Two streams of ash result from the combustion of municipal solid waste: fly ash and bottom ash. Studies have consistently found fly ash to contain levels of heavy metals, dioxins, acid gas constituents and other pollutants in excess of regulatory standards, while the bottom ash stream is generally within acceptable limits for those same pollutants.
Research and development activities focused on the reuse of bottom ash as a lightweight aggregate have shown very promising opportunities for recycling a portion of this waste stream. The industry approach thus far has been to combine the two streams to assure that the fly ash is not classified as a hazardous or toxic material under present and proposed Federal Environmental Protection Agency (EPA) standards. However, with the increasing regulatory demand for the removal of hydrochloric acid (HC1) and sulfur dioxide (SO2), the technology for the removal of potentially harmful substances from waste conbustion fuel gases will improve, and more heavy metals, dioxins, and so forth will be concentrated in fly ash, rendering it less useful in recycling applications.
It is suggested that the two separate ash streams can no longer reasonably be considered one and should be analyzed and treated separately. This will increase and enhance the opportunity to recycle the bottom ash (75%) and to focus on a reduced quantity of fly ash material (25%) for fixation and disposal. Such action would conform to the principles of, and the present regulatory trend towards, waste minimization.
Implementation of both a processed refuse fuel (PRF) bottom ash reuse study being conducted by Energy Answers Corporation, and preliminary results of a fly ash fixation process will be discussed as a rationale for mandatory separation of fly ash and bottom ash streams.