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18 The types of municipal solid waste (MSW) and sewage sludge byproducts can be classified as: ⢠MSW â Bottom ash â Boiler ash ⢠Combined ash (most common in the United States) ⢠Sewage sludge combustion ash. For clarification, the use of combinations of byproducts in the United States versus the separation of byproducts in Europe might be included in any discussion of these byproducts. Municipal Solid WaSte Background Minnesota defines MSW as any garbage, refuse, and other solid waste from residential, commercial, industrial, and com- munity activities that the generator of the waste aggregates for collection, but does not include auto hulks, street sweepings, ash, construction debris, mining waste, sludges, tree and agri- cultural wastes, tires, lead-acid batteries, motor and vehicle fluids and filters, and other materials collected, processed, and disposed of as separate waste streams (Minnesota Stat- utes § 115A.03, Subd. 21). MSW combustion ash is the end result of burning this waste material in solid waste combus- tion facilities. Figure 5 shows a general schematic of a typi- cal solid waste combustion facility and indicates the MSW byproduct collection locations within the facility. MSW fly ash, as with coal combustion fly ash, is ash removed from the air pollution control system, which consists of the scrubber and fine particle removal system. In the United States, most facilities combine the air pol- lution control system ash byproducts into the combined ash collection location (RMRC 2008). In Europe, most facilities separate and separately manage the MSW bottom ash and MSW fly ash streams. The two basic types of MSW combustion facilities in the United States are mass burn and refuse-derived fuel (RMRC 2008). The mass burn facilities combust unsorted solid waste, whereas the refuse-derived fuel facilities burn preprocessed waste. The preprocessing consists of shredding solid waste and removing ferrous metal and certain non-ferrous metals prior to burning. Currently, about 15% of the total ash frac- tion is recovered metal material and only about 5% of all non- ferrous metal is recovered from the pre-combustion MSW. Because of the difference in the waste streams being burned, the byproduct composition and characteristics will be depen- dent on the type of combustion facility producing the MSW byproducts. Other MSW byproduct differences are associated with the age of the various combustion facilities. The newer facili- ties incorporate more advanced furnace designs and emissions controls. For example, newer facilities will add lime or lime- based reagents into the pollution control system to remove the acid gases from the gas stream. This results in both reacted and unreacted lime in the MSW fly ash. Newer emissions control systems are also more efficient at capturing finer particles in the exhaust gases, which result in changes in the physical and chemical composition of the MSW fly ashes. literature Review Summary The only MSW information found for material handling was related to separating the raw MSW prior to use as a refuse- derived fuel. The only reference to material preparation of sew- age sludge was found in the research that used this byproduct to burn with clay to produce lightweight aggregates. For this usage, the sewage sludge was dried and crushed to 0.15-mm sieve size before sintering (burning) with the clay. A Korean study used dried and crushed sewage sludge and combined it with various percentages of clay in a rotary kiln to produce a synthetic lightweight aggregate. It was interest- ing to note that the chemical composition of the dried sewage sludge was found to be similar to that of clay. The toughness of the synthetic aggregate was as good, or better, than that of commercially available European lightweight aggregate. Most of the trace metals of concern were not detectable in leach- ate testing of the synthetic aggregates. Other testing of MSW focused on the evaluation of heavy metals and increased pH. agency Survey Responses for Municipal Solid Waste Of the 30 states with a potential source for MSW combustion ash byproducts, only Kentucky, North Dakota, and Wisconsin indicated they had experience with using MSW byproducts in highway applications in 2009 (Table 3). Both Kentucky and chapter four non-coal coMBuStion BypRoductS
19 Refuse for burning Boiler Scrubber Stack Boiler Ash Combined Ash Fine Particle Removal System Scrubber Ash Precipitator Ash Turbine Generator Bottom Ash FIGURE 5 Schematic for MSW combustion process (after RMRC 2008). Byproducts Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embank. Flowable Fill HMA Surface Treatment PCC Soil Stability MSW Bottom Ash 0 0 0 2 (KY, ND) 1 (WI) 0 0 0 0 MSW Combination Ash 0 0 0 0 0 0 0 0 0 Combustion Ash, Unknown Type 0 0 0 0 0 0 0 0 0 Embank. = embankment. TABlE 3 SUMMARy OF NUMBER OF STATES USINg MSW BypRODUCTS IN HIgHWAy ApplICATIONS Combustion Air Auxiliary Fuel Scrubber Dewatered Sludge Solids 20 to 25% Volatiles 60% Ash 40% Stack Incinerator Sewer Sludge Ash FIGURE 6 Schematic of sewage sludge combustion process (RMRC 2008).
20 tral rotating shaft. The dewatered sludge, usually with about 20% solids, is introduced into the furnace. Cooling air is used to prevent overheating; spent air is recirculated (i.e., com- bustion air; Figure 6). The flue gases are scrubbed as a part of the air pollution control system that removed the particles in the air flow. The fluidized bed facility configuration con- sists of a vertical cylindrical vessel with a grid in the lower portion to support a bed of sand. The dewatered sludge is introduced into the vessel above the sand bed and combus- tion air flows upward and fluidizes the mixture of hot sand and sludge (RMRC 2008). agency Survey Responses for Sewage Sludge Minnesota was the only agency that reported having explored sewage sludge ash in flowable fill. North Dakota used the byproduct in embankment applica- tions and Wisconsin used it in flowable fill. SeWage Sludge Background Sewage sludge ash is the byproduct generated by the com- bustion of dewatered water treatment plant sewage sludge in one of two types of incinerator facilities. One type of facility is the multiple hearth, and approximately 80% of the sys- tems in the United States are this type. The second type of system, which is less frequently used in the United States, is a fluidized bed configuration (RMRC 2008). The multiple hearth facility is typically comprised of a circular steel fur- nace with a number of solid refractory hearths and a cen-