The majority of Utilities and Agencies in the United States (U.S.) that incinerate sludge treat this practice as a disposal method and do not maximize the recovery of energy available in the sludge and hence, do not contribute to the reduction in greenhouse gas (GHG) emissions. The Metropolitan Council Environmental Services (MCES) have used incineration at the Metro Wastewater Treatment Plant (MWWTP), St. Paul, MN since the 1930s, using multiple hearth furnaces (MHFs) and a variety of conditioning and dewatering technologies. In the late 1990s, MCES began planning the replacement of their MHFs, which were reaching the end of their useful life, as well as a thermal conditioning system. MCES selected incineration as their longterm method of managing solids produced at the MWWTP and took the visionary approach that the incineration system must include energy recovery and electricity production and the process trains must be equipped with state-of-the-art emissions control technology.
The resulting facility went into operation in late 2004. It comprises a new solids management building (SMB) that incorporates seven (7) high solids centrifuges, three incineration processing trains and an electricity generation system. The incineration processing trains include fluid bed incinerators (FBIs), waste heat recovery boilers and state-of-the art air pollution control equipment. With respect to reduction of GHG emissions, the design anticipated that the FBIs would operate autogenously, thus reducing natural gas consumption that was required to operate the MHFs. Steam generated in the waste heat boilers would be used to generate electricity and to provide plant heating, further reducing and displacing fossil fuels. In addition to the long-term environmental benefits, this system was expected to reduce solids processing costs, compared to the previous system.
An analysis of GHG emissions reductions comparing pre-start-up and post start-up operations indicated that the facility reduced MWWTP GHG emissions by about 18%.