Most high-rate digesters are operated in the range 30 to 38°C (86 to 100°F). Bacteria that grow in this temperature range are called mesophilic. Another group of microorganisms, called thermophilic bacteria, grow in the temperature range 50 to 57°C (122 to 135°F). Thermophilic anaerobic digestion has been studied since the 1930s, at both the laboratory and plant scales (U.S. EPA, 1979).
In general, advantages claimed for thermophilic anaerobic digestion over mesophilic digestion are:
• Faster reaction rates, which permit increased volatile solids destruction
• Increased destruction of pathogens
Disadvantages of thermophilic anaerobic digestion include:
• Higher energy requirements for heating
• Lower-quality supernatant, containing large quantities of dissolved materials
• Higher odor potential
• Poorer process stability because thermophilic bacteria are more sensitive than mesophilic bacteria to temperature fluctuations
• Poor dewaterability
U.S. federal regulations controlling land application of sludge classify ther-mophilic digestion, along with mesophilic digestion, as a process to significantly reduce pathogens (PSRP). That is, although there may be greater reduction of pathogens levels in thermophilic digestion, it is not classified as a process to further reduce pathogens (PFRP). Therefore, single-stage ther-mophilic digestion is limited in its application.
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