Previous Corner.

The LDR Dilution Prohibition

EPA has always been uneasy about allowing hazardous wastes to be diluted prior to disposal. This is a valid concern, because one of the easiest ways to meet a concentration-based treatment standard would be to simply dilute the waste. Therefore, the LDR program includes a dilution prohibition in §268.3 stating that dilution as a substitute for adequate treatment is prohibited.

This dilution prohibition applies only where an element of land disposal is involved. For example, if listed wastewater is treated in tanks and discharged under an NPDES permit, no land disposal is involved, no LDR treatment standard applies, and the LDR dilution prohibition is not triggered. If a hazardous sludge is generated in the wastewater treatment process, the dilution prohibition would apply from its point of generation to its point of disposal.

In some circumstances, dilution occurs in conjunction with adequate treatment, as discussed in the next two sections.

Aggregation for centralized treatment

For some large-scale operations, it is common to mix different wastes together prior to treatment. One such situation is illustrated in Figure 1, where different process wastes are consolidated and delivered to a RCRA-permitted incinerator. As each of the process wastes is consolidated, it is diluted. In some cases, this dilution might be sufficient to meet LDR treatment standards even though no treatment has occurred.

EPA terms this type of system “aggregation for centralized treatment.” The only way to avoid the inherent dilution in this system would be to treat each waste stream individually—this would clearly be uneconomical in large plants. Instead, the agency allows this inherent dilution as long as the type of treatment ultimately provided will remove or destroy the contaminants being diluted. Referring to Figure 1, incineration will destroy benzene, trichloroethylene, and methylene chloride in the combined waste; hence, dilution of these waste streams is permissible. On the other hand, incineration will not destroy P011—arsenic pentoxide. If this waste were consolidated with the rest, the agency would consider it impermissible dilution.

During a site cleanup, contaminated soil from various onsite locations is consolidated. Could this be considered impermissible dilution?

“If mixing occurs through the normal consolidation of contaminated soil from various portions of a site that typically occurs during the course of remedial activities or in the course of normal earthmoving and grading activities, then the agency does not consider this to be intentional mixing of soil with nonhazardous soil for the purposes of evading LDR treatment standards. Therefore, it is not viewed as a form of impermissible dilution….

“Some situations may require soil mixing, as part of a pretreatment process, to facilitate and ensure proper operation of the final treatment technology to meet the LDR treatment standards. If the mixing or other pretreatment is necessary to facilitate proper treatment in meeting LDR standards, then dilution is permissible. For example, addition of less-contaminated soil may be needed to adjust the contaminated soil Btu value, water content, or other properties to facilitate treatment. These adjustments would be for meeting the energy or other technical requirements of the treatment unit to ensure its proper operation. The agency views this type of pretreatment step as allowable, provided the added reagents or other materials produce chemical or physical changes and do not 1) merely dilute the hazardous constituents into a larger volume of waste so as to lower the constituent concentration, or 2) release excessive amounts of hazardous constituents to the air.” [EPA/530/R-02/003 (cited earlier), RO 14338]

Dilution as a consequence of treatment

Figure 2 illustrates another type of permissible dilution: adding large volumes of treatment reagent (stabilization agents) to a waste. Any dilution inherent in this process is permissible as long as the reagents are effective in treating the waste. In this example, the stabilization agents would have to reduce the leachability of the waste. If the reagent is sand, no effective treatment would be occurring, and the agency would consider the process to be impermissible dilution. [November 7, 1986; 51 FR 40592, July 8, 1987; 52 FR 25778]

The regulations specifically identify a case of impermissible dilution of this sort: “It is a form of impermissible dilution, and therefore prohibited, to add iron filings or other metallic forms of iron to lead-containing hazardous wastes in order to achieve any land disposal restriction treatment standard for lead.” [§268.3(d)]

When organic-containing hazardous wastes are solidified, would EPA consider this to be a form of impermissible dilution?

Maybe. Stabilization is a well-recognized process for treating metal-containing wastes. Stabilizing organic wastes, however, may be considered to be impermissible dilution when:

1. During stabilization, hazardous organic constituents are released to the air in an uncontrolled manner at levels that pose a risk to human health and the environment.
2. Organics in the waste prevent the stabilization reagents from bonding with metals in the waste.
3. Stabilization reagents such as lime or cement kiln dust increase the pH, solubility, and leachability of arsenates, chromates, and other metals.
4. Analytical tests after stabilization do not show adequate binding of organic constituents. Chosen stabilization reagents (such as clays, silica, alumina, zeolites, and/or activated carbon) must bond with organics so they are not extracted after stabilization. Provided an appropriate analytical method with proper QA/QC is chosen for “before” and “after” testing, EPA will not consider impermissible dilution to be the cause of concentration reductions because of the known adsorptive capability of the reagents mentioned earlier.
5. Site-specific leachate properties at the disposal site break down the adsorptive capacity of the reagents over time.

This information is extracted from “Draft Interpretive Memorandum on Stabilization of Organic-Bearing Hazardous Wastes to Comply with RCRA Land Disposal Restrictions,” September 2001, available at http://www.understandrcra.com/rccd/stabilizationoforganics.pdf.

In November 2016, EPA provided additional guidance on this topic: “If a solid waste is mixed with a characteristic hazardous waste, the solid waste must provide a useful and effective contribution to decharacterizing the hazardous waste (i.e., possess a unique property to remove the hazardous characteristic from the hazardous waste instead of merely diluting it).” [81 FR 85756]

McCoy’s dilution diagram

EPA’s rules and policies regarding when dilution is permissible are fragmented. Therefore, we have prepared the logic diagram in Figure 3 to simplify the issue. The diagram is self-explanatory and allows the user to determine when dilution is allowable and when it is prohibited. Pertinent references to the regulations or Federal Register preambles are included. Dilution examples using this diagram are given in Case Study 1.