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                       238                       Waste Management Practices: Municipal, Hazardous, and Industrial
                       8.12 MSW COMPOSTING BY ANAEROBIC PROCESSES
                       Anaerobic digestion of low-solids (4 to 10%) wastewater has been carried out for decades at pub-
                       licly owned treatment works and in industrial facilities. A number of waste management facilities
                       in the United States and Europe, however, now employ the so-called high-solids reactors, contain-
                       ing up to 30% or greater solids content. This technology allows for the anaerobic digestion of high-
                       solids MSW, specifically the organic fraction.
                          Anaerobic digestion is described by the following equation:

                         organic MSW   H O → CO   CH  NH   H S   mixed solids   new cell biomass  (8.8)
                                         2      2      4    3   2
                          The desired end products of this process include methane and sludge water. Other products are
                       carbon dioxide and trace quantities of ammonia and hydrogen sulfide. The sludge water is dewatered
                       to produce a supernatant and a filter cake. The filter cake serves as a soil conditioner. The filtrate can
                       be mixed with the organic MSW to create a slurry feedstock or it can be fed directly to the digester.
                       The liquids can also be used as fertilizer. A simple schematic of anaerobic digestion of MSW appears
                       in Figure 8.13.
                          High-solids anaerobic digestion (HSAD) of the MSW organic fraction occurs in three phases:

                         1. Hydrolysis. High-molecular-weight compounds are converted into low-molecular weight
                             compounds by microbial action (e.g., hydrolyzing bacteria); for example, polysaccha-
                             rides are hydrolyzed to monosaccharides, lipids to fatty acids, proteins to amino acids,
                             and nucleic acids to purines and pyrimidines. These products subsequently serve as sub-
                             strate for new populations of microorganisms.




                          Municipal       Food                         Municipal
                            solid       processing      Industrial      waste-
                           waste          waste          waste          water


                           Shred                       Pretreatment  Pretreatment


                                                Feed
                                               mixing
                                                tank
                                                                          Drying    Gas        Pipeline
                                             High-solids                            scrubber   methane
                                              anaerobic
                                                                Biogas
                         Liquid                digester
                        recycling                                         Flare

                                               Digested
                                                sludge
                                                           Compost
                                                storage
                                                 tank


                                                Liquid
                                               fertilizer
                       FIGURE 8.13 Schematic for a high-solids anaerobic digestion facility.
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