Frequent Questions

Why are emission standards established for both metal volatility groups (semivolatile metals and low volatile metals) as well as particulate matter given that these metals are controlled by controlling particulate matter?

To ensure MACT control of metal hazardous air pollutants, the rule establishes standards for both semivolatile metals and low volatile metals as well as particulate matter. A particulate matter standard is needed to ensure MACT control of metals because these metals are condensed to the solid, particulate phase in the emission control system. Removing particulate matter removes these metals. In addition, however, we decided that separate emission standards are needed for particularly toxic metals- the 2 semivolatile metals (lead and cadmium) and the 3 low volatile metals (arsenic, beryllium, and chromium). This will help ensure MACT control because hazardous waste can contain virtually any level of these metals. Thus, under a particulate matter standard only, the emitted particulate matter could be comprised of a relatively large percentage of these particularly toxic metals.
 
Note also that emissions of these metals are controlled by controlling the feedrate of these metals as well as particulate matter. Consequently, the emission standards for semivolatile and low volatile metals are based on both a MACT level of feedrate control and particulate matter control.

For more information, see http://www.epa.gov/epaoswer/hazwaste/combust/

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