Waste classification, general purpose

This Report is concerned with classification of hazardous waste for purposes of disposal. However, the principles and concepts embodied in the waste classification system could be applied in classifying hazardous materials for any other purpose. The classification system is intended to be applied to hazardous waste prior to disposal. It is not intended to be applied to screening or ranking of contaminated sites, including existing hazardous waste disposal sites, because these activities involve site-specific considerations that cannot be included in a generally applicable waste classification system. However, any wastes exhumed from contaminated sites that then require disposal would be included in the waste classification system. 

For the purpose of developing the waste classification system described in Section 1.4.1, a simple method of evaluating risks to the public posed by radionuclides and hazardous chemicals in waste is needed. The term risk generally refers to the probability of harm, combined with the potential severity of that harm. In the context of hazardous waste disposal, risk is the probability of a response in an individual or the frequency of a response in a population taking into... 

The waste classification system developed in this Report includes a general class of exempt waste. Waste in this class would contain sufficiently small amounts of hazardous substances that it could be managed in all respects as if it were nonhazardous (e.g., as household trash). NCRP intends that exempt materials could be used or disposed of in any manner allowed by laws and regulations addressing disposition of nonhazardous materials. However, exempt waste would not necessarily be exempt for purposes of beneficial use without further analysis of the risks associated with anticipated uses. Materials could be exempted for purposes of disposal or beneficial use based on similar considerations of acceptable risk. However, based on differences in exposure scenarios for the two dispositions, limits on the amounts of hazardous substances that could be present in exempt materials intended for beneficial use could be substantially lower than the limits for disposal as exempt waste. Thus, disposal may be the only allowable disposition for some exempt materials based on considerations of risk. In addition, some exempt materials may consist of trash, rubble, and residues from industrial processes that would have no beneficial uses and must be managed as waste. 

For radionuclides, NCRP reaffirms use of a best estimate (MLE) of the response probability obtained from a linear or linear-quadratic model as derived from data in humans, principally the Japanese atomic-bomb survivors. This model essentially is linear at the low doses of concern to waste classification. Specifically, for purposes of health protection of the public, NCRP reaffirms use of a probability coefficient for fatal cancers (probability per unit effective dose) of 0.05 Sv 1 (ICRP, 1991 NCRP, 1993a). Although this probability coefficient is less rigorous for intakes of some long-lived radionuclides that are tenaciously retained in the body than for other exposure situations, such as external exposure or intakes of short-lived radionuclides (Eckerman et al., 1999), it is adequate for the purpose of generally classifying waste, especially when the lack of data on cancer risks in humans for most chemicals is considered. 

Limits on amounts of hazardous substances in each waste class would be calculated based on values of the so-called risk index for each hazardous substance in the waste and the composite risk index for mixtures of hazardous substances. For the purpose of describing the recommended framework for a risk-based hazardous waste classification system, the risk index is generally defined as ... 

In general, calculation of the risk or dose from waste disposal in the numerator of the risk index in Equation 6.2 or 6.3 involves the risk assessment process discussed in Section 3.1.5.1. As summarized in Section 6.1.3, NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites should be used in calculating risk or dose for purposes of waste classification. Implementation of models describing exposure scenarios for inadvertent intruders at waste disposal sites and their associated exposure pathways generally results in estimates of risk or dose per unit concentration of hazardous substances in waste. These results then are combined with the assumptions about allowable risk discussed in the previous section to obtain limits on concentrations of hazardous substances in exempt or low-hazard waste. 

The risk index defined in Equation 6.1 (see Section 6.2.1) is intended to provide a measure of the potential risk that arises from disposal of any waste that contains hazardous substances. In Section 6.3, the general definition of the risk index is elaborated and recommendations on suitable approaches to calculating the risk index for individual hazardous substances are presented. For purposes of developing a comprehensive and risk-based waste classification system, a simple method of calculating the risk from disposal of mixtures of hazardous substances is needed. The method must take into account that the allowable concentrations of particular hazardous substances in waste of a given class generally will be lower when multiple substances are present than when only a single substance is present. Such a method is presented and discussed in this Section. 

The proposed framework for risk-based classification of all radioactive and hazardous chemical wastes developed in Section 6.2.2 represents waste classification in its broadest, most general terms. Thus, this classification system can be viewed as the highest level of a possible hierarchy of hazardous waste classifications (e.g., see Figure 4.2). Further subclassification of these broadly defined waste classes may be desirable for such purposes as protection of workers during waste operations, protection of public health and the environment following waste disposal, and development of efficient methods of waste management taking into account the characteristics of actual wastes. 

Development of a comprehensive and risk-based hazardous waste classification system, in which waste classes are defined in relation to types of disposal systems that are expected to be generally acceptable in protecting public health, would not obviate the need to establish waste acceptance criteria at each disposal site based on the characteristics of the site and engineered disposal facility and the properties of wastes intended for disposal therein. The primary purposes of a hazardous waste classification system are to facilitate cost-effective management and disposal of waste and effective communication on waste matters. 

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