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In the United States, hazardous wastes are subject to regulations mandated by the Resource Conservation and Recovery Act (RCRA). Every month, we provide clear, in-depth guidance on a different aspect of the RCRA regulations. The information presented here is an excerpt from McCoy’s RCRA Unraveled, 2019 Edition.

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Air Emission Standards for Process Vents—Subpart AA

When hazardous wastes containing organics are treated or recycled in chemical/physical separation equipment (such as solvent stills), high levels of air emissions may be produced. Consider the simple solvent still shown in Figure 1. Spent solvent is heated in the batch still so that volatile organics are vaporized. The solvent vapors leave the still in what is typically called the “overhead stream” and pass into a condenser. Condensed liquids (recovered solvent) along with noncondensable gases flow into a distillate receiver, where the gases and recovered solvent separate into two phases. The liquids flow to a storage tank (product storage) and the noncondensed gases are simply vented to atmosphere through an open vent pipe. The purpose of Parts 264/265, Subpart AA is to limit the quantity of organic emissions from process vents such as the one shown on the distillate receiver in Figure 1.

Figure 1

Applicability of the Subpart AA regulations

Figure 2 identifies the conditions under which the Subpart AA air emission control standards for process vents will apply to a hazardous waste management operation. Important details referenced in this diagram are discussed below.

Figure 2

Perhaps the most useful reference available from EPA on Subpart AA applicability is EPA/450/3-89/021, Technical Guidance Document for RCRA Air Emission Standards for Process Vents and Equipment Leaks, July 1990. This document can be obtained online at http://nepis.epa.gov/EPA/html/Pubs/pubtitleOAR.html by downloading the report numbered 450389021.

Units handling hazardous waste

Note that the Subpart AA standards apply only to units handling hazardous waste. For example, if a steam stripper is being used to treat contaminated ground water, but the ground water does not contain a listed waste and does not exhibit a characteristic of hazardous waste, Subpart AA control requirements would not apply. Subpart AA also does not apply to process operations associated with manufacturing (e.g., fractionation columns used to separate components during manufacture of a chemical product).

Only specific separation processes are subject to controls

Only certain types of hazardous waste processing equipment having the potential to emit large quantities of volatile organics are regulated under Subpart AA. These types of processes are defined in §§264/265.1031 as follows:

  1. Distillation—“an operation, either batch or continuous, separating one or more feed stream(s) into two or more exit streams, each exit stream having component concentrations different from those in the feed stream(s). The separation is achieved by the redistribution of the components between the liquid and vapor phase as they approach equilibrium within the distillation unit.”
  2. Fractionation—“a distillation operation or method used to separate a mixture of several volatile components of different boiling points in successive stages, each removing from the mixture some proportion of one of the components.”
  3. Thin-film evaporation—“a distillation operation that employs a heating surface consisting of a large diameter tube that may be either straight or tapered, horizontal or vertical. Liquid is spread on the tube wall by a rotating assembly of blades that maintain a close clearance from the wall or actually ride on the film of liquid on the wall.”
  4. Solvent extraction—“an operation or method of separation in which a solid or solution is contacted with a liquid solvent (the two being mutually insoluble) to preferentially dissolve and transfer one or more components into the solvent.”
  5. Air stripping—“a desorption operation employed to transfer one or more volatile components from a liquid mixture into a gas (air) either with or without the application of heat to the liquid. Packed towers, spray towers, and bubble-cap, sieve, or valve-type plate towers are among the process configurations used for contacting the air and a liquid.”
  6. Steam stripping—“a distillation operation in which vaporization of the volatile constituents of a liquid mixture takes place by the introduction of steam directly into the charge.”

These definitions follow conventional processing terminology and do not have any hidden meaning or nuances of particular importance.

EPA clarified in Federal Register preamble language that portable equipment would be subject to Subpart AA control standards as well as stationary units. [December 8, 1997; 62 FR 64639]

Organic concentration of 10 ppmw triggers requirements

The Subpart AA regulations apply only where organic concentrations of at least 10 ppmw are involved. [§§264/265.1030(b)] The details of determining whether a waste meets the 10-ppmw limit are rather complex and are codified with the testing procedures in §§264/265.1034(d). When direct measurement (as opposed to process knowledge) is used, all waste streams entering one of the processes described above must be identified. At least four grab samples of each of these waste streams must be taken when the organic concentrations are expected to be at a maximum.

For wastes generated onsite, the grab samples must be taken before the waste is exposed to the air. Each sample is analyzed using SW–846 Method 9060—Total Organic Carbon. The four (or more) results for each waste stream are averaged and then a time-weighted, annual-average total organic concentration for all waste streams processed in the unit is calculated. If this time-weighted annual average equals or exceeds 10 ppmw, the unit is potentially subject to Subpart AA controls.

If the time-weighted annual average is <10 ppmw (i.e., the unit is not subject to Subpart AA), wastes that are generated continuously must be reanalyzed at least annually or whenever the wastes change or the processes that generate or treat the wastes change. [§§264/265.1034(e)]

Up-to-date information on whether or not a process vent is subject to regulation must be recorded in the facility operating record. [§§264/265.1035(f)]

Knowledge-based determinations

Rather than using direct measurement to determine the organics concentration, an owner/operator can use his/her knowledge of the waste’s composition to determine if the organic content is less than 10 ppmw. If a knowledge-based determination is made, §§264/265.1034(d)(2) specify that documentation of the waste determination is required.

The first example of using knowledge in §§264/265.1034(d)(2) discusses understanding of the process and allows an owner/operator to document that “no organic compounds are used” in the production process. In some instances, knowing that your production process does not use organics may be sufficient. However, using knowledge of the process should also entail a detailed look at all the chemical constituents formed in the production process.

Other examples in §§264/265.1034(d)(2) utilize knowledge of the process which stems from analytical testing. In the second example, an owner/operator may use information from waste generated by a process at another facility provided that the processes are identical and the other facility has previously demonstrated, by direct measurement, that the waste stream has a total organic content less than 10 ppmw. In the third example, prior speciated analytical results may be used for a waste stream for which an owner/operator can document that no process changes have occurred since that analysis that could affect the total organic concentration. In both the second and third examples, if analytical results were obtained for another regulatory purpose, or the analytical results may not have considered organics formed in the production process, direct measurement may be the most conservative approach.

Only certain categories of units are regulated

Not all units as described above are subject to Subpart AA. Only units that fit into three categories are regulated. First, units that are subject to the permitting standards of Part 270 (i.e., they either are in interim status or have a RCRA permit) are subject to Subpart AA. Such units are not very common because most of the processes described above are recycling processes, and recycling units are not subject to permitting per §261.6(c)(1). The following units that are exempt from RCRA permitting are also exempt from Subpart AA:

  • Manufacturing process units (such as distillation columns that generate hazardous waste still bottoms; these units are not subject to Subpart AA because they are not managing hazardous wastes)—see exemption in §261.4(c).
  • Totally enclosed treatment facilities—see exemption in §270.1(c)(2)(iv).
  • Elementary neutralization units—see exemption in §270.1(c)(2)(v).
  • Wastewater treatment units—see exemption in §270.1(c)(2)(v).
  • Closed-loop recycling units—see exemption in §261.4(a)(8).

Second, if a recycling unit is located at a facility that has a RCRA permit (e.g., for hazardous waste storage tanks), the recycling unit is subject to Subpart AA. [§261.6(d)] On the other hand, if the recycling unit is located at a generator’s facility that does not have or need a RCRA permit for any treatment, storage, or disposal activities, the recycling unit is not subject to Subpart AA.

Third, 90-day units (that are not recycling units) are subject to Subpart AA. This applicability provision is unlikely to be triggered because 90-day units are typically not used for distillation, fractionation, solvent extraction, thin-film evaporation, etc. One situation where Subpart AA applies is when a 90-day tank is used to air strip wastewater for organics removal and the treated water is disposed by underground injection.

The following examples should help clarify the applicability of Subpart AA. [EPA/450/3-89/021, RO 14461]

A large quantity generator (LQG) stores spent solvent in a 90-day tank, processes it in a batch still, and then reuses the recovered solvent in a manufacturing process. No other hazardous waste management activities occur at the site. Is the batch still subject to Subpart AA?

No. Recycling units at facilities that don’t have interim status or a RCRA permit for some other hazardous waste management activity are not subject to Subpart AA.

In the preceding example, the spent solvent is stored in a tank for more than 90 days prior to distillation. Is the still subject to Subpart AA?

Maybe. If the solvent recovery system meets the closed-loop recycling with reclamation criteria of §261.4(a)(8) or qualifies as reclamation under the control of the generator per §261.4(a)(23), the still is not processing solid or hazardous waste and Subpart AA does not apply. On the other hand, if §§261.4(a)(8) and 261.4(a)(23) do not apply, the still is subject to Subpart AA because storing spent solvents for more than 90 days requires a RCRA storage permit. Recycling units at facilities subject to RCRA permitting are subject to Subpart AA.

A generator processes spent solvent (D001) in a still without prior storage. Still bottoms are accumulated in containers for less than 90 days before being shipped offsite. Somewhere else on the site, hazardous wastewater is stored in a surface impoundment. Does Subpart AA apply to the still?

Yes. Because a RCRA permit is required for the surface impoundment, Subpart AA applies to the still.

At an industrial facility, hazardous ground water is pumped to a feed tank and then to an air stripping column for organics removal. The ground water contains >10 ppmw organics. Treated ground water is piped directly to the facility’s NPDES-permitted outfall point. Does Subpart AA apply to the air stripper?

Probably not. The air stripper probably meets the definition of a tank and qualifies for the wastewater treatment unit exemption, as would the feed tank. [§§264.1(g)(6), 265.1(c)(10), 270.1(c)(2)(v)] Subpart AA does not apply to wastewater treatment units as defined in §260.10.

In the previous example, treated ground water is reinjected rather than being discharged under an NPDES permit. Does Subpart AA apply to the air stripper?

Yes. Under these circumstances, the air stripper is not a wastewater treatment unit and may be subject to permitting unless it is operated as a 90-day tank. However, 90-day tanks used for air stripping (and that are not used for recycling) are subject to Subpart AA.

An offsite solvent recycler stores spent solvents in tanks prior to processing them in a fractionation column. Does Subpart AA apply to the vents associated with the column?

Yes. The offsite facility needs a RCRA permit to store the solvent in tanks (i.e., the tanks are not 90-day units because the recycler is not the generator of the spent solvent). Fractionation units recycling hazardous wastes at facilities having a RCRA permit are subject to Subpart AA.

A generator excavates hazardous soil containing >10 ppmw organics and plans to put the soil in a roll-off box fitted with air sparging equipment. Is this activity subject to Subpart AA?

Probably. This sounds like air stripping, even though it doesn’t meet the definition in §264.1031. The roll-off box is probably a 90-day container (otherwise, the generator would need a treatment permit). Air stripping units that are 90-day containers are subject to Subpart AA.

Only certain process vents are regulated

Subpart AA controls emissions from “process vents,” which are defined as “any open-ended pipe or stack that is vented to the atmosphere either directly, through a vacuum-producing system, or through a tank (e.g., distillate receiver, condenser, bottoms receiver, surge control tank, separator, or hot well) associated with hazardous waste distillation, fractionation, thin-film evaporation, solvent extraction, or air or steam stripping operations.” [§§264/265.1031]

Particular attention needs to be directed to the term “associated with” in this definition. The preamble to the original Subpart AA rule [June 21, 1990; 55 FR 25461] stated:

“Thus, the final vent standards apply to: 1) vents on distillation, fractionation, thin-film evaporation, solvent extraction, and air or steam stripping processes and vents on condensers serving these processes; and 2) vents on tanks (e.g., distillate receivers, bottoms receivers, surge control tanks, separator tanks, and hot wells associated with distillation, fractionation, thin-film evaporation, solvent extraction, and air or steam stripping processes) if emissions from these processes are vented through the tank. For example, uncondensed overhead emitted from a distillate receiver (which fits the definition of a tank) serving a hazardous waste distillation process unit is subject to these [Subpart AA] air controls. On the other hand, emissions from vents on tanks or containers that do not derive from a process unit specified above are not covered by these rules. For example, if the condensed (recovered) solvent is pumped to an intermediate holding tank following the distillate receiver mentioned in the above example, and the intermediate storage tank has a pressure-relief vent (e.g., a conservation vent) serving the tank, this vent will not be subject to the process vent standards.” [Emphasis in original.]

Figure 3 illustrates these concepts. Four vents are located on the storage tanks and distillation system. Vents #1 and 4 are not associated with the distillation system and are not subject to Subpart AA. Uncondensed overhead vapors from the distillation process are not being released from these vents. These vents are probably conservation vents designed to protect the mechanical integrity of the tanks. On the other hand, Vents #2 and 3 are releasing uncondensed overhead vapors from the distillation process and are subject to Subpart AA. [EPA/450/3-89/021]

Figure 3

Process vents in compliance with CAA requirements are exempt

To eliminate regulatory overlap between the CAA (which may also regulate emissions from vents) and RCRA, EPA codified an exemption at §§264.1030(e)/265.1030(d). In the Federal Register preamble establishing this exemption, the agency stated that if Subpart AA-regulated process vents are equipped with and operating air emission controls in accordance with CAA requirements in 40 CFR Part 60—Standards of Performance for New Stationary Sources, Part 61—National Emission Standards for Hazardous Air Pollutants, or Part 63—National Emission Standards for Hazardous Air Pollutants for Source Categories, this constitutes compliance with Subpart AA. Two caveats exist, however, for this CAA exemption:

  1. “[T]oday’s exemption is only available at a facility where each and every process vent that would otherwise be subject to Subpart AA is equipped with, and operating, air emission controls in compliance with an applicable CAA standard under Parts 60, 61, or 63…. [T]o comply with the requirements at paragraphs [§264.1030(e) or §265.1030(d)], the emissions from each Subpart AA process vent must be routed through an air emission control device; a vent that is in compliance with a CAA standard under an exemption from control device requirements is not in compliance with those provisions of Subpart AA.” [December 8, 1997; 62 FR 64638–9]
  2. “A process vent that is in compliance with a CAA standard under an exemption from control requirements (i.e., is not equipped with and operating a control device) does not meet the criteria established in the provisions paragraph §264.1030(e) or §265.1030(d) of Subpart AA. Therefore, a unit that does not use the required air emission controls but is in compliance with a NESHAP through an ‘emission averaging’ or ‘bubbling’ provision does not qualify for the exemption. Similarly, if the Clean Air Act standard for the particular unit is no control (for example, because the Maximum Achievable Control Technology (MACT) floor for the source category is no control and the agency decided not to apply controls more stringent than the floor), the exemption from the RCRA standards under §264.1030(e) or §265.1030(d) of Subpart AA would not apply since the unit would not actually be controlled (i.e., equipped and operating air emission controls) under provisions of the MACT standard.

“To take the above example a step further, at a facility where all but one of the Subpart AA process vents are equipped with air emission controls for compliance under CAA rules and the one uncontrolled Subpart AA process vent is also in compliance with a CAA regulation but is not controlled for air emissions, the facility’s Subpart AA process vents do not meet the applicability exemption criteria as stated in Subpart AA and thus are not exempt from the rule under §264.1030(e) or §265.1030(d). Despite this restriction, EPA considers this alternative to provide the facility owner or operator with a broader degree of compliance flexibility, and less extensive monitoring, recordkeeping, and reporting requirements under RCRA.” [EPA Region 4, CAA and RCRA Overlap Provisions in Subparts AA, BB, and CC of 40 CFR Parts 264 and 265, October 2000, available at http://www.trainex.org/web_courses/subpart_x/TopicSearch pdf files/pdf docs ABC/Final Overlap Provisions.pdf]

Complying with Subpart AA

The preceding discussion serves to identify process vents that are subject to Subpart AA. Once affected vents are identified, the next step is to determine how to comply with Subpart AA control standards.

Subpart AA emission limits

Once an owner/operator has identified process vents subject to Subpart AA, he/she must determine the organic emission rate for each vent and then aggregate the emissions for the entire facility. Emission rates may be determined via engineering calculations or direct source tests. The regulations [§§264/265.1035(b)(2)(ii)] specify:

“[D]eterminations of vent emissions…must be made using operating parameter values (e.g., temperatures, flow rates, or vent stream organic compounds and concentrations) that represent the conditions that result in maximum organic emissions, such as when the waste management unit is operating at the highest load or capacity level reasonably expected to occur. If the owner or operator takes any action (e.g., managing a waste of different composition or increasing operating hours of affected waste management units) that would result in an increase in total organic emissions from affected process vents at the facility, then a new determination is required.”

The aggregated facility emission rate is compared against both a short-term limit [3 lb/hr (1.4 kg/hr)] and a long-term limit [3.1 tons/year (2.8 Mg/yr)]. If the organic emissions from all affected vents at the facility never exceed either limit, no controls are required.

If the aggregated facility emission rate exceeds either limit, two options are available [§§264/265.1032(a)]:

  1. Reduce facility-wide emission rates below both limits, or
  2. Install control devices capable of reducing facility-wide, aggregated emissions by 95 weight percent [enclosed combustion devices such as fume incinerators, boilers, or process heaters may meet alternative performance standards per §§264/265.1033(c)].

Note that emission reductions have to be met on a facility-wide, aggregated basis. [EPA/530/K-02/003I, October 2001, available from http://www.epa.gov/sites/production/files/2015-09/documents/air.pdf] In other words, if a facility has 10 affected vents, nine of which have very low emissions and one that has high emissions, installing a control device on only the high-emission source might succeed in meeting the emission limits. In this case, no controls would be necessary for the other nine vents.

Subpart AA control devices

If emissions must be reduced, the reduction may be accomplished via changes in operating practices or via the use of add-on control devices. Add-on control devices would be connected to the process vent via a “closed-vent system,” which is defined as “a system that is not open to the atmosphere and that is composed of piping, connections, and, if necessary, flow-inducing devices that transport a gas or vapor from a piece or pieces of equipment to a control device.” [§§264/265.1031]

The regulations do not require the use of any specific type of control equipment; however, performance requirements are specified for vapor-recovery systems [condensers and carbon adsorbers—see §§264/265.1033(b)], enclosed combustion devices [§§264/265.1033(c)], and flares [§§264/265.1033(d)]. Other types of control devices may be used as long as they meet the emission limits cited previously.

In order to control emissions from a distillation column, a facility wants to utilize a vapor-recovery system that includes the primary condenser attached to the column. How does the use of this condenser figure into the 95% recovery efficiency requirement?

According to EPA, “[v]apor-recovery systems whose primary function is the recovery of organics for commercial or industrial use or reuse (e.g., a primary condenser on a waste solvent distillation unit) are not considered a control device and should not be included in the 95-percent emission reduction determination.” [June 21, 1990; 55 FR 25463]

EPA/450/3-89/021 gives detailed information on various control devices for complying with the Subpart AA regulations.

Inspection and monitoring

If control devices are used to comply with Subpart AA requirements, the devices must be routinely inspected and monitored in accordance with §§264/265.1033(f) and (i). In general, vent stream flow rates must be recorded at least every hour, process-specific parameters (e.g., combustion chamber temperatures in fume incinerators) must be monitored continuously, and the monitoring devices must be inspected at least once each operating day. If a problem is found, corrective measures must be implemented immediately.

Closed-vent systems conveying vapors to the control device are also subject to inspection per §§264/265.1033(k–m). These systems must either operate with no detectable emissions (<500 ppmv above background) or operate under negative pressure. After an initial inspection, annual reinspection is required; reinspection is also required whenever equipment components are changed or repaired. If a leak is detected, emissions must be controlled as soon as practicable, but within 15 calendar days at most.

Recordkeeping and reporting

According to EPA:

“The RCRA air emission standards are meant to be self-implementing. Consequently, EPA does not play an active role in ensuring that facilities are in compliance with the regulations on a day-to-day basis. Instead, the agency verifies that owners and operators are complying with the regulations through facility inspections and audits. Subpart AA [§§264/265.1035] requires owners and operators to keep detailed records in the facility’s operating log to demonstrate compliance with the standards. Design documentation and monitoring, operating, and inspection information for each unit subject to the Subpart AA standards must be kept up-to-date and in the facility’s operating log for at least three years.

“Subpart AA also requires that, every six months, owners and operators of permitted facilities report to the Regional Administrator any instances during that time period when a control device exceeded or operated outside of its design specifications (as indicated by monitoring devices) for a period of longer than 24 hours without being corrected (§264.1036). The report must indicate the duration of each exceedance and any corrective measures taken to remedy the situation. If during the six-month period no exceedances occur at the facility, the owner and operator need not submit a report to the Regional Administrator.” [EPA/530/K-02/003I]

Note that the only difference between the Subpart AA regulations in Part 264 and Part 265 is that Part 264 includes this reporting requirement for permitted facilities. Interim status facilities, regulated under Part 265, are not required to submit the report.


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This document addresses issues of a general nature related to the federal RCRA regulations. Persons evaluating specific circumstances dealing with the RCRA regulations should review state and local laws and regulations, which may be more stringent than federal requirements. In addition, the assistance of a qualified professional should be enlisted to address any site-specific circumstances.