A large number of TSDFs use combustion, the controlled burning of substances in an enclosed area, as a means of treating and disposing of hazardous waste. Approximately 11% of the hazardous non-wastewater generated in the United States in 1999 was treated using combustion. As a hazardous waste management practice, combustion has several unique attributes. First, if properly conducted, it permanently destroys toxic organic compounds contained in hazardous waste by breaking their chemical bonds and reverting them to their constituent elements, thereby reducing or removing their toxicity. Second, combustion reduces the volume of hazardous waste to be disposed of on land by converting solids and liquids to ash. Land disposal of ash, as opposed to disposal of untreated hazardous waste, is in many instances both safer and more efficient.
Combustion is an intricate treatment process. During burning, organic wastes are converted from solids and liquids into gases. These gases pass through the flame, are heated further, and eventually become so hot that their organic compounds break down into the constituent atoms. These atoms combine with oxygen and form stable gases that are released to the atmosphere after passing through air pollution control devices.
The stable gases produced by combustion of organics are primarily carbon dioxide and water vapor. Depending on waste composition, however, small quantities of carbon monoxide, nitrogen oxides, hydrogen chloride, and other gases may form. These gases have the potential to cause harm to human health and the environment. The regulation of these emissions is the primary focus of the RCRA combustion unit standards.
The management or disposal of metals and ash, other by-products of the combustion process, also causes concern. Ash is an inert solid material composed primarily of carbon, salts, and metals. During combustion, most ash collects at the bottom of the combustion chamber (bottom ash). When this ash is removed from the combustion chamber, it may be considered hazardous waste via the derived-from rule or because it exhibits a characteristic. Small particles of ash (particulate matter that may also have metals attached), however, may be carried up the stack with the gases (fly ash). These particles and associated metals are also regulated by the combustion regulations, as they may carry hazardous constituents out of the unit and into the atmosphere. Since combustion will not destroy inorganic compounds present in hazardous waste, such as metals, it is possible that such
compounds may also end up in bottom ash and fly ash at harmful concentrations. Ash residue is subject to applicable RCRA standards and may need to be treated for metals or other inorganic constituents prior to land disposal (Figure 12.8).
In the early years of RCRA, U.S. EPA's idea was to combust as much hazardous waste as possible and landfill the resultant ash. This process destroyed the majority of the waste, thus reducing the volume requiring disposal. However, it was determined that incomplete or improperly conducted combustion had the potential to present a major public health risk and therefore, became the topic of much public outcry. This public concern, coupled with U.S. EPA's advancements in assessing potential risks arising from combustion, caused a shift in U.S. EPA's strategy on combustion. This shift in thinking resulted in the increasing stringency of combustion requirements over time.
In September 1999, U.S. EPA issued a joint CAA/RCRA rule that upgraded the emission standards for hazardous waste combustors, based on the maximum achievable control technology (MACT) approach commonly employed under the CAA. This process develops technology-based, emission limits for individual hazardous air pollutants. Much like the BDAT concept for LDR, the MACT emission standards are based on the performance of a technology. U.S. EPA researches available pollution control technologies to determine which available technology is the best at controlling each pollutant to determine allowable emission limits. The regulated community may then use any technology to meet the numeric emission standards set by U.S. EPA.
Consistent with U.S. EPA's trend of gradually increasing the stringency of standards over time, this joint rule promulgated more stringent emission standards for dioxins, furans, mercury, cadmium, lead, particulate matter, hydrogen chloride, chlorine gas, hydrocarbons, carbon monoxide, and several low-volatile metals. After the promulgation of this rule, a number of parties representing the interests of both industrial sources and the environmental community requested a judicial review of this rule.
In July 2001, the U.S. Court of Appeals for the District of Columbia Circuit vacated the challenged portions of the rule. When it made its decision, the Court invited any of the parties to request either that the current standards remain intact or that U.S. EPA be allowed time to publish interim standards. Acting on this initiative, U.S. EPA and the other parties jointly asked the Court for additional time to develop interim standards, and the Court granted this request. On February 13, 2002, U.S. EPA published these interim standards that temporarily replace the vacated standards. The interim standards will remain in place until U.S. EPA issues final "replacement" standards that comply with the Court's opinion. U.S. EPA has also completed other actions agreed to in the joint motion, such as extending the compliance date by one year to September 30, 2003, and finalizing several amendments to the compliance and implementation provisions by February 14, 2002.
Hazardous wastes are combusted for various purposes. The purpose of combustion is directly related to the type of unit used. There are two classes of combustion units, those that burn waste for energy recovery and those that burn waste for destruction.
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