Commercial and Industrial Solid Waste Incineration Units 

(CISWI); Reconsideration and Final Rule Amendments (SAN 5105.1)

4. Removal of the Definition of “Homogeneous Waste”

The EPA included in the final 2011 CISWI Rule a definition 

of homogenous waste and a process for evaluating claims that a 

particular waste stream is homogenous. The definition was added 

to the 2011 CISWI rule in response to comment. Because the 

determination of homogeneity of a waste stream is relevant to 

applicability of CAA section 129 to qualifying small power 

producers and qualifying cogeneration facilities, we determined 

it was reasonable to include a definition of “homogenous waste” 

and a process by which sources could obtain a determination that 

a waste stream is homogenous from the EPA. 

In the 2011 CISWI Rule, the EPA stated that a determination 

concerning whether a waste is homogeneous is made on a case-bycase basis. The EPA added provisions to the CISWI final rule 

that require source owners or operators seeking the exemption to 

submit a request for a homogeneous waste determination to the 

EPA, and that they support their request with information 

describing the materials to be combusted and why they believe 

the waste is homogeneous. The 2011 CISWI rule also stated that 

the determination of what constitutes a homogeneous waste is not 

delegable to the state or local agencies. In the December 23, 

2011, reconsideration proposal, we proposed for comment the 
definition of “homogeneous waste” and the provisions for making 

homogeneous waste determinations that were included in the 2011 

CISWI rule.

Commenters generally did not agree with the proposed 

definition and provisions for making a homogeneous waste 

determination, arguing that the definition and provisions 

introduced ambiguities and stipulations that would prevent 

classification of many materials (including fossil fuels) as 

being “homogeneous.” We reevaluated the definition and 

provisions in light of the comments and determined that the 

definition and provisions could be interpreted in a manner that 

would be unduly restrictive; however, we also determined that 

commenters proposed alternative definitions and provisions were 

equally problematic. Therefore, the final rule does not include 

a definition of “homogeneous waste”. We are also removing the 

requirement that qualifying small power producers and qualifying 

cogeneration facilities that combust solid waste obtain a 

determination from EPA that such waste is homogenous. Because 

the final rule does not include a homogenous waste definition or 

a process to obtain a determination from EPA, we believe that it 

is appropriate to inform the EPA when a unit qualifies as a 

small power generator or cogeneration facility as defined under 

section 129 because the site specific fact patterns for 

different types of waste may vary considerably. Therefore, the 
final rule requires qualifying small power producers and 

qualifying cogeneration facilities that combust solid waste 

notify the EPA that such waste is homogeneous. (40 CFR 60.2020 

and 40 CFR 60.2555).

Section 129 states, in part, that the term “solid waste 

incineration unit” does not include:

...qualifying small power production facilities, as 

defined in section 796 (17)(C) of title 16, or 

qualifying cogeneration facilities, as defined in 

section 796 (18)(B) of title 16, which burn 

homogeneous waste (such as units which burn tires 

or used oil, but not including refuse-derived 

fuel) for the production of electric energy or in 

the case of qualifying cogeneration facilities 

which burn homogeneous waste for the production of 

electric energy and steam or forms of useful 

energy (such as heat) which are used for 

industrial, commercial, heating or cooling purposes...

CAA Section 129(g)(1)(B) (emphasis added) 

We believe that the parenthetical contained in the 

exemption that prohibits refuse derived fuel, which is made from 

municipal solid waste, from qualifying as homogenous waste and 

allows tires and used oil to qualify as homogenous wastes 

provides guidance on what constitutes a homogenous waste. We do 
not accept industry’s assertion that any waste from a common 

source is homogeneous, or that in all cases combining two 

homogeneous wastes results in a homogeneous waste, as doing so 

could result in almost any waste stream being homogenous. We do 

not believe that is consistent with the statute. Instead, we 

believe Congress intended this exemption to apply only when the 

waste stream has a consistent makeup that allows the source and

the enforcement authority to predict the range of emissions from 

the combustion of the waste on an ongoing basis. 

In keeping with this interpretation, we maintain that the 

homogeneous wastes are generally material specific (e.g. tires 

or used oil). We believe this means that a homogeneous waste is 

of known origin and that it can be identified as a specific 

material or materials – using the example in the Act, certain 

used oils or scrap tires. By contrast, municipal solid waste can 

be identified as municipal solid waste as a general term, but it 

is not composed of only one or two specific type of waste; e.g. 

municipal solid waste cannot be identified as one specific 

material or group of materials. Regarding variability of the 

composition of homogeneous waste throughout, homogeneous waste 

may have variations in composition, but it should generally be 

within the range of operations which produce the waste (e.g. 

size, contaminant levels, state of matter.) We also believe 

that off-spec materials may be homogeneous, even if they are not 
homogeneous to the on-spec material, and that, if combusted 

together, both the on-spec and off-spec materials may require 

separate homogenous waste determinations. We also believe that 

homogeneous waste should have predictable known contaminant 

levels, even if those contaminant levels vary within a range. 

We may question the homogeneity of a specific material if it is 

adulterated such that it takes on the characteristics of a 

different type of waste (e.g. used oil which is so contaminated 

with PCB’s from a leaking heat exchanger, such that the used oil 

takes on the characteristics of a waste PCB stream as opposed to 

a used oil stream) or where the BTU value of a waste is so 

altered that other fuels must be introduced to ensure combustion 

and preserve the purpose of combustion under the exemption, i.e. 

to produce energy.