How it works

Hover over the numbered circles below to view photos and learn about how solids are processed at the Metro Plant.

  • 1

    Centrifuge

    How it Works: Centrifuge

    Thickened sludge is pumped into a dewatering centrifuge which spins at 2,600 rpm to increase solids concentration from 5% to 28% to a consistency similar to moist soil.

  • 2

    Cake Bin

    How it Works: Cake Bin

    Dewatered “cake” falls into a cake bin. A sliding frame and an extraction screw conveyor feeds cake into the cake pump.

  • 3

    Cake Pump

    How it Works: Cake Pump

    A hydraulically powered piston pump feeds cake through pipes to the fluidized bed incinerator.

  • 4

    Polymer

    How it Works: Polymer

    Emulsion or mannich polymers make the thickened sludge particles adhere to each other assisting in the dewatering process in the centrifuges.

  • 5

    Fluid Bed Incinerator

    How it Works: Fluid Bed Incinerator

    The cake combusts at a temperature of 1,375 degrees Fahrenheit in a bubbling sand bed. Combustion reduces the volume of cake by 95% and eliminates bacteria. The cake has enough energy that the incinerator normally burns cake without the need for supplemental natural has fuel.

  • 6

    Primary HEX

    How it Works: Primary Heat Exchanger

    Hot flue gas leaving the incinerator is recovered to preheat the fluidizing air entering the bottom of the incinerator.

  • 7

    Waste Heat Boiler

    How it Works: Waste Heat Boiler

    The waste heat boiler recovers heat from the flue gas by converting water pumped through hundreds of metal tubes into steam.

  • 8

    Secondary HEX

    How it Works: Secondary Heat Exchanger

    Powdered activated carbon is injected into the flue gas to remove mercury.

  • 9

    Carbon Tower

    How it Works: Carbon Tower

    Carbon is injected into the flue gas to remove mercury.

  • 10

    Baghouse

    How it Works: Baghouse

    The baghouse uses 816 filter bags to remove particulates which include injected carbon and heavy metals. The particles collected on the outside of the bags fall to bottom in the form of ash.

  • 11

    Wet Scrubber

    How it Works: Wet Scrubber

    Water is sprayed into the wet scrubber to cool the flue gas and remove remaining particulates. Caustic is added to neutralize acid gases.

  • 12

    Wet ESP

    How it Works: Wet Electrostatic Precipitator

    The wet electrostatic precipitator uses electrically charged metal rods to remove any remaining very fine particulates and heavy metals from the flue gas.

  • 13

    Stack

    How it Works: Stack

    Emissions leaving the stacks are clean, odorless, colorless, and have no visible plume.

  • 14

    Fluidizing Air Blower

    How it Works: Fluidizing Air Blower

    The fluidizing air blower pushes air through the primary heat exchanger and 1,300 tuyeres to fluidize the sand inside the incinerator.

  • 15

    Induced Draft Fan

    How it Works: Induced Draft Fan

    The induced draft fan pulls the flue gas from the incinerator, energy recovery, and air pollution control systems and pushes the clean flue gas through the secondary heat exchanger and out the stack.

  • 16

    Ash Conveyance

    How it Works: Ash Conveyance

    Ash transporters under the baghouse and waste heat boiler collect ash and use compressed air to push ash to storage silos.

PROJECT OVERVIEW

Metro Plant Solids Management Facility Plan

 

The Metro Plant, located southeast of downtown Saint Paul, treats 180 million gallons of wastewater every day for 66 communities and processes 850 wet tons of solids every day for 73 communities.  The Metro Plant needs additional solids processing capacity to preserve existing wastewater treatment plant infrastructure and serve regional population growth.  MCES proposes to construct a fourth incinerator, found to be the most cost-effective and sustainable alternative to meet the region’s wastewater needs.  

How it Works

Processing Wastewater Solids at the Metro Plant


Wastewater solids are a renewable, carbon-based fuel source.  The goals of processing wastewater solids are to reduce volume, recover energy, and reduce or eliminate pathogens.  Solids at the Metro Plant are collected in settling tanks, thickened, and then dewatered before being sent to incineration. 

How it Works

The Incineration Process at the Metro Plant


The Metro Plant has been processing wastewater solids with incineration since 1938 and with fluid bed incinerators since 2005. Incineration of wastewater solids is an efficient thermal combustion process that reduces the quantity of solids by 95% and eliminates pathogens.

How it Works

Energy Recovery for Incineration at the Metro Plant


The Metro Plant incineration facilities can take what most people consider a waste and recover valuable energy from it. The energy recovered is used within the plant and reduces our utility bills by $2.5 million per year, ultimately passing those savings on to our customer communities.

How it Works

Air Pollution Control for Incineration at the Metro Plant


The Metro Plant incineration facilities have one of the most advanced, highest performing, and state-of-the-art air pollution control systems in the country. The existing incinerators have an exceptional track record of environmental compliance. They operate at well below the new, most stringent standards established by the U.S. Environmental Protection Agency for new incinerators.