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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.