November 11, 2015

Determining If a Waste Is a Liquid

Since the criteria for ignitable waste determinations in §261.21 are different for liquids versus nonliquids, the generator must first determine if his/her waste is a liquid. There are a number of ways this can be accomplished. A knowledge-based determination may be made: “EPA believes that, for purposes of the characteristics of ignitability and corrosivity, it will generally be obvious whether or not the waste is a liquid.” [April 30, 1985; 50 FR 18372] For instance, a generator can use knowledge to determine that waste paint thinner or solvent is a liquid. For wastes with both a liquid and solid phase (semisolids), however, it can be more difficult to make such a determination.

For these materials, generators commonly run the Paint Filter Liquids Test (Method 9095B in SW–846). This test, which was added to SW–846 in 1985 [50 FR 18370], is used to determine the presence of free liquids in a representative sample of a waste. [RO 13601] The sample is placed in a conical paint filter and allowed to drain for 5 minutes; if even one drop of liquid passes through the filter in that time frame, the sample contains a free liquid. Although the paint filter test was added to the agency’s test methods to ensure no free liquids were disposed into landfills, EPA noted that “this test provides a practical method of testing ignitable and corrosive materials to determine the presence of liquids, and assists the regulated community in complying with the Part 261 requirements until further evaluation is done.” [50 FR 18372]

Chapter 7 of SW–846 (which discusses the four hazardous waste characteristics and how they should be evaluated) currently includes the following statement: “Use Method 9095…to determine free liquid.” EPA proposed in 1993 to modify that language by replacing the paint filter test with the pressure filtration technique specified in Method 1311 (the toxicity characteristic leaching procedure or TCLP). [August 31, 1993; 58 FR 46052] Although the agency never finalized that proposal, the pressure filtration technique remains its preferred method:

“The definitive procedure for determining if a waste contains a liquid for the purposes of the ignitability and corrosivity characteristics is the pressure filtration technique specified in Method 1311. However, if one obtains a free liquid phase using Method 9095, then that liquid may instead be used for purposes of determining ignitability and corrosivity. However, wastes that do not yield a free liquid phase using Method 9095 should then be assessed for the presence of an ignitable or corrosive liquid using the pressure filtration technique specified in Method 1311.” [January 13, 1995; 60 FR 3092]

The pressure filtration technique specified in Method 1311 defines a “liquid” as the material that passes through a 0.6- to 0.8-micron filter when a representative sample of the waste is subjected to a 50-psi differential pressure. Clearly, this is a more stringent procedure than the gravity-based paint filter test, but EPA has never codified this requirement in the regulations or in SW–846—it is just guidance. In fact, two weeks after the agency issued the above quoted preamble language preferring the pressure filtration technique over the paint filter test, it released guidance noting that, until it finalizes the proposed change in SW–846, “the paint filter test is the method to use to determine if a free liquid is present for ignitability determination.” [RO 11935]

In other guidance found in EPA’s RCRA FAQ Database, EPA takes the position that if the paint filter test does not produce free liquid, the generator must use the pressure filtration test. Note that the FAQ Database guidance changed the word “should” in the above quote from the 1995 Federal Register to “must.” In any case, the guidance on which methodology to use is conflicting. The most conservative approach would be to use the pressure filtration technique.


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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.