Replacement of a Multiple Hearth by a Fluid Bed Incinerator The Ypsilanti Sludge Disposal Case History

The incinerator at the Ypsilanti Community Utilities Authority (YCUA) wastewater treatment plant in Ypsilanti, Michigan was built to replace a 21 year old multiple-hearth, to satisfy the YCUA wastewater treatment plant expansion from 29 MGD to 46 MGD and therefore to comply with the State's updated more stringent air requirements. In 2000, an internal YCUA Committee concluded a study evaluating options for future municipal sewage sludge reuse and disposal. The study included reuse processes: agitated bay composting, pelletization, liquid land applications, alkaline stabilization, and two disposal processes: landfilling of dewatered sludge and incineration. The consensus was to continue incineration, replacing the existing multiple hearth incinerator (MHI) with new fluid bed incinerator (FBI) with modern air pollution control (APC) system. The decision was based on both economic and environmental reasons. Design of the new FBI system began in January 2003 and equipment delivery for the incineration system began in February 2004. Initial operation occurred in January 2006. Primary and secondary sludge, dewatered to 25% dry solids by belt filter presses, is delivered by piston pumps to the 24 foot freeboard ID fluid bed. A shell and tube primary heat exchanger recaptures heat from the exhaust gas and preheats the combustion ambient air to 1230° F, resulting in minimal auxiliary fuel use. The APC devices are a venturi scrubber followed by an impingement cooling tray tower for particulates and acid gas control, a wet electrostatic precipitator to polish particulate emission and a GAC adsorber for mercury and dioxins removal. The FBI system is also equipped with a secondary heat exchanger recovering the heat from the flue gas to preheat ambient air for stack plume suppression and for reheating the moisture laden exhaust gas prior to introduction to the GAC conditioner. Ash coming out of the air pollution control system in a slurry-form is pumped to an existing ash lagoon for settling and dewatering. The system includes a state of the art PLC system for computer control of the operation. YCUA is the first municipal plant in North America to use a GAC adsorber to remove mercury and dioxins from the FBI exhaust gas. Characteristics and performance of the GAC system are discussed in this paper.