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The Metropolitan Council Environmental Service (MCES) owns and operates eight wastewater treatment plants in the Greater Metropolitan Minneapolis/St. Paul area. The largest of these, the Metropolitan Wastewater Treatment Plant (MWWTP), has been incinerating sewage sludge for six decades, using multiple hearth incineration technology developed over a century ago. The MWWTP treats approximately 225 million gallons per day (mgd) on an average day and processes approximately 200 tons of dry solids. Most equipment components of the existing solids processing system, which include thermal conditioning, dewatering and multiple hearth incineration, are 20 years old. Due to the age of the equipment, MCES is faced with increasing maintenance costs required to keep equipment operational and continue meeting regulatory requirements.

In 1993, MCES embarked on a program to chart the MWWTP's wastewater treatment course for the next 20 years and beyond. The first step in the program was the development of a Master Plan (MP) to determine future needs for treating wastewater and solids in the MWWTP service area. The MP reviewed and evaluated 12 solids management alternatives and determined that fluidized bed incineration was the preferred method of solids management, with heat drying and alkaline stabilization as other options. The next step was to prepare a Facility Plan (FP) to confirm the technology selection, develop the conceptual design of the preferred alternative to a sufficient level for obtaining an air emissions permit amendment, and prepare a permit application for that amendment.

The preferred alternative recommended by the FP consisted of fluidized bed incineration as the primary process with alkaline stabilization for processing peak solids production and production during incinerator maintenance.

Air emissions from the fluidized bed incinerators will be controlled through a system of air pollution control equipment designed to remove greater than 99 percent of all particulate matter. The air pollution control system will also achieve 70 percent mercury removal via an activated carbon injection system in combination with the high level of particulate removal. The fluidized bed incinerators will operate without the use of supplemental fuel to burn the sewage solids, except for the initial heating required to start the incinerators. Indeed, sewage solids contain sufficient BTUs to burn without supplemental fuel when dewatered to 30 percent dry solids, and will provide excess energy for facility heating and electricity production. The process achieves the goal of reducing greenhouse gas emissions, and is more sustainable than those alternatives dependent upon natural gas fuel to stabilize sewage solids. The beneficial reuse of incinerator ash continues to be the MCES goal. Further, the biosolids product from the alkaline stabilization system will be applied to agricultural land as a soil amendment.

The new facility will include sludge storage and blending, centrifuge dewatering, and sludge cake pumps to feed either the fluidized bed incinerators or the alkaline stabilization facility.

This paper will identify the design criteria established for this project and the steps required to obtain an air permit amendment for the new facility. The paper will discuss details of the solids processing system and will highlight the innovative features of the incineration and air pollution control train. There are some 120 publicly owned treatment works (POTWs) in the US that operate multiple hearth incinerators, many of which will require replacement over the next two decades. The state-of-the-art fluidized bed incineration and air pollution control process described in this paper will be of interest to POTWs who find themselves in similar positions.
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Document Type: Research Article

Publication date: 2002-01-01

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