Dual digestion is aerobic thermophilic digestion as a first stage followed by mesophilic anaerobic digestion in a second stage. This system has been used extensively in Europe. Residence time in the aerobic digester is typically 18 to 24 hours at the temperature range 55 to 65°C. Residence time in the anaerobic digester is about 10 days. Hydrolysis in the aerobic digester results in increased degradation during subsequent anaerobic digestion and gas production. Advantages of dual digestion are (1) increased levels of pathogen reduction, (2) improved volatile solids reduction, (3) increased methane gas production in the anaerobic reactor, (4) fewer odors produced by the stabilized sludge, and (5) one-third less tankage required than for a single-stage anaerobic digester.
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