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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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De Minimis Wastewater Exemptions

Under §261.3(a)(2)(iv)(A–G), mixtures of very small quantities of certain listed hazardous wastes and wastewater managed in a Clean Water Act (CWA) wastewater treatment system are exempt from regulation as hazardous wastes. These so-called “de minimis” wastewater exemptions prevent the mixture rule from applying to large-volume, nonhazardous wastewaters that 1) get mixed with small quantities of listed hazardous wastes that are not principal waste streams, and 2) are managed in a facility’s onsite wastewater treatment system. Accordingly, if wastewater mixtures meet EPA’s criteria (described below), they are not hazardous wastes once they reach the headworks of the wastewater treatment system, and they will not produce wastewater treatment sludges that are listed hazardous waste. This is the primary reason the de minimis wastewater exemptions were promulgated: to protect wastewater treatment sludge from carrying listed waste codes.

EPA believes that these mixtures of wastewater and small amounts of listed wastes do not pose a substantial threat to human health and the environment because the relatively small amounts of listed wastes in these mixtures are at very low (often parts-per-billion) concentrations. Furthermore, the listed wastes are treated in the plant’s onsite wastewater treatment system, further reducing their hazardousness. [November 17, 1981; 46 FR 56583, October 4, 2005; 70 FR 57771]

The §261.3(a)(2)(iv) de minimis wastewater exemptions are sometimes referred to as the “headworks exemptions” because they exempt eligible wastewater mixtures from the mixture rule when they reach the headworks of the facility’s wastewater treatment system. [RO 11116, 11614] For instance, if a facility’s wastewater treatment system leaks waste that meets a listing description before it reaches the headworks, the leaked material is classified as listed hazardous waste. [RO 14095] So where is the headworks of a specific wastewater treatment system? EPA says that it is where “final combination of raw or pretreated process wastewater streams typically takes place.” [October 4, 2005; 70 FR 57775] “[H]eadworks can include a central catch basin for industrial wastewaters, a pump station outfall, equalization tank, or some other main wastewater collection area that exists in which transport of process wastewaters stops and chemical or biological treatment begins.” [April 8, 2003; 68 FR 17242]

The de minimis wastewater exemptions cover seven different types of listed wastes, but only if they are mixed with wastewater that is subject to regulation under Section 307(b) or 402 of the CWA. (Section 307(b) deals with discharges to POTWs; Section 402 deals with NPDES-permitted discharges.) Facilities that have eliminated their wastewater discharge as a result of NPDES or pretreatment program requirements (i.e., zero dischargers) may also take advantage of these exemptions. The agency has noted that facilities that have eliminated the discharge of wastewaters using permitted Class I injection wells can claim the de minimis exemptions: “EPA continues to believe that underground injection wells can meet the headworks’ definition of zero discharge if the injection well is being used for the purposes of complying with a NPDES permit, other applicable effluent guideline, or pretreatment program requirements.” [October 4, 2005; 70 FR 57777]

The seven specific exemptions are discussed in the following sections.

Listed spent solvents

Two exemptions are provided for spent solvents that are discharged to a plant’s wastewater system:

  1. Listed spent solvents that are carcinogens—Four F001/F002/F005 spent solvents (benzene, carbon tetrachloride, tetrachloroethylene, and/or trichloroethylene) in wastewater mixtures at a concentration of no more than 1 ppm are not hazardous. [§261.3(a)(2)(iv)(A)]
  2. Listed spent solvents that are toxic—Fourteen F001–F005 spent solvents (methylene chloride, 1,1,1-trichloroethane, chlorobenzene, o-dichlorobenzene, cresols, cresylic acid, nitrobenzene, toluene, methyl ethyl ketone, carbon disulfide, isobutanol, pyridine, chlorofluorocarbon solvents, and/or 2-ethoxyethanol) in wastewater mixtures at a concentration of no more than 25 ppm are not hazardous. [§261.3(a)(2)(iv)(B)]

These two regulatory sections exempt wastewater containing certain spent solvents listed in §261.31 if the appropriate concentration limit is not exceeded. What EPA has in mind here are spills or incidental losses from degreasing, maintenance, or manufacturing operations in which small amounts (not principal waste streams) of spent solvents are washed or otherwise released into a sump or drain and are subsequently managed in the onsite wastewater treatment system. [RO 11845] The agency believes that these small releases of organic solvents can be reasonably and efficiently managed by the chemical or biological wastewater treatment system associated with most affected facilities.

The exemption also applies to scrubber water generated during combustion of these spent solvents if it is discharged to the combustion facility’s wastewater treatment system. Extending the de minimis exemption to such scrubber waters is a departure (see October 4, 2005; 70 FR 57777) from the agency’s previous position (see RO 11116, 11845). EPA justified adding scrubber waters derived from such combustion, noting that “these scrubber waters would be comparable in expected constituents and concentration levels with the already exempted F-listed solvents.” [70 FR 57777]

However, the de minimis exemption does not apply to spent solvent-contaminated sludges that are mixed or otherwise commingled with wastewater or to such sludges that generate a wastewater through dewatering. [RO 12283]

Two options are allowed to demonstrate compliance with the 1- or 25-ppm limits:

  1. The concentration limit may be calculated from the maximum total weekly usage of these solvents (other than the amounts that can be demonstrated not to have been discharged to wastewater) divided by the average weekly flow of wastewater into the headworks of the facility’s wastewater treatment system. Facilities must be able to prove by mass flow calculations that the applicable limits are not exceeded. The solvent-usage numbers can be determined via an audit of records already maintained at most facilities (e.g., invoices of solvent purchases, logs showing the quantities of solvents that were distributed to various locations throughout the plant, spent solvent quantities shipped offsite, and other operating records). New audits or calculations must be made whenever a change in the facility’s operations could affect the amount of spent solvents in the wastewater. [November 17, 1981; 46 FR 56585]
  2. The concentration limit may be determined by direct measurement of solvent concentrations, on an average weekly basis, at the headworks of the facility’s wastewater treatment system. This option is conditioned on 1) the facility being subject to a CAA regulation (or an enforceable limit in a federal operating permit) that minimizes fugitive process or wastewater emissions, 2) the facility identifying the headworks of its wastewater treatment system, and 3) a sampling and analysis plan being developed and submitted to EPA or the state.

The agency noted that “it is not necessary for the receiving wastewater treatment unit itself to be subject to CAA regulations. However, EPA stresses that the process streams and wastewater streams that lead up to the headworks point must be subject to CAA regulations, or an enforceable limit federal operating permit, that minimizes fugitive emissions.” [October 4, 2005; 70 FR 57774]

Wastewaters containing no more than 1 ppm benzene are exempt only if they are managed in an aerated biological wastewater treatment system that does not contain any unlined surface impoundments before secondary clarification.

Facilities may alternate between the two compliance options noted above or may use both methods and report the result of either method. [70 FR 57775]

Case Study 1 gives two examples of determining compliance with these spent solvent de minimis exemptions.

Case Study 1

Refinery wastes

Mixtures of one or more of the following K-wastes and wastewater are not hazardous if the wastes are discharged to the refinery’s oil recovery sewer upstream of primary oil/water/solids separation: K050 heat-exchanger bundle cleaning sludge, K169 crude oil storage tank sediment, K170 clarified slurry oil tank sediment, K171 spent hydrotreating catalyst, and K172 spent hydrorefining catalyst. [§261.3(a)(2)(iv)(C)]

This provision exempts wastewaters from refinery cleaning operations in certain situations. The exemption is not designed to allow the discharge of the entire waste stream (e.g., tank sediments or spent catalysts) into the wastewater collection and treatment system; rather, dilute wastewaters generated during tank or unit cleanout and dewatering operations are covered. [August 6, 1998; 63 FR 42120] The listed wastes excluded under this exemption are discussed below:

  • K050 heat-exchanger bundle cleaning sludge—This hazardous waste is generated during periodic backflushing and/or hydroblasting of heat-exchanger bundles. The sludges from these routine maintenance operations (listed as hazardous due to the presence of hexavalent chromium) are usually discharged to the refinery’s sewer system. EPA believes that mixtures of K050 sludges and nonhazardous wastewater do not pose a threat to human health and the environment because the hexavalent chromium from the sludge is almost completely reduced to the trivalent state by reducing agents, such as sulfides, in the raw wastewater. Furthermore, the chromium is present in very low concentrations.
  • K169 crude oil storage tank sediment and K170 clarified slurry oil tank sediment—Wastewater containing these wastes is typically generated by dewatering (e.g., centrifuging) the sediment removed from tanks and by final rinsing of the tanks themselves. However, these residues are dilute and contain low levels of the listed wastes.
  • K171 spent hydrotreating catalyst and K172 spent hydrorefining catalyst—Application of the headworks exemption to these catalysts allows refiners to continue to use water to cool and wash out spent catalysts from process units prior to further management. Wastewater recovered from “drilling out” the catalyst, steam stripping or washing, or pad drainage are all covered by the de minimis exemptions.

EPA noted that, although the wastewaters generated from the refinery cleaning operations noted above are excluded from hazardous waste management, any wastes discharged to the wastewater collection system that were not previously counted toward the total annual benzene quantity under the benzene NESHAP rule (40 CFR Part 61, Subpart FF) will have to be included in those calculations.

De minimis losses of commercial chemical products and F- and K-wastes

EPA recognizes that small amounts of products that are being produced by, or used as raw materials in, a manufacturing process are often unavoidably lost during normal material handling operations. Many of these materials are P- and U-listed hazardous wastes in §261.33 when discarded. Additionally, small amounts of F- and K-wastes are also inadvertently lost during normal material handling activities. All of these materials are typically disposed by draining or washing them into floor drains leading to the wastewater treatment system, a “reasonable and practical means of disposing of these lost materials.” [November 17, 1981; 46 FR 56586]

Since these small quantities of listed wastes can be managed and treated in the facility’s wastewater treatment system without posing a substantial hazard to human health and the environment, the agency allows mixtures of wastewater (which will be discharged under the CWA) and de minimis losses of §261.31 F-wastes, §261.32 K-wastes, and §261.33 P- and U-chemicals to be exempt from hazardous waste management. [§261.3(a)(2)(iv)(D)] The de minimis exemption for F- and K-wastes was added on October 4, 2005. [70 FR 57769] Examples of the de minimis exemption as it applies to commercial chemical products are included in Case Study 2.

Case Study 2

Sometimes, a chemical is produced at a plant, not as an end product for sale, but as an intermediate in the manufacture of a different chemical. This wastewater exemption applies to de minimis losses of chemicals listed in §261.33(e) and (f), whether they are raw materials, end products, or chemical intermediates.

What constitutes a de minimis loss? First, when EPA promulgated the exemption in 1981, the Federal Register preamble contained the following discussion:

“Data provided by [the Chemical Manufacturers Association] for several chemical manufacturing operations…show that the incremental amounts of §261.33 materials reaching the wastewater treatment system as a result of spills, leaks, maintenance, and laboratory activities usually constitute a small percentage (in all cases except one, less than one percent) of the total amount of such materials contained in the wastewater influent. For example, a plastics manufacturing plant using acrylonitrile [a commercial chemical product listed in §261.33(f)] discharges into its wastewater treatment system only 8 lb per day of discarded acrylonitrile resulting from equipment leakage and cleanup, relief-device discharges, and line rinsings, whereas the quantity of acrylonitrile introduced into its wastewaters from the manufacturing process per se amount to 800 lb per day.” [November 17, 1981; 46 FR 56583]

Beyond this preamble language, the regs only provide a qualitative description. De minimis losses of F-, K-, P-, and U-wastes include “those from normal material handling operations (e.g., spills from the unloading or transfer of materials from bins or other containers, leaks from pipes, valves, or other devices used to transfer materials); minor leaks of process equipment, storage tanks, or containers; leaks from well maintained pump packings and seals; sample purgings; relief-device discharges; discharges from safety showers and rinsing and cleaning of personal safety equipment; and rinsate from empty containers or from containers that are rendered empty by that rinsing.” [§261.3(a)(2)(iv)(D)]

Elsewhere, the agency said that large material losses would void the de minimis quantity exemption. [RO 14095] EPA added “inadvertent releases” to the §261.3(a)(2)(iv)(D) de minimis losses qualitative description on October 4, 2005. At that time, the agency clarified that de minimis losses “must be minor and must result from normal operating procedures at well-maintained facilities…. [A]ny large intentional losses of these wastes will not be considered as de minimis and, accordingly, will not be exempted under §261.3(a)(2)(iv)(D)…. [L]osses, which result from mismanagement, neglectfulness, or carelessness during normal operating procedures are not (and have never been) included in the exemption.” [70 FR 57778–9] Other than the above guidance, the dividing line between large and de minimis losses is probably going to be decided by state policy.

The de minimis losses exemption does not apply to wastewater mixtures created as a result of:

  • Discarding off-specification §261.33 materials; or
  • Discarding §261.33 materials during abnormal manufacturing operations (e.g., plant shutdowns or operational malfunctions resulting in substantial spills, leaks, or other releases). [46 FR 56586]

The agency believes that the above materials are not de minimis losses but can reasonably be segregated from wastewater and shipped offsite as hazardous wastes.

Note that EPA provided some disconcerting guidance about a de minimis loss being “rinsate from empty containers or from containers that are rendered empty by that rinsing.” On January 31, 1991, the agency noted that only the “third rinse in triple rinsing” would meet the definition of rinsate from containers that are rendered empty by that rinsing. [56 FR 3869]

In addition to adding small losses of F- and K-wastes to the §261.3(a)(2)(iv)(D) de minimis exemption, the October 2005 final rule also expanded the types of facilities that are eligible to claim the exemption. Previously, this exemption was available only to manufacturing facilities. Now, the federal RCRA program allows nonmanufacturing plants, such as raw material storage terminals and hazardous waste TSD facilities, to claim the exemption. [70 FR 57772] (States must adopt the October 4, 2005 rule into their authorized programs for facilities to take advantage of this expansion.)

However, before a manufacturing facility can claim a de minimis exemption for any F- or K-waste and before a nonmanufacturing facility can claim a de minimis exemption for any F-, K-, P-, or U-waste, the facility must identify all expected constituents in these wastes that may be released at the CWA discharge. For de minimis releases of an F- or K-waste, the list of constituents includes: 1) all constituents in Appendix VII of Part 261 for which the waste was listed, and 2) all constituents of concern in the §268.40 table of LDR treatment standards associated with the waste code. For de minimis losses of a P- or U-waste, this list of constituents includes: 1) the specific chemical associated with the P- or U-code, and 2) all constituents of concern in the §268.40 table of LDR treatment standards associated with the waste code. These identified constituents must be submitted to the NPDES permitting authority (or pretreatment control authority if wastewater discharge is to a sewer line flowing to a POTW), and this notification must occur before the expanded portions of the de minimis exemption may be claimed. Once notified, the NPDES permit writer or pretreatment control authority will determine if permit limits or pretreatment standards need to be added to the CWA permit to account for the new de minimis losses into the facility’s wastewater treatment system.

Laboratory wastes

Laboratory wastewaters containing listed wastes that were listed due to their toxicity are eligible for the de minimis exemption if: 1) the annualized average flow of laboratory wastewater does not exceed 1% of total wastewater volumetric flow into the headworks of the facility’s wastewater treatment system, or 2) the combined annualized average concentration of toxic §§261.31, 261.32, and 261.33 wastes resulting from laboratory operations does not exceed 1 ppm at the headworks. Toxic wastes used in laboratories that are demonstrated not to have been discharged to wastewater do not have to be included in this calculation. [§261.3(a)(2)(iv)(E)] See Case Study 3 for examples of the laboratory de minimis exemption.

Case Study 3

The laboratory de minimis wastewater exemption applies to incidental losses of listed hazardous waste (e.g., small amounts of listed spent solvents, listed wastes brought in for analysis, §261.33 chemicals used during the analysis, etc. that are added essentially unavoidably to large volumes of water) from laboratory operations only. Examples include laboratory spills washed into a sink drain and residues from washing of glassware that are carried into the sewer. Deliberate bulk discharges of chemicals that are not part of laboratory operations are not covered. “The introduction of other listed wastes into the plant wastewater system [outside the conditions set forth in §261.3(a)(2)(iv)(E)] may void the exclusion for the facility.” [RO 11727; see also RO 12257]

Both analytical and research and development laboratories may qualify for this exemption. However, EPA noted that a pilot plant is not considered to be a laboratory operation, and wastes generated from such a unit are not exempted under this section. [November 17, 1981; 46 FR 56587]

Facilities may prove compliance with the 1% wastewater flow limit by measuring (EPA’s preferred approach) or conservatively calculating the annual average wastewater discharge from the laboratory and the annual average wastewater flow entering the wastewater treatment system.

EPA recognizes that, even though some laboratories may exceed the 1% limit, they still may not discharge enough listed hazardous wastes into their drains to warrant regulation under the mixture rule. As a result, the agency allows the 1-ppm compliance test to be used in lieu of the 1% wastewater flow limit. If a facility chooses this option, compliance may be demonstrated using an audit of laboratory chemical purchases, an estimate of the aggregate amounts of toxic §§261.31, 261.32, and 261.33 materials disposed, and estimates of wastewater flow into the headworks of the treatment or pretreatment facility. “Facilities must make the worst-case assumption that all listed hazardous wastes used in the laboratories will be discarded to wastewater, unless they can demonstrate through appropriate records that these materials were disposed of elsewhere.” [November 17, 1981; 46 FR 56587]

Carbamate production wastes

De minimis losses of certain wastes (K156 and K157) from the production of carbamates and carbamoyl oximes are also exempt when they are mixed with wastewater and pass through the headworks of a facility’s wastewater treatment system. [§261.3(a)(2)(iv)(F–G)] These exemptions operate similarly to those for losses of listed spent solvents discussed above; that is, the exemptions are predicated on meeting certain chemical concentration limits at the headworks of the wastewater treatment system.

Miscellaneous issues

The de minimis wastewater exemptions apply only to mixtures that are sent to a facility’s onsite wastewater treatment system discharging under Section 402 or 307(b) of the CWA. Any mixtures of listed wastes and wastewater that are sent offsite must be managed as hazardous waste, including the use of a manifest. [RO 14181]

The exemptions in §261.3(a)(2)(iv) do not limit the manner in which the listed wastes are transported to the wastewater treatment plant. Therefore, the wastes can be transported via truck, for example, in addition to direct discharge to a sewer and still qualify for the exemption. [RO 13488, 14181]

The de minimis wastewater exemptions do not apply to facilities which discharge into privately owned treatment works. However, the privately owned treatment works may qualify for a de minimis exemption if its own discharge is subject to regulation under an NPDES permit or pretreatment program and any listed wastes that it generates meet the §261.3(a)(2)(iv) exemption criteria. [November 17, 1981; 46 FR 56584]

If a facility meets the criteria for one of the §261.3(a)(2)(iv) de minimis wastewater exemptions, does the exemption also apply to sludge produced by the wastewater treatment plant that treats the wastewater? Sludge generated from a wastewater that meets all of the criteria for a de minimis exemption would also be exempted from the hazardous waste listing. Since the wastewater is not listed at the headworks of the treatment facility (by virtue of the exemption from the mixture rule), the derived-from rule would not apply the listing to the resulting sludge. However, if the sludge exhibits a hazardous waste characteristic, it would be considered a hazardous waste for that reason. Additionally, if the influent wastewater meets a listing description not addressed by the de minimis exemptions, the sludge would be hazardous via the derived-from rule. [RO 13419, 13784]

As noted above, the de minimis wastewater exemptions apply to mixtures of listed hazardous wastes and wastewater at the headworks of the facility’s wastewater treatment system. However, the listed wastes are still hazardous at their point of generation. By implication, all RCRA hazardous waste recordkeeping requirements apply to these wastes until they reach the headworks. For example, the wastes would be subject to the one-time LDR notification required in §268.7(a)(7). [April 8, 2003; 68 FR 17242, RO 11727]


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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.