was raised ‘‘with reasonable

specificity during the period for public comment.’’ 42 U.S.C.

§ 7607(d)(7)(B). Moreover, we may invalidate a rule because

of procedural errors only if: (1) the agency’s failure to

observe the required procedures was arbitrary or capricious,

id. § 7607(d)(9)(D); and (2) the error was ‘‘so serious and

related to matters of such central relevance to the rule that

there is a substantial likelihood that the rule would have been

significantly changed if such errors had not been made,’’ id.

§ 7607(d)(8); see id. § 7607(d)(9)(D); Chemical Mfrs. Ass’n

v. EPA, 28 F.3d 1259, 1262 (D.C. Cir. 1994).

standards it sets for small MWC units in Class I are substantially

the same as those the 1995 Rule set for large MWC units. See

Petitioners’ Br. at 15–16. As EPA points out, however, CAA § 129

does not mandate that the standards for small MWC units must

necessarily differ from those for large MWCs. Instead, the section

establishes statutory criteria and the methodology that EPA is to

use in applying those criteria to calculate standards. As long as the

Agency separately analyzes the two statutory categories, the fact

that the ultimate standards are equivalent need not concern us.

Indeed, as EPA explains, it is not surprising that emission standards would be the same for both existing Class I units and large

units, since the best performing units in both groups already use

similar control technology. Respondent’s Br. at 22–23.
18

a. Absence of Rationale

Industry Petitioners first attack EPA for failing to set

forth a rationale for its classification of units based on aggregate plant capacity. According to petitioners, even if EPA

may legally distinguish among MWCs based on this characteristic, it did so here without explaining why such classification was appropriate. Although we would ordinarily consider

a challenge to an agency’s rulemaking rationale as a form of

substantive attack, in this case petitioners level only a procedural charge. That is, they do not contend that it would be

substantively unreasonable for the Agency to distinguish

among MWCs based on the aggregate capacities of the plants

at which they are located. Rather, they simply contend that

the Agency has failed altogether to proffer a rationale for so

doing. Industry Petitioners assert that EPA’s failure violates

the requirement of CAA § 307(d) that each proposed and

promulgated rule be accompanied by a ‘‘statement of its basis

and purpose’’ that includes a summary of ‘‘the major legal

interpretations and policy considerations underlying’’ the rule.

42 U.S.C. § 7607(d)(3), (d)(6)(A).

EPA responds by claiming that petitioners are foreclosed

from making this charge because they did not satisfy the

exhaustion requirement of CAA § 307(d)(7)(B), 42 U.S.C.

§ 7607(d)(7)(B), by objecting with reasonable specificity to

EPA’s failure to articulate a rationale during the public

comment period. We disagree for two reasons. First, a

number of commenters plainly did challenge the Agency’s

failure to explain its subcategorization rationale during the

rulemaking.12 Second, as we held in Appalachian Power Co.

12 See Comments of Dutchess and Islip (J.A. 1830); Comments of

Institute of Clean Air (J.A. 1841); Comments of Illinois Environmental Protection Agency (J.A. 2089). It is sufficient that an issue

was raised by any commenter; the party petitioning for judicial

review need not have done so itself. See Reytblatt v. Nuclear

Regulatory Comm’n, 105 F.3d 715, 721 (D.C. Cir. 1997); accord

Cellnet Communication, Inc. v. FCC, 965 F.2d 1106, 1109 (D.C. Cir.

1992) (‘‘Consideration of the issue by the agency at the behest of

another party is enough to preserve it.’’).
19

v. EPA, the EPA at all times ‘‘retains a duty to examine key

assumptions as part of its affirmative burden of promulgating

and explaining a nonarbitrary, non-capricious rule,’’ and

therefore must justify its basic ‘‘assumption[s] even if no one

objects TTT during the comment period.’’ 135 F.3d 791, 818

(D.C. Cir. 1998) (quoting Small Refiner Lead Phase–Down

Task Force v. EPA, 705 F.2d 506, 534–35 (D.C. Cir. 1983))

(internal quotation marks omitted). As there is no question

that the validity of the distinction between large and small

aggregate plant capacities was a key assumption underlying

the 2000 Rule, EPA was duty-bound to set forth its rationale

for subcategorizing on that basis.

We thus turn to the underlying question: Did EPA explain

its decision to establish subcategories based on aggregate

plant capacity? We are, frankly, stunned to find that it did

not. As the Agency concedes, there is not one word in the

proposed or final rule that explains why the Agency chose to

distinguish among small MWCs on the basis of the aggregate

capacities of the plants at which they are located. Indeed,

other than arguing that petitioners are barred from raising

the issue, the text of EPA’s brief does not even respond to

Industry Petitioners’ argument that the failure to provide a

rationale dooms the 2000 Rule.

In a footnote to its brief, EPA does assert that the Agency

‘‘articulated its rationale for distinguishing among MWC units

based on aggregate capacity when it proposed the first comprehensive MWC regulations in 1989.’’ Respondent’s Br. at

32 n.31 (citing Emission Guidelines: Municipal Waste Combustors, 54 Fed. Reg. 52,209, 52,219–20 (Dec. 20, 1989)). That

rationale, which was contained in the preamble to a rule that

EPA proposed but never adopted, stated as follows:

The proposed capacity aggregation is necessary because

of the common practice within the MWC industry of

constructing multiple MWC’s at the same location. This

aggregation ensures that similar MWC plants with similar emission potential are subject to the same emission

guidelines regardless of the number of individual MWC’s

at the plants. Because multiple MWC’s can have the
20

same emission quality impacts as a larger single MWC, it

is reasonable to apply the proposed emission guidelines

to all existing MWC’s at the same locationTTTT

54 Fed. Reg. at 52,219–20. At oral argument, EPA further

asserted that this 1989 rationale was ‘‘incorporated’’ into the

2000 Rule, and thus was sufficient to satisfy the requirement

of CAA § 307(d).

We are not persuaded. While an express statement of

intent to incorporate a rationale contained in another, specific

document might satisfy the Agency’s statutory obligation,

EPA made no such statement here. The sum and substance

of the statement in the 2000 Rule that the Agency regards as

incorporating the 1989 preamble reads as follows:

Docket No. A–98–18 [1998] and associated Docket Nos.

A–90–45 [1990] and A–89–08 [1989] contain supporting

information for the emission guidelines. The dockets are

available for public inspection and copyingTTTT

65 Fed. Reg. at 76,378; see 64 Fed. Reg. at 47,234 (identical

statement in proposed rule). That statement does not expressly ‘‘incorporate’’ anything, let alone refer interested

parties or the courts to a specific document containing the

Agency’s rationale. At best, it is an invitation to search

through a mountain of documents, contained in three rulemaking dockets stretching back over a decade, in pursuit of

‘‘supporting information.’’

Such a vague reference cannot possibly satisfy § 307(d)’s

instruction that each proposed and promulgated rule ‘‘shall be

accompanied by’’ a ‘‘statement of basis and purpose’’ that

‘‘shall include a summary of TTT the major legal interpretations and policy considerations underlying the proposed rule.’’

42 U.S.C. § 7607(d)(3) (emphasis added); see id.

§ 7607(d)(6)(A). A rationale buried in a document published

in 1989 simply does not ‘‘accompany’’ a rule proposed and

promulgated more than a decade later. Nor can such a

reference satisfy the fundamental requirement of nonarbitrary administrative decisionmaking: that an agency set forth

the reasons for its actions. See Motor Vehicle Mfrs. Ass’n of
21

United States, Inc. v. State Farm Mut. Auto. Ins. Co., 463

U.S. 29, 48–50 (1983); Appalachian Power Co., 135 F.3d at

818; see also Small Refiner, 705 F.2d at 551 (‘‘A rule without

a stated reason is necessarily arbitrary and capricious.’’).

Without a readily accessible statement of the agency’s rationale, interested parties cannot comment meaningfully during

the rulemaking process. Nor can they, or the courts, determine whether the agency has acted capriciously or whether

its statutory interpretation is reasonable under Chevron’s

second step.

Although EPA’s failure to set forth its rationale requires us

to remand the 2000 Rule for further consideration, see State

Farm, 463 U.S. at 57, that defect does not require us to

vacate the rule. See Allied–Signal, Inc. v. Nuclear Regulatory Comm’n, 988 F.2d 146, 150 (D.C. Cir. 1993) (‘‘An inadequately supported rule TTT need not necessarily be vacated.’’).

We decline to do so for several reasons. First, the 1989

rationale pointed to by EPA is sufficient to persuade us that

the Agency ‘‘may be able to explain’’ the subcategorization

decision it made in 2000. Id. at 151. That rationale also

militates against a finding that the error was ‘‘so serious TTT

that there is a substantial likelihood that the rule would have

been significantly changed’’ if it had not been made. 42

U.S.C. § 7607(d)(8). At the same time, there is no doubt that

‘‘the consequences of vacating’’ would be ‘‘quite disruptive.’’

Allied–Signal, Inc., 988 F.2d at 151. Indeed, it was concern

over just such disruption of EPA’s pollution control program

that ultimately persuaded us to remand rather than vacate

the 1995 large-unit regulations, originally invalidated in

Davis. See Davis County Solid Waste Mgmt. v. EPA, 108

F.3d 1454, 1458 (D.C. Cir. 1997) (expressing concern ‘‘that

vacating the standards for large units could have significant

deleterious effects on MWC emissions control’’). Accordingly, rather than vacate, we remand the 2000 Rule to EPA ‘‘for

it to develop a reasoned’’ explanation for its decision to

subcategorize on the basis of aggregate plant capacity. Allied–Signal, Inc., 988 F.2d at 151; see, e.g., Radio–Television

News Directors Ass’n v. FCC, 184 F.3d 872, 888–89 (D.C. Cir.
22

1999); American Mining Cong. v. EPA, 907 F.2d 1179, 1190

(D.C. Cir. 1990).

b. Response to Comments

We next consider Industry Petitioners’ second procedural

charge: that EPA failed to respond to significant comments,

as required by CAA § 307(d)(6)(B). That provision requires

that ‘‘[t]he promulgated rule shall also be accompanied by a

response to each of the significant comments TTT submitted in

written or oral presentations during the comment period.’’

42 U.S.C. § 7607(d)(6)(B); see Appalachian Power Co. v.

EPA, 249 F.3d 1032, 1051 (D.C. Cir. 2001) (‘‘While we generally uphold the EPA’s authority to make emission projections

and set emission limitations accordingly, we do so only where

the EPA adequately responded to comments and explained

the basis for its decisions.’’).

Petitioners assert, first, that EPA failed to respond to

comments complaining about the high cost of retrofitting acid

gas controls for certain units. This assertion fails on its facts.

The Agency did respond to those complaints, explaining that

the Clean Air Act does not permit it to take cost into account

in setting MACT floors, and that (in its view) the beyond-thefloor standards strike the correct balance between cost and

emissions reductions. EPA Response to Comments at 52

(J.A. 2199).

Industry Petitioners also claim that EPA failed to respond

to comments asserting that § 129(a)(2) of the Clean Air Act

bars the Agency from subcategorizing MACT floors on the

basis of aggregate plant capacity. We need expend no further effort in analyzing that charge, however, as we have

already decided to remand the 2000 Rule to EPA so that the

Agency may explain its rationale for such subcategorization.

See supra Part II.A.2.a. During the course of that remand,

the Agency will have ample opportunity to respond to the

cited comments and to cure this procedural failure as well.

c. Adequate Notice

The third procedural challenge is raised only by Industry

Petitioner Northeast Maryland Waste Disposal Authority.
23

Northeast Maryland contends that, because the 2000 Rule

differs from the rule EPA originally proposed, the Agency

failed to provide the advance notice required by the Clean Air

Act. See 42 U.S.C. § 7607(d)(3) (requiring EPA to publish a

notice of proposed rulemaking ‘‘as provided under’’ 5 U.S.C.

§ 553(b)); 5 U.S.C. § 553(b)(3) (requiring each agency to

publish notice of proposed rulemaking that includes ‘‘either

the terms or substance of the proposed rule or a description

of the subjects and issues involved’’).

As we have discussed, the final 2000 Rule subcategorizes

small MWC units into two classes based on aggregate plant

capacity: Class I units are those located at plants with

aggregate capacities greater than 250 tpd; Class II units are

those located at plants with aggregate capacities equal to or

less than 250 tpd. By contrast, the proposed rule established

three classes for existing small MWCs: it subcategorized

them not only on the basis of aggregate plant capacity, but

also based on whether a unit utilized ‘‘refractory’’ or ‘‘nonrefractory’’ technology. 64 Fed. Reg. at 47,237.13 The three

proposed classes were: Class A, for nonrefractory units

located at plants with aggregate capacities of more than 250

tpd; Class B, for refractory units located at plants with the

same aggregate capacities; and Class C, for all units located

at plants with aggregate capacities equal to or less than 250

tpd. Id. The proposed rule subjected the Class A units to

the most stringent emission standards.

EPA initially distinguished between refractory and nonrefractory units based on its belief that refractory units were

less amenable to pollution control technology. The Agency

thought that refractory units generated significantly more

exhaust (flue gas) per ton of waste burned, and that emission

control devices were less efficient at removing pollutants from

larger air volumes with lower concentrations. Id. However,

13 The refractory-nonrefractory distinction hinges on a unit’s cooling technology. After combustion, refractory units are cooled by

circulating excess air. Nonrefractory MWC units are lined with

water-filled steel tubes. Cool water flows through the tubes to

remove heat and protect the unit’s structure.
24

after receiving public comments opposing the subcategorization scheme and reanalyzing the issue, EPA ultimately concluded that any difference in flue gas flow rates between

refractory and nonrefractory units was insufficient to justify

the imposition of different emission standards. 65 Fed. Reg.

at 76,380. Accordingly, in the final rule, EPA collapsed Class

A and Class B into a single class — Class I — composed of all

units located at plants with aggregate capacities exceeding

250 tpd. Id.

Because the proposed rule indicated that EPA was considering three subcategories, while the final rule establishes only

two, Northeast Maryland contends that it was deprived of

proper notice of the Agency’s intentions. As petitioners

recognize, however, EPA is not required to adopt a final rule

that is identical to the proposed rule. Indeed, ‘‘[i]f that were

the case, [EPA] could learn from the comments on its proposals only at the peril of subjecting itself to rulemaking without

end.’’ First Am. Discount Corp. v. Commodity Futures

Trading Comm’n, 222 F.3d 1008, 1015 (D.C. Cir. 2000) (internal quotation marks omitted); see American Water Works

Ass’n v. EPA, 40 F.3d 1266, 1274 (D.C. Cir. 1994). Agencies,

are free — indeed, they are encouraged — to modify proposed rules as a result of the comments they receive. See

Arizona Pub. Serv. Co. v. EPA, 211 F.3d 1280, 1300 (D.C. Cir.

2000) (noting that ‘‘the Agency’s change of heart TTT only

demonstrates the value of the comments it received’’); Kooritzky v. Reich, 17 F.3d 1509, 1513 (D.C. Cir. 1994) (‘‘It is an

elementary principle of rulemaking that a final rule need not

match the rule proposed, indeed must not if the record

demands a change.’’).

In light of these considerations, we have held that an

agency satisfies the notice requirement, and need not conduct

a further round of public comment, as long as its final rule is

a ‘‘logical outgrowth’’ of the rule it originally proposed. First

Am. Discount Corp., 222 F.3d at 1015; Arizona Pub. Serv.

Co., 211 F.3d at 1299. A rule is deemed a logical outgrowth if

interested parties ‘‘should have anticipated’’ that the change

was possible, and thus reasonably should have filed their

comments on the subject during the notice-and-comment
25

period. City of Waukesha v. EPA, 320 F.3d 228, 245 (D.C.

Cir. 2003); see First Am. Discount Corp., 222 F.3d at 1015;

National Mining Ass’n v. Mine Safety & Heath Admin., 116

F.3d 520, 531 (D.C. Cir. 1997); Kooritzky, 17 F.3d at 1513.

We conclude that the final 2000 Rule, which merely collapses the proposed rule’s three categories into two, is a logical

outgrowth of the proposed rule. By announcing that it

proposed to distinguish between refractory and nonrefractory

units, EPA invited comments on both the pros and cons of

that distinction. It thus effectively served notice that, if

persuaded that the latter outweighed the former, the distinction might not survive. Nor did the interested parties misread either the invitation or the stakes involved. Numerous

commenters — including two that are among the Industry

Petitioners here — filed comments that were critical of the

distinction between refractory and nonrefractory units.14 On

the other side, Northeast Maryland’s predecessor, WEP, filed

comments that supported the distinction. Comments of WEP

at 1 (J.A. 2093). Accordingly, we reject Northeast Maryland’s contention that the evolution of the rule deprived it of

adequate notice and an opportunity to comment. See Appalachian Power Co., 135 F.3d at 816 (finding that a rule was a

logical outgrowth where commenters ‘‘clearly understood’’

that a matter was under consideration, since ‘‘the agency

received comments on [the matter] from several sources’’).

d. Late Docketing of Materials

Next, Industry Petitioners assert that, in promulgating the

2000 Rule, EPA improperly relied on documents added to the

docket after the close of the comment period and too late for

effective rebuttal. While the docket for this rulemaking

closed on October 29, 1999, EPA docketed a number of

materials in late September 2000, approximately two months

14 See, e.g., Comments of Dutchess and Islip at 2 (J.A. 1832); see

also, e.g., Comments of Institute of Clean Air Companies at 1 (J.A.

1917); Comments of Wasatch Clean Air Coalition at 1 (J.A. 1921);

Supplemental Comments of Institute of Clean Air Companies at 1

(J.A. 1839).
26

before the rule’s December 6, 2000, publication. Industry

Petitioners specifically complain about Document IV–B–5, an

EPA-drafted memorandum that set forth EPA’s rationale for

eliminating the refractory-nonrefractory distinction. EPA

Combustion Group Mem. (docketed Sept. 28, 2000) (J.A.

1801).

In this case, as Industry Petitioners concede, all of the

documents at issue were docketed by the time the 2000 Rule

was promulgated. Hence, EPA did not violate the letter of

CAA § 307(d)(6)(C), which bars EPA from basing a rule on

data ‘‘which has not been placed in the docket as of the date

of [the rule’s] promulgation.’’ 42 U.S.C. § 7607(d)(6)(C).

Nevertheless, as petitioners correctly point out, our cases

hold that EPA violates ‘‘the structure and spirit of section

307’’ if it ‘‘submit[s] so late as to preclude any effective public

comment’’ a document ‘‘vital to EPA’s support for its rule.’’

Sierra Club v. Costle, 657 F.2d 298, 398 (D.C. Cir. 1981); see

Small Refiner, 705 F.2d at 540.

But Document IV–B–5 is not the kind of document to which

our cases refer. Document IV–B–5 expressed EPA’s response to, and agreement with, public comments that it had

received indicating there was no significant difference in flue

gas flow rates between refractory and nonrefractory units.

In effect, then, the memorandum was little more than a

statement of the Agency’s response to comments and of its

rationale for eliminating the proposed distinction between

Classes A and B. It is thus the kind of statement that would

ordinarily not appear until the notice of final rulemaking, and

the fact that EPA placed it in the docket in advance of that

notice cannot be regarded as a procedural defect. See Costle,

657 F.2d at 352–53 (‘‘It is entirely proper and often necessary

for the agency to continue its deliberations and internal

decisionmaking process after the close of public comment in

order to assimilate those comments and arrive at a policy

choice.’’).

e. Petition for Reconsideration

Finally, we address Industry Petitioners’ claim that EPA

erred in refusing to ‘‘convene a proceeding for reconsidera-
27

tion,’’ pursuant to CAA § 307(d)(7)(B), 42 U.S.C.

§ 7607(d)(7)(B), in response to a petition for reconsideration

that WEP filed after publication of the final rule. That

petition sought reconsideration principally on the grounds

that the final rule was not a logical outgrowth of the proposed

rule, and that reliance on the late-docketed Document IV–B–5

violated the Clean Air Act. As we have concluded that

neither of those claims of procedural error has merit, there is

no ground for holding that a reconsideration proceeding was

required, see 42 U.S.C. § 7607(d)(7)(B), or that the denial of

WEP’s request had a prejudicial effect, see id. § 7607(d)(8).

B. Sierra Club Challenges

Sierra Club challenges the 2000 Rule on three grounds.

We address each ground in turn.

1. Permit–Based MACT Floors

for Existing Small Units

First, Sierra Club challenges EPA’s decision to base existing small unit MACT floors on the emission limits contained

in state permits. As in the 1995 Rule, in the 2000 Rule EPA

based the MACT floor on the limits set for state-permitted

MWC units in the particular subcategory. For each pollutant, EPA calculated the MACT floor by averaging the most

stringent 12% of state permit limits in each class.15 For

pollutants for which there were too few permitted units, EPA

assigned a ‘‘default’’ emission level, namely, the estimated

emission level of a totally uncontrolled unit. Sierra Club

contends there is nothing in the record to demonstrate that

state permit limits or the uncontrolled default levels reflect

‘‘the average emissions limitation achieved by the best performing 12 percent of units in the category,’’ the floor required by § 129(a)(2), 42 U.S.C. § 7429(a)(2). We agree with

Sierra Club and conclude that the MACT floors for existing

small units must therefore be remanded.

15 EPA extracted the permit limits from its 1995 rulemaking

database. EPA Permit Basis Mem. at 2 (J.A. 1757).
28

In Sierra Club v. EPA, 167 F.3d 658 (D.C. Cir. 1999), the

court rejected EPA’s similar use of state permit limits to set

the MACT floor for medical waste incinerators (MWIs). The

court recognized that CAA § 129 may permissibly be construed ‘‘to permit the use of regulatory data’’ but only ‘‘if they

allow EPA to make a reasonable estimate of the performance

of the top 12 percent of units.’’ 167 F.3d at 662. The court

rejected the use of such data in that case because ‘‘[a]lthough

EPA said that it believed the combination of regulatory and

uncontrolled data gave an accurate picture of the relevant

MWIs’ performance, it never adequately said why it believed

this.’’ Id. at 663. EPA fares no better here. It offered the

following justification for deciding to use state permit limits:

The EPA used a permit approach to determine the

MACT floors in the 1995 emission guidelines (40 CFR

part 60, subpart Cb) and believes that using the permit

approach is appropriate for this rulemaking. Permit

limits and regulatory limits provide a reasonable estimate of the actual performance of the best performing

units under the worst reasonably foreseeable circumstances, making this approach consistent with the court

opinion in the Sierra Club case. Permits include a

margin for compliance and must be achievable.

EPA Response to Comments at 75 (J.A. 2222). As in Sierra

Club, EPA here stated only that it ‘‘believes’’ state permit

limits reasonably reflect the actual performance of the best

performing units without explaining why this is so. There is

also evidence here that the MWCs, like the MWIs in Sierra

Club, ‘‘might be substantially overachieving the permit limits,’’ that is, ‘‘the regulatory limits are in fact much higher

than the emissions that units achieve in practice,’’ 167 F.3d at

663. See Sierra Club’s Br. at 22 (asserting, with record

evidence, that EPA’s testing data show MWCs in general

(and small MWCs in particular) ‘‘routinely overachieve their

permit limits’’). Given the absence of evidence that the

permit levels reflect the emission levels of the bestperforming 12 percent of existing MWCs and the affirmative
29

evidence that they do not, we cannot uphold the MACT floors

for existing units under the CAA.

In support of using state permit levels, EPA points to its

determinations that emission levels are inherently variable,

EPA Response to Comments (1995 Rule) (J.A. 1570), and that

basing MACT floors on the Agency’s test data would not

accurately reflect this variability, id. at J.A. 1633 (noting ‘‘it is

not unusual for one or more of the annual tests to produce

emissions that fall within the best 12–percent data, while the

remaining annual test data fall outside this range’’). Even

assuming actual testing data should not be used for setting

MACT floors, EPA must still justify selecting state permit

and uncontrolled default levels as alternative bases for the

floors.

2. Technology–Based MACT Floors

for New Small Units

The CAA requires that the MACT floor for new small units

be set at the ‘‘emissions control that is achieved in practice by

the best controlled similar unit.’’ 42 U.S.C. § 7429(a)(2). To

satisfy this requirement, EPA must ‘‘demonstrate with substantial evidence — not mere assertions’’ that the chosen

floors ‘‘represent ‘a reasonable estimate of the performance of

the [best-performing] units.’ ’’ Cement Kiln Recycling Coalition v. EPA, 255 F.3d 855, 866 (D.C. Cir. 2001) (quoting

Sierra Club v. EPA, 167 F.3d 658, 662 (D.C. Cir. 1999))

(alteration in original). To set the floors for new small MWC

units, EPA (1) reviewed available MWC emissions test data

associated with all types of combustors and all types of

emission control technologies currently used to control emissions of specific pollutants, (2) identified the best controlled

unit and reviewed the performance of its associated control

technology and (3) set the floor for each pollutant at the level

of emissions that units equipped with that technology can

continuously achieve in practice (based on 24–hour averaging

periods or, if continuous emission monitoring was unavailable,

on annual stack tests). See 59 Fed. Reg. at 48,214–16.

Sierra Club asserts EPA has not demonstrated that that

technology alone, without regard to other technologies or to
30

non-technology factors, achieves ‘‘the emissions control that is

achieved in practice by the best controlled similar unit,’’ as

CAA § 129(a)(2) requires. We agree that EPA has not

shown that the technology-based approach will achieve a

reasonable estimate of the emission level achieved by the best

performing MWC unit and, accordingly, remand to the Agency to establish MACT floors for new units that do. Because

we remand for new MACT floors, we need not consider

Sierra Club’s alternate contention that the Agency should

have considered how factors other than the chosen technology

affect emissions.

In setting the MACT floor, the EPA reasoned that ‘‘[b]ecause MACT must be achievable and there is inherent variation in emissions among MWC units, TTT the floor emission

levels are set at levels that are demonstrated to be achievable

by the population of MWC units with the best technology.’’

EPA Response to Comments at 31 (J.A. 2178). This is

precisely the rationale we rejected in Cement Kiln. As we

explained in Cement Kiln, ‘‘[w]hile standards achievable by all

sources using the MACT control might also ultimately reflect

what the statutorily relevant sources achieve in practice, EPA

may not deviate from [the statute’s] requirement that floors

reflect what the best performers actually achieve by claiming

that floors must be achievable by all sources using MACT

technology.’’ Cement Kiln, 255 F.3d at 861 (citing Chevron

U.S.A. Inc. v. Natural Res. Def. Council, Inc., 467 U.S. 837,

842–43 (1984)). EPA has once again improperly invoked

achievability (incorrectly relying on the emission variability of

all MWCs that use the technology rather than on the variability of the best performing unit) to gloss over the actual

achievement requirement.

3. Beyond-the-Floor Standards

Finally, Sierra Club raises three objections to EPA’s beyond-the-floor standards. First, Sierra Club asserts that in

deciding whether to set beyond-the-floor standards for certain pollutants — namely Hazardous Air Pollutant (HAP)

metals (mercury, lead and cadmium) and dioxins — EPA
31

failed to consider ‘‘nonair quality health and environmental

impacts,’’ such as the impacts of deposition, persistence and

bioaccumulation, as required under 42 U.S.C. § 7429(a)(2).

Second, Sierra Club contends EPA failed to require precombustion separation of pollutants from the waste as required by CAA § 129(a)(3), which provides that standards

‘‘shall be based on methods and technologies for removal or

destruction of pollutants before, during, or after combustion.’’

42 U.S.C. § 7429(a)(3). And third, Sierra Club challenges

EPA’s decision to set ‘‘no-control’’ floors and beyond-the-floor

standards for nitrogen oxide emissions from new and existing

Class II MWC units. These no-control standards, according

to Sierra Club, violate 42 U.S.C. § 7429(a)(4) and (a)(2), as

well as our holding in National Lime Ass’n v. EPA, 233 F.3d

625 (D.C. Cir. 2000). In light of our remand of all of the

MACT floors, we need not address these objections at this

time. As Sierra Club’s counsel acknowledged at oral argument, the Agency’s beyond-the-floor determinations cannot be

evaluated if, as we have concluded, the MACT floors themselves were improperly set.

III.

For the foregoing reasons, we grant the petitions in part

and deny them in part and remand to EPA to: (1) explain its

decision to subcategorize small MWC units according to the

aggregate capacities of the plants at which they are located;

(2) establish new MACT floors for new and existing small

units; and (3) readdress the beyond-the-floor standards as

required.

So ordered.