Modern WTE plants use sophisticated air pollution-control systems to ensure the air-pollutant levels are much lower than those of past waste-combustion methods. EPA regulates emissions from WTE facilities under the Clean Air Act.

Currently, 75 facilities in the United States recover energy from the combustion of municipal solid waste. These facilities exist in 25 states, mainly in the Northeast. A new facility was built in Palm Beach County, Florida, in 2015 (EPA 2025).

The amount of ash generated ranges from 15-25 percent (by weight) and from 5-15 percent (by volume) of the MSW processed. Generally, MSW combustion residues consist of two types of material: fly ash and bottom ash. Fly ash refers to the fine particles removed from flue gas and includes residues from other air pollution control devices, such as scrubbers. Fly ash typically amounts to 10-20 percent by weight of the total ash. The remaining MSW combustion ash is called bottom ash (80-90% by weight). The main chemical components of bottom ash are silica (sand and quartz), calcium, iron oxide, and aluminum oxide. Bottom ash usually has a moisture content of 22-62 percent by dry weight. The chemical composition of the ash varies depending on the original MSW feedstock and the combustion process. The ash that remains from the MSW combustion process is sent to landfills (EPA 2025).

Energy recovery from waste is essential in the development of sustainable energy policies. EPA continues to develop regulations that encourage energy recovery from materials that might otherwise be disposed of as solid waste. The 2011 non-hazardous secondary material (NHSM) final rule under the Resource Conservation and Recovery Act (RCRA) identifies which non-hazardous secondary materials are, or are not, solid wastes when burned in combustion units. This determines which Clean Air Act emission standards a combustion unit must meet (EPA 2025).

Waste minimization, as defined in the RCRA statute, includes both source reduction and certain environmentally sound recycling practices. EPA's highest priority is to achieve reductions through source reduction. However, if this is not achievable, then environmentally sound recycling is also an Agency priority Conventional waste treatment activities (such as burning for energy recovery) for municipal solid waste do not constitute waste minimization or recycling. Also, treatment for destruction or disposal is not part of waste minimization; rather, it is an activity that occurs after waste minimization opportunities have been pursued. (EPA 2025).

If MSW combustor ash exceeds the toxicity characteristic regulatory limit at Title 40 of the Code of Federal Regulations Section 261.24 using the Toxicity Characteristic Leaching Procedure (TCLP), it is identified as a hazardous waste due to the risks it poses to groundwater contamination under a worst-case mismanagement scenario. Non-hazardous MSW combustor ash may still pose risks via other pathways, such as inhalation, ingestion, or dermal (skin) contact. These risks should also be considered during transport, disposal, and/or beneficial reuse of the ash as a non-hazardous secondary material (EPA 2025). WTE facilities have established operational procedures that allow for the reuse of ash from combustion while protecting the health and safety of the staff, the surrounding community, and the environment.