CO-DISPOSAL Heat energy, generated by incineration of sludge and prepared solid waste, is recovered and converted to steam. High pressure steam is used to power certain process equipment and air pollution control devices. Low pressure steam generated by this process is used to heat the plant. This process conserves over 3,000,000 gallons of fuel oil annually. 1. Fluidized bed incineration: Sludge and solid waste are burned in the fluidized bed incinerators. These large furnaces have a sand bed suspended in the incinerator by a powerful air blower. 1a. Over 15,000 cubic feet per minute of air drawn from the odorous air collection system is used as combustion air. The odorous air is collected from the high-strength odor sources throughout the plant. 1b. Refuse Derived Fuel, RDF, is pneumatically transported into the fluidized bed at four points. Back-up fuel, (waste-wood chips) is used to supplement fuel needs when required. The combination of high air velocities and high temperature makes the special sand bed "fluid." Fuel and air is turbulently mixed for complete combustion. 1c. High temperature combustion gases and ashes from incinerated sludge and solid waste pass through reactor to the ash collection system, air pollution control equipment and energy recovery system The sand bed also acts as a heat reservoir to maintain the incinerators' operating temperature between 1400 and 1600 degrees Fahrenheit. 2. Energy recovery system: Heat energy generated by the incineration of refuse and sludge is captured in the waste heat boiler. In the boiler, heat energy is converted to high pressure steam to power equipment drive turbines. Over 1600 horsepower in electrical savings is possible. Low pressure turbine exhaust is used to heat the plant.

SOLID WASTE PROCESSING The biological wastewater treatment process generates 200 wet tons of sludge a day. The District prepares raw municipal refuse (garbage) for use as a fuel to incinerate sewage sludge, a process which conserves fossil fuels and reduces overall operating costs. 1a. Gatehouse: All solid waste delivered for processing in commercial packer trucks is weighed. The haulers pay tipping fees based upon weight. The tipping fee pays much of the costs to process solid waste, operate the landfill and other programs. 1b. Household hazardous waste building: The Regional Household Hazardous Waste program operates this facility receiving and processing hazardous waste from resident's homes. 2. Tipping floor: Mixed municipal solid waste is processed to make a fuel for sludge incineration. Garbage from households and businesses is dumped onto the tipping floor. A loader is used to "feed" refuse into the infeed conveyor. 3. Primary disc screen: The first processing step removes sand, glass and other undesirable ash forming materials from the refuse prior to further processing. Rejected materials are transported to the refuse waste disposal facility for burial. Materials pass a visual picking station before entering the shredder. 4. Shredder: Refuse from the primary disc screen is conveyed to a 1,000 H.P. shredder which reduces 90% of the refuse into particles that are less than 4 inches in size. 5. Magnetic separator: Ferrous metals are pulled from the shredded waste stream for recycling. 6. Air knife and sizing disc screen: Refuse from the shredder is passed through an air knife to remove heavy non-combustibles and then through a sizing disc screen to remove oversized materials. The end product of this process is called refuse derived fuel or RDF. 7. RDF storage silo: Refuse derived fuel or RDF is then fed into the storage silo. Fuel is reclaimed as needed in the fluidized bed incinerators.

Incineration of Municipal Solid Waste, MSW along with sludge requires a fluid bed incinerator similar to the one processing sludge alone but with the following additions:

1. a sorting system for rejection of non-combustible waste to land-fill or recycle
2. a shredder system to reduce size of combustible material to less than 4"
3. a storage and feeding system for refuse derived fuel
4. a flue gas cleaning system treating for acids, heavy metals, dioxin etc.

Refuse is shredded to less than 4" particle size after removal of most non-combustibles. This Refuse Derived Fuel, RDF replaces auxiliary fuel required to incinerate sludge because of it's high water content. Both sludge and refuse are fed to the fluidized bed via the reactor freeboard where a near zero pressure is maintained by an induced draft fan. This fan can also discharge the flue gas to the atmosphere via a heat exchanger which heats flue gas and eliminates any plume from the stack.

Data obtained from the Western Lake Superior Sanitary District of Duluth, Minnesota shows that 35 Dry tons per day of sewage sludge along with 130 tons per day of RDF were continuously disposed of using an 18' diameter fluidized bed incinerator. It required 15,000 cfm of air through the bed with about 1000 cfm flowing over the bed for secondary combustion.

Although metals were removed from the refuse, metal build up occurred in the fluidized bed and would cause defluidization if not removed. Removal of 30 tons per day of bed material maintained an acceptable metal concentration in the bed. This material was cooled, screened and sent back to the reactor. The removed lumps consisted of bed material (sand particles) bound together with melted metal.